Unless stated otherwise the seminars take place in Physics C12 at 15:45 with the student session in CAPT A113 at 15:00. Click on an event for more information.
The standard model of cosmology is very successful in describing many key observations with only very few parameters and at high significance. However, this phenomenological description of the Universe is no real attempt to understand the true nature of its most dominant components today, dark matter and dark energy. More importantly, tension between theoretical predictions and observations arises when entering the astrophysical realm of dark matter haloes that decoupled from the Hubble flow. The observed mass function of galaxy clusters, the observed density profiles and substructure contents of objects spanning all mass ranges are not quite in line with theoretical predictions. This is why we are most interested in the non-linear regime of structure formation. While for many on-going and planned cosmological surveys this regime is a mere nuisance which needs to be controlled in order to obtain unbiased cosmological parameters, its detailed study is also one of the most promising ways to achieve deeper insight into the mechanisms of structure formation. In my presentation I will show how we can use gravitational lensing to unveil and study non-linear structure in the sky. I will present novel mass mapping techniques which combine many different probes into a joint reconstruction of the density field. In order to optimally characterise such maps beyond the usual approach of 2-point statistics and 1D parametric density profiles, we deploy a number of machine learning techniques from the field of computer vision, which allow tighter inference on the most degenerate cosmological models. All these techniques are based on the use of modern many-core computational architectures (such as GPUs), which I will also introduce, and which are able to harvest upcoming large data sets efficiently. I will present results from earlier lensing surveys such as CLASH and show our current efforts within the on-going KiDS survey. I will end my presentation with our plans to fully exploit the potential of the upcoming Euclid satellite.
Accretion discs around rotating black holes are often assumed to be aligned with the black hole spin. Relaxing this assumption by inclining the disc means that the rotation of the black hole is able to affect the disc evolution, leading to previously unexplored structures in the disc. I will present three-dimensional simulations investigating the evolution of such tilted discs at a range of inclinations. In the small inclination case we identify an oscillatory disc profile, in agreement with previous analytical predictions. In the high inclination case we confirm disc breaking and tearing, where the disc is torn in independently precessing rings of gas. I will then consider applications of these structures to observational features, considering the currently unexplained phenomenon of quasi-periodic oscillations. Turning to geometrically thick accretion flows we consider a torus (with a donut shaped profile), such as would form from a tidal disruption event. Simulating a torus in three-dimensions is difficult as they may be unstable to the non-axisymmetric hydrodynamic Papaloizou-Pringle instability. I will present our simulations of an idealised, circularised torus that has similar parameters to a previously simulated tidal disruption remnant and show that i) it is unstable to this instability and ii) this instability is capable of driving angular momentum transfer. Our simulations imply that the initial accretion rate (and hence lightcurve) of tidal disruption events may be constrained by this instability rather than the magneto-rotational instability in the case of extremely weak initial magnetic fields.
Species of animals are disappearing from Earth at a rate not seen since the extinction of the dinosaurs. Protecting biodiversity is vital for a multitude of environmental and societal reasons. To form effective conservation strategy detailed understanding of wildlife populations is needed. However ecosystem knowledge is still commonly based on data gathered through foot surveys. Drones offer a major advantage for ecosystem surveying, animal tracking and monitoring for poachers as they can survey large areas or difficult terrain quickly with minimal disturbance to wildlife. But this comes with its own challenges, such as optimizing observing strategy and analysing the ever increasing quantity of data produced. In a world first collaboration between astronomers and ecologists, we are adapting techniques used in astrophysics to help revolutionise conservation ecology through the use of thermal equipped drones. I will discuss the solutions we are developing and some of our recent successes.
The current frontier in cosmology is the charting of the accelerating expansion rate of the universe with time, and thereby characterising the phenomenology of dark energy. Doing so requires ambitious new surveys of galaxies over unprecedented volumes. The Dark Energy Survey is the largest ever galaxy survey and hopes to unlock the potential of gravitational weak lensing, amongst other methods, as a probe for cosmology. In this seminar, I will cover the early constraints that have been achieved on cosmology parameters from science verification data taken with the new DECam. A particular challenge for weak lensing analysis is in understanding the distances to the millions of survey galaxies. I will cover our approach taken during the science verification analysis and the future challenges we face in this aspect of the experiment.
The Epoch of Reionization signals the end of the Dark Ages of the Universe and the birth of the first stars. The race is on to make the first statistical detection of this epoch however the foregrounds swamp the cosmological data by several orders of magnitude and their removal remains a significant challenge for both current and future telescopes. I will speak broadly about the foreground mitigation techniques currently being used with EoR data and take a closer look at the efforts being made by LOFAR with blind foreground removal methods.
Lyman Break Galaxies (LBGs) are the largest population of high-redshift (z > 3) star forming galaxies and as such provide valuable insights into the mass assembly of normal galaxies during the first few Gyr of the Universe. They have moderate star formation rates (~10s-100s Msun/yr) and ~10-100x higher space densities than the more extreme submillimetre galaxy population, thus they should comprise a significant portion of the far-infrared background at high redshift. But determining their true contribution to the far-infrared background has been difficult due to the rather large uncertainties that go hand-in-hand with deriving dust corrected UV luminosities and star formation rates. The only reliable way of measuring the dust content of LBGs is directly through submillimetre observations. In this talk I will present some new results on LBGs selected at z~3, 4, and 5 from a series of different studies using SCUBA-2, Herschel and ALMA.
I will present new results regarding the first ~2 Gyrs of cosmic time using very wide-field Lyman-alpha (Lya) narrow-band surveys, including a large, matched Lya-Halpha survey to investigate how Lya and Lyman-continuum (LyC) photons escape from typical star-forming galaxies at high-redshift. We find that large Lya halos are ubiquitous in star-forming galaxies, and that the typical escape fraction of Lya and LyC photons is typically below a few percent. However, the escape fractions of Lya selected sources are significantly higher. We also find a much higher space density of very luminous Lyman-alpha emitters all the way from z~2 to z~7 than previously assumed, which we confirm spectroscopically with Keck, VLT and WHT. Many of our sources show high-ionisation lines in the rest-frame UV (CIII], CIV, HeII), and some have clear Lya blue wings. At z~7 our sources (e.g. CR7) show signatures of PopIII-like stellar populations (extremely metal poor) and/or direct collapse black holes and provide interesting challenges ahead of the launch of JWST. Our results also show that the steep drop in the Lya luminosity function into the epoch of re-ionisation happens only for the faint Lya emitters, while the bright ones likely ionise their own local bubbles very early on, and thus are visible at the earliest cosmic times.
I will briefly recap the motivation for, and progress towards, numerical modelling of the formation and evolution of the galaxy population - from cosmological initial conditions at early epochs through to the present day. I will focus in particular on the EAGLE simulations. They represent a significant development in this arena, since they broadly reproduce key properties of the evolving galaxy population, and do so using energetically-feasible feedback mechanisms. I shall present a broad range of results derived from EAGLE analyses, concerning the evolution of galaxy masses, their luminosities and colours, and their atomic and molecular gas content. I hope to convey some of the strengths and limitations of the current generation of numerical models.
The far-infrared Herschel Space Observatory has opened our eyes to the cold dusty Universe. Far-IR wavelengths provide arguably the best tracers for star-formation in active galactic nuclei (AGN), since luminous nuclear activity is rather inefficient at keeping dust cold. I will report on studies that bring together the very best modern multi-wavelength survey datasets, from the X-rays to the optical to the far-IR, aimed towards developing a coherent view of the growth of supermassive black holes (in AGN) in relation to the growth of stellar content in galaxies (through star-formation). These studies build on the newest advances in our knowledge of galaxy evolution across most of the Universe's history. I will demonstrate that a positive relationship between star-formation and AGN activity is now clearly seen to z > 2. However, the nature of this relationship supports weak or stochastic co-evolution, driven more by the smooth increase of gas content in normal galaxies over time rather than a dominant role of short, intense episodes, such as star-bursts or mergers. This has important implications for the connections between galaxies and the black holes that reside at their hearts.
Galaxies contains dust, and this complicates our view of the stellar populations in optical and UV light. The position of the Sun close to the galactic plane does not provide a better viewpoint of our own galaxy compared to external galaxies - on the contrary, the situation is even worse. Nonetheless, both the outer and inner view of galaxies are needed. A powerful route to the inner make-up of galaxies is to decode their panchromatic spectral energy distribution, taking into account the direct and dust-reradiated components of stellar light. In this talk I will show recent progress in self-consistently modeling radiative processes in galaxies and in the Milky Way, including the high energy gamma-ray emission, in an attempt to lift the veil on our understanding of the fundamental process of galaxy formation and evolution.
Cosmological simulations predict that massive dark-matter haloes are home to a spectrum of smaller, less massive haloes and gaining access to these low mass haloes gives important clues to the fundamental nature of dark matter. Measurements of the subhaloes are naturally only possible through observations of baryonic tracers. I will discuss two different tracers that will shed some light on the properties of the Local Group’s subhalo population. First, tidal streams are dynamically cold structures that are sensitive to close encounters with dark matter subhaloes. I will discuss recent progress in the modelling of tidal streams, how gaps in streams caused by subhaloes can be efficiently modelled and the current constraints from data. Secondly, many of the larger subhaloes are expected to be the hosts of the dwarf spheroidal (dSph) galaxies in the Local Group. I will discuss progress on non-spherical modelling of dSphs and how flattening can enhance or diminish the expected dark-matter annihilation signals from dSphs. I will close by discussing recent results on the shapes and alignments of the Local Group dSph population.
Over the past ~20 years the high-redshift Universe has been increasingly opened to scrutiny at far-infrared wavelengths, where cool dust emission from star-formation dominates. The dusty star-forming galaxies (DSFGs) and submillimeter galaxies (SMGs), selected at these wavelengths likely represent an important, but short-lived phase in the growth of massive galaxies. These DSFGs often have star-formation rates in excess of ~1000 solar masses per year and are confirmed beyond z~6, although their redshifts and high dust contents make them faint and difficult to study at other wavelengths. Using data from the Herschel Space Observatory I have obtained mid-infrared spectra of SMGs, probing the conditions in their ISM. I will present the results from this spectroscopy program as well as discuss other results from our systemically-selected sample of gravitationally lensed SMGs.
Distant galaxy clusters are powerful laboratories for observing the hierarchical growth of large-scale structure, constraining cosmological parameters, and for studying the formation of galaxies. However, distant (z>1.5) clusters are extremely rare and faint, so locating and studying them poses a significant observational challenge. In this seminar, I will review the theory of cluster formation and present recent advances we have made in detecting and studying distant galaxy clusters.
Universities and funding bodies now tend to agree that public engagement is something to be encouraged. But delivering successful public outreach while holding down a research career isn’t always an easy balance to strike. Marek Kukula, Public Astronomer at the Royal Observatory Greenwich explores some of the ways in which astronomers can engage with a wide variety of audiences including the general public, schools, specialist groups and the media, and looks at how museums and galleries can help to provide a platform for researchers via exhibitions, talks and cross-disciplinary approaches from high art to popular culture. He'll also try to address the question of how public engagement can help early-stage researchers to develop their careers both in outreach and in academia and, time permitting, will reveal how he teamed up with writer Simon Guerrier to explore The Scientific Secrets of Doctor Who.
Biography: Marek completed his PhD in Radio Astronomy at the University of Manchester's Jodrell Bank Observatory and then carried out research into quasars and galaxy evolution at Liverpool John Moores University, the University of Edinburgh and the Space Telescope Science Institute in Baltimore. He subsequently worked for the University of Edinburgh’s Office of Lifelong Learning and as Project Manager for Researchers in Residence, a UK-wide scheme to support early-career academics to work with secondary schools. As the Public Astronomer at the Royal Observatory Greenwich he is part of a team of astronomy education and outreach specialists who engage with the public, schools and the media. The role involves working with scientists, historians and artists to explore the cultural as well as the scientific impact of astronomical research. Marek is a judge on the Insight Astronomy Photographer of the Year competition and has curated several exhibitions, including Visions of the Universe, on the history and impact of astrophotography, at Royal Museums Greenwich in 2013 and dark frame/deep field at the Breese Little gallery in 2015, juxtaposing the work of six contemporary artists with vintage NASA photographs. He is the author of The Intimate Universe and the co-author of The Scientific Secrets of Doctor Who, both published in 2015.
I will give an overview of what we learned in our understanding of galaxy evolution thanks to integral-field spectroscopy.
Active galactic nuclei are among the most powerful astronomical objects in the universe. Though usually located in the core of their host galaxies, their feedback has the potential to influence the dynamics and energetics of their hosts on large scales, even affecting their star formation history. Radio-loud sources, in particular, can efficiently transport energy, through relativistic jets, up to megaparsec scales, influencing cluster gas dynamics and cooling times. I will try to summarise the properties of these objects, why they are important to understand galaxy and cluster evolution, and what surveys and methods we are currently using to study them.
Emission line galaxies trace the cosmic star formation history and moreover, they are being used as tracers of the dark matter distribution up to z~1. We have use a state-of-the-art model of galaxy formation and evolution to study the properties of model emission line galaxies, in particular [OII] emitters. To mimic a range of surveys, we select model [OII] emitters by making cuts in [OII] line flux, optical magnitudes and colours. The model [OII] emitters have luminosity functions in reasonable agreement with observations at z~1. 95% of these model galaxies are centrals hosted by haloes with M halo > 10^11 Msun/h . In the model very few haloes contain more than one [OII] emitter that is a satellite galaxy. Confronting this result to available observations, suggests the need to revise the modelling of hot gas stripping in satellite galaxies. The mean halo occupation distributions of these model galaxies have a global shape typical of star forming galaxies. For central [OII] emitters we have identified that we can split the contribution of central galaxies to the mean halo occupation distribution into an asymmetric Gaussian for central disks and a step function plateauing below one for central spheroids.
The "Lambda cold dark matter" (LCDM) cosmological model is one of the great achievements in Physics of the past thirty years. Theoretical predictions formulated in the 1980s turned out to agree remarkably well with measurements, performed decades later, of the galaxy distribution and the temperature structure of the microwave background radiation. Yet, these successes do not inform us directly about the nature of the dark matter. Indeed, there are competing (and controversial) claims that the dark matter might have already been discovered, either through the annihilation of cold, or the decay of warm, dark matter particles. In astrophysics the identity of the dark matter manifests itself clearly in the properties of dwarf galaxies, such as the satellites of the Milky Way. I will discuss predictions from cosmological simulations assuming cold and warm (in the form of sterile neutrinos) dark matter and show how astronomical observations can conclusively distinguish between the two.
Galaxy clusters are the largest gravitationally bound structures in the Universe, and we know that early type galaxies (ETGs) are more common towards their centres. Clusters of galaxies are increasingly rare at early times, but are essential for understanding the formation of these massive structures and how they alter the fate of their member galaxies. However, long integration times on large telescope are required to constrain the stellar properties of these distant cluster ETGs. Now with the advent of the multiplexed near-infrared integral field instrument, the K-band Multi-Object Spectrograph (KMOS) on the Very Large Telescope, we can target the ETGs in these valuable high-redshift clusters more efficiently than ever. The KMOS guaranteed observing program, the KMOS Cluster Survey (KCS; P.I.s Bender & Davies), has enabled a study of cluster galaxies in overdensities spanning z = 1–2 through absorption-line spectroscopy obtained from 20-hour integrations. I will present KMOS spectra for 16 galaxies and Hubble Space Telescope photometry of the furthest KCS overdensity, JKCS 041, an ETG-rich cluster at z = 1.80. To determine stellar ages of the cluster galaxies, a scaling relation called the fundamental plane (FP) was constructed for seven galaxies in JKCS 041; the highest redshift FP for ETGs in a single cluster. The relative velocities of the galaxies derived from the spectra indicated an infalling group of galaxies, and we determined that these were significantly younger than the rest of the cluster members. Based on the galaxy dynamics, cluster morphology, and galaxy stellar age results, we concluded that JKCS 041 is in formation and consists of two merging groups of galaxies. Our results could link galaxy ages to large-scale structure for the first time at this redshift.
From 2009 to 2014, the Baryon Oscillation Spectroscopic Survey (BOSS) used the SDSS telescope to obtain spectra of 1.5 million galaxies to get very accurate measurements of the Baryon Acoustic Oscillations (BAO) scale at redshift z ~0.5. At the same time, BOSS observed over 184 000 high redshift quasars (z>2.15) with the goal of detecting the BAO feature in the clustering of the intergalactic medium, using a technique known as the Lyman alpha forest (LyaF). In this talk I will overview the final results from the LyaF working group in BOSS, including the measurement of BAO at z=2.4 both from the auto-correlation of the LyaF (Bautista et al. 2017), and from its cross-correlation with quasars (du-Mas-des-Bourboux et al. 2017). From the combination of these studies we are able to measure the expansion rate of the Universe 11 billion years ago with a 2% uncertainty. Starting in 2019, the Dark Energy Spectroscopic Instrument (DESI) will increase this data set by an order of magnitude. DESI will provide an exquisite measurement of the expansion over cosmic time, while at the same time addressing other interesting questions: the sum of the mass of the neutrino species, properties of dark matter particles, tests of general relativity and the shape of the primordial power spectrum of density fluctuations.
Galactic plane surveys of the Milky Way in a variety of gas or dust tracers give us different perspectives on how the physical conditions of the interstallar medium vary throughout the Galaxy. The Herschel Infrared Galactic Plane Survey (Hi-GAL) covers the peak of the spectral energy distribution of dense, cold dust and thus supplies an essential part of the observational description of the conditions necessary for star formation in the Milky Way. With a catalogue of over 100000 compact Hi-GAL sources, I will discuss how star formation varies as a function of Galactocentric radius and proximity to spiral arms. This allows us to revisit several long-standing questions about the effect large-scale Galactic properties have on star formation on parsec scales. Moreover, with a comprehensive profile of the Milky Way over kiloparsec scales, these results provide a new detailed context in which to understand star formation in external galaxies.
Late last year there was much excitement within the planetary science community, and in the press, about the first macroscopic interstellar object to be discovered passing through our Solar System. I'll tell the story of the discovery of 1I/'Oumuamua, the scramble to observe it before it left, and what we know about it after a couple of months of rapid publication of surprising results.
Variability in Active Galactic Nuclei has been seen as a defining feature for many years, and has long been seen as problematic for accretion disc theory. However, recent observations of extreme variabiity - factors of many over years - have brought these problems to a crisis. I will summarise the key observations, and examine in turn three possible explanations - microlensing events, tidal disruption events, and accretion disc instabilities.
Using examples from the XMM Cluster Survey, the Dark Energy Survey, ITV's "Take Me Out", and BBC Three's "Don't Tell the Bride", I will demonstrate the importance of clusters of galaxies to modern Observational Cosmology and some of their down sides. Recent publications to be featured include: The redMaPPer Galaxy Cluster Catalog From DES Science Verification Data; Comparing Dark Energy Survey and HST-CLASH observations of the galaxy cluster RXC J2248.7-4431: implications for stellar mass versus dark matter; The XMM Cluster Survey: The Halo Occupation Number of BOSS galaxies in X-ray clusters; The XMM Cluster Survey: evolution of the velocity dispersion -- temperature relation over half a Hubble time; Crowdsourcing quality control for Dark Energy Survey images; OzDES multifibre spectroscopy for the Dark Energy Survey: first-year operation and results; The XMM Cluster Survey: testing chameleon gravity using the profiles of clusters; Galaxies in X-ray Selected Clusters and Groups in Dark Energy Survey Data I: Stellar Mass Growth of Bright Central Galaxies Since z~1.2. No Likey? No Lightie [where "Lightie" in this context translates to "well, you can always try using weak lensing, but that's a whole other can of worms"]!
A galaxy's visual morphology is primarily determined by its stellar orbits and thus its assembly history. Stars in ordered rotation likely originate from the circular orbits of a gas-disk, while the disordered orbits in bulges reflect more complex formation histories. But while late-type galaxies are easily identified, subtle differences between early-type galaxies (ETGs) are not: a near face-on axisymmetric disk of old stars in a lenticular (S0) looks similar to a genuine ellipsoidal distribution of stars in an elliptical (E). However, integral field spectroscopy can distinguish the kinematics of disks (Fast Rotators, FRs) from ellipsoids (Slow Rotators, SRs): the SAURON & ATLAS3D surveys found that 66% of visually-classified Es contained ordered disk-like rotation and were thus misclassified S0s. With ETGs more common in denser environments, we extend this kinematic classification to the Coma and Abell 1689 galaxy clusters. Although SRs are found in the cluster centres, virtually none are found in the outer regions. Remarkably, the total SR:ETG ratio in each cluster was the same as in the (ATLAS3D) field, just 15%. Revisiting Dressler's original visual galaxy morphologies in 55 clusters we show that a similar result holds: the total E:ETG fraction in each cluster is 30%. Using the statistics from ATLAS3D, we show that these results are equivalent.
The talk will present new expected and unexpected results from the Dark Energy Survey beyond cosmological studies, including solar system objects, Milky Way companions, galaxy evolution, galaxy clusters, high-redshift objects and gravitational wave follow ups (Reference: arXiv:1601.00329).
The Kepler space mission revolutionised exoplanet science by discovering thousands of transiting planets, but the loss of two of its four reaction wheels put an end to the original mission a little over 2 years ago. Luckily, the satellite has been given a new lease of life, in the form of the K2 mission. K2 uses the Kepler satellite to observe 4 fields per year, near the Ecliptic plane, with only slightly reduced photometric precision compared to the original Kepler data, enabling it to survey thousands of particularly interesting targets for planetary transits: bright FGK stars, low mass M-stars, and stars in young open clusters. I am particularly interested in the latter, as it is the first time that we can perform efficient transit surveys in open clusters, and each detected system provides a unique constraint on evolutionary models for low-mass star or planets. In my talk I will give an overview of how we detect, confirm and characterise transiting exoplanets and young binaries with K2. The main challenges include dealing with the systematics effects caused by the reduced pointing accuracy of the satellite, the enhanced variability of young stars, and the challenges of measuring precise radial velocities for planet candidates around active stars. I will give a brief overview of the young eclipsing binaries and candidate planets found by K2 so far, and outline the rich prospects for open cluster science with future missions such as TESS and PLATO.
Understanding the process responsible for transforming star forming galaxies into passive and quiescent systems is currently one of the hottest topics in astronomy. I will discuss recent observational results probing different mechanisms at work in different galaxies and at different epochs. I will present multi-wavelength observations providing evidence that powerful starburst-driven and AGN-driven outflows have a profound impact on the evolution of galaxies, both locally and at high redshift; however such massive outflows may not be able to completely quench star formation in galaxies and actually, in some cases, such outflows can even boost star formation. I will show that the analysis of the stellar metallicities in large samples of local galaxies reveals that “starvation” (or “strangulation”, i.e. the lack of gas inflows) is actually responsible for quenching star formation in most galaxies. I will discuss the possible mechanisms responsible for the starvation of galaxies. I will also present some recent results from the ongoing Manga-SDSSIV survey, which is delivering integral field spectroscopy for thousands of galaxies. Based the analysis of the initial sample of several hundred galaxies I will show evidence that the quenching process in galaxies occurs predominantly inside-out, and I will show that even this observational finding can be explained in terms of “starvation”.
As the faintest galaxies we are able to observe in the Universe, the dwarf spheroidals can be thought of as the fundamental galactic unit. Within our Local Group, we are able to study these objects in extremely high detail, resolving their mass profiles, chemistries, and evolutionary histories. These measurements have led to several surprising results. One is that the masses of these systems appear to be lower than predicted by cold dark matter simulations. Additionally, dwarf galaxies are not distributed isotropically around their hosts, as naively expected in the current cosmological paradigm. In this talk, I will discuss these observational peculiarities, and how we may account for them, using examples of interesting dwarf galaxies in the Andromeda system, and results from recent hydrodynamical simulations.
The exploration of young and metal-poor galaxies across cosmic history is key to deciphering how fundamental processes such as enrichment, feedback, and star formation operated in the first galaxies. However, the spatially-resolved spectroscopic observations that are needed to constrain these physical mechanisms are currently extremely difficult, if not impossible, to perform because the objects are so faint and small. In this talk I will describe several new observing campaigns that resolve this issue by connecting low-metallicity star-forming galaxies in the nearby Universe with distant strongly-lensed galaxies as proxies for the earliest galaxies, including a newly discovered population of young, extremely metal-poor blue diffuse galaxies. I will discuss how the maps of star formation, ionisation, metallicity, and gas kinematics obtained from these observations currently constrain galaxy formation models (including the so-called `bathtub' and `galactic fountain' models), and I will highlight future observational capabilities that will push our understanding of galaxy evolution to even smaller physical scales and earlier times.
I will start with an overview of the state-of-the-art in galaxy formation and large-scale-structure numerical simulations from cosmological initial conditions. On the one hand we wish to simulate large volumes to gain representative samples of galaxies and to understand the cosmological implications of forthcoming survey data from LSST, Euclid and DESI. On the other, we also want to maintain very high resolution to resolve highly non-linear astrophysical processes and internal kinematics for forthcoming galaxy IFU studies like MaNGA. These two requirements result in a tension on how to best spend limited computer time. I will argue that a new approach to simulations, in which we use statistical models to tailor cosmological initial conditions for different questions, can help relieve this tension. I will show a range of applications from large scale power spectrum estimation to galaxy quenching.
Post-starburst galaxies have long been known as an intriguing class of galaxies, identified through their unusual optical spectra as having had a recent burst of star formation that has since quenched. What caused the starburst and what caused the quenching? Originally identified in clusters (so-called E+A or K+A galaxies), they are now known to exist in all environments. They are rare at low-redshift, but increasingly common at high redshift. I will review recent progress in studying post-starburst galaxy properties, focussing on the question of whether they are true “transition” species i.e. galaxies transiting from the blue cloud to the red sequence, and how important they could be for accounting for the growth of the red sequence since z~2.
Gravitational lensing has seen a surge of interest in the past few years. Lensing is one of the very few probes capable of mapping dark matter halo distributions. Lensing also provides independent cosmological parameter estimates and enables the study of galaxy populations that are otherwise too faint for detailed study. The handful of strong lensing systems known in the year 2000 has now been replaced with hundreds, thanks to innovative multi-wavelength selection, and there is an imminent prospect of thousands of lenses from Herschel and other sub-millimetre surveys. I will show how Euclid and the Square Kilometre Array promise tens or even hundreds of thousands of strong lensing systems, and some early results from our long-term project on the 10m Southern African Large Telescope, which is extremely well placed to have an enormous impact in follow-up observations of foreground lenses and background sources.
Active Galactic Nuclei (AGNs) are amongst the most powerful individual objects in the Universe. Powered by material accreting onto supermassive black holes at the centres of galaxies, the energy released by AGNs is widely thought to have played a major role in shaping today's galaxies. Despite their importance, fundamental questions still surround many aspects of AGNs, not least what governs their accretion rate (and thus power output) and how - or indeed whether - they influence their host galaxies. In this talk, I will summarise the exciting progress that has been made in both these areas of AGN astronomy over the past decade, while highlighting what key questions still remain to be addressed.
It is now clear that the conversion of interstellar gas to stars, along with the subsequent feedback from massive star formation are fundamental agents in the shaping, evolution, and formation of galaxies. Thanks to a wealth of new multi-wavelength observations from the ground and space our empirical understanding of these processes is being revolutionised, yet our understanding of the underlying physical processes which trigger and regulate large-scale star formation remains embryonic. This talk will review what we have learned in the past decade about the demographics and diversity of star formation in galaxies, its evolution over cosmic time, and the empirical correlations and scaling laws that offer clues to deeper underlying physical processes of star formation and galaxy evolution.
One of the best ways of studying compact objects in the Universe, such as white dwarfs, neutron stars, stellar-mass black holes, exoplanets and Solar System objects, is through their brightness variations. These tend to occur on timescales of seconds and below, and hence require specialised astronomical instrumentation. In this talk, I shall review the design and scientific highlights of the high-speed cameras ULTRACAM, ULTRASPEC and HiPERCAM, the first two of which have been in operation for over a decade, and the last of which is due to see first light next year on the largest optical telescope in the world.
In a hierarchical Universe clusters grow via the accretion of galaxies from the field, groups and even other clusters. As this happens, galaxies lose their gas reservoirs via different mechanisms, eventually quenching their star-formation. One of the most effective mechanisms in clusters is ram-pressure stripping by the intra-cluster medium. I will present recent results from multi-wavelength observations of z<0.2 clusters that are helping to constrain the efficiency, time-scale, and consequences of stripping. In particular, I will present a phase-space analysis of HI-stripping and star formation in cluster galaxies, and show recent MUSE/VLT observations of the most extreme examples of stripping, the so-called "jellyfish galaxies".
Observing galaxies in the Far-Infrared (FIR) gives us a unique window into the star formation rates of very high redshift, dusty galaxies. These galaxies are generally thought to be forming stars at a prodigious rate, heating their dust content, which then radiates in the FIR. However, observing this glow is difficult, even with a space-based telescope such as the Herschel Space Observatory, as the resolution of the images returned is quite poor. It is often assumed that a bright source in the FIR belongs to a single, highly star forming galaxy, but this is impossible to verify with low resolution images. In this talk I will discuss a new method of unpicking a single FIR detection into individual galaxies, and what the results of this unpicking imply for our understanding of galaxies in the early Universe.
I will present a selection of results and ongoing projects in radio weak lensing. I will present forecasts which suggest that the SKA will be capable of world-leading weak lensing cosmology, potentially providing additional information to help remedy a number of systematics which could limit optical and near-IR weak lensing. In addition, cross-correlating cosmic shear measurements between optical and radio bands gives comparable constraints and should be free of many wavelength-dependent systematics. In order to achieve this promise, many challenges need to be solved and I will discuss a few of them. Outstanding questions include how to measure shapes from radio interferometer data, how to model the source galaxy population at faint micro-Jy fluxes, and how to estimate their redshifts. I’ll also discuss a number of radio weak lensing pathfinder surveys on which we are developing and training the required new analysis techniques.
Recent observations have shown that the properties of the dense intracluster gas cooling in the cores of clusters of galaxies are significantly affected by AGN activity in the central brightest galaxy. I will review these observations and focus on the most nearby example of this AGN Feedback from NGC1275 at the core of the Perseus cluster.
In this talk I will describe how mapping the dynamics of molecular clouds in the centre of galaxies can help us to constrain a wide range of astrophysical problems. From the enigmatic relation between galaxies and their supermassive black holes, the suppression of star-formation in dying galaxies, and the puzzling variation of the stellar initial mass function, molecules provide an ideal probe that can help us make progress. I will show how high resolution observations (with CARMA and ALMA) can be used to estimate the masses of supermassive black holes in galaxies across the Hubble sequence, and describe the WISDOM project, that aims to use this technique to constrain the importance of accreting SMBHs in galaxy quenching. I will show that the deep potential wells of massive galaxies can play an important role in quenching star-formation, transitioning galaxies as they grow from star-forming to "red and dead". Finally I will show how one can use molecules to probe the controversial topic of variation in the stellar initial mass function.
Studying galaxies at high redshift (z > 6) provides a unique insight into the early stages of galaxy formation and evolution. The samples of star-forming galaxies discovered during this epoch have largely come from deep HST surveys, with wider-area ground-based data providing key constraints at the bright end. I will present our results from two degree scale near-infrared surveys (UltraVISTA/COSMOS and UDS/SXDS), looking at the bright-end of the luminosity function at z = 6 and 7. Studying the galaxies in more detail with Spitzer and HST, we find evidence for strong rest-frame optical emission lines such as [OIII] and a clumpy/merger-like morphology for the brightest objects. I will end with a discussion of our new analysis of the strong Lyman-alpha emitter, and potential Pop III candidate, CR7.
The top recommendation for a large space mission in the US 2010 Decadal Survey was the Wide Field Infrared Survey Telescope (WFIRST). Similarities in hardware requirements between proposed dark energy, exoplanet microlensing, and near infrared surveyor missions allowed for a single mission that would accomplish all three goals. The gift of an existing 2.4 meter telescope to NASA by another US government agency allowed for the addition of a coronagraph that will take images and spectra of nearby exoplanets; this instrument will be a technological stepping stone to imaging other Earths in the 2030s. I will give an overview of WFIRST's proposed instrumentation, science goals, and implementation plan.
In recent years there has been tremendous progress in identifying large samples of distant galaxies, but many crucial aspects of galaxy formation remain poorly understood. In particular, we still do not understand why star formation was abruptly quenched in many massive systems at high redshift. It is also unclear if the same processes are linked to the morphological transformation of galaxies, to produce the Hubble Sequence we see today. I will discuss recent observational and theoretical progress in this area, and present new evidence suggesting that the key transformative processes are intimately linked; for the most massive galaxies at least, the quenching of star formation appears to occur during (or very shortly after) the event that forms the compact proto-spheroid.
Mapping Nearby Galaxies at Apache Point Observatory (MaNGA, part of the fourth incarnation of the Sloan Digital Sky Surveys or SDSS-IV), is partway through it's ~6 year programme to obtain spatially resolved spectral maps for ~10,000 nearby galaxies selected from the SDSS Main Galaxy Sample. These data will unwrap the layers of local galaxies - revealing their stellar and gas dynamics, as well as the ages and chemical make-up of their constituent stars, and locations of current star formation. MaNGA began observations on the Sloan Telescope at APO in July 2014 and is now the largest sample of resolved spectroscopy in the world, with ~3300 galaxies observed to date. MaNGA will provide an amazing census of the stellar and ionized gas content of galaxies for a representative sample of nearby galaxies. However, complementary information about the cold gas content is crucial for a number of applications, but especially understanding the physical mechanisms that regulate gas accretions and quench galaxy growth. In this seminar I will describe the HI-MaNGA project which is one of the follow-up projects for MaNGA focused on learning about cold gas components. For HI-MaNGA we have been awarded almost 1000 hours of time on the Robert C. Byrd Green Bank Telescope (in West Virginia) to obtain 21cm HI (neutral hydrogen) global profiles of ~600 MaNGA galaxies. I will explain how measuring the total HI content of MaNGA galaxies can add to our understanding of the physical mechanisms regulating star formation in galaxies, as well as show some interesting early results from the project.
The first detections of gravitational waves from binary black hole mergers have opened up new opportunities and challenges in astrophysics and fundamental physics. I will describe these recent discoveries and discuss advances in the analysis and interpretation of gravitational-wave observations. I will focus on my group's efforts to extract the astrophysical evolution of massive stellar binaries from observations of gravitational waves emitted during mergers of the stellar remnants.
Galaxy clusters are the most massive structures in the universe and their mass growth provides a unique test for cosmological models of structure formation. Clusters are also the location where many galaxies have their star formation strongly truncated, and this process is still poorly understood. I will present new results on the growth of stellar mass in clusters over ~10 Gyr of cosmic time which shows they are highly concentrated at early times and are growing in an inside-out manner, something that is not seen in most simulations. I will also present new constraints on the timescale and location for quenching of galaxies in the cluster environment at early times. These timescales are showing us that the process by which clusters quench star formation is likely evolving over cosmic time, and that we clearly need to invoke much more sophisticated models of environmental quenching and feedback in galaxies if we are to truly understand how galaxies evolve in high-density environments.
Observations of molecular gas in distant galaxies are experiencing a coming-of-age, transitioning from a "discovery" to a "survey" mode. New and upgraded facilities are now making it possible to survey molecular gas efficiently in large galaxy samples, and these observations are proving to be critical in refining our general picture of galaxy evolution. In this talk, I will review recent results from the two largest surveys for molecular gas in normal star-forming galaxies, the z=0 IRAM-30m COLD GASS survey and the z=1-2 IRAM-PdBI PHIBSS survey, and show how they combine to lend strong support in favor of the "equilibrium" model for galaxy evolution, under which most of galaxy evolution is regulated by gas supply and the efficiency of the star formation process.
Stellar halos contain information about various stages of galaxy formation, from the incipient stages of in situ star formation to the later episodes of accretion and tidal disruption of dwarf galaxies. Recent gas-dynamical simulations suggest that the inner halo may be formed from the destruction of a previous galactic disc by merging with satellite galaxies or, less violently, by quasi-secular rearrangement of a proto-disc. The various mechanisms of halo formation leave different signatures in the present-day mass distribution of halo stars or in the spatial distribution of their chemical abundances. I will review the global properties of stellar halos from several recent gas-dynamical simulations and discuss the level of agreement among these and with the available observational data.
We illuminate the thermal and ionization history of the intergalactic medium (IGM) by confronting cosmological hydrodynamical simulations of the IGM with the latest observational constraints on the IGM temperature, the statistics of Lyman-alpha emitters and the opacity of the Lyman-alpha forest, as well as with the latest Planck measurements of the Thomson scattering optical depth towards the CMB.
In this talk, I will discuss the result of a study of extremely isolated galaxies residing in voids in the Galaxy and Mass Assembly (GAMA) survey. While the majority of these galaxies are blue, star forming objects with (u-r) < 1.9, we identify a number of void galaxies with optical colours consistent with no ongoing star formation. A line strength analysis reveals these galaxies to have nuclear spectra consistent with old stellar populations. However, when the mid-IR colours of these galaxies are examined, we find that only void galaxies with masses > 10^10 Msun have truly passive stellar populations. Given their isolation, these highest mass void galaxies have likely undergone mass quenching.
Galaxy luminosity functions are used to measure the distribution of galaxy masses and star formation rates, and are thus critical for measuring galaxy growth and for constraining galaxy formation models. Though functions have been measured in many wavelengths, very little research has focused on what actually shapes the galaxy luminosity function, and as a result, many functions are fitted empirically rather than with physically motivated functional forms. To address this issue, we have measured luminosity functions which trace galaxy stellar mass, as well as functions which trace current star formation, as functions of galaxy morphology and of galaxy optical colour. As dark matter halo mass and stellar mass have a strong correlation, we show that functions of the two share a similar form. Star formation does not have such a clear correlation, and we propose a new method for fitting star forming luminosity functions by convolving models of the star formation rate duty cycle with functions of stellar mass.
Mergers of compact binaries involving neutron stars lie at the intersection of several key problems in astrophysics. They are widely thought to lead to short-duration gamma-ray bursts; to be an important production site for the nucleosynthesis of r-process heavy elements; and to emit strong gravitational wave (GW) signals that are the most promising for detection by the next "advanced" generation of detectors. Recently, the first evidence for kilonova emission, predicted to be produced by the radioactive decay of species created during such a merger, was found, associated with sGRB 130603B. I will review this discovery together with other observational constraints on the nature of sGRBs, and consider the prospects for kilonovae as electromagnetic signatures of GW events.
Biomedicine and astronomy face similar challenges in dealing with increasingly large and complex data-sets which can include multi-modal, multi-spectral and multi-dimensional measurements. This presentation will address potential synergistic overlap of biomedical and astronomical signal processing approaches. I will particularly focus on magnetic resonance imaging (MRI) and magnetoencephalography (MEG), which are the techniques that underpin the research carried out at the Sir Peter Mansfield Imaging Centre, touching on data processing methods used in functional MRI, quantitative susceptibility mapping and measurement of resting state brain networks.
The last few years have seen an explosion in our knowledge of extra-solar planetary systems. We now know that exoplanets have an extraordinary range of properties, and almost every conceivable planetary architecture seems to exist in nature. Planets form in cold discs of dust and gas around young, newly-formed stars, and in this talk I will try to explain how such a diverse population of planets formed from these relatively homogenous initial conditions. I will first review the physics of protoplanetary disc evolution, and discuss the conditions under which planets form and migrate. I will show how disc evolution and dispersal influences migrating planets, leading to "deserts" and "pile-ups" in the distribution of exoplanets. I will then consider the new class of compact planetary systems discovered by Kepler, and discuss under what conditions it is possible to build these systems through migration. Finally I will present models of disc evolution in binary systems, and consider the fo rmation and dynamics of circumbinary planets such as Kepler-16b.
The Hubble Frontier Fields (HFF) initiative constitutes the largest commitment ever of HST time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. This program devotes 140 orbits of HST time to deep imaging observations of each of six cluster lenses reaching m~29 (AB) uniformly in all pass-bands (10-30 orbits per filter - 3 ACS and 4 WFC3 pass-bands). The full set of data on Abell 2744 (z=0.308) has been taken in October-November 2013 with WFC3, and May-July 2014 with ACS. The second target, MACSJ0416.1-2403 (z=0.397) has been observed with ACS in January-February 2014, and with WFC3 in July-August 2014. I will present the new gravitational lensing pictures of these two complex systems using this exquisite set of data coming from the HFF program. We have demonstrated that we are now able to 'weight' these clusters' cores down to the percent level precision (recently published works), serving our quest for the high-redshift Universe.However, while the depth of these dataset makes these clusters amazing Cosmic Telescopes, it also enables us to get an unprecedented understanding of the cluster physics. Therefore, presenting the case of MACSJ0416 & Abell 2744, I will demonstrate the importance of such high-quality data to analyse the merging/dynamical history of the clusters themselves while comparing dark matter, light and gas distributions.
I will discuss observations of environmental quenching using spectroscopic redshift surveys from the local universe to redshift ~ 1.5. Ultimately, I will use these observations, together with numerical simulations, to infer how long it takes satellite galaxies to quench at a range of redshifts. This evolution of satellite quenching times gives important insights into the physical mechanism driving this quenching and into the general baryon cycle of galaxies.
The WISE all-sky survey has discovered some of the most luminous dusty galaxies in the sky. Very powerful AGN, they appear to be surrounded by an excess of other powerful dusty galaxies that are similar to the submillimetre-selected galaxies SMGs. I will describe some of the features of the WISE-detected objects, and the potential for investigating them using ALMA and probing their role in reacting to and shaping their environments.
I will outline our past views regarding what it takes to find the mass of molecular hydrogen gas in galaxies, and what maintains the thermal and kinematic state of this, most important fuel for star-formation in the Universe. Then I will discuss how these views are now being dramatically revised, especially for vigorously star-forming galaxies, and outline the new paths of Interstellar Medium (ISM) research that have opened up as a result. ISM novices are especially welcome, for now is the time to start anew....
I will discuss the evolution of morphological and star-formation properties of galaxies focusing on two aspects: (1) the effect of environment on local group galaxies and (2) the frequency of major mergers vs. disk instabilities in high-z galaxies.
How the first generations of galaxies build up and how they bring the reionisation of the neutral gas in the intergalactic medium during the first Gyr of cosmic time remains one of the biggest questions in extragalactic Astronomy. While the launch of the James Webb Space Telescope will soon provide us with many new insights into this early epoch, we can already learn a lot about the properties of the earliest galaxies with use of current facilities. I will discuss how we can use extremely deep Hubble and Spitzer photometry in combination with rest-frame UV spectroscopy to further our understanding of early galaxy formation processes and the sources of ionising photons in the reionisation era.
In traditional models of galaxy evolution, feedback associated with an active galactic nucleus (AGN) have been invoked as the standard channel to regulate stellar mass growth at the high end of the mass function. We have been investigating a sample of massive, compact galaxies that exhibit ultra-fast gas outflows (up to 2500 km/s) with no evidence of significant AGN activity. Recently we have shown that in at least one of these galaxies a significant amount of molecular gas is being driven out at speeds of up to 1000 km/s to scales of 10 kpc (Geach et al., 2014, Nature). I will discuss how this sample demonstrates that stellar feedback can be an effective channel for curtailing stellar mass growth in massive galaxies, and in particular the role of stellar radiation pressure as a mechanism for launching galaxy scale multi-phase super winds from high density star forming regions.
I will provide an overview of the current understanding of cosmology and galaxy evolution from radio continuum surveys, and show how this field will be revolutionised over the coming years, on the lead up to the SKA.
The nucleosynthesis of the lightest elements of the periodic table, hydrogen, helium, lithium and their isotopes, during the first few minutes of our Universe history is one of the cornerstones of the standard model of cosmology and particle physics. In the last few years, it has become possible to determine the primordial abundances of these elements with high precision, finally realising their long-appreciated potential for measuring the cosmic density of ordinary matter. In this seminar, I shall review the latest developments in the determination of the primordial abundance of deuterium in particular, using the technique of QSO Absorption Line Spectroscopy. The 'punch line' is that independent measures of the density of baryons at different cosmic epochs are in excellent mutual agreement. Such concordance places interesting limits on the existence of relativistic particles beyond the standard model of physics.
In this talk I will discuss which feedback mechanisms are needed to reproduce realistic stellar masses and galaxy morphologies in the present day Universe and argue that the black hole feedback is necessary for the quenching of massive galaxies. I will then demonstrate how black hole - host galaxy scaling relations depend on galaxy morphology and colour, highlighting the implications for the co-evolutionary picture between galaxies and their central black holes. In the second part of the talk I will present a novel method that permits to resolve gas flows around black holes all the way from large cosmological scales to the Bondi radii of black holes themselves. I will demonstrate that with this new numerical technique it is possible to estimate much more accurately gas properties in the vicinity of black holes than has been feasible before in galaxy and cosmological simulations, allowing to track reliably gas angular momentum transport from Mpc to pc scales. Finally, I will also discuss if AGN-driven outflows are more likely to be energy- or momentum-driven and what implications this has for the redshift evolution of black hole - host galaxy scaling relations.
The INT Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS) started in 2004 and is now complete apart from some updates to even out the data quality. The companion blue survey, UVEX, started a couple of years later and at the end of 2011 coverage of the Southern Plane, via VPHAS+ running on the VST, got underway. Between them, these surveys provide imaging in u,g,r,i and H-alpha of the complete Galactic Plane within the latitude range |b| < 5 degrees, down to at least 20th magnitude. In this talk, I will give some of the background on these surveys before focusing on a number of results from them that relate to the large scale properties of the plane of the Milky Way. These will include extinction mapping of the northern Plane and a comprehensive search for massive stars in the south.
Diverse astrophysical and cosmological observations indicate that most of the matter in the Universe is cold, dark and non-baryonic. Weakly Interactive Massive Particles (WIMPs) are generically a good dark matter candidate and particle physics provides us with a well-motivated WIMP candidate in the form of the lightest supersymmetric particle. WIMPs can be detected indirectly (via the products of their annihilation) or directly (via elastic scattering in underground detectors). After an introduction to WIMPs and their detection I will focus on direct detection experiments, in particular astrophysical uncertainties and how they can be addressed.
On the largest of scales, the Universe is organised in to a so-called "cosmic web" - with galaxies being funnelled along filaments that surround voids and in to the nodes of the web -- clusters of galaxies. As they do so, they evolve and change. The question of how galaxies form and evolve over time within the cosmic web is one that has been with us for many years now and research in this area has only accelerated and thrown up more issues as problems get solved and raise new ones. In this talk, I will outline my recent contributions to this field by examining how galaxies in the local Universe are evolving (or not) and how biases in certain datasets can affect the degree of confidence we have in galaxy evolution. I will show new results on the HI content of galaxies in the cosmic web taken from a unique marriage of the 6dFGS survey and HIPASS radio data and highlight our recent work on the Coma cluster that suggests a number of studies may be biased when using this cluster as a redshift=0 baseline to compare other higher redshift clusters to.
I will review what we know about the accretion flow in the stellar mass black hole binary systems, and show how we can use them to test Einstein's General Relativity in strong gravity. The resulting accretion flow models and their associated jet can be scaled up to the supermassive black holes to give some physical insight into the zoo of different types of AGN and Quasars.
The discovery that supermassive black holes reside in the centers of most if not all massive galaxies has emphasized the importance of Active Galactic Nuclei (AGN) in galaxy evolution. Despite this, the processes that trigger Active Galactic Nuclei remain poorly understood. While low luminosity AGN require fuel supplies low enough to allow fueling through so-called secular processes, the gas masses required to power luminous AGN are so large that major mergers of gas-rich galaxies are likely the only triggering mechanisms. However, the observational evidence for a connection between mergers and AGN remains weak. I will present a HST CANDELS study analyzing AGN host galaxy morphologies in a sample of moderate redshift (z=0.5-0.8) AGN spanning a wide range of luminosities. I will discuss if and how the importance of mergers changes as a function of AGN luminosity.
The history of disk galaxy simulation is dotted with remarkable successes, tempered by frustrating impasses, including an inability to recover anything remotely similar to the Milky Way. Recent advances suggest that we might have made a breakthrough by generating essentially bulgeless disks. I will examine the evidence for this new-found optimism and identify where the shortcomings suggest we should be concentrating our future efforts.
Some of the most extreme star formation in the Universe occurs in a population of dusty galaxies at z~2, selected at submillimeter wavelengths (SMGs). Resolved studies of the star forming gas and the fuel for the star formation in SMGs have provided extensive information about the origin of their properties and imply that these systems may be the progenitors of the massive galaxies in the local Universe. I will present observations of SMGs from Integral Field Units, used to study the ionised gas morphologies and dynamics, testing if they consist of merging components. I will then present the results from observing SMGs using millimeter interferometry to probe the molecular gas and the cold neutral interstellar medium, constraining the physical conditions within the SMGs. I will discuss the implications of these results on models of the evolution of massive galaxies. I will then explore the next stage of this evolution, from a merger induced starburst (the SMG phase) to a UV luminous quasar, by discussing observations of reddened quasars which may represent this transition phase.
We present FIR−CO luminosity relations (i.e., log LFIR = a log L′CO + b) for the full CO rotational ladder continuously from J = 1 − 0 up to J = 13 − 12 for a sample of 76 (Ultra) Luminous Infra- red Galaxies (LIR > 1E11Lsun) using date from Herschel SPIRE-FTS and ground-based telescopes. We extend our sample to high redshifts (z > 1), by including 49 (sub)-millimeter selected dusty star forming galaxies from the literature with robust CO observations, and sufficiently well- sampled FIR/sub-millimeter spectral energy distributions (SEDs) so that accurate FIR luminosities can be deduced. The luminous starbursts at high redshifts enlarge the range of the FIR−CO luminosity relations towards the high IR-luminosity end while adding more mid-J/high-J CO line data (J=5–4 and higher) that have been scarce until the advent of Herschel. The now much enlarged dataset (both in terms of IR luminosity and J-ladder) reveals linear FIR−CO luminosity relations (i.e., a ≃ 1) for J = 1 − 0 up to J = 5 − 4, with a nearly constant normalization (b∼2-2.5). This is expected from the also linear IR-(molecular line) relations recently found for the dense gas tracer lines, as long as the dense gas mass fraction does not vary strongly within our (merger/starburst)-dominated sample. However from J = 6 − 5 and up to the J = 13 − 12 transition we find increasingly sub-linear slopes and higher normalization constants the higher the J-ladder. We argue that these are caused by a new warm (∼100K) and dense (> 104 cm−3) gas component whose thermal state is not maintained by the SF-powered far-UV radiation fields (and thus is no longer tied to the SFR) and one that is increasingly present towards the high LIR end. The IR-normalized global CO SLEDs that remain mostly flat from J = 6 − 5 up to J = 13 − 12, and are a generic feature of the (U)LIRGs in our sample, further support the presence of such a gas component.
It is well known that stars observed in the local universe form with something very close to a universal initial mass function (IMF) where the vast majority are of low mass. However, this cannot be the case for the very first stars to ever form as a similar IMF would yield many surviving to the present day, contradicting the fact that none have ever been observed. Theory and simulation have also suggested that the first stars likely had a more top-heavy IMF owing to their unique chemical makeup. This implies that a transition in star formation modes must have taken place at some point in the history of the universe. Like the formation of the first stars, this critical epoch exists outside of the range of direct observation and, as such, has been primarily the domain of theory. I will give a review of research into the formation of the first (Population III) stars and theories of the transition from Population III to modern-day star formation. I will then present new results from a set of simulations designed to directly simulate the conditions of this period. Finally, I will conclude with a discussion of the yt simulation analysis toolkit, whose aim is to become a lingua franca for astrophysical simulations by allowing researchers to focus on physical objects instead of files on disk, regardless of the simulation code they use.
In principle galaxy clusters offer powerful constraints on the dark energy equation of state, and opportunities to test gravity theory. These exciting goals can only be achieved at a useful level of precision if the mass of galaxy clusters can be measured accurately. One of the main aims of the Local Cluster Substructure Survey is to calibrate clusters as cosmological probes as part of the global effort to characterise dark energy. I will present new results on cluster mass measurement from our programme of lensing, X-ray, and infrared observations of clusters at z=0.2, and discuss their implications for cluster cosmology. I'll also mention our work on galaxy evolution in clusters if time allows.
Relativistic jets from active galactic nuclei are now known to play an important role in galaxy evolution in the nearby Universe, with their role at higher redshifts remaining uncertain. In the near future, deep extragalactic surveys with next-generation radio telescopes will lead to an unprecedented view of the low-luminosity radio jet population to high redshifts. Translating radio-galaxy population statistics into a robust understanding of the evolving role of radio jet feedback in galaxy evolution requires solving decades-old uncertainties in the physics, energetics and environments of radio-loud AGN. I will discuss recent advances in this subject, driven by the powerful combination of X-ray and radio observations, including early results from the Low-Frequency Array (LOFAR).
The current wisdom is that stars form in molecular clouds as a consequence of turbulent fragmentation. I will discuss what constraints are placed on this theory by observation and what successes the theory has had in satisfying the constraints, the roles played by thermodynamics and feedback in regulating star formation, and the many problems that remain to be solved. I will illustrate, with SPH simulations, some of the processes that may be at work.
Studies of weak gravitational lensing on a cosmic scale are beginning to provide interesting results which are adding to our knowledge of cosmology, and there is promise of precise measurements of the properties of dark energy and modified gravity with future surveys. In this talk, I will show recent results related to CMB lensing and galaxy lensing, showing on the one hand beautiful agreement with theory, and on the other, an indication that things may be more complicated than we think.
In recent years, the concept of the Virtual or Digital Earth has gained currency and a new international society has been set up to develop it further. However, attention has focussed on the surface of the planet and it could be argued that the virtual Earth is hollow. Geologists are working through international initiatives such as the Group on Earth Observations to add the third dimension. The techniques used and the observations made are increasingly available for other planetary bodies, such as Mars, for which we have even less subsurface information. By developing methods to populate the Earth’s third dimension, we also develop ways to do this on Mars. The talk will set out some of this context and then describe ongoing and planned work in NGI and partner organisations that aims to address these challenges.
One of the most important, but still highly debated, issues in contemporary cosmology, is the formation and evolution of massive spheroidal galaxies. Extensive semi-analytic galaxy formation models (SAMs), still debate whether mergers have played a major role in the assembly of ellipticals, or other "in-situ" processes, such as strong, dissipative early bursts of star formation, and/or clumpy accretion, have played an equally important role. In this talk, I will discuss the global evolution of massive spheroids, adopting state-of-the-art SAMs, as well as advanced semi-empirical models. In particular, I will show that, at variance with several previous attempts, hierarchical models can faithfully reproduce the overall shape, normalization, and scatter of the local size-stellar mass relation for massive early-type galaxies as measured in SDSS. I will then move on discussing the role of progenitor bias and environment in the overall structural evolution of massive galaxies. I will conclude with the additional constraints obtainable from lensing measurements on the global profile of galaxies.
I will present results from bulge-disk decompositions of massive galaxies at \(1 \lt z \lt 3 \) in the CANDELS survey and discuss the implications of our findings within the context of some of the current models of galaxy evolution and quenching. By decomposing the galaxies in our sample according to their H(F160W)-band light fractions and extending this analysis across multiple bands, we have been able to conduct SED fitting of the separate components. In addition to morphological properties, this has provided us with individual component stellar-mass and star-formation rate estimates. These decompositions have allowed us to explore the evolution of the galaxies in our sample split into their bulge and disk, star-forming and passive sub-samples, and has provided new insight into the links between quenching and morphological transformations within these systems.
Using new data from the Keck telescope I report on the scaling relations of small stellar systems, ie globular clusters, ultra compact dwarfs, dwarf and compact ellipticals. The elevated mass-to-light ratios seen in some of these systems may indicate either the presence of dark matter, central black holes or a bottom-heavy IMF. Various scaling relations are investigated to shed light on which of these interpretations is most likely. The origin of cEs and UCDs is also discussed.
One of the most important questions in astronomy today is the source of the ionising photons that caused a predominantly neutral Early Universe to evolve into the ionised one we see today. I have investigated this Epoch of Reionisation with a a new suite of high resolution hydrodynamical simulations, created within the DRAGONS group. These simulations show that the galaxies observed at early times represent merely the tip of an iceberg; with a hidden population of faint galaxies that can Reionise the Universe with ease. Although the global star formation history of the early universe is strongly constrained by current observations, we actually know little about the nature of star formation at this time (as given by the specific star formation rate). I will demonstrate that we can understand these objects but need to push our observations deeper, a goal that will likely have to await the James Webb Space Telescope.
The most massive galaxies (M_stellar > 10^11 M_sun) are still very mysterious objects because they display extremely small sizes (~1 kpc) at z > 1.5 and therefore a remarkable change in their observational properties is necessary to match them with their local Universe counterparts. In particular, their size-mass relation has been subject to a great debate because high redshift observations may lose the light from the extended low surface brightness galaxy outer parts. Thanks to the depth, resolution and careful data reduction of the Hubble Ultra Deep Field 2012 programme we are able to detect extended stellar haloes for the six massive Early-Type Galaxies (ETGs) located in this image at \(z \lt 1\). We are able to measure reliably their surface brightness profiles out to ∼31 mag arcsec^−2 , which translates into >25 effective radii or >100 kpc for some objects in our sample, making our observations comparable to the nearby Universe studies but at much larger cosmic distances. Once the faint component is included in our analyses, it has only a reduced impact in the structural parameter estimations, reinforcing the reported compactness of the massive galaxy population at high-z. The presence of these extended stellar haloes, along with dim tidal features and a large number of galaxy satellites seem to be common for massive galaxies. HUDF images are not only useful for investigating the highest redshift galaxies but they also open up a window for the comprehension of the local Universe.
Bayesian inference provides a self-consistent method of model comparison, provided that i) there are at least two models under consideration and ii) all the models in question have fully-specified and proper parameter priors. Unfortunately, these requirements are not always satisfied in astronomy and cosmology: despite the existence of exquisitely-characterised measurements and quantitative physical models (i.e., sufficient to compute a believable likelihood), these models generally have parameters without well-motivated priors, making completely rigorous model comparison a formal impossibility. Still, huge advances have been made in cosmology, in particular, in the last few decades, implying that model comparison (and testing) is possible in practice even without fully specified priors. I will discuss the above principles and then illustrate some test cases of varying rigour, outlining some schemes for formalising heuristic approaches to model testing within a Bayesian framework.
I'll be presenting a study of galaxies in the Herschel-ATLAS local volume \(z \lt 0.05\). We have found that most of the galaxies in the local volume are low surface brightness, blue and dust rich, with very cold dust temperatures. The average gas fraction of the dust selected sample is 0.5, which is very much higher than the average gas fraction of typical optically selected samples. I'll describe the properties of these sources and ask how representative they may be of galaxies in the early Universe.
The Diffuse Gamma-Ray Background (DGRB) is the radiation that remains after the contribution of Galactic gamma-ray emission and extragalactic sources is subtracted from the total gamma-ray flux. The DGRB collects the radiation of all those sources that are too faint to be resolved individually and, thus, it represents an essential tool to study faint gamma-ray emitters like star-forming or radio galaxies and the exotic Dark Matter. I will review our current knowledge of the nature of the DGRB, presenting the strategies that have been proposed to study it. I will focus, in particular, to what we can learn from the measurement of its angular anisotropies.
The polarization of the Cosmic Microwave Background (CMB) has had significant attention this year in light of results from the BICEP2 experiment. I will review CMB polarization and describe how it can be used to test cosmic inflation via gravitational wave signatures. It can also be used to probe other features of the primordial universe, and to trace the cosmic web of dark matter via its gravitational lensing signal. I will describe the ACTPol experiment, currently in operation in Chile. It measures the CMB polarization at high resolution and overlaps with several optical large-scale structure surveys. I will show first results from data measured in 2013, and discuss prospects for cosmology. I will also describe preparations for the next stage Advanced ACTPol project, which will map the CMB polarization over half the sky at multiple wavelengths.
Powerful radio jets launched by a central supermassive black hole pump a substantial amount of energy into their surrounding galaxies and cluster environment. This active galactic nucleus feedback is now thought to be the essential mechanism in galaxy formation models regulating galaxy growth by suppressing gas cooling and star formation. But many key questions remain, including how the black hole is fuelled, how the heating can be distributed over large scales yet closely coupled to the gas cooling rate and the role of the cold molecular gas apparently cooling from cluster atmospheres. I will present ALMA Early Science observations of molecular gas in the central galaxies of A1664 and A1835 which show spectacular 10 billion solar mass outflows driven out by the radio jets and a massive gas inflow settling into a disk around the nucleus.
Black holes in our Galaxy, such as those in the microquasars SS433 and Cygnus X-3, demonstrate dynamic behaviour in accretion and dramatic mass outflows. We observe winds and relativistic plasma jets to emerge from these objects which resemble the modes of mass-loss in the supermassive black holes of powerful quasars in the distant universe. Time-resolved observations of the accretion and subsequent mass-loss from microquasars offers great rewards in terms of information about the nature of these remarkable phenomena in the Universe. I will describe how combined multi-wavelength strategies across the electromagnetic spectrum continue to yield new discoveries and discuss how time-resolved observations have led to the discovery of a further mode of mass-loss in SS433 via a circumbinary disc. I shall present the exquisitely detailed behaviour of these modes of mass-loss before, during and after a major flare event in this object, with reference to comparable behaviour seen in similar objects.
The origin of today’s Hubble sequence and the associated galaxy scaling relations are salient issues in modern astrophysics. In this talk I overview the current state of computational galaxy formation and discuss recent advances in the field. I will focus on results from recent state-of-the-art cosmological simulations where the locations of massive star clusters are beginning to be resolved. I illustrate the sensitivity of galaxy evolution to how star formation and stellar feedback proceeds on small scales in the interstellar medium, and discuss the role of “feedback regulated" star formation. In this context I demonstrate the role of stellar feedback, and emerging galactic winds, in controlling observables such as the baryon content of galaxies, galaxy sizes, gas and stellar metallicities, and the Kennicutt-Schmidt relation over cosmic time.
I will present results from two spectroscopic surveys of the LMC supergiant HII region 30 Doradus (Tarantula Nebula), namely the VLT-FLAMES Tarantula Survey (VFTS) in which the physical, binary and kinematical properties of 800 massive stars have been obtained, plus a HST/STIS census of the central R136 ionizing cluster. I will also compare our spatially resolved stellar census with the integrated properties of the giant HII region and R136 cluster - as would be witnessed if viewed from Mpc distances, relevant to determinations of ages and stellar mass functions of extragalactic star forming regions.
The warm dark matter (WDM) model is a promising alternative cosmological scenario. I will talk about WDM structure formation, addressing both numerical simulations and analytical approaches.
Star formation is a key problem in astronomy which impacts on galaxy formation and evolution, and sets the initial conditions for planet formation. I will talk about our current understanding of local, low-mass star formation. Firstly, how we think that bound star clusters form from smaller substructure. And also how binary observations suggest that star formation might vary from region to region even though the IMF stays the same. The conclusion is that - right now - I don't think we really understand how stars form...
Star formation is a key problem in astronomy which impacts on galaxy formation and evolution, and sets the initial conditions for planet formation. I will talk about our current understanding of local, low-mass star formation. Firstly, how we think that bound star clusters form from smaller substructure. And also how binary observations suggest that star formation might vary from region to region even though the IMF stays the same. The conclusion is that - right now - I don't think we really understand how stars form...
The astronomy group runs a weekly lunchtime talk programme with staff, postdocs, students and visitors giving short talks on a subject of interest to the group. Talks begin at 1pm on Thursdays in room A113 (unless stated otherwise) in the Centre for Astronomy and Particle Theory.
The talks have a duration of approximately 20-30 minutes, and are followed by a short question and answer session.
Click on an event for more information.
Event organiser: Alex Griffiths
Every week, a student or post-doc presents a recent paper that caught his/her attention. Staff are not allowed at these meetings to encourage students to discuss freely. More details can be found on the wiki hosted at PB Works (password required).
The Journal Club takes place every Tuesday, at 1pm in room A113 of the Cripps Building.
Event organiser: Lizzie Elmer
This Colloquium series hosts prominent scientists who will discuss some of the newest developments in physics and astronomy. It is aimed at a general physics audience at a level accessible to physics PhD students.
All Colloquia will be held in Physics building at 4pm followed by refreshments in room C10.
Event Organiser: Juan P. Garrahan (Condensed-matter theory), Peter Beton (Experimental CM/Nanoscience), Josef Granwehr (Magnetic Resonance), Lucia Hackermuller (Cold Atoms), David Maltby (Astronomy), Tony Padilla (Particle Theory)