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7.3 The central engine

  • An object that fluctuates in brightness on a timescale Δt can have a radius no greater than RcΔt.

  • The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System.

  • The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M and Schwarzschild radius
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5.1 Introduction

So far we have seen how the properties of the central engine of the AGN can be accounted for by an accreting supermassive black hole. Though there are many questions still to be resolved, this model does seem to be the best available explanation of what is going on in the heart of an AGN. But of course all AGNs are not the same. We have identified four main classes and in this section we will attempt to construct models that reproduce the distinguishing features of these four classes.


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4.7 Jets

You have seen that two kinds of active galaxies – quasars and radio galaxies – are often seen to possess narrow features called jets projecting up to several hundred kiloparsecs from their nuclei. If these are indeed streams of energetic particles flowing from the central engine, how do they fit with the accretion disc model? How could the jets be produced?

The answers to these questions are not fully resolved, but there are some aspects of the model of the central engine which prob
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3.5 Blazars

Blazars appear star-like, as do quasars, but were only recognised as a distinct class of object in the 1970s. They are variable on timescales of days or less. All are strong and variable radio sources. There are two subclasses.

BL Lac objects are characterised by spectra in which emission lines are either absent or extremely weak. They lie at relatively low redshifts. At first, they were mistaken for variable stars until their spectra were studied. (Their name derives from BL Lac
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3.1 Introduction

Active galaxies have occupied the attention of an increasing number of astronomers since the first example was identified in the 1940s. By one recent estimate, a fifth of all research astronomers are working on active galaxies, which indicates how important this field is. In this section you will learn about the observational characteristics of the four main classes of active galaxies: Seyfert galaxies, quasars, radio galaxies and blazars. This will set the scene for subsequent sections in wh
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Active galaxies

Figure 10 shows the spectral energy distribution of an active galaxy.

Figure 10
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Normal galaxies

Figure 8 shows schematically the broadband spectrum of a normal spiral galaxy. It resembles that of the Sun, although the peak occurs at a slightly longer wavelength and there are relatively greater spectral flux densities at X-ray, infrared and radio wavelengths.

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2.3 Broadband spectra

The broadband spectrum is the spectrum over all the observed wavelength ranges. To plot the broadband spectrum of any object it is necessary to choose logarithmic axes.

  • Why is it necessary to use logarithmic axes?

  • Because both the spectral flux density, Fλ, and the wavelength vary by many powers of 10.

Author(s): The Open University

Learning outcomes

By the end of this unit you should be able to:

  • explain how and why the optical spectrum of an active or starburst galaxy differs from that of a normal galaxy;

  • explain how and why the broadband spectrum of an active or starburst galaxy differs from that of a normal galaxy;

  • describe briefly the observed features of starburst galaxies and the four main classes of active galaxies (quasars, radio galaxies, Seyfert galaxies and blazars);


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References

References and further reading
Bodmer, W. (1985) The Public Understanding of Science, London, Royal Society.
Bown, W. (2005) Time to disengage. Editorial. Research Fortnight, 14 September 2005.
Council for Science and Technology (2005) Policy through dialogue: informing policies based on science and technology
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6.2 How DEMOCS works

DEMOCS games involve groups of around six to eight participants and take a couple of hours to play. They come as self-contained kits, which can be downloaded from nef's website. To register and log in for access to DEMOCS games, see http://www.neweconomics.org/gen/z_sys_DemocsRegister.aspx?destination=/gen/democsdownload.aspx, accessed 13 March 2007. Topics covered so far include stem cell research, over-the-counter genetic testing kits, xenotransplantation, pre-implantation genetic diagnosis
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6.1 Introduction

Reading 5 ends with a call for a move towards a more ‘deliberative democracy’ in which public engagement takes place in parallel with the development of new technologies, so that opportunities are provided for ongoing dialogue and influence between the two. To help to achieve this, the authors argue, ‘… now is the right time to start experi
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4 The GM Nation? debate

The UK's GM Nation? debate took place in the summer of 2003 – a time when both the political mood and scientific innovation had moved on a good deal in the eight years since the first UKNCC. Despite the relatively non-controversial introduction of the first GM products (notably a GM tomato paste and GM cheese) in the shops in the mid-1990s, public concerns grew in subsequent years. One particularly influential event was the arrival in the late 1990s of US and Canadian soya beans in Europe,
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9.2 Other hazards

9.2.1 Stumbles, falls, etc.

Such minor accidents are always possible, especially on rough or uneven terrain. It is recommended, therefore, that boots should be worn to protect the feet and ankles, and that outer clothing should be of a suitable nature to minimise the chance of cuts, scratches, and abrasions being sustained. Wearing gloves will minimise damage to hands.


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8.1 Syringes and sharps

  • Observe good laboratory practice by wearing gloves; latex or nitrile gloves are best.

  • Do not re-sheath needles or sharps unless unavoidable due to the nature of the work.

  • Remove scalpel blades with a specific scalpel-blade-removing tool.

  • Dispose of all syringes and sharps in designated containers (labelled ‘sharps’); when these are full they should be taken for commercial disposal
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3.1 Insect resistance

We will now look briefly at the science underlying the traits introduced into commercial crops, which you explored in Activity 1; a useful place to start is by considering how the property of resistance to insects is acquired by crops.

Insect damage causes huge losses of agricultural crops each year. For example, without co
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2.2 Using A. tumefaciens to genetically modify plant cells

Genetic engineers have capitalised on the fact that part of the DNA from the Ti plasmid of A. tumefaciens is integrated into the plant genome during the infection process. Ti plasmids can be isolated and a foreign gene spliced in at an appropriate point, making it possible to transfer the novel gene into the plant.

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Introduction

In recent years, scientists have made huge gains in their understanding of how genes can be altered and transferred from one organism to another – but that knowledge has been acquired amidst controversy and concern. The deep ethical concerns that have resulted from the emergence of genetic manipulation are explored in this unit. We begin with an examination of the basic structure and function of genes. A number of pioneering examples and techniques are explored, helping to explain why our p
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5 Summary of unit

We have seen in this unit that, despite having a high natural abundance, iron is in very short supply because of the insolubility of its oxides and hydroxides. A result of this is that organisms have developed methods for the uptake, transport and storage of iron. Bacteria, in particular, secrete very powerful iron chelators known as siderophores. Of all the iron–siderophore complexes, the iron(III)–enterobactin complex has the exceptionally high stability constant of 1049 mol<
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