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Topic outline

 

  • Time: 20 hours
    Level: Intermediate

 
 

Introduction

  • Introduction Resource
  • Active galaxies provide a prime example of high-energy processes operating in the Universe. This unit introduces the evidence for activity from the spectra of some galaxies, and the concept of a compact...
 

1 Overview

  • 1 Overview Resource
  • Even in images taken with the most modern equipment on a large telescope, it can be difficult to pick out the galaxies now known as ‘active’ from the other more normal galaxies. But if your telescope were...
 

2 The spectra of galaxies

  • 2.1 What contributes to the spectra of galaxies? Resource
  • This section reviews what you may already know about the spectra of galaxies. The topic will later be developed further to help you appreciate the spectra of active galaxies.
  • 2.2 Optical spectra Resource
  • Normal galaxies are made up of stars and (in the case of spiral and irregular galaxies) gas and dust. Their spectra consist of the sum of the spectra of these components.
  • 2.3 Broadband spectra Resource
  • 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.
 

3 Types of active galaxies

  • 3.1 Introduction Resource
  • 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...
  • 3.2 Seyfert galaxies Resource
  • In 1943 the American astronomer Carl Seyfert (1911–1960) drew attention to a handful of spiral galaxies that had unusually bright point-like nuclei. Figure 12 shows NGC 4051, one of the first Seyfert galaxies...
  • 3.3 Quasars Resource
  • One of the most unexpected turns in the history of astronomy was the discovery of quasars. When first recognised, in 1963, quasars appeared at radio and optical wavelengths as faint, point-like objects...
  • 3.4 Radio galaxies Resource
  • Radio galaxies were discovered accidentally by wartime radar engineers in the 1940s, although it took another decade for them to be properly studied by the new science of radio astronomy. Radio galaxies...
  • 3.5 Blazars Resource
  • 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....
  • 3.6 A ‘non-active’ class – the starburst galaxies Resource
  • We end this section by drawing a distinction between the classes of active galaxy that are described in the previous subsections and the starburst galaxies mentioned earlier. As you have seen, starburst...
 

4 The central engine

  • 4.1 Introduction: the active galactic nuclei (AGN) Resource
  • From Section 3 you will have discovered that one thing all active galaxies have in common is a compact nucleus, the AGN, which is the source of their activity. In this section you will study the two properties...
  • 4.2 The size of AGNs Resource
  • AGNs appear point-like on optical images. It is instructive to work out how small a region these imaging observations indicate. Optical observations from the Earth suffer from ‘seeing’, the blurring of...
  • 4.3 The luminosity of AGNs Resource
  • It is instructive to express the luminosity of an AGN in terms of the luminosity of a galaxy like our own. The figure may then be converted into solar luminosities, if we adopt the figure of 2 × 1010L⊙...
  • 4.4 A supermassive black hole Resource
  • A black hole is a body so massive and so small that even electromagnetic radiation, such as visible light, cannot escape from it. It is its combination of small size and very strong gravitational field...
  • 4.5 An accretion disc Resource
  • What will happen to matter that comes near a black hole? Consider a gas cloud moving to one side of the black hole, such as cloud A in Figure 29.
  • 4.6 Accretion power Resource
  • Calculations based on the above accretion disc hypothesis show that if a mass m falls into the black hole, then the amount of energy it can radiate before it finally disappears is about 0.1 mc2, or about...
  • 4.7 Jets Resource
  • 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...
 

5 Models of active galaxies

  • 5.1 Introduction Resource
  • 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...
  • 5.2 The obscuring torus Resource
  • If an AGN consisted solely of the central engine, observers would see X-rays and ultraviolet radiation from the hot accretion disc (accounting for the ‘the big blue bump’ in Figure 17) and, apart from...
  • 5.3 The broad- and narrow-line regions Resource
  • In our model, the engine is surrounded by gas clouds (Figure 32). You have already seen how common these are in our own and other galaxies, so it is reasonable to expect them to be present in at least...
  • 5.4 Unified models Resource
  • You are now familiar with the main components for building models of AGNs: a central engine powered by an accreting supermassive black hole (with or without jets), clouds of dust, clouds of gas and accretion...
 

6 Outstanding Issues

  • 6.1 Introduction Resource
  • The active galaxy model is very attractive. Indeed, it is so attractive that it is easy to overlook the many problems that remain. We will now consider some of the outstanding questions about the origin...
  • 6.2 Do supermassive black holes really exist? Resource
  • One outstanding feature of the black-hole model is that the black hole must be supermassive. Can one at least detect the presence of a massive central object?
  • 6.3 Where are they now? Resource
  • At the beginning of this unit we asked whether active galaxies really are in a class of their own or whether most galaxies go through an active stage at some point in their lives. We can shed some light...
 

7 Unit Summary

  • 7.1 The spectra of galaxies Resource
  • The spectrum of a galaxy is the composite spectrum of the objects of which it is composed.
  • 7.2 Types of active galaxy Resource
  • All active galaxies have a compact, energetic nucleus – an AGN.
  • 7.3 The central engine Resource
  • An object that fluctuates in brightness on a timescale Δt can have a radius no greater than R ∼ cΔt.
  • 7.4 Models of active galaxies Resource
  • The standard model of an AGN consists of an accreting supermassive black hole (the engine) surrounded by a broad-line region contained within a torus of infrared emitting dust and a narrow-line region....
  • 7.5 Outstanding issues Resource
  • Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes.
  • 7.6 Questions Resource
  • Suppose that a galaxy has emission lines in its optical spectrum. A line of wavelength 654.3 nm is broadened by 2.0 nm. Estimate the velocity dispersion of the gas giving rise to the broadened spectral...
 

References and Acknowledgements

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