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ASTRO110 An Overview of Active Galaxies - OPEN University
Mark A Garlick: www.space-art.co.uk©
Course Description
This unit begins by studying evidence leading to
our basic hypothesis that active galactic nuclei (AGN) are accreting,
supermassive black holes. It also covers some physics of radiation
which you will need to be able to interpret the observed emission of
AGN, and includes an examination of AGN.
You
will be studying a young subject and there are fundamental issues which
are still being vigorously debated by the experts and are subject to
current research activity. Consequently, it is not possible to give
clear definitive explanations of all aspects of the subject.
You
may already have become accustomed to reading unfamiliar words and
phrases. In this unit, you will not only encounter new and specialised
vocabulary, you will meet ideas which are currently being shaped and
tested. Do not be dismayed if you fail to immediately grasp the
underlying principles behind some of the material you will read: it is
possible no-one has yet elucidated them!
By the end of this unit you should be able to:
- recognise the terminology which is used to describe the properties and behaviour of active galactic nuclei (AGN);
- manipulate numbers, algebraic symbols and mathematical functions in equations.
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Introduction
 Introduction Resource- This unit begins by studying
evidence leading to our basic hypothesis that active galactic nuclei
(AGN) are accreting, supermassive black holes. It also covers some
physics of radiation which you will...
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1 Meet your first active galactic nuclei
- 1 Meet your first active galactic nuclei Resource
- Figure 1 compares two nearby
spiral galaxies of similar distance and type. NGC 5548, on the left,
has a brighter nucleus than that of NGC 3277, on the right. This extra
emission from the central...
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2 Black holes: a reminder
- 2 Black holes: a reminder Resource
- You may have previously met the
formation of a black hole at the end of the life of a massive star.
Accreting black holes, which were formed in this way, are members of
close binary star systems.
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3 AGN reside at the centres of galaxies
- 3.1 AGNs Resource
- It is clear that the objects
studied by Fath and Seyfert, such as those shown in Figure 1 and Figure
2, are bright nuclei at the centres of apparently otherwise normal
galaxies. For many AGN this fact...
- 3.2 QSO spectra Resource
- It only became apparent that
these quasi-stellar objects were not stars when their spectra were
examined. At first astronomers could not interpret their spectra
because the spectral lines did not appear...
- 3.3 Luminosities Resource
- The issue of whether QSO
redshifts are of cosmological origin was unambiguously settled by the
work illustrated in Figure 5 (particularly the two leftmost panels).
When sensitive enough observations are...
- 3.4 The Eddington Limit Resource
- Thus the observations require
that a luminosity of around 100 times that of the entire Milky Way
Galaxy be generated within a region with a diameter only about 1000
times that of the Earth's orbit! (A...
- 3.5 Example 1 Resource
- (a) A gravitationally bound
uniform density sphere, of radius r, is composed of a large number of
subelements, with total mass M. Use the virial theorem,
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4 Black holes at the centres of ordinary galaxies
- 4.1 The Milky Way Resource
- Figure 1 showed two spiral
galaxies: NGC 5548, which has an active nucleus, and NGC 3277 which
does not. If we accept that AGN are the result of accretion on to
supermassive black holes at the centres...
- 4.2 Activity 3: Stars orbiting our local supermassive black hole Resource
- Read through the following notes, and then watch the animation below.
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5 Distances in extragalactic astronomy
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6 The key questions
- 6 The key questions Resource
- Some of the most important issues which astronomers working in this field are currently trying to resolve are:
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7 Continuum emission processes
- 7.1 Blackbody radiation Resource
- Interpretation of the light
astronomers collect from AGN depends on understanding the physical
processes leading to the emission of that light. Because the conditions
in the emitting regions of AGN are...
- 7.2 Free-free radiation Resource
- The blackbody spectrum is emitted
when thermally emitting matter is optically thick. Optically thin
matter can also emit thermal radiation. Whenever a charged particle is
accelerated it emits electromagnetic...
- 7.3 Polarization of electromagnetic radiation Resource
- So far we have described
electromagnetic radiation in terms of its wavelength, frequency and
speed. It has another, sometimes important, property: polarization.
Figure 10 shows the electric and magnetic...
- 7.4 Faraday depolarization Resource
- Radiation of wavelength λ which
starts off linearly polarized in a particular direction and travels
through a plasma has its direction of polarization rotated by an angle
- 7.5 Emission from spiralling electrons: synchrotron radiation Resource
- In the very first reading
(Activity 1) we encountered the term ‘non-thermal’ describing the
spectrum of light emitted from AGN. In this subsection we will learn
more about the most important type of non-thermal...
- 7.6 Producing synchroton radiation in a laboratory Resource
- Synchrotron radiation can, of
course, be produced in a laboratory by arranging for electrons to be
deflected by a magnetic field. Figure 13 shows a beam of synchrotron
radiation produced at the European...
- 7.7 Radiation detection Resource
- In astronomy we detect the
radiation from large numbers of electrons, rather than being able to
distinguish the contributions of individual electrons. The electrons
will have a range of velocities and...
- 7.8 Example 2 and questions Resource
- Example 2
- 7.9 Compton scattering Resource
- Electromagnetic radiation
interacts strongly with electrons. If a photon encounters an electron,
there is a high probability that a scattering interaction will occur.
In the low-energy non-relativistic...
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8 Basic properties and historical perspective
- 8.1 Continuum spectra Resource
- Read Section 1.3 of Peterson, up to and including the first two paragraphs of Section 1.3.1 by clicking the link below.
- 8.2 Extended radio sources Resource
- In section 7.5 we studied the
spectrum of the synchrotron emission, i.e. how the flux density of
radiation depends on the frequency or wavelength of the radio emission.
Using radio telescopes such as the...
- 8.3 Line spectra: ions and spectral lines Resource
- For obvious reasons optical
astronomy developed earlier than radio and X-ray astronomy, and
astronomers are able to learn many things from analysis of optical
emission. Just as in the radio band, optical...
- 8.4 Line spectra: line flux and equivalent width Resource
- In general, astronomical objects
emit both continuous emission and lines superimposed on this continuous
emission. The equivalent width is a useful way of describing the
relative strength of a line compared...
- 8.5 Line spectra: Activity 7 Colours and broad lines Resource
- Read Peterson Sections 1.3.3 and 1.3.4 by clicking the link below.
- 8.6 Line spectra: Activity 8 Quasar redshifts Resource
- Read Peterson section 1.3.5 (pages 16 and 17) by clicking the link below.
- 8.7 Luminosity functions Resource
- Samples of galaxies can be biased due to the flux limit of the sample that is observed. This is the so called Malmquist bias.
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9 Summary
- 9 Summary Resource
- Active galactic nuclei (AGN) are
powered by accretion onto supermassive black holes. The masses of these
central engines of AGN can be estimated by means of the virial theorem,
using the size of the nucleus...
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References and Acknowledgements
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Copyright 2007,
by the Contributing Authors.
Cite/attribute Resource.
administrator. (2010, January 03). ASTRO110 An Overview of Active Galaxies - OPEN University. Retrieved July 31, 2010, from Free University Courses OCW Courses OpenCourseWare Freeversity Foundation Web site: http://www.freeversity.org/science-and-mathematics/astronomy/astro110-an-overview-of-active-galaxies-open-university.
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