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[
Joint Center for Astrophysics]
Introduction to AGN
What is an Active Galactic Nucleus ?
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At optical wavelengths, the
emission from most galaxies is dominated by that from
the stars.
However in at least 10% of galaxies, additional
intense emission
is also detected from the center (nucleus) of the galaxy.
This emission often far out-shines that from the surrounding
stars (often by factors >102).
In such galaxies, observations in other wavebands
(radio, IR, UV, X-rays & Gamma-rays) often
also reveal emission indicating a variety of
non-stellar processes are present.
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A composite of images from the Galaxy Centaurus A in the
radio, optical and X-ray bands.
[Image Credit:
CXC]
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Artist's Impression
[Image Credit:
Imagine!]
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Furthermore the emission from the nucleus invariably
has temporal and
spectral characteristics
which indicate the source of the emission is
not the result of stellar processes.
- the emission varies too fast to be due to a collection of stars
- the overall spectral energy distribution of the emission is
very different to that for stars
- the spectral contain features not observed in stars
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Such galaxies are called "active galactic nuclei"
(AGN), or sometimes simply "active galaxies".
The more distant (highly luminous) objects are usually referred to a
quasars.
As stated above, in other wavebands,
the difference between "normal galaxies"
and AGN is often far more obvious.
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In the radio band a subset of AGN exhibit
jets, often ending in lobes far outside the
galaxy. These jets eminate from the center of the galaxy, which is
often visible a point source of emission.
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In the optical & UV bands, spectral
emission lines are usually present with
characteristic shapes and intensity ratios
(along with absorption lines and/or jets in some sources).
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In the X-ray band the emission from
"classic" AGN
is totally dominated by a point source at the center of
the galaxy (although diffuse, extended emission
and jets are sometimes visible, along with a
number of low-luminosity X-ray sources within the galaxy
itself).
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Images from the Galaxy Centaurus A in the
radio, optical and X-ray bands.
[Image Credit:
CXC]
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Traditionally AGN are divided into numerous
(often overlapping)
sub-classes
based on various differences in
their observed characteristics.
However the current paradigm is that the ultimate
energy source in all AGN is similar, and that the
observed characteristics are secondary effects
of their environment and our viewing angle.
Why are AGN Interesting ?
The copious energy output from AGN
(typically 109 - 1013 Lsun)
is thought to arise from the
accretion of material onto a
supermassive black hole
(typically 106 - 109 Msun).
This is supported by
- Efficiency Arguments: Convertion of PE into KE
(by accretion) and hence
radiation most efficient process we know of.
- Variability Studies: The sources vary fast
implying a small region (size limited by speed of light).
- Spectral features: the shapes of several spectral
features are consistent with being formed in the
accretion disk, most notably the
Fe Kalpha fluorescent line.
- Jets: While theories not fully worked out,
an accretion disk/black hole sytem
is our current best-guess at how the jets might
form
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Artist's Impression of a Black Hole & inner Accretion Disk
[Image Credit:
NASA/GSFC]
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Thus the inner regions of AGN may some of the most
extreme laboratories in the universe.
Thus we can use them to test our
physical laws & understand
(including General Relativity) in extreme conditions
(which cannot be produced on Earth).
Comments or questions to ian.george@umbc.edu
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