Active Galaxies and Quasars
Active galaxies are
galaxies which have a small core of emission embedded in an otherwise typical
galaxy. This core may be highly variable and very bright compared to the rest
of the galaxy. Models of active galaxies concentrate on the possibility of a supermassive
black hole which lies at the center of the galaxy. The dense central
galaxy provides material which accretes onto the black hole releasing a large
amount of gravitational energy. Part of the energy in this hot plasma is
emitted as x-rays and gamma-rays.
For "normal" galaxies, we can think of the total energy they emit
as the sum of the emission from each of the stars found in the galaxy. For
the "active" galaxies, this is not true. There is a great deal more
emitted energy than there should be... and this excess energy is found in the
infrared, radio, UV, and X-ray regions of the electromagnetic spectrum. The
energy emitted by an active galaxy (or AGN) is anything but "normal".
So what is happening in these galaxies to produce such an energetic output?
There are several types of active galaxies: Seyferts, quasars, and
blazars. Most scientists
believe that, even though these types look very different to us, they are
really all the same thing viewed from different directions! Quasars are active
galaxies which are all very, very, very far away from us. Some of the quasars
we have seen so far are 12 billion light-years away! Blazars are very
bright in the radio band, which results from looking directly down a
jet which is emitting in synchotron radiation. On the other hand, if the jet is not pointing toward you
at all, and the dusty disk of material which lies in the plane of the
galaxy is in the way, you would see just
what we see from the Seyferts. By measuring their redshifts, we find
that Seyferts are much closer to us than
quasars or blazars.
Active galaxies are intensely studied at all wavelengths. Since they
can change their behavior on short timescales, it is useful to study
them simultaneously at all energies. X-ray and gamma-ray observations have
proven to be important parts of this multiwavelength approach since many
high-energy quasars emit a large fraction of their power at such
energies. X-rays can penetrate outward from very near
the center of a galaxy. Since that is where the "engines" of AGN are
located, X-rays provide scientists with unique insights into the physical
processes occurring there. In addition, gamma-ray observations alone can
provide valuable information on the nature of particle acceleration in the
quasar jet, and clues as to how the particles interact with their
surroundings.
A diagram of an active galaxy, showing
the primary components.
Seyfert Galaxies
Of the two types of Active Galactic Nuclei (AGN) which emit gamma-rays,
Seyfert galaxies are the low-energy gamma sources.
Seyfert galaxies typically emit most of their gamma-rays up to energies of about
100 keV
and then fade as we obeserve them at higher energies. Early gamma-ray observations of Seyfert
galaxies indicated that photons were detected up to MeV energies, but more
sensitive observations have cast doubt on this possibility. At these low
gamma-ray energies, the emission is usually a smooth continuation of the X-ray
emission from such objects. This generally indicates that the physical
processes creating the gamma-rays are thermal processes similar to those
responsible for emission from galactic black hole sources. As a result,
gamma-ray studies of the high-energy spectrum and variability can give
scientists important information about the physical environment in the AGN.
Observations of Seyfert galaxies in gamma-rays are also important for
studies of the cosmic gamma-ray background. Even in regions of the sky where
there are no point sources, a faint gamma-ray glow is detectable. It may be
that this glow is the sum of many faint galaxies or perhaps a more exotic
process. Studies of individual Seyfert galaxies can be combined with a model
of how such objects are distributed in the Universe to compare to the diffuse
gamma-ray background. In this way, astronomers not only learn about the
interesting AGN phenomena, but learn more about the general nature of the
Universe as a whole.
An artists concept of an active galactic
nuclei
Quasars
One of the most remarkable trends in gamma-ray astronomy in recent
years has been the emergence of high-energy gamma-ray quasars as an
important component of the gamma-ray sky. At gamma-ray energies, these
active galaxies are bright; they are highly variable at all energies.
Unlike the Seyfert type AGN, most of these sources are
preferentially detected at high energies, usually 100 MeV or more.
In fact, they have been detected above 1 GeV, and some up to several TeV!
Given the large distances to these objects and the strong emission of
high-energy gamma-rays, these are the most powerful particle accelerators in
the Universe. Over 50 high-energy quasars are known at this time.
Some appear as fuzzy stars that can be seen with large amateur telescopes.
Many astronomers believe that Seyfert galaxies and high-energy
quasars are basically the same type of objects, but we are simply viewing
them differently. Radio observations of AGN often show powerful jets, streams
of particles coming from the central source -- like water from a spigot.
Charged particles are accelerated to nearly the speed of light in these jets. In the unified view of active galaxies,
high-energy quasars are being viewed with the jet pointed towards us
which allows us to see the resulting energetic radiation. With Seyfert
galaxies, we are viewing from the side and do not see the very high-energy
radiation which is traveling down the jet.
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The region of the sky containing one of the high-energy quasars,
PKS 0528+134, is shown at two different times using the EGRET
instrument on the Compton Gamma Ray Observatory.
These active galaxies are highly variable, strongly emitting gamma-rays
sometimes, disappearing at other times.
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Blazars
The AGN's observed at higher energies form a subclass of
AGNs known as blazars; a blazar is believed to be an AGN which has
one of its relativistic jets pointed toward the Earth so that
what we observe is primarily emission from the jet region. They are
thus similar to quasars, but are not observed to be as luminous. The visible
and gamma-ray emission from blazars is variable on timescales from
minutes to days. Although theories exist as to the
causes of this variability, the sparse data do not yet allow any of the
ideas to be tested.
To date more than 60 blazars have been detected by the EGRET experiment
aboard the Compton Gamma-Ray Observatory. All these objects appear to
emit most of their bolometric
luminosity at gamma-ray energies and, in
addition, are strong extragalactic radio sources.
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