Active Galaxies and Quasars - Introduction
Active Galaxies and QuasarsActive 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 synchrotron 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 GalaxiesOf the two types of Active Galactic Nuclei (AGN) which emit gamma rays, Seyfert galaxies are the low-energy gamma-ray sources.
Seyfert galaxies typically emit most of their gamma rays up to energies of about 100 keV and then fade as we observe 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 nucleus
QuasarsOne 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.
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.|
BlazarsThe AGNs 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.