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1. INTRODUCTION

In 1963, Maarten Schmidt of the California Institute of Technology took a spectrum of a "star" that was associated with the newly discovered radio source 3C273. To his amazement, he found that it was not a star at all, but it was the most distant object that had been seen in the universe to that date. At that distance, 3C273 had the luminosity of 2 x 10¹³ Suns -- 100 times the luminosity of the entire Milky Way galaxy! Schmidt called 3C273 a "Quasi-Stellar Radio Source," a name that was soon shortened to quasar. Soon afterwards, astronomers found several other quasars, some substantially more luminous and distant than 3C273.

Today, 35 years later, we are almost certain that this is the case. Moreover, we now know of thousands of galaxies that contain supermassive black holes at their centers. They are not all quasars: some are called Seyfert galaxies; some are called radio galaxies, some are called Blazars. Since believe that they are all variations of the same phenomenon, we call them Active Galactic Nuclei, or AGN for short.

Moreover, we suspect that many -- perhaps most -- other galaxies also have supermassive black holes in their centers. They are fairly quiet now, but they may once have been as luminous as quasars and they may blaze again at some future time. Indeed, the Milky Way contains such a monster, as we have already discussed in Lesson 9.

An excellent place to read more about Active Galaxies and Quasars is Gravity's Fatal Attraction (1996: Scientific American Library), by Mitchell Begelman and Martin Rees.

As a warm-up, you might want to start with Active Galaxies and Quasars from NASAs Imagine.

There's a delicious irony about the process of scientific discovery. Optically, quasars look like faint stars. The brightest, 3C273, has visual magnitude 12.8, and most are much fainter. No astronomer would bother to take a spectrum of such a star -- there are millions in the sky -- unless it had some peculiarity. But in 1960, radio astronomers in Cambridge, England, published the 3rd Cambridge Catalogue of radio sources, and a few of these radio sources were located at exactly the same position as some of these faint stars. The radio emission led Maarten Schmidt to the discovery of quasars. But now we know that 95% of quasars* have no significant radio emission.

*Some astronomers reserve the word quasar for the 5% of such objects that are strong radio sources, calling the radio-quiet ones Quasi-Stellar Objects, or QSOs. But I think it's OK to call them all quasars.

In the early 1960s, radio astronomers from England completed the 3rd Cambridge Catalog of radio sources. As a result, most of the radio sources in the sky are designated with a label 3C...* They found that many of these radio sources were coincident with distant galaxies. Now, with the much better angular resolution of the VLA and VLBA radio telescope arrays, we can see that these radio sources often consist of point sources at the center of the galaxy and extended sources that extend thousands or even millions of light years beyond the galaxy into intergalactic space.

*An important astronomical source can have several different names that arise from the different surveys by which it is observed. For example, the giant elliptical galaxy M87 in the Virgo Cluster is also called: NCG 4486, from the New General Catalog; Virgo A, because it is the brightest radio source in the constellation Virgo; and 3C 274, from the 3rd Cambridge Catalog.

 The image on the left is Cygnus A, one of the brightest radio sources in the sky and a classical example of a double-lobed radio galaxy. The radio emission comes primarily from two giant lobes in intergalactic space, each displaced more than 250 million light years from the central galaxy. The tiny spot in the center of the image lies at the center of the galaxy. You can see faint jets of radio emission extending from this point to both lobes. They are narrow beams of relativistic electrons (i.e., electrons moving at nearly the speed of light). At the bright radio lobes, the beams are stopped by collisions with intergalactic gas, as illustrated here: Jet Near Light Speed. Powerful turbulent flows generate electrical currents and magnetic fields, which cause the electrons to spiral and emit polarized synchrotron radiation.

On the right is a HST optical image of the central region of the giant elliptical galaxy M87 (Virgo A). The scale is much smaller here: the jet extending in the 2 o'clock direction from the point-like nucleus of the galaxy has a length of about 2 kpc. You can see the same core-jet structure in radio and X-ray images of M87, and time-lapse images show that the bright knots in the jet are moving at a substantial fraction of the speed of light.

Radio astronomers at Manchester University have invented the acronym, DRAGNs, meaning Double Radio Sources Associated with Galactic Nuclei. I think it's a great name, because these DRAGNs are certainly breathing fire! You should look at Atlas of DRAGNs from Manchester University, where you can find an excellent summary of radio galaxies and many images. You can also find some more good Images of Radio Galaxies and Quasars by Alan Bridle of the National Radio Astronomy Observatory.

Recently, the Hubble Space Telescope has observed a few radio galaxies and has found very complex optical structures. See Hubble Observes Radio Galaxies. The optical counterparts of most radio galaxies appear to be either elliptical galaxies (e.g., M87) or colliding galaxies (e.g., Cygnus A, Centaurus A).

We think the difference between the two types of Seyfert galaxies is just a matter of viewing aspect. If we see the galaxy nearly face-on, we can see broad emission lines from very rapidly moving gas near the center; but if the galaxy is tilted so that the center is obscured by dust, we only see narrow emission lines from more slowly moving gas that is more distant from the center, as illustrated in this outline summary: Seyfert galaxies.

We believe that Seyfert galaxies, like all the objects we discuss in these lessons, are powered by supermassive black holes at their centers. Recently, the Hubble Space Telescope has found more evidence that NGC4151 has such a black hole; see Fireworks in a Black Hole near the core of Seyfert galaxy NGC4151.

Until recently, quasars were unresolved star-like objects. Because a quasar is hundreds or thousands of times more luminous than an entire galaxy, and the quasars are at great distance, it was almost impossible to see whether there was a galaxy around a quasar with ground-based telescopes. But now, with the Hubble Space Telescope, astronomers have seen galaxies around quasars. The galaxies are often highly disturbed and may be interacting galaxies. See Hubble Surveys the "Homes" of Quasars.

With very long baseline interferometry (VLBI), radio astronomers can get a much closer look at the center of 3C273, as illustrated by the movie clip on the right. They see radio "blobs" moving away from the central object in the same direction as the jet. The blobs seem to move a transverse distance of about 30 light years in 3 years, apparently at a speed of 10 times the speed of light. This "superluminal expansion" is an illusion resulting from the fact that the jet is actually moving toward us. The true speed of the jet is about 95% of the speed of light -- still pretty fast, but no violation of Einstein's theory.

Most quasars are so distant that their light has been travelling for billions of years on its way to our telescopes. So, when we watch quasars today, we are seeing them as they actually were billions of years ago. We call this time delay look-back time. (This point is discussed in the box at the top of pp. 554-555 of your text. But the numbers given there for look-back time depend on the age of the universe, which we do not know very well. In fact, the numbers quoted there are probably low by about 25%.)

We see quasars with large redshifts as they were when the universe was much younger. Therefore, when we plot the numbers of observed quasars as a function of their redshifts, we are actually measuring the density of quasars in the universe as a function of the age of the universe. The amazing result of such exercise is that there were thousands of times as many quasars in the universe when it was about 2 billion years old as there are today. See A glimpse into the time before quasars were born. What happened to these quasars? We will see the answer in section 7 below.

Before you leave this section, you may want to check this excellent outline review: Quasars by Barbara Ryden.

Besides for radio galaxies (DRAGNs), Seyfert galaxies, and quasars, astronomers have found it convenient to invent another term, Blazars, to describe the most luminous and violent active galactic nuclei. Blazars are also called BL Lac objects, or Optically Violent Variables. They have the following distinguishing characteristics:

The image above is The Gamma Ray Sky as observed by the Compton Gamma Ray observatory. The bright sources above and below the Milky Way are blazars. For example, the brightest one, at about 1 o'clock, is the blazar 3C279. (Astronomers don't always distinguish blazars from quasars.) Its brightness can increase or decrease by factors of four in a few days, as you can see in this time history of gamma rays from 3C279.

In Lesson 9, we have already seen the evidence that the Milky Way has a supermassive black hole at its center. As we shall see here, astronomers are finding that many galaxies also have such monsters in their centers.

Above on the left is an optical image taken by the Hubble Space Telescope of the central region of the elliptical galaxy NGC 4261 in the Virgo cluster. You are looking at a tilted disk of gas and dust with a diameter of about 800 light years, evidently swirling around a central object of mass about 4 x 10⁸ Suns (click on the image for more details). See NGC 6251 for another HST image of a disk at the center of a galaxy.

Evidence is accumulating that many other relatively quiet galaxies have such supermassive black holes. See Massive black holes dwell in most galaxies from the Space Telescope Science Institute and John Kormendy's Search for Black Holes in the Centers of Galaxies for a list.

The most convincing evidence of a supermassive black hole in the center of a galaxy comes from radio observations of spots of very bright spectral line emission from water molecules in the galaxy NGC 4258 (a spiral galaxy also known as M106). One can measure the velocities of the clouds from the redshifts and blueshifts of the spectral lines. In Homework 5, you found that these velocities corresponded to the motion that Kepler's Third Law would predict if the central object had a mass of about 35 million Suns.

By now, you shouldn't be surprised to learn that active galaxies also have supermassive black holes at their centers. For example, the Hubble Space Telescope measured the spectrum of a rapidly rotating gas disk in M87 and found that this galaxy has the most massive central object yet found: about 3 x 10⁹ Suns!

The energy source: Einstein's famous formula, E = mc², gives the energy that can be produced as radiation if one can convert mass to energy. Stars do this by burning hydrogen into helium, helium into carbon and oxygen, and ultimately into iron. But the fraction of the original hydrogen mass that can be converted into energy by such fusion reactions is only about 1%. The other 99% of the mass is locked up as iron, the "ash" of the reactions, which has no more energy to release by fusion.

Most of the luminosity is generated by gas flowing in the accretion disk within a few times the Schwarzschild radius of the black hole. The Schwarzschild radius of a 10⁸ solar mass black hole is 2 Astronomical Units (see Lesson 8).

Feeding the monster: But what is the source of this mass flow? We have already seen several examples of relatively quiet galaxies with disks of gas and dust around supermassive black holes. But the disks we see with the HST are orbiting at distances of tens of light years, hundreds of thousands of times greater than the Schwarzschild radius. Evidently, most of the gas in these disks is simply orbiting the black hole, not spiraling in.

The Eddington Limit: There is a limit to the luminosity of an active galactic nucleus. This limit was originally derived by Sir Arthur Eddington in another context. Eddington realized that if a star became luminous enough, the radiation leaking out from the center of the star would exert an outward force on the star's envelope that would exceed the inward force due to gravity. Then the star would simply fly apart. Eddington calculated that there could be no star more massive than about 100 Suns as a result of this outward force, and that seems to be the case.

Jets: As we have seen, many active galaxies, including M87, DRAGNs, Seyfert galaxies, and 3C273, have narrow jets of electrons that move close to the speed of light and extend far into intergalactic space. We do not have a very good understanding of the mechanism that causes these jet-like outflows, but we can see that the jets are perpendicular to the accretion disks and that they are formed very near the center of the disks. We suspect that magnetic forces play an important role in confining the jet outflows to the polar directions and in accelerating the electrons to relativistic speeds.

Spectra: AGNs are luminous because of the energy released by gas as it flows into a supermassive black hole. But what accounts for the variety of different spectra that we see in AGNs? We believe that much of this variety can be explained by the orientation of the accretion disk.

X-ray emission: The gas in the inner disk near the Schwarzschild radius is very hot and radiates mostly X-rays. Typically, the spectrum X-rays from such hot gas has a strong emission line at energy 6.4 keV due to iron atoms in the gas. In 1995, Japanese astronomers using the ASCA X-ray satellite observed the X-ray spectrum of a type I Seyfert galaxy called MCG-6-30-15. They found a very broad emission line with a peak at 6.4 keV and a broad redshifted wing extending down to about 4 keV.

The X-ray line profile is shown in the above figure (courtesy of Chris Reynolds). The top image is an optical picture of MCG-6-30-15. The middle image is a theoretical model for the inner part of the accretion disk that is producing the X-ray emission line. A black hole is at the center. The X-ray emission is much brighter on the left because the disk is rotating counterclockwise and the gas on the left is moving toward us at almost 1/3 the velocity of light. The iron line emission extending down to 4 keV comes from gas in the disk that is not moving toward us. Its redshift is due to a combination of its high velocity and gravitational redshift (see Lesson 8) due to the fact that it is emitted so close to the black hole (about 3 times the Schwarzschild radius). The red curve is a theoretical model for the line profile that would be emitted by such a system. The fact that it agrees well with the data is consistent with the idea that the luminosity of Seyfert galaxies comes from gas in a disk near a supermassive black hole.

If you want to see a great little movie illustrating an X-ray flare near a black hole, click here.

Optical emission: Gas somewhat further from the center of an active galaxy will absorb of the X-rays that shine on it. The X-rays will heat and ionize the gas, causing it to emit optical and ultraviolet emission lines that we see in the spectra of quasars and Seyfert galaxies. If we can see near to the center of the active galaxy, we will see broad emission lines due to gas moving with velocities of several thousands of km/s. Such galaxies are quasars and Type I Seyfert galaxies. (Quasars are probably the same phenomenon as Seyfert galaxies, except that they are more luminous.)

Radio emission: Most of the radio emission from active galaxies comes from the beams of electrons going in the polar directions. We don't see much radio emission from most quasars and Seyfert galaxies because the beams are not pointed in our direction. DRAGNs are an exception to this rule: we see them because the beams are colliding with gas, causing a buildup of magnetic fields and enhanced synchrotron emission in all directions at the lobes where the collision takes place.

In blazars, one of the beams points almost directly at us. In that case, the radio, optical, and X-ray continuum radiation from the relativistic particles in the beam is so powerful that we don't notice the weaker emission line radiation produced by gas near the AGN.

A beginners guide to active galactic nuclei;
Quasar tour; and
Unified Model of AGN.

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Last modified May 1, 1998
Copyright by Richard McCray