Active Galactic Nuclei and Supermassive Black Holes
Until recently, the underlying physics of active galactic nuclei (AGN) were somewhat of a mystery to cosmologists. They seemed to produce forces and energies that were so large and in such a small volume that current cosmological theories could not account for them. Recent discoveries while studying inactive galactic nuclei, the center of normal galaxies like our own Milky Way, have allowed for theories to be put forward explaining AGN and the formation of all galaxies.
Active Galactic Nuclei come in several different types: Seyfert galaxies, quasars, and blazars. To understand how all of these seemingly different and disparate phenomena can be grouped together under a single category, we must first understand the history of each of these individual classes of object.
First investigated by Vanderbilt University astronomer Carl K. Seyfert (for whom they are named) in the 1940s, Seyfert galaxies are otherwise normal galaxies which have an uncharacteristically bright point source of electromagnetic radiation in the 100 keV gamma-ray band (1). This point source can be variable with a very small period, which indicates that the volume of the object emitting the radiation was on the order of one parsec (1). While these objects were found to be not uncommon in the universe and shown to be extragalactic due to their redshift, a viable theory explaining the object observed in the nucleus of these galaxies was absent for many years.
In the late 1950s and early 1960s, a new class of radio source objects was first catalogued. These sources of radio waves were soon matched to what appeared to be very dim stars in the visual band (2). When the redshift of one of these objects, 3C273, was calculated in 1962 by M. Schmidt and discovered to be 0.158, astronomers discovered that they were, in fact, very distant galaxies. This gives them the name quasar, which is short for QUAsi StellAr Radio source (2). In the 1960s, it was discovered that quasars have spectra very similar to the nuclei of Seyfert galaxies. This indicated that quasars were the same objects as Seyfert galaxies, only they were much further away (and consequently, much younger) and that the nucleus outshines the rest of the galaxy by a factor of 10-1000 (1). Radio observations of quasars show that they emit twin jets of particles and energy at relativistic speeds. We cannot detect these jets directly, as they are not pointed at us. These jets are ‘seen’ from earth as the energy given off as these extremely high-energy particles collide with the intergalactic gas and dust that permeates space (3).
Blazars are objects that emit very high amounts of electromagnetic radiation across all bands of the EM spectrum. Their redshift indicates that they, too, are extragalactic and occur towards to edge of the observable universe. Some characteristics of blazars are that their light exhibits high optical polarization and that they have high variability with periods of less than a few days. This very rapid variability indicates that the object emitting this polarized radiation is very small for an extragalactic object (1). The fact that blazars are extragalactic and emit extremely high amounts of energy from a small volume leads cosmologists to believe that they too are AGN. The crucial difference between blazars and Seyfert galaxies/quasars is that blazars are oriented in such a way that their relativistic jets of radiation and particles are pointed at the Earth. In the case of blazars, these jets need not be inferred from their interaction with the contents of intergalactic space, but can be viewed directly. This theory also accounts for the much higher luminosity and the much wider range over which this luminosity is observed (3). To date, more than 80 blazars have been catalogued by the EGRET experiment on board the Compton Gamma-Ray Observatory (1).
There are two subclasses of blazars that have been defined. Some blazars have spectra that exhibit emission lines similar to those of quasars. These are called Flat Spectrum Radio Quasars (FSRQ). The other classification encompasses blazars that have featureless optical spectra. These are known as BL Lac objects. Approximately 20% of blazars appear to be BL Lac objects, while the rest are FSRQs (1).
Bringing It All Together
When these phenomena were initially discovered no one had any idea that Seyfert galaxies, quasars, and blazars were all different manifestations of the same type of object. As they were all studied following their initial discovery, however, certain clues came to light that lent themselves to the modern theory of AGN. Firstly, the redshift of these objects, when applied to Hubble’s Law, indicated that they were all objects that resided outside our own galaxy. Thus far, the only objects luminous enough to be observed by us and reside outside the Milky Way were other galaxies. This idea fit well with Seyfert galaxies, but quasars and blazars appeared to be point sources, and the period of variability of all of these objects indicated that they occupied a very small volume. The connection between Seyfert galaxies and quasars was made when the spectra of these objects were compared. The spectra of these objects indicated that they were generated by a similar process. The redshift of the spectra indicated that the quasars were much further away from us than the Seyfert galaxies. Obviously, the quasars were Seyfert galaxies whose nuclei were much more active. So active, in fact, that they outshone the rest of the galaxy so that the nuclei were the only part that was visible from so far away. The blazars were connected to the Seyfert galaxies/quasars when it was observed that most blazars had emission lines similar to quasars, only with much more intensity. It was theorized that the bipolar jets emitting particles at relativistic speeds observed indirectly in Seyfert galaxies and quasars were pointed directly at Earth in the case of the blazars, accounting for their unmatched luminosity (1)(2)(3).
A Workable Theory
Once it was determined that Seyfert galaxies, quasars, and blazars were all in fact instances of the same phenomenon, henceforth referred to as AGN, the race to explain their peculiar behavior was on. As stated earlier, AGN exhibited both exceptionally intense luminosity and variability with a period on the order of a few days or weeks. Their luminosity coupled with their distance from Earth indicated that the process driving the AGN was extraordinarily intense, as they were radiating very large amounts of energy. The short variability period indicated that the mechanism of this process was housed in a very small volume, only a few light-weeks across. What could possibly emit so much energy in such a small volume? The only thing cosmologists had to offer were black holes containing several billion times the mass of our sun.
The theory of black holes and accretion disks has been around since 1971, when Cygnus X-1 was first proposed as the first empirical evidence for the singularity predicted by Special Relativity. Since light cannot escape their pull, the theory goes, it is impossible to observe black holes directly. Cygnus X-1 made direct observation unnecessary by being a binary system. The gravitational effects of the black hole were indicated by the other member of the system, a rather unremarkable B0 type star that was orbiting around a seemingly invisible partner. Additionally, this partner was siphoning stellar material off of the star into an accretion disk. As the material in this disk moved toward the center, it gained velocity and energy from friction. This energy was given off as X-rays.
Explaining the massive energy production of AGN was just a matter of applying the idea of accretion disks on a much larger scale. Instead of consuming a star, the supermassive black hole at the center of an AGN was consuming a galaxy. The accretion disk surrounding this black hole would be of such a scale as to explain the energy output of AGN. Bipolar jets are another feature of accretion disks surrounding black holes, so this strange feature is also explained by the supermassive black hole theory.
A Startling Discovery
Cosmologists now had a working theory to explain the seemingly unique features of the galaxies located furthest from earth. Because they are the furthest, they are also the youngest galaxies we can see. It would seem that most young galaxies have a black hole billions of solar masses at their center. This line of reasoning begs a very important question: what happened to these black holes in more mature galaxies like our own? An answer to that question would come as quite a shock to Alan Dressler in 1983 (4).
Intending to solidify the existence of a supermassive black hole in AGN NGC1068, Dressler devised an experiment in which he would measure the Doppler shift of material on either side of the galactic core in order to determine the velocity at which that material was orbiting. If it was moving fast enough, that would be evidence for the existence of a supermassive black hole in that galaxy. In order to have a basis for comparison, he also measured the Doppler shift of material orbiting the core of our galactic neighbor, Andromeda. While he was not able to get a precise enough measurement of NGC1068 to support his hypothesis, the results from Andromeda were quite clear: there was a supermassive black hole at the center of the Andromeda galaxy! Since then, similar experiments on other nearby galaxies confirm that it is quite common for a galaxy to have such an object at its core and that they may actually be intrinsic to the formation of all galaxies (4).
(1): The Imagine Team/Dr. Jim Lochner. Active Galaxies. http://imagine.gfsc.nasa.gov/docs/science/know_l2/active_galaxies.html
(2): Christian, Eric and Masetti, Maggie. The Discovery of Quasars. http://imagine.gfsc.nasa.gov/docs/ask_astro/answers/980316b.html
(3): The Imagine Team/Dr. Jim Lochner. Active Galaxies and Quasars. http://imagine.gfsc.nasa.gov/docs/science/know_l1/active_galaxies.html
(4): British Broadcasting Corporation. Supermassive Black Holes (Transcript). http://www.bbc.co.uk/science/horizon/massivebholes_transcript.shtml
And this concludes another exciting episode of Node Your Homework