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16.16 Active Galaxies

The placid exteriors of some galaxies conceal events of great violence. While sources of potentially deadly high-energy events are diverse — ranging from gamma-ray bursts to cataclysmic variables to black hole related events — the most powerful objects in the universe are quasars. The most luminous of these objects give off the same amount of energy as 1 trillion suns. This means these central objects can give off far more energy than the sum of all other energy outputs of the surrounding galaxy. Quasars are the most luminous of a class of physically similar objects called active galaxies. Astronomers believe that all galaxies contain super massive black holes, which act as gravitational "engines" to cause a wide range of high energy phenomena in the centers of galaxies.

To understand active galaxies, it's necessary to understand the basic structures at the hearts of galaxies. Observations seem to indicate that all large galaxies (and perhaps all galaxies) contain super massive black holes in their centers. If any material gets too close to this many million- to billion-sun sized black hole, it can get pulled in (just like a comet getting pulled into the sun), but as it spirals to its doom, it will get torn apart. If enough material falls in at once, it will form a hot disk called an accretion disk. It is the interplay between this accretion disk of material and the black hole that changes a galaxy from "normal" to "active," and galaxies can transition between these states as their black holes go from states of eating to starvation. Black holes are truly black — nothing can escape from inside the event horizon — but their intense gravity accelerates material and heats it up, leading to the apparent paradox of black holes appearing very bright!

Active galaxies were first discovered in 1908 by Edward Fath, who discovered intense emission lines coming from the central regions of the bright galaxy NGC 1068. Vesto Slipher and Edwin Hubble discovered other galaxies with similar lines. By the 1940s, Carl Seyfert had studied active galaxies in detail and noted their common features: a bright compact nucleus, strong and very broad absorption and emission lines, intense radio emission, and a peculiar/disrupted morphology (structure). How strongly we see (and sometimes don't see) each of these aspects depends on observing angle and other factors. Most importantly, black holes only seem to be active for a small percentage of time, less than 1%. So if we observe a hundred galaxies, only one will have a blakc hole that's in its active phase.

When an active galaxy is viewed such that spectra can capture light emitted from material near the super massive black hole (for instance when a spiral is viewed face on or mostly face on, ), very broad emission can be observed. Systems with these lines are called Seyfert galaxies, after their discoverer. In a normal galaxy, hot gas in the disk reveals itself through emission lines. These lines are broadened by an amount corresponding to the rotation of the galaxies disk, thus revealing the velocity of the gas. The total range in gas velocity is typically several hundred km/s. Gas moving towards us is blue shifted and gas moving away from us is red shifted as it orbits, and the result is a smeared-out emission line. In Seyfert galaxies, by contrast, the emission lines are much broader, indicating a gas velocity of thousands of km/s. Under normal conditions, gas at such a high velocity could not be gravitationally bound — it would fly away from the center. There are two possible explanations for the high velocity of this gas: either the gas is actually being ejected from the nucleus of the galaxy, or it is orbiting very close to a dark massive object in the galaxy's core. In either case, something unusual is going on in the nuclear regions. Many Seyfert galaxies look like normal galaxies — typically disturbed spiral galaxies — in images, and their unusual nature is only revealed in spectra. There are exception however; many Seyfert galaxies have bright star-like nuclei.

About 1% of all galaxies (and 10% of all active galaxies) have extraordinary levels of radio emission. By 1944, the amateur astronomer Grote Reber had detected strong radio sources in the constellations of Sagittarius, Cassiopeia, and Cygnus. The Sagittarius radio source corresponded to the Galactic Center, and the Cassiopeia source to a supernova remnant. However, the position of the Cygnus source could not be specified accurately until 1951, when Walter Baade and Rudolf Minkowski located a faint, distorted-looking galaxy at the position on the sky where the radio emission was centered. Cygnus A, the brightest radio source in the constellation of Cygnus, was the first known radio galaxy. Its radio luminosity is 10 million times that of the Milky Way. This elliptical galaxy has a bright radio nucleus at the center corresponding exactly to the position of the optical galaxy. Cygnus A also has a pair of radio jets, one of which is only dimly visible, joining the nucleus to two enormous radio lobes. The lobes have a complex structure, with large regions of wispy emission and intense emission at the outer edges. Cygnus A is 230 Mpc, or 750 million light-years away. Yet it emits radio waves strong enough to be detected by amateur astronomers with backyard equipment!

Today, we find radio emission associated with both large spiral and elliptical galaxies, with emission in some cases being primarily associated with the galactic nuclei, and in other cases with material shooting away from the system in dramatic jets (and if you look with very sensitive radio telescopes, radio emission is also found associated with star formation, but star formation doesn't make a galaxy an active galaxy). The intense radio waves that come from some galaxies are examples of non-thermal radiation, synchrotron radiation in particular. Synchrotron radiation is emitted by particles accelerating on a curved path by a magnetic field. In active galaxies, extremely powerful magnetic fields can be generated by the heated, and typically electrically charged, material orbiting in the accretion disk. As material accelerates along field lines, it gives off synchrotron radiation. This is how a radio transmitter works; electrons racing up and down a wire generate a radio wave. If energy is delivered to a hot gas threaded by a magnetic field, electrons spiral around the magnetic lines of force at nearly the speed of light and lose energy by emitting synchrotron radiation. (Protons are massive and not nimble enough to spiral very fast, so they emit little synchrotron radiation.)

Synchrotron radiation is variable — when the energy source for accelerating electrons varies, the amount of synchrotron radiation released can also vary by a large factor. When energy is quickly added to particles by a magnetic field, they emit a broad spectrum of radiation with no emission peak at any particular wavelength. The shortest wavelength of the non-thermal spectrum depends on the highest energy of the electrons, with wavelength decreasing as energy increases. For highly relativistic electrons, with speeds 99.9% of the speed of light, the spectrum can extend all the way to X-rays or gamma rays. Nature's particle accelerators in the centers of galaxies can do far better than human devices such as the Large Hardon Collider at CERN in Geneva.

In addition to broad emission lines and strong radio emission, active galaxies often have a peculiar morphology. The classic example is the irregular galaxy M 82, which harbors large amounts of nuclear gas and dust and has chaotic filaments of excited gas streaming out from the nucleus. The high-resolution imaging of the Hubble Space Telescope shows that the Seyfert galaxy NGC 1275, a peculiarly shaped elliptical in the Perseus cluster, has young and blue globular clusters. Their formation is apparently connected with the violent events occurring in the nucleus. A close-up view of the peculiar galaxy Arp 220 reveals gigantic young star clusters near the nucleus. It is important not to associate a particular morphology — spiral or elliptical — with nuclear activity too strongly, but peculiar and disrupted features are common. These morphological oddities can result from galaxies interacting with one another and with the material between galaxies. Some apparently normal galaxies also have active nuclei. But astronomers have discovered a fairly strong statistical connection between galaxy interactions (as revealed by peculiar morphology) and nuclear activity. It appears that interactions can drive material into the galaxies central regions and fuel the super massive black hole, transforming it into an active galactic nucleus.