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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.