Long ago when galaxies were young, the stars in their cores were very closely packed. Star collisions and mergers occurred, giving rise to a single massive black hole (MBH) with perhaps 106 to 109 Mo. Gas from the galaxy’s interstellar medium, from a cannibalized galaxy, or from a star that strays too close, falls onto the MBH. As in X-ray binary star systems, an accretion disk forms, emitting huge amounts of light across the electromagnetic spectrum (infrared to gamma-rays). The MBH plus accretion disk produces the phenomena seen in active galactic nuclei (AGN). Below you see optical and radio images of the active galaxy NGC 4261. The central object, accretion disk, and lobes are all visible. The different types of AGN are variations on this theme. Many galaxies today (including our Galactic center??) may have a quiet MBH which happens not to have recently accreted gas. Seyfert galaxies have accretion onto a moderate-mass MBH, while the more luminous quasi-stellar objects (i.e. quasars) have accretion onto a high-mass MBH. In ~10% of the AGN, the MBH + accretion disk somehow produce narrow beams of energetic particles and magnetic fields, and eject them outward in opposite directions away from the disk. These are the radio jets, which emerge at nearly the speed of light. Radio galaxies, quasars, and blazars are AGN with strong jets, which can travel outward into large regions of intergalactic space. Many of the apparent differences between types of AGN are due to our having different orientations with respect to the disk. With Blazars and Quasars, we are looking down the jet. For Seyferts, we are viewing the jet broadside. Consider NGC 4151, a spiral galaxy 15 Mpc away. Photographs by Carl Seyfert in the 1940s showed a very bright point-like nucleus. Its spectrum is very unusual: in addition to continua + absorption lines from normal stars, Seyfert galaxy nuclei have very strong emission lines. Some are common lines (e.g. H-alpha, H-beta) but others are weird (e.g. twice-ionized oxygen lines), requiring hot gas far out of equilibrium. The lines are very broad, requiring that the gas be Doppler shifted in all directions up to ~20,000 km/s. The nuclei vary in brightness on timescales of months, requiring them to be < 1 parsec in size. The total luminosity can be equivalent to 1010 Lo! Later in the 1940s, astronomers began scanning the skies with radio telescopes. They found strange radio structures on opposite sides of radio galaxies, plus a tiny source of radio emission at the nucleus. The nuclei of these radio galaxies shoot out narrow beams of extremely energetic electrons and magnetic fields, producing radio synchrotron radiation. The radio components include: the compact core at the galaxy nucleus, jets, lobes, and a hot spot where the jet slams into the interstellar medium. . . . . .
PS. . . SUCK IT!
