Quasars, Active Galaxies, and super-massive black holes Lecture 38: Quasars, Active Galaxies, and super-massive black holes

discovery of ‘Quasars’ 3C 48 and 3C 273 look like normal Galactic stars in optical images, but were odd in that they were detected in the radio also, their spectral lines are redshifted, showing that they are at large distances (z=0.367 and z=0.158, or 1300 and 620 Mpc) these distances implied that the objects are incredibly luminous – hundreds to thousands of times brighter than our entire galaxy!

Characteristic Luminosities Sun: 4 x 1026 W Milky Way Galaxy: 1037 W Quasars: 1038-1042 W

weird quasar fact #1: quasars appear ‘point-like’ (star-like) but are incredibly luminous

Average Quasar Spectrum

weird quasar fact #2: quasars emit radiation over a very broad range of wavelength/frequency – their spectra are much ‘flatter’ that those of stars or galaxies

time variability

quasar size and time variability observed quasar brightness changes over timescales of weeks to months the time scale of variability places limits on the size of the region producing the radiation for example, an object one light-year in diameter cannot vary in brightness over a period of less than one year.

weird quasar fact #3: all that energy is produced in a region with a diameter about equal to that of our Solar System!

weird quasar fact #4: the epoch of quasar activity seems have been several billions of years in the past – no quasars are found at low redshift (nearby)

X-ray

Radio Galaxies Cygnus A

M87 (Virgo Cluster)

Centaurus A

Seyfert galaxies

Quasar ‘host’ galaxies

summary of strange beasts: quasars: very compact, energetic sources with non-blackbody spectra (at high redshift) radio galaxies: normal looking galaxies with radio emission, often with very extended ‘lobes’ (at all redshifts) Seyfert galaxies (active galaxies, AGN): galaxies with faint quasars in their middles (at low redshift)

questions… what do all of these objects have in common, and how are they connected? what could produce so much energy in so small an area?

supermassive blackholes? In 1968, Lynden-bell suggested that accretion of gas onto supermassive blackholes at the centers of galaxies could fuel quasars, radio galaxies, and Seyfert galaxies black hole accretion produces energy very efficiently – 10 to 40 percent of the rest mass is converted to energy!

accretion disk and jet surrounding a supermassive black hole

twisted magnetic field lines emerging from the accretion disk could produce the radio jets

observed disk in radio galaxy NGC4261

supermassive black holes in galaxies there is now very solid observational evidence, from the motions of gas and stars in galaxies, that almost every galaxy has a supermassive black hole in its center the larger the bulge, the larger the mass of the black hole

M31