Active Galaxies PHYS390 Astrophysics Professor Lee Carkner Lecture 22

Active Galactic Nuclei (AGN)   Often associated with galaxies   Caused by central black hole   Need a unified model of a black hole system than can explain all types of AGNs

Seyfert Galaxies   Seyfert 1  show broad lines (~3000 km/s) and narrow lines (500 km/s)   Seyfert 2  show just narrow lines  X-rays weak and heavily absorbed

Radio Galaxies   Like Seyferts, there are two kinds:  Broad-line (BLRG)  Narrow-line (NLRG)   compact core and large halo  flanking lobs and jets

Lobes  Radio lobes are very large   can be larger than galaxy source   Radio energy many times greater than energy of non- AGN galaxy   Radio emission is synchrotron  From electrons in magnetic fields

Jets   Each about 70 kpc long   Shocked clumps of ejecta  Some jets are bent  Galaxy moving through intercluster gas

BL Lac  Point source with very rapid variability   Originally thought to be stars, but distance is extra- galactic   No radio lobes or jets   Sometimes called Blazars

Quasar  Quasi-stellar radio source   L ~ 5X10 29 W  ~ times as bright as normal galaxy   Have high redshift   Some quasars have weak radio emission  Quasars can be radio-load or radio-quiet

Quasar Distance  Quasar distances often given by redshift parameter, z z = ( obs - rest )/ rest =    The radial speed away from us z = [(1+(v r /c))/(1-(v r /c))] ½ -1  d =cz/H 0  Where d is in Mpc, c is in km/s and H 0 = 71 km s -1 Mpc -1  Quasar redshifts go up to ~5.5

AGN Spectra  AGNs produce emission over a wide range of frequencies   Radio is synchrotron emission   Big blue bump of thermal emission from hot accretion disk

Central Engine  The power source must be small (few AU)   The fastest the change can occur is the speed of light   About 10 8 M sun in a radius of 2 AU  AGNs are powered by accretion on to supermassive black hole

Energy  Matter falling into a strong gravitational field releases energy as, L =  Mc 2    is the efficiency   More energy generated per kg than nuclear fusion

Disk  Structure of accretion disk is uncertain   Inner disk is thick and hot  ~10 4 K   Outer disk is thin and heated by inner disk   Similar to PSM star jets  Beyond thin disk is optically thick torus

Line Production   Consists of clouds of gas heated by the disk to produce emission   T~10 4, n = m 3  Narrow line region must lie outside the torus   Is less dense and can show forbidden lines  May be clouds pushed out form center by wind or jets

Unified Model  Edge-on   Seyfert 2, narrow-line radio galaxy  Inclined   Seyfert 1, broad-line radio galaxy, quasar  Face-on   BL Lac

Why AGNs?   AGNs are fueling the black hole due to gravitational interaction or merger   Gravitational interaction causes gas to fall into the center   Quasars are from the early universe 

Next Time  Read 27.1, 27.3  Homework: 27.1, 27.3, 27.9, 27.13