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 ~ 5X1029 W ~100000 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, zz = (obs-rest)/rest =

The radial speed away from us

z = [(1+(vr/c))/(1-(vr/c))]½-1

d =cz/H0 Where d is in Mpc, c is in km/s and H0 = 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 108 Msun 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 = Mc2

is the efficiency

More energy generated per kg than nuclear fusion

Disk Structure of accretion disk

is uncertain

Inner disk is thick and hot ~104 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~104, n = 1015 m3

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