Download redshift parameter, z

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 nonAGN 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 extragalactic

 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, z
z = (lobs-lrest)/lrest = Dl/l

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 = hMc2

h 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