redshift parameter, z
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Transcript 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