Active Galaxies PHYS390 Astrophysics Professor Lee Carkner Lecture 22.
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Transcript of Active Galaxies PHYS390 Astrophysics Professor Lee Carkner Lecture 22.
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