Active Galactic Nuclei
Ay 16, April 8, 2008
AGN
DEFINITION
PROPERTIES
GRAVITATIONAL LENSES
BLACK HOLES
MODELS
WHAT IS AN AGN?
Objects that emit significant amounts of
radiation from non-thermal sources
(i.e. not stars or dust or thermal gas).
In order of discovery:
Seyfert Galaxies
Radio Galaxies
Quasars
BL Lac Objects
LINERS (Low Ionization Emission Regions)
Early History
Carl Seyfert & strange emission line galaxies
(1943)
Birth of Radio Astronomy & discovery of
strong sources = radio galaxies (40-50’s)
Greenstein & Schmidt & QSO’s (1963)
Surveys of Extremely Blue Objects (60-70’s)
Surveys at other Wavelengths (70’s > )
SY1
AGN
Spectra
SY2
QSO
Mean UV QSO Spectrum
LBQSO Spectrum
NGC 4151
Typical
SY1
Seyfert 1 galaxy and quasar spectra are
dominated by broad line regions, lines
often showing doppler velocities of 0.1c
Seyfert 2 galaxies spectra are dominated
by narrower lines, ~ 1-2000 km/s wide
LINER spectra are dominated by stars plus
emission lines from multiple ionization
states indicative of non-thermal excitation
BL Lac spectra show almost no features but
strong continua
M87
Core
In
X-rays
Markarian 231
Markarian 421
BL Lac in an E
Galaxy
TeV  Source
BL LAC
•
SED’s
Radio Galaxies
M87 = Virgo A
M87 in
the
Radio
(VLA)
M87
Central
Jet
M87
Jet
In 3 bands
Centaurus
A
Radio on
Optical
Image
Cygnus A
(VLA Conway & Blanco)
Synchrotron Power
 = related to kinetic enegy of electron
= (1 - (v/c)2)-1/2 = (1 - 2)-1/2
where  = v/c
Synchrotron Spectra
AGN
NORMAL
LINERS
Baldwin, Phillips
& Terlevich
Ly Alpha Forest
Eddington Luminosity
Largest Luminosity that can pass through
a gas in hydrostatic equilibrium :
Outward Force of Radiation Pressure
> Inward Force of Gravity
Gravity
dP/dr = -g = -GM/r2
Luminosity dP/dr = -(T/mpc) (L/4r2)
T = Thomson Cross section
LEdd = 4GMmpc/T
= 3.3 x 104 L(M/M)
Gravitational Lensing
Deflection of Light passing at a distance b
by an object of mass M:
 = 4GM/bc2
Bending of light by the Sun:
r = 6.96 x 1010 cm M = 1.989 x 1033 g
 = 8.47 x 10-6 radians = 1.74”
Gravitational lensing:
2237+0305 = Einstein’s Cross
Gravitational Redshift
Photons lose E as the move out of a
gravity well.
For a photon E = hc/, “m” = E/c2 = h/c
Gravitational Potential E = -GMm/r
E = -GMh/cr
/ = GM/rc2
which generally is pretty small except for NS and BH
Black Holes
Curvature around a point mass has a
critical radius = Schwarzchild Radius
Inside this is a singularity
RS = 2GM/c2
In units we know
RS = 3.0 km (M/M)
Gravitational redshift from RS = 
NGC 4261 X-ray vs Optical
V = 1100 km/s
M87’s central mass:
Given the HST observations, what’s the
enclosed mass?
v = 1100 km/s  ~ 1” D = 16.1 Mpc
R = 0.078 kpc (78 pc)
GMm/R = 1/2 mV2 for circular orbits
M = 0.5 V2R/G
G = 4.309 x 10-6 kpc (km/s)2 / M
Mcore ~ 1010 M
Reverberation Mapping
NGC5548
Lag ~ 20 days Size ~ 20 light days ~ 0.02 pc
Maggorian Relation (Nuker team)
Almost every large galaxy has a central BH, and the BH
mass is directly correlated with the galaxy’s total bulge
mass!
A Unified Model of AGN’s
We have come to believe that all AGN are
essentially driven by the same
phenomena --- a central black hole
surrounded by an accretion disk, hot
clouds of gas and dust. The disk is hot,
105 K plus. The AGN is powered by
accretion onto the BH.
And what you actually see is driven by the
viewing angle.
Unified AGN Model
BH + Disk + Torus
SY2
SY1
BL Lac
edge on
tilted
down the pipe