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Active Galactic Nuclei
Chapter 26
Revised 2007
Active Galactic Nuclei
Come in several varieties;
Starburst Nuclei – Nearby normal galaxies with unusually
high star formation rates in the nucleus.
Seyfert Galaxies – identified by Carl Seyfert in 1943.
Essentially normal, nearby galaxies, with
unusually bright nuclei
Quasars – quasi-stellar objects, distant, high luminosity
first recognized as galaxies by Martin Scmidt
in 1963.
NGC 7742 – A Seyfert Galaxy
3C 48 – A Quasar
To learn more we have to understand
the emission spectrum
A typical quasar spectrum
But, of course, it’s redshifted…
So redshifted that astronomers
must use the relativistic form of
the Doppler Equation
/ = √ [(1 + v/c)/ (1- v/c)]
-1
For the quasar 3C 273
/ = 2 so,
v/c = 0.16 or 16% of the speed of light
Thus, the quasar must be very far away, using v = Ho d
d = 900 Mpc
Quasar Luminosities are very high.
Quasars are observed with redshifts as high as v = 0.8c
The fact that they are so far away and so bright must mean that
they have very high luminosities ~ 1011Lo which is 1000 times
more luminous than a normal spiral galaxy.
The most distant quasars have the
largest redshifts and very broad
emission lines.
We also see evidence for Jets
Jets power lobes of radio emission
A closer look at the nucleus
reveals disks
All of which leads to a standard
model for AGN’s
and the broad emission lines are radiated
from the disk which allows an estimate
of the black hole mass
MBH
r
m
v
Balancing forces;
mv2/r = G MBH m /r2
which re-arranges to
MBH = 2.32 x 105 v2(km/s) r(kpc)
Mo
Time Variability
Some quasars are observed to change in brightness
on timescales of days, which can be used to set a
limit on the size of the emitting region.
Correlating the continuum variations with the
emission line variations provides the light travel time
t which yields the size of the emitting region
r~ct
r
Variability observed on a timescale of a day leads to the
following size;
r = 3 x 108 m/s 24 hrs/day 60min/hr 60s/min
r ~ 1013 – 1014 m or 100 – 1000 AU !
If the width of the broad lines, typically 10,000 km/s wide,
reflects the rotational velocity of an accretion disk around a
super massive black hole, then the line width together with a
size for the emitting region leads to an estimate for the mass
of the black hole using,
MBH = 2.32 x 105 v2(km/s) r(kpc)
Mo
MBH = 2.32 x 105 (10,000)2 (1 x 1014 m / 3 x 1019 m/kpc)
MBH ~ 108 Mo
The large mass inferred from the broad lines combined
with the small size inferred from the rapid time variability
combined with the high luminosities inferred from the large
redshifts all point to an exotic object at the heart of a quasar
- a Massive Black Hole
Basic Black Hole Physics
Escape Velocity
The escape velocity from a Black Hole is calculated the same
way as the escape velocity from any massive object
m v2esc = GMBH
2
r
(just
K.E. = P.E.)
In the case of a Black Hole, the maximum escape speed is c, the
speed of light, so
c2 = 2 GMBH/R
which can be re-arranged to make R the subject of the formula
R = 2GMBH/ c2
also known as the Schwarzschild radius
or “the event horizon”
The Schwarzschild radius for a typical quasar is
R = 2GMBH/ c2
R = 2 . (6.67 x 10-11) 108 (1.99 x 1030 )/ (3 x 108)2
R = 2.94 x 1011 m
or
~ 2 AU for a 108 Mo Black Hole
Energy Source
It is the release of gravitational potential energy as matter
falls into a black hole that drives the high luminosities observed
from quasars
P.E. = - G MBH
Rs
L = G MBH dm/dt
Rs
where dm/dt is the mass inflow rate
For a Typical Quasar
L ~ 1012 L, MBH ~ 108 M , Rs ~ 2 AU then
dm/dt = L Rs = 1012 L 2.94 x 1011
G MBH
6.67 x10-11 108 M
and
L/M = 1.99 x 1030 kg/3.9 x 1026 W
Then
dm/dt ~ 2.72 x 1029 kg/yr
Or
~ 0.1 M/yr which is a trivial amount !
Conversion Efficiency
The conversion of gravitational PE into light is not expected
to be 100% efficient. But, even if the efficiency is ~ 10%,
then dm/dt increases to only 1 M/yr which is still a trivial amount.
So, the high luminosity is not due to a high infall rate but rather
is due to the enormous depth of the gravitational potential well
that the material is falling into.
One final word on singularities. Black holes have finite mass but
zero volume, so the density is infinite, and the laws of physics as
we know them break down.
A common misconception among the general public is that black
holes have infinite mass – they don’t. The mass is finite.
Quasar Evolution
Quasars are observed only
at high redshifts – at large
lookback times, so they existed
in the distant past. There are
no nearby quasars.
The lower luminosity AGN’s,
the Seyfert and starburst
galaxies, bridge the gap
between us and the more
distant quasars
There is evidence for a super-massive Black
Hole at the center of our Galaxy
Ghez, A, 1998, ApJ...509..678
MBH = (2.6 +/- 0.2) x 106 M
Tidal disruption
Stars passing closer than a distance
d = r (3MBH/M*)1/3
are torn apart by the enormous gradient in the gravitational field
strength between the front-side of the star facing the BH and the
back-side of the star furthest from the BH.
The next nearest supermassive black
hole is in M81
Which has a curious nuclear spiral…
The gas provides a means to determine a mass for the nucleus by measuring the
rotational velocity of the gas using the Doppler effect.
Devereux, N., 2003, AJ 125, 1226
MBH = (7.0
+2
-1)
x 107 M
The principle evidence that the mass
concentration is a black hole is that
if the mass were stars, the nucleus of
the galaxy would be 5.3 magnitudes, or
about a factor of 130 times, brighter
than is actually observed!
model
observed
Finally,
Black Hole
the movie ……….