Slide 1
Active Galactic Nuclei
For further information see:
Quasar Astronomy by Dan Weedman
Active Galactic Nuclei by Ian Robson
Accretion Power in Astrophysics (2nd
edition) by Frank, King and Raine
4C15 - High Energy Astrophysics
emp@mssl.ucl.ac.uk
http://www.mssl.ucl.ac.uk/
Slide 2
Introduction
• Apparently stellar
• Non-thermal spectra
• High redshifts
• Seyferts (usually found in spiral galaxies)
• BL Lacs (normally found in ellipticals)
• Quasars (nucleus outshines its host galaxy)
Active Galactic Nuclei are the central
engines of distant galaxies. They are
apparently stellar on the sky (although
deeper observations often reveals
evidence of the underlying host galaxy),
their spectra extend from the radio up to
X-rays and gamma-rays and seem to be
dominated by a flat, non-thermal
component, and they have high redshifts,
up to z~5 when the Universe was one
tenth of its present age.
There are three main types of objects:
Seyfert galaxies - normally found in
spiral galaxies
BL Lacs - usually observed in ellipticals
Quasars - highly luminous AGN which
outshine the underlying host galaxy.
Slide 3
Quasars
• Animation of a quasar
This animation takes you on a
tour of a quasar from beyond
the galaxy, right up to the edge
of the black hole.
It covers ten orders of magnitude, ie the last frame covers a
distance 10 billion times smaller than the first.
Slide 4
Quasars - Monsters of the Universe
Artist’s impression
Slide 5
It is commonly accepted that AGN are all
manifestations of accretion onto a
supermassive black hole. This is deduced
from:
1. The very high luminosities involved,
up to ~1e47 erg/s or more
2. Fast variability on timescales of only
days or even shorter for BL Lacs and
optically-violent variables.
AGN Accretion
Believed to be powered by accretion onto
supermassive black hole
high luminosities
highly variable
Eddington limit
=> large mass
small source size
Accretion onto
supermassive black hole
The fast variability implies a very small
source size for the nucleus. If the time
taken to vary significantly is 10,000
seconds (for example), then the size of
the source must be no more than 1e4
times the speed of light, which is 3e12m.
This corresponds to the Schwarzchild
radius of a 1e9 solar mass black hole.
Slide 6
Radio superluminal expansion
Features appear to move at v>c!
At time t=0, the source, which is 2 light
years away, emits a blob of material at
20degrees from our line of sight at a
speed 0.9 times the speed of light.
20 
1.8cos(20)
2 light years
rs
ty e a
ligh
1.8
v=0.9c
0.6ly
t=0
after 1 ly
after 2 ly
0.3ly
after 2.3ly
After 2 light years, the light from the
source has reached us but emission from
the blob is still 2.0-1.8cos(20)=0.3 light
years away.
So 0.3 light years later when this
emission has reached us, it has appeared
to move across the sky by
1.8sin(20)=0.62 light years in 0.3 years ie at twice the speed of light - but in fact
it is only moving at 0.9c.
Thus the apparently superluminal speeds
are actually an optical illusion.
Slide 7
Slide 8
Slide 9
Accretion disk and black hole
• In the very inner regions, gas is believed to
form a disk to rid itself of angular
momentum
The disk is about the size of our Solar
System. It is geometrically thin and
optically-thick and radiates like a
collection of blackbodies, very hot towards
the centre (emitting soft X-rays) and cool
at the edges (emitting optical/IR).
Slide 10
Accretion rates
Calculation of required accretion rate:
L  10 40 J / s
.
M
L
10 40

2
c
0.1 3  108


2
 10 24 kg / s  3  10 31 kg / yr  10 M Sun / yr
Slide 11
Accretion disk structure
The accretion disk (AD) can be considered as
rings or annuli of blackbody emission.
Dissipation rate, D(R)
0 .5
3 GM M 
R  

1   *  
3
8 R
 R  

R
= blackbody flux
 T 4 ( R )
Slide 12
Disk temperature
Thus temperature as a function of radius T(R):
 3GM M
T (R)  
3
 8 R 
When R  R*
 3 GM M
T*  
3
 8 R * 



1/ 4
  R  0.5  
1     
  R*   
1/ 4
T  T* R / R* 
3 / 4
It is assumed that the disk is
geometrically-thin and optically-thick in
the z-direction. Thus each annular
element of the disk radiates roughly as a
blackbody with a temperature T(R) ,
where :
Sigma x T^4(R) = D(R)
Where D(R) is the dissipation rate and
sigma x T^4 is the blackbody flux.
R_* is the radius of the black hole (or
compact object).
Dissipation through the disk is
independent of the viscosity in the disk –
and the dissipation rate is the energy flux
through the faces of the thin disk. Thus if
the disk is optically-thick in the zdirection, we are justified in assuming
that the dissipation rate is equivalent to
the blackbody emission.
Substituting the blackbody flux equation
into the dissipation equation gives the
temperature of the disk as a function of
radius. At radii larger than the radius of
the compact body, the temperature is
given by the equation shown. Note that
the temperature decreases with radius
with a power –0.75.
Slide 13
The total disk spectrum is the sum of the
emission from each of the annuli that
make up the disk. The emission is
dominated by the hottest regions ie from
the annuli closest to the black hole. At
long wavelengths therefore, the spectrum
has the form of the Rayleigh-Jeans tail
where the flux rises with the square of the
frequency. At short wavelengths, the
Wien form dominates and the flux falls
with e^-nu.
Disk spectrum
Flux as a function of frequency,  -
Log *F
Total disk spectrum
Annular BB emission
Log 
Slide 14
Black hole and accretion disk
The innermost stable orbit occurs at :
rmin 
6GM
c2
T  T* R / R* 
When R  R*
3 / 4
Slide 15
High energy spectra of AGN
Log *F
Spectrum from the optical to medium X-rays
Low-energy
disk tail
Balmer cont,
FeII lines
optical
14
UV
15
Comptonized
disk
high-energy
disk tail
EUV soft X-rays X-rays
16
Log 
17
18
The distance to the inner edge of the
accretion disk is proportional to the mass
of the central black hole. The
temperature, on the other hand, decreases
as the radius increases. Thus the inner
edges of large mass black holes are
relatively cool, while those of low mass
black holes are relatively hot. This
means that disks around black holes in
AGN are much hotter than those around
Galactic black hole candidates.
Moving from low frequencies up to Xrays, these features are known as:
The small blue bump – emission from the
Balmer series which forms an excess
above the underlying continuum at the
Balmer limit.
The big blue bump – a rise towards high
frequencies above an extrapolation of the
lower energy spectrum, believed to be
due to the outer edges of the accretion
disk.
The soft X-ray excess – an excess of flux
above an extrapolation of the
medium/hard X-ray spectrum (2-50keV).
This has a mean slope of about -2 (ie
Flux, F_nu is proportional to nu^-2) as
measured by the ROSAT observatory in
the 0.1-2keV range.
The medium to hard X-ray spectrum has
a mean slope of about –1 (ie Fnu is
proportional to 1/nu) as measured by
EXOSAT and ASCA. It is thought to be
due to the inverse Compton scattering of
photons from the accretion disk in a hot,
100 million degree corona which
surrounds the disk.
There is also a strong FeKalpha
fluorescence line observed at about
6.7keV in Seyfert galaxies (not seen yet
in quasars) which is believed to emitted
from the very inner regions of the
accretion disk, close to the black hole
itself.
Slide 16
FeK line
Fluorescence line observed in Seyferts – from
gas with temp of at least a million degrees.
X-ray
FeK
e-
An X-ray photon collides with an Fe ion,
removing an electron from an inner K or
L shell. The ion may return to a lower
energy state by emitting an electron from
a higher shell (this is known as the
‘Auger effect’) – or by a radiative
transition. The relative probability of a
radiative transition is known as the
fluorescence yield.
The energy of the Kalpha line depends on
the number of electrons present and it
increases as the line becomes more
highly ionized, reaching 6.9keV for
FeXXVI.
The ionization state observed indicates
gas temperatures which are relatively
cool (about a million degrees) and the
strength (ie the equivalent width) is quite
high, indicating that the gas producing
the Fe line has a high covering factor.
FeKalpha emission from an accretion
disk fits these observations very well,
providing a high covering factor to the
source of X-rays (probably the accretion
disk corona) without obscuring our line
of sight. It also has the right temperature
in the inner regions, where the line is
thought to originate.
Slide 17
Source of fuel
• interstellar gas
• infalling stars
• remnant of gas cloud which originally
formed black hole
• high acc rate necessary if z cosmological
- otherwise not required if nearby
Black holes could accrete this much
material from the interstellar gas, or in
the form of stars (disrupted by the
gravitational field of the black hole) or
from the remnant of the original rotating
gas cloud from which the black hole is
thought to have formed.
Large amounts of accreting matter are
required to explain the observed
luminosities mainly due to the
assumption that their measured redshifts
indicate the cosmological distances of the
AGN. The emitted power would be less
however if AGN are actually nearby
objects but their spectra are redshifted by
some other mechanism.
Slide 18
The Big Bang and redshift
All galaxies are moving
away from us. This is
consistent with an
expanding Universe,
following its creation
in the Big Bang.
Slide 19
Cosmological redshift
zem
zab2
z ab3
flux
• Continuity in luminosity from Seyferts to
quasars
• Absorption lines in optical spectra of
quasars with z abs  zem
zem

z ab1 z z
ab1 ab 2 z ab3
There are two important pieces of
evidence which support the theory of
cosmological redshift for AGN:
1. The AGN in Seyferts are seen
surrounded by a host galaxy and features
in the galaxy have the same redshift as
the AGN located within. The transition
from Seyfert to quasar in terms of
luminosity is continuous and indeed the
dividing line between Seyferts and
quasars is a subjective one. Thus quasar
redshifts are believed to be cosmological.
2. Absorption lines are often observed in
quasars at redshifts which are different to
those of the emission lines. At low
velocities, these are due to intervening
clouds moving towards us or away from
us, but associated with the quasar itself
(z(ab2) and z(ab3)). At much lower
velocities, absorption features are also
observed and these are due to systems of
clouds along the line of sight between us
and the quasar (z(ab1)).
Slide 20
Alternative models
• Supermassive star
- 108 solar mass star radiating at 10 39 J/s or
less does not violate Eddington limit. It
would be unstable however on a timescale
of approx 10 million years.
• May be stabilized by rapid rotation
=> ‘spinar’ - like a scaled-up pulsar
There are alternative models to the black
hole hypothesis for the source of power
in AGN (although the black hole model is
widely accepted).
A supermassive star could exist in
principle, although there are problems
with stabilizing such a star (assuming that
it is supported by gas pressure). It would
be unstable on a timescale of about 10
million years - it can be stabilized by
rapid rotation and such an object is
known as a spinar. The radiation from
this type of object may be by a scaled-up
version of the pulsar mechanism.
However such a large mass also incurs a
general relativistic instability setting in and the supermassive star would then
evolve into a black hole anyway.
Slide 21
• Also, general relativity predicts additional
instability and star evolves into black hole.
• Starburst nuclei
- a dense cluster of massive, rapidly
evolving stars lies in the nucleus,
undergoing many SN explosions.
• Explains luminosity and spectra of lowluminosity AGN
Slide 22
• BUT SN phase will be short (about 1
million years) then evolves to black hole
• radio observations demonstrate wellordered motions (ie jets!) which are hard to
explain in a model involving random
outbursts
Slide 23
Radio sources
• Only few % of galaxies contain AGN
• At low luminosities => radio galaxies
• Radio galaxies have powerful radio
emission - usually found in ellipticals
• RG
1038- 1043 erg/s = 1031- 1036 J/s
• Quasars 1043 - 10 47erg/s = 10 36- 1040J/s
We have already considered a few types
of AGN (quasars, Seyferts, BL Lac
objects) for which there is a very large
energetic output over the whole
electromagnetic spectrum, ie from the
radio to gamma rays. Fast variability in
these objects suggests the presence of a
massive black hole as the central engine.
Note though that only a few percent of
galaxies have very active central regions.
At the low end of the luminosity
distribution of AGN are found radio
galaxies. These have very powerful radio
emission and are associated usually with
elliptical galaxies.
About 10% of quasars are radio-loud these are a more powerful version of
radio galaxies.
Radio-quiet quasars, on the other hand,
are more similar to Seyfert galaxies in
their properties.
Slide 24
Jets: focussed streams of ionized gas
lobe
jet
energy carried out
along channels
material
flows back
towards
galaxy
hot
spot
Slide 25
Electron lifetimes
Calculating the lifetimes in AGN radio jets.
For  = 108 Hz (radio), ~ 4.17x10 36 E2 B
E 2 B = 2.5x10-29 J 2 Tesla
 syn= 5x10-13 B-2 E-1 sec
Jets appear to be channels along which
energy is carried out to the large scale
lobes and hot spots. The jets are actually
narrow, focussed streams of ionized gas
emanating from the AGN, ie they carry
high energy electrons and magnetic field
flux. The advancing jet pushes the
interstellar (ie galactic) matter out of the
way. At the end of the jet, the material
moves more slowly and energy
accumulates here, forming a hot spot.
The jet material then flows back towards
the galaxy and this inflates the large radio
lobes.
Thus in the largest radio galaxies at least,
electrons from the core cannot power the
lobes all the way out; the electrons must
be accelerated in the lobes, by shock
waves.
For B = 10 -3Tesla, syn ~3x106 sec, ~ 1 month
For B = 10-8 Tesla, syn~ 1014 sec, ~ 3x10 6 yrs
Slide 26
Shock waves in jets
Lifetimes short compared to extent of jets
=> additional acceleration required.
Most jet energy is ordered kinetic energy.
Gas flow in jet is supersonic; near hot spot gas
decelerates suddenly => shock wave forms.
Energy now in relativistic e- and mag field.
The gas flows in the jet at supersonic
velocity. Near the hot spot, it decelerates
suddenly and this causes a shock wave to
form across the jet.
Before reaching the shock wave, most of
the energy is ordered kinetic energy. The
passage through the shock converts this
into relativistic electron energy and
magnetic field energy.
Slide 27
Equipartition of energy
Relative contributions of energy
Energy in source
particles
magnetic field
What are relative contributions for minimum
energy content of the source?
Slide 28
• Assume electrons distributed in energy
according to power-law:
N ( E )  kE 
Total energy density in electrons,
E max


k
2 
Emax
2 
N ( E ) EdE 
0
Must express k and E max as functions of B.
Slide 29
We observe synchrotron luminosity density:
E max
L
 N (E)P
syn
dE
0
And we know that:
Psyn  k ' E 2 B 2
We know that the energy in the magnetic
field is proportional to the square of the
magnetic field strength, B. We want to
express the energy in particles as a
function of magnetic field strength as
well.
We begin by assuming that the electrons
are distributed in energy according to a
power law so that the total energy density
in electrons is given by the expression
shown. Thus we must express the
quantities k and Emax as functions of B.
Slide 30
Hence:
E max
L
 kE

k ' E 2 B 2 dE 
0
So:
k
kk ' B 2 3
Emax
3 
(3   ) L
3
k ' B 2 Emax
And the total energy
density
in electrons then becomes:

(3   )
L
( 2   ) k ' B 2 Emax
Slide 31
Finding Emax
Find E max by looking for max :
2
 max  const  BEmax
So:

Emax  k ' ' B 1/ 2 1/ 2
(3   )
L
 aB 3 / 2
1/ 2
(2   ) k ' B 2 k ' ' B 1/ 2 max
Slide 32
The energy density in the magnetic field is:
B2
 bB 2
2 0
Thus total energy density in source is:
T  aB 3 / 2  bB 2
For T to be minimum with respect to B:
T
0
B
We can have an idea of Emax by looking
for the maximum frequency at which
synchrotron radiation occurs,max).
Slide 33
Thus:
T
3
  aB 5 / 2  2bB  0
B
2
b
So:
3 7 / 2
aB
4
3
T  aB 3 / 2  aB 3 / 2
4
particle
magnetic field
Slide 34
And finally,
energy density in particles 
energy density in magnetic field
4
1
3
This corresponds to saying that the minimum
energy requirement implies approximate
equality of magnetic and relativistic particle
energy or equipartition.
Slide 35
Equipartition in radio sources
• Example: Cygnus A
minimum energy requirement ~ 1052 J
magnetic field, B ~ 5x10-9 Tesla
observed luminosity ~ 5x1037 J/s
• This implies a source lifetime ~ 10 7 years
=> require electron acceleration in lobes
In the large-scale lobes of classical
double radio sources such as Cygnus A,
the minimum energy requirement is
found to be that shown
Slide 36
Maximum frequency observed is 1011 Hz.
 m  4.2 10 36 E 2 B
E 2 B  2.5  10 26
E 2  5 10 18 J 2  E  1010 eV    105
 syn  5 10 13 B 2 E 1
 1013 sec  3 10 5 yrs
Thus electron acceleration is required in the lobes.
Slide 37
Small-scale jets
• Small-scale morphology probed by VLBI
• Most cases - only one jet is observed
Two phenomena are at work in these cases:
1. Superluminal expansion
2. Relativistic beaming
Slide 38
Relativistic Beaming
Plasma appears to radiate preferentially along
its direction of motion:
Photons emitted in a
cone of radiation and
Doppler boosted
towards observer.
Thus observer sees only jet pointing towards
her - other jet is invisible.
A cloud of plasma radiating photons in
all directions will appear to be shining
preferentially in its direction of motion.
The effect is pronounced when the
plasma moves relativistically. Consider
the plasma which gives the illusion of
superluminal expansion: most of the
photons radiated by the blob are in a cone
facing the direction of motion. Moreover,
photons in the cone are made more
energetic by Doppler shift (blueshift for
photons coming towards the observer).
Slide 39
One-sided jets
Superluminal expansion + relativistic
beaming => only one jet is visible.
When motions are not relativistic however,
these cannot apply - perhaps other jet was
unstable and this instability accelerated
electrons and increased radio in visible jet.
Slide 40
Jet collimation
• Nozzle mechanism
hot gas inside large, cooler cloud which is
spinning: hot gas escapes along route of
least resistance = rotation axis
=> collimated jet
• But VLBI implies cloud small and dense
and overpredicts X-ray emission
Slide 41
Supermassive Black Hole
• Black hole surrounded by accretion disk
• Disk feeds jets and powers them by
releasing gravitational energy
• Black hole is spinning => jets are formed
parallel to the spin axis, perhaps confined
by magnetic field
For the nozzle mechanism, hot gas exists
inside a large, cooler cloud which is
spinning. The hot gas wants to escape
along the path of least resistance, ie along
the rotation axis, thus a collimated jet
forms.
However the VLBI structure of jets
indicated that this nozzle must be only a
few light years across and the cloud must
be sufficiently dense in this region to
collimate the flow, that it would predict
many more X-rays than are actually
observed.
Slide 42
Geometrically-thick disk
• Black hole + disk; acc rate > Eddington
• Disk puffs up due to radiation pressure
• Torus forms in inner region which powers
and collimates jets
• Predicted optical/UV too high however, but
still viable
If the mass supply to the black hole via
the accretion disk is high, the radiation
pressure puffs-up the innermost parts of
the disk so that a torus forms. The jets are
powered by this radiation pressure and
collimated by the funnel-shaped profile
of the disk.
However, the optical and UV
luminosities predicted by this model are
higher than those actually observed…
although aspects of the model are still
viable.