Active Galaxies
Chapter 21
Centaurus A
What are Active Galactic Nuclei (AGN)?
Galaxies with a source of non-stellar light coming from the nucleus
(excessive ultraviolet, infrared, radio and X-ray light), sometimes
showing jets and variability
Central Supermassive Black Hole
accreting material from surrounding
host galaxy (Lynden-Bell 1969)
Quasars – luminous (MB<-23) objects with broad emission lines,
distant, many are strong radio sources
Seyferts – fainter (MB>-23), most identified locally, spiral hosts
Sey 1s – broad emission lines (e.g. Hydrogen)
Sey 2s – narrow emission lines
Radio Galaxies – excess radio emission and jets
Starbursts – extreme star-formation in addition to black hole
accretion in the nucleus
ULIRGS – ultra-luminous infrared galaxies
Quasars (or QSOs)
By 1960, several hundred radio sources were cataloged with no obvious
optical counterparts (due to poor positional accuracy).
In the early 1960’s, positional improvements allowed for the detection of
optical counterparts for two radio sources: 3C48 and 3C273.
Both sources looked like normal stars – “quasi-stellar objects or QSOs”
They appeared bluer than normal stars with strong, broad emission lines.
Quasars - Discovery
Maartin Schmidt was the first to recognize
that these lines were normal Hydrogen
emission lines redshifted by a large amount
and indicating high velocities and great
distances (according to Hubble’s Law).
D =
660 Mpc (2.2 billion light years) for 3C273
1340 Mpc (4.4 billion light years) for 3C48.
The large distances implied large luminosities:
L = 20 trillion Lsun (or 1000 Milky Way’s) for
3C273
These point-like sources were the most luminous objects that had been
found in the Universe at that time!
Within ~2 years, quasars were discovered that were 10 billion light years
away and L  100 trillion Lsun. The most distant quasar known today is
12.7 billion light years away!
Quasars – Host Galaxies
The bright emission (light) from quasars is actually
embedded in a “host galaxy”
•difficult to detect due to the brightness of nuclear quasar emission
•appear to be a mixture of galaxy types - from disturbed galaxies to
normal E’s and early type spirals – brightest QSOs tend to be in
E’s
Quasars – Broad Absorption lines
• Some quasar spectra not only show broad emission lines
but also broad absorption lines
• (BAL) Quasars: normal quasars viewed at angle along the
line-of-sight of intervening, fast-moving material.
Trump et al. 2006
Quasars – Lyman Alpha Forest
QSO spectra also reveal
some absorption lines
not associated with the
quasar at all.
These narrow absorption
lines are caused by
intervening galaxies
(halos) between us and
the QSO.
Redshifts of the material will be equal to or less than the QSO.
Lyman alpha is the lowest excitation level for Hydrogen gas.
Seyfert Galaxies
First identified by Carl Seyfert in 1943
Defined class based on observational characteristics:
• Almost all the luminosity comes from a small (unresolved)
region at the center of the galaxy – the galactic nucleus.
• Nuclei fainter than QSOs –
generally L ~ 108 to 1012 Lsun
• Unusual spectra
• Light from the nucleus is
variable on timescales of
months
NGC 4151
Seyfert Galaxies - spectra
Seyfert galaxy spectra fall into two classes
•Broad emission line spectra (Seyfert type 1; similar to quasars)
•Narrow emission line spectra (Seyfert type 2)
Seyfert 1s:
Broad and
narrow
lines
(BLAGNs)
Seyfert 2s:
Only narrow
lines
(NLAGNs)
Seyfert Galaxies - spectra
Seyfert 2 galaxies (Narrow-Line AGN)
•can be differentiated from normal emission line galaxies through the
flux ratios of certain emission lines.
•shape of the underlying ionizing source determines how many
photons are available to produce particular emission lines.
Kewley et al 2006 – red line shows extreme starburst, dashed line separates
Active galaxies (AGN/Seyfert) from normal, star-forming galaxies (HII)
Seyfert Galaxies - variability
• QSOs and Seyfert nuclei have long been recognized as variable
• Optical flux changes occur on timescales of months to years
• Cause of variability? – instabilities in accretion disk, SN or starbursts,
microlensing…..
Quasar light curve ~25 years
Seyfert light curve over ~11 months
Hawkins 2002
Variability occurs at most wavelengths - X-rays through radio
This indicates that the fluctuations are originating from a very tiny object.
Why does rapid variability indicate small physical size of the
emitting object?
Consider an object like the Sun. Any instantaneous flash
would appear “blurred” in time by  t = RSun / c.
RSun
observer
RSun
Time Delay =  t = RSun / c
700,000 km / 300,000 km/s = 2.3 sec
Seyfert continuum luminosity varies significantly in less than a year
(some variation occurs on timescales of days or weeks).
This implies an emitting source less than a few light-weeks across!
BL Lac objects
• Bright radio sources
• Variability faster and higher amplitude than normal quasars and
Seyferts
• Look like QSOs with extremely bright nucleus and faint fuzz “host”
galaxy surrounding
• Rapid and strong variability “floods” emission lines so that they
can’t be detected
• Variability light curve indicates emitting region less than one lightday across (or ~200 AU)
Spectrum
Light Curve
Which AGN are the brightest?
QSOs (Quasi-Stellar Objects) or quasars
Which AGN are fainter than the QSOs?
Seyferts
What’s the difference between Type 1 and 2 Seyferts?
Type 1 have broad & narrow emission lines, Type 2 only
have narrow emission lines
Which AGN are the most variable?
BL Lac objects
Which AGN generally have high redshifts (are at great distances)?
QSOs
Which AGN are brightest at radio wavelengths?
Radio galaxies of course!
Radio Galaxies
•Emit most light at radio wavelengths (~107 to
1011 Lsun)
•Radio emission is highly polarized
synchrotron radiation
•Morphology can be extended or compact
•Host galaxies are generally Ellipticals
Radio morphology first classified by Fanaroff & Riley (1974)
•FR I: less luminous, 2-sided jets brightest closest to
core and dominate over radio lobes  compact
•FR II: more luminous, edge-brightened radio lobes
dominate over 1-sided jet (due to Doppler effect
“brightening” of approaching jet over receding jet) 
extended
Radio Galaxies
FR I - 3C 47
FR II - 3C 449
Radio Galaxies
Radio “Light”
FR II radio galaxies: most
emission comes from lobes
Centaurus A
Visible Light
The radio lobes span about 10 degrees on the sky!
Lobes consist of material ejected from the nucleus.
Radio Galaxies
Radio image of the FR II radio galaxy
Cygnus A.
The lobes occur where the jets plow into intracluster gas.
Radio Galaxies
FR I radio galaxy: most of the energy comes from a small nucleus
with a halo of weaker emission around the nucleus and jets close to
nucleus.
Visible image of the FR I radio galaxy M87.
This giant elliptical (E1) galaxy is ~100 Kpc across.
Radio jet is visible in radio, optical and X-ray light
Radio Galaxies
Close-up view of the jet in M87 at radio wavelengths.
galaxy nucleus,
i.e. the radio core
The jet is apparently a series of distinct “blobs”, ejected by the
galaxy nucleus, and moving at up to half the speed of light.
Radio Galaxies: Superluminal Expansion
Some radio “blobs” in jets appear to move
faster than the speed of light – superluminal
For a blob moving distance r below:
x = r cos , y = r sin  and t = r/v
Light from P takes x/c less time
to reach us than light emitted
from O.
Time the observer sees for
object to travel from O to P is
tapp = t - x/c
tapp = (r/v) - (r/c) cos 
tapp = (r/v) (1 -  cos )
Apparent velocity across sky
vapp = y/tapp
radio jet in 3C273
vapp = (v sin )/(1 -  cos )
Radio Galaxies: Superluminal Expansion
vapp = (v sin )/(1 -  cos )
For v << c,  is close to 0 and vapp = v sin
For v close to c, vapp can be much more than v and even greater than c
To see this, find the angle that gives the
maximum vapp for a given v by dividing
this equation by v, differentiate wrt theta
and set equal to zero
Substitute this into the above
equation gives
(vapp/v)max = 1/(1- 2)1/2
What is the maximum apparent
velocity that could be observed for a
“blob” moving at 0.95c? What is
the angle required to get this velocity?
Radio Galaxies: M87
Mini-spiral at the center
of the galaxy
Radius of central disk:
r = 16 pc = 4.9 x 1017 m
Inclination of disk:
i = 42 degrees
Doppler shifted spectrum
reveals rotation rate:
vr = vc sin i = 460 km/s
So that vc = 690 km/s
MBH = vc2 r / G = 4 x 1039 kg = 2 x 109 Msun
ULIRGs (ultra luminous IR galaxy) - Starburst or AGN?
A starburst galaxy:
• May result from a galaxy collision/merger
• Gas streams converge from different directions causing shocks which
compress material and trigger star formation
• Gas which loses enough angular momentum falls into the galaxy center
 bar formation  funnels more gas inward  violent star formation near
center of disk and further out
ULIRGS are:
•Galaxies that emit most of their light
in the IR - LIR > 1012 Lsun
• more common beyond z > 1
• nearly all are undergoing mergers
• IR light is a combination of dust
reprocessed AGN emission and star
formation
•Some AGN may appear as ULIRGs
Nuclear close-up (HST) of NGC
during different stages of galaxy
1808 starburst galaxy. Galaxy
evolution.
has barred-spiral morphology.
Structure of AGNs and Unified Theory
Central Black Hole
Rs = 3x1011 m or 2 AU
for 108 Msun
Masses of a million to
a billion Msun
Obscuring Torus
Dusty structure that
blocks view of inner
region
Accretion disk
UV/visible light from
region 1012 - 1013m (Xrays probably come
from a corona that
surrounds the disk)
Broad Line Region
• dense (109 /cm3) gas
clouds in area a few lightdays to months across
•outer edge defined by
dust sublimation radius
(1500K)
Jets – ionized gas
from accretion disk
spirals along magnetic
field lines away from
the disk
Narrow Line Region
• less dense (103/cm3)
gas clouds located 10 to
1000 lyrs out
•part of the host galaxy
ISM near galaxy center
Structure of AGNs and Unified Theory
Seyfert 2
Observer is looking at
blackhole “edge-on”
through the surrounding
dusty torus - does not see
broad emission lines
produced by gas near BH
Quasar/Seyfert 1
Observer is looking into
the center of the
accretion disk, viewing
motions of gas near
blackhole - sees broad
emission lines
Observer sees mostly synchrotron emission from jet - highly
variable with hardly any emission lines/absorption lines
/BL Lac
What Powers Active Galactic Nuclei??
(1) A compact central black hole
provides a very intense
gravitational field.
MBH = 106 - 109 Msun
(2) Infalling gas forms an
accretion disk around the
black hole.
(3) As the gas spirals inward,
friction heats it to extremely
high temperatures (T>104 K);
emission from the accretion
disk at different radii
accounts for optical thru soft
X-ray continuum.
(4) Some of the gas is driven out
into jets focused by the
magnetic field.
How efficient
is the energy
production?
Before entering the black hole, some fraction of the
mass is converted into energy. Matter is heated to
high temps by dissipation in the accretion disk and
radiates away its gravitational potential energy.
BH radius is Rs=2GMbh/c2 = 0.25 M8 light hours.
What energy is available (via gravitational potential
energy) for a mass m falling from far away to the Rs?
Emax = GMbhm/Rs
Substituting equation for Rs
Emax = (1/2)mc2
Half the rest energy of the infalling mass is converted to kinetic energy. If the mass is
decelerated (via the accretion disk), KE can be converted to thermal energy and then
photon energy.
Ephot = ηmc2 where η is efficiency and should be ≤ 0.5
The efficiency of this conversion is typically ~10% (η=0.1) for AGN.
Since the luminosity is dE/dt, then the AGN luminosity is
L = η(dm/dt)c2
If we know an AGN’s luminosity, we can deduce infall rate
Example:MW “AGN” luminosity is1000 Lsun
 implying dm/dt = 10-9 Msun per year)
What would be the luminosity if the infall rate (dm/dt) is one solar
mass per year with 10% efficiency?
L = η(dm/dt)c2 = (0.1)(2 x 1033g / 3.15 x 107s)(3 x 1010 cm/s)2
= 6 x 1045 ergs/s = 1.6 x 1012 Lsun
 100 times brighter than the entire Milky Way Galaxy
So...can you just shovel in mass at higher and higher rates to create a
superluminous AGN?
The Eddington Limit – the maximum L for a given black hole mass
(any higher and radiation pressure will blow away surrounding gas!)
Force caused by outward flow of photons balances gravitational force of
infalling matter (assuming pure ionized H gas)
σeL / (4πr2c) =
GMbhmp / r2
(eq. 21.12)
LE = 4πcGMbhmp/σe = 3.3x1012L(Mbh/108M)
If L is greater than LE, ionized gas will be accelerated outward and
accretion will cease. This leads to a maximum accretion rate for black
holes:
.
ME = LE / ηc2 = 2M/yr (Mbh/108M) (0.1/η)
.
.
. .
The Eddington ratio is then m = M / ME = Lbol/LE where m = dm/dt
Black Hole Mass - Bulge Mass Relation
• Evidence for SMBHs has been found in most galaxies with a significant spheroidal
component. It is now generally believed that all galaxies contain SMBHs.
• A clear relationship between the mass of the spheroidal component of a host galaxy
and the black hole mass has been found.
• indicates connection between galaxy formation (star formation) and growth/evolution
of central black hole
(Gebhardt et al. 2000; Ferrarese & Merritt 2000)
Active Galaxies as part of Galaxy Evolution
Quasars are relatively short-lived phenomena – only need 20 Myr to grow largest BHs known
Number density of quasars peaked when universe was about 3 Gyr old
Where do those BH’s go? Become dormant – go into “hibernation”?
•As small galaxies merge
to form larger ones, black
holes may form at the
nucleus (ULIRG stage?)
•With plenty of fuel
available early on, the
galaxy light is dominated
by emission of the black
hole (Quasar). Feedback
from AGN radiation may
squelch star formation in
the host galaxy – setting
spheriodal component size
•Additional mergers and depletion of fuel may result in radio galaxies and Seyfert galaxies.
•Further fuel depletion results in a normal galaxy with a dormant black hole at the nucleus.
Active Galaxies as part of Galaxy Evolution
A closer look at how the AGN phase may be the result of galaxy mergers
Hopkins et al. 2008