Lec. 10
III-2 Active Galactic Nuclei
(Main Ref.: Lecture notes; FK p. 703;
Sec. 21-5, 6, 7; 24-1 through 5; Suppl. III;
CD photos shown in class)
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III-2a. Introduction – Discovery of Quasars
(Ref.:
Lecture notes; FK p. 703, Sec. 24-1; Suppl. III)
Quasars look like stars but have huge redshifts
After World War II, radio-emitting objects from outside the solar system
were discovered, radio sources mostly associated with galaxies –
Radio Galaxies.
Discovery of Quasars:
In 1963 M. Schmidt of Caltec discovered an optical `point source’ at
the site of one of these radio sources. Its optical emission lines were
found to be highly redshifted (see Fig. III-40a, 40b). Also, the
spectrum is NOT blackbody, but powerlaw (see Fig. III-42).
Redshift z is found to be cosmological –i.e., due to the expansion of the
universe.
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• To be seen at such large distances, quasars must be very
luminous, typically about 1000
times brighter than an ordinary
galaxy
• These redshifts show that
quasars are several hundred
megaparsecs or more from
the Earth, according to the
Hubble law
Recall that According to
Hubble Law:
Fig. III-39: The Quasar 3C 48
v c = H0 d
z = (– 0) / 0 =   / 0 = vc / c
Eqn(III-4)
Eqn(III-5)
(non-rel.)
3
where vc is receding velocity due to cosmological redshift; d is
distance, and H0 is Hubble Constant. From Eqns (III-4) and (III-5),
we get:
d = v c / H 0 = z c / H0
(a)
Eqn(III-6)
(b)
Fig. III-40 :Redshift of emission lines of (a) 3C 273, and (b) PKS 2000-0304
Then, large redshift z means large d, i.e., further away.
Subsequently, more objects like this have been found. Since
the spectrum is power-law, not blackbody, this opitcal pointsource looks blue. These highly redshifted blue star-like
radio-emitting objects are called `Quasi-Stellar Radio
Source’ or QSR.
Since then, these highly redshifted, blue star-like objects are
found which are not radio sources, i.e., Radio Quiet. These
radio-quiet quasi-stellar sources are called
Quasi-Stellar Object’, or QSO
Both are now called Quasars. See class notes for Further details.
EX 55: Radio galaxy Cyg. A: z = 0.056;
QSR 3C273: z = 0.158, d = 620 Mpc.
QSR 3C 48: z = 0.367, d = 1300 Mpc.
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Eqn(III-5) applies if non-relativistic, i.e., vc << c.
When vc  c we must use relativistic equation:
z = Square root of [(c + vc)/( c – vc )] – 1.
Eqn(III-7)
Then, as vcc, z   (infinity)! (See Fig. III-41).
Rearranging Eqn (III-7), the correct equation to find vc from z
becomes:
vc / c = [(z + 1)2 – 1] / [(z + 1)2 + 1].
Eqn(III-8)
Edge of the observable universe at vc=c, z = 
See class notes for the explanation.
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Fig. III-41: Redshift vs velocity relationn
Fig. III-42: Powerlaw vs blackbody spectrum
EX 56: Quasar PKS 2000-330 (See Fig. III-40(b) for the spectrum).
Lyman  line: real o = 121.6 nm; but measured  = 582,5 nm. From
Eqn(III-7) z = 3.79; and vc = 0.916 c from Eqn(III-8), i.e., velocity is
91.6% of velocity of light.
So, must use the relativistic equation! (Optional for non-science
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majors.)
See class notes for details.
Study carefully the relation between z, v, and d in Table III-2
Rise and Fall of Quasars: Fig. III43 tells that quasars were the most
abundant when redshift is about 3.
That means the quasar activity
peaked around that time. The
implication will be discussed when
we get to GUT (Grand Unified
Theory) of AGN.
FK
(*)
(*)
(*) Edge of the observable universe.
Table III-2: Redshift and distance
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Fig. III-43: Rise and fall of quasars
Note: Most quasars (~ 90%) are QSOs. Only ~10% are QSRs.
III-2b. Main Members of AGN
(Ref.: Lecture notes; FK
Sec. 24-1 though 5; Suppl. III)
V)
(i) Quasars: Most luminous of AGN.
Total Luminosity LQ ~ 1011–15 L☉ (compare with ~ 108–10 L☉ for
Spiral galaxies,105–11 L☉ For elliptical galaxies.)
(ia) QSO: Radio quiet quasars,
~90% of quasars. Typical
(1) IR bump
spectrum is `bumpy’, without
(8)
Powerlaw
the radio component, and
Radio
complicated.
(7)
IR
I
UV bump
(ib) QSR: Radio-loud quasars,
QSR
(5)
~10%. Typical spectrum is
UV
optical
(6)
bumpy, with extra radio
X-Ray
QSO Photon energy
Gamma-ray
component, and complicated.
See Fig. III-44. I = photon intensity.
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Fig. III-44: Typical Continuum Spectrum of Quasars
• Bumps
are thermal emission; Powerlaw is non-thermal emission.
• In addition, superimposed onto the continuum, many broad and narrow
emission lines.
• Parent galaxies discovered by HST, etc. Closer ones are Spiral, more
distant ones are elliptical galaxies.
See class notes for further details.
(ii) Seyferts = Mini QSO
(iia) Seyfert I: ~ QSO, but less luminous.
Total Luminosity LS ~ 108–12 L☉ ~ 0.001LQ. Parent galaxy mostly spiral.
(iib) Seyfert II: ~ Seyfert I, but no (or only weak) broad emission lines.
(iii) Radio Galaxies
radio
•LR ~ 1011–13 L☉.
Strong radio galaxy LR ~ LQ
• Parent galaxy ~ elliptical – giant
elliptical for strong radio galaxy.
• Spectrum ~ smooth – typical
synchrotron radiation (see Fig. III-45).
I
Gamma-ray
Photon energy
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Fig. III-45: Spectrum of Radio Galaxies
Table III-3: Properties of AGN
¤Strong radio galaxy has huge radio lobes (sometimes both
outer and inner lobes).
¤ Often X-ray (closest to the center) and optical jets come
out of the central galaxy
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(See Fig. III-46).
____________ d ___________
Elliptical galaxy
•
Black hole
Radio lobes
Inner
outer
lobe
lobe
V
X-ray jets
Optical jets
Radio lobes
Fig. III-46: Strong radio galaxy
•
(iv) Blazers and BL Lacs
Their luminosity is ~ weak LQ,
but NO (or only very weak) broad emission lines.
The spectrum is smooth, no bumpy spectrum, but typically like
spectrum of radio galaxies = smooth synchrotron spectrum like Fig.
III-45.
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AGN Enviromnent: See Fig.III-47
(8)
°°
Fig. III-47: AGN Environment
See class notes for the explanation.
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III-2c. Supermassive Black Hole (SMBH) and
Grand Unified Theory (GUT) of AGN
(Ref.: Lecture notes; FK Sec. 21-5, 6, 7, 24-2 to 5; Suppl. III; CD photos)
(i) `Best-Buy’ Model – Grand Unified
Theory (GUT) of AGN
Hard X-rays (6)
¤Central
Engine of AGN
Accretion-Powered AGN – QSO and
Seyferts: Energy source is the
potential energy released by
accreting gas. See Fig. III-48.
Black Hole
Cool disk
Soft X-rays (7)
Hot corona
••
Rotation-Powered AGN – Radio
Fig. III-48: Accretion-Powered AGN
Galaxies: Energy source is the
Magnetic field lines
rotational energy released from a
rotating black hole which is spinning Ergoregion
down. See Fig. III-49. Nothing can
Ergosphere
come out of a black hole event
Event horizon
Black Hole
horizon (surface), but when a black hole
is rotating, there is another outer surface,
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called ergosphere (see FK Sec. 22-7). Fig. III-49: Rotation-Powered
AGN
Magnetic fields penetrate through the space between event horizon and
ergosphere, called ergoregion, and magnetic fields can extract
rotation energy from ergoregion, and energy is deposited at X-ray
jets, optical jets, and radio lobes – e.g. like a generator and radiators
in an electric circuit.
See class notes for further details.
Evolution of AGN:
*Accretion-powered AGN – QSO and Seyferts: Early stages of
evolution. In the early universe, when the redshift z ~ 3, the quasar
activity is strongest, probably because of supply of a lot of gas from
galaxy collisions and merging (a lot of evidence for such activity –see
CD photos). Theory predicts that a SMBH was already there (formed
earlier). Due to a lot of gas supplied, the black hole eats up the
accreting gas at high accreting rate. The infalling gas radiates X-rays
and gamma rays from regions closest to the hole, while radio, IR and
optical radiation comes from regions further out. See Fig. III-44, 47,
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and 48.
¤ *Rotation-powered AGN – Strong Radio Galaxies:
Late stages of evolution. During the earlier quasar phase, not only
mass but also angular momentum will be swallowed by the black
hole, and so the black hole will spin up – rotate faster with time.
Eventually, the gas supply will be exhausted as time goes on, and
the quasar activity ends. Then, the black hole will start spinningdown, i.e., start losing rotational energy. Although nothing can
come out of a black hole, energy can be extracted from a rotating
black hole, from ergoregion, through magnetic fields penetrating
that region. The particles (electrons and ions) and their energy are
carried along the field lines along the rotation axis to regions further
out, and deposited as X-rays and optical in their jets, and then radio
emission from radio lobes.
• What about other AGN?
They can be explained as the effects of viewing angle, and/or as the
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intermediate stages of evolution.
• *QSR: They are objects in an intermediate stage, i.e., the quasar
activity is still going on, but the gas supply decreases, the spinning
down of the black hole starts, and the rotation energy starts getting
lost by radiation from jets, etc., which emit non-thermal synchrotron
radiation (which goes from radio to gamma-rays.) Therefore, the
observed spectrum is the combination of the typical QSO radiation +
synchrotron radiation from jets, etc. (with radio component now
present) (see Fig. III-44 and 47).
*Blazers and BL Lacs: These objects are explained as due to the effect of
viewing angles, i.e., they are QSR or radio galaxies viewed head-on in the
direction of the rotation axis. The particles in the jets are traveling at relativistic
speeds (i.e., v ~ c). Then the radiation from the jets is enhanced as 2, where  is
Lorenz factor. So, although the radiation from the central engine (QSR) or parent
elliptical radio galaxy is too faint to see (due to far away distance) the enhanced
radiation from the jets (which is synchrotron) still can be seen. So, the spectrum of
these objects are smooth typical synchrotron radiation (see Fig. III-45), and the
broad emission lines and various bumps in the continuum spectrum typical of quasar
spectrum from the central engine are too faint to be seen.
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See Fig. III-50, 51.
Fig. III-50: Unified Model of AGN
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EX 57: e.g., if  = 10, observed jet
emission LJO = 100LJR
(= real jet luminosity).
So, if real LJR ~ LQ(=
(radiation from quasar in
the central engine), it is
too faint to be seen.
Only jet L can be seen.
•
Your eye
Radio lobe
Line of sight
jet
Black hole
Elliptical galaxy
Fig III-51: BL Lacs and Blazer
Seyfert II: Seyfert II is a. Seyfert I viewed ~
edge-on, or with a large angle away from the Seyfert II
rotation axis. So, the huge ion torus blocks the
optical broad line radiation from the broadline region (BLR) within the torus (see Fig.
III-52). The BLR is, therefore, hidden from
Molecular
Torus
our view, and hence its emission is absent in
the observed spectrum. X-rays, however, can
penetrate through the molecular torus, and so
we can see them.
See class notes for further details.
Seyfert I
Hidden
central
engine
Narrow line
region - NLR
Fig III-52: Seyfert I 
Seyfert II 
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ii) Why SMBH?
The `best-By Model’ of AGN requires the presence of a supermassive
black hole (SMBH) in the central engine of AGN – Why?
(iia) Black hole mass Mbh:
Accretion-powered AGN (Quasars & Seyferts):
How to estimate black hole mass Mbh for accretion-powered AGN (e.g.,
QSO and Seyferts) from accretion rate M/t? Use Eqn(III-9):
Mbh = (M/ t) 
Ean(III-9)
where M/t = accretion rate;  = life time of QSO/Seyfert activity.
M/t ~ 0.01 – 0.1M☉/year for Seyferts(see EX 60);
M/t ~ 1M☉/year for QSO, from accretion models.
 ~ 107–8 years for Seyferts; ~ 106 for QSO; from statistics.
See class notes.
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EX 58: Seyfert nucleus: From Eqn(III-9), for M/t ~ 0.1M☉/yr and  ~ 108
years,
we get Mbh ~ 107 M☉.
******************************************************************
Rotation-powered AGN (Radio galaxies): Use Eqns (III-10) and (III-11), where:
d = v t
Eqn(III-10)
Mbh = t L / ( c2)
Eqn(III-11)
where d = distance from the center of the parent galaxy to the edge of the radio
lobe; v = the velocity with which the outer edge of the radio lobe is going away
from the central galaxy (see Fig. III-46); t = time it takes for particles to
travel from the center of the galaxy to the edge of the radio lobe, L is the
observed radio luminosity, and  = efficiency of converting mass to radio-wave
radiation ~ 0.01.
See class notes for derivation.
EX 59: Radio galaxy: Distance from the central galaxy to the edge of the radio
lobe = 0.5 Mpc; velocity of the outer edge away from the galaxy v = 1000
km/sec,  = 0.01, L = 1012 L☉. Note: 1pc = 3.086 x 1013 km, c = 3 x 108 m/s,
L☉= 3.86 x 1026 W, M☉ = 2 x1030 kg.
Then, from Eqn(III-10) t = 1.5 x 1016 sec, and from Eqn(III-11), we get
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Mbh = 3.2 x 109 M☉
(iib) Accretion Rate M/t: How to find M/t for acccretion-powered AGN?
Use Eqn(III-12)
L =  (M/t) c2
Eqn(III-12)
where  = efficiency of converting mass to radiation ~ 0.1, L = luminosity.
See class notes for derivation.
EX 60 Seyfert nucleus with L = 1011 L☉. 1 year = 3 x 107 sec. Then, Eqn(III-12)
gives
M/t ~ 0.065 M☉/yr.
(iic) Time Variability
see Fig. III-53.
R~ct
Eqn(III-13)
where R is the size of the emission
region, c is the light speed, and
 t = time scale of X-ray variability.
Reason: See FK 24.
For Seyfert nucleus, typical t ~ hours.
Fig.III-53: time variability of AGN
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Now, radius of a stationary black hole Rbh = Rs from Lec.9, so that:
Rbh = Rs = 3 ( Mbh / M☉ ) (km).
Eqn ( III-14)
EX 61: For Seyfert of Ex 58, if t ~ 1 hour, Eqn(III-13) R ~ 109
km.
From Eqn(III-14), Rbh = 3 x 107 km.
So,
R/Rbh ~ 33.
 The region emitting X-rays is only about 33 times the black hole
radius!
• Note: If QSO/Seyfert X-rays come from star clusters or a
supermassive star, a star cluster or a supermassive star of the size of ~
33Rbh will collapse to a black hole quickly! They cannot stay as a
stable system for any length of time.
Conclusion: So, there must be a black hole there, and the X-rays are
emitted from gas close to the black hole!
See class notes for further details.
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(iid) Strong Radio Galaxies: Circumstantial evidence.
Jets and radio lobes along the same direction, along the rotation axis of the central
rotating black hole, like a gyroscope. See Figs. III-46, 65, 66, and 69.
See class notes for further details.
(iie) Motion of stars and gas near the
center of galaxy:
* Study Fig. III-54 for a typical rotation
curve, to show the presence of a central
concentration – a black hole.
* Mass measured by the rotation curve:
Our Milky Way Galaxy
Mbh = 3 x 106 M☉.
Andromeda Galaxy
Mbh = 3 x 107 M☉.
FK
Fig. III-54: The rotation curve
of the core of M31
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Examples:
Seyfert galaxies seem to be
nearby, low-luminosity, radioquiet quasars
Seyfert galaxies are spiral galaxies
with bright nuclei that are strong
sources of radiation
Fig. III-55: A Seyfert Galaxy NGC 7742
Quasars are the ultraluminous centers of distant galaxies
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Fig. III-56: Quasars and Their Host Galaxies
Fig. III-57: BL Lac
Fig. III-58: The Inner Edge of an Accretion Disk
Fig. III-59: A Dusty Torus Around a SMBH
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Fig. III-60: Quasar 3C 273 and Jets
Fig. III-61: A Quasar Jet
Fig. III-62: M 87
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Fig. III-64: Quasar 3C345
in X-rays (taken from Newton,
Fig. III-63: Central engine of AGN, showing BLR
Vo. 3, No.2, p.32, 1983, by S.
Tsuruta)
(broad line region), innermost accretion diskcorona emitting UV bump and X-rays, shocks and
jets (taken from Newton, Vol.3, No.2, p.32,1983, by
S. Tsuruta)
28
Fig. III-66: Jets from a SMBH
Fig. III-65: At the Core of an AGN
Fig. III-67 The Radio Galaxy Cen A
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(a)
(b)
(c)
Fig. III-68: (a) Strong radio galaxy Centaurus A, with the outer and inner radio
lobes, and the central giant elliptical galaxy NGC 5128 (in optical) superimposed;
(b) Radio galaxy NGC 6521with a straight radio jet; (c) Quasar 3C273 in optical,
with a jet coming toward upper left (taken from Newton, Vol. 3, No. 2, 1983, by
30S.
Tsuruta)
Fig. III-69: Strong radio galaxy Cygnus A (3C 405), with huge radio lobes and the
central peculiar galaxy in optical, which looks like two galaxy in the process
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of collision.
Fig. III-70: Providing Fresh “Fuel” for a SMBH
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Fig. III-71: A Radio Galaxy
Fig. III-72: The Head-Tail Radio Galaxy NGC
1265
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