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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)

Lec. 10

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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: vc = H0 d Eqn(III-4)

z = (– 0) / 0 = / 0 = vc / c Eqn(III-5)

(non-rel.)

Fig. III-39: The Quasar 3C 48

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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 = vc / H0 = z c / H0 Eqn(III-6)

Fig. III-40 :Redshift of emission lines of (a) 3C 273, and (b) PKS 2000-030

(a) (b)

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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 point-source 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 QSOBoth 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 vcc, 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-42: Powerlaw vs blackbody spectrum Fig. III-41: Redshift vs velocity relationn

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 majors.) See class notes for details.

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Study carefully the relation between z, v, and d in Table III-2 Rise and Fall of Quasars: Fig.

III- 43 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.

Table III-2: Redshift and distance Fig. III-43: Rise and fall of quasars

FK

(*)

(*) Edge of the observable universe.

(*)

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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 spectrum is `bumpy’, without the radio component, and complicated.(ib) QSR: Radio-loud quasars, ~10%. Typical spectrum is bumpy, with extra radio component, and complicated.See Fig. III-44. I = photon intensity.

I

Photon energy

(8) Radio

(5)

optical

IR

UV(6)

X-Ray Gamma-ray

(1) IR bump

(7)

UV bump

Fig. III-44: Typical Continuum Spectrum of Quasars

Powerlaw

QSR

QSO

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• 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•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).

Fig. III-45: Spectrum of Radio Galaxies

I

Photon energy

radio

Gamma-ray

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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 (See Fig. III-46).

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Radio lobesRadio lobes

Inner outer

lobe lobe

Elliptical galaxy

X-ray jets

Optical jets

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.

____________ ___________d

V•

Black hole

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AGN Enviromnent: See Fig.III-47

Fig. III-47: AGN Environment

See class notes for the explanation.

° °

(8)

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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

¤Central Engine of AGN

Accretion-Powered AGN – QSO and Seyferts: Energy source is thepotential energy released by

accreting gas. See Fig. III-48.

Rotation-Powered AGN – Radio Galaxies: Energy source is the rotational energy released from a rotating black hole which is spinning down. See Fig. III-49. Nothing can come out of a black hole event horizon (surface), but when a black hole is rotating, there is another outer surface, called ergosphere (see FK Sec. 22-7).

••Cool disk

Hot corona

Hard X-rays (6)

Soft X-rays (7)

Black Hole

Fig. III-48: Accretion-Powered AGN

Ergosphere

Black Hole

Magnetic field lines

Fig. III-49: Rotation-Powered AGN

Event horizon

Ergoregion

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Magnetic fields penetrate through the space between event horizon and ergosphere, called ergoregion, and magnetic fields can extractrotation 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, and 48.

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¤ *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 spinning-down, 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 intermediate stages of evolution.

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• *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.

See Fig. III-50, 51.

18Fig. III-50: Unified Model of AGN

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EX 57: e.g., if = 10, observed jet

emission LJO = 100LJ

R

(= 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. 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 rotation axis. So, the huge ion torus blocks the optical broad line radiation from the broad-line region (BLR) within the torus (see Fig. III-52). The BLR is, therefore, hidden from 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. Fig III-52: Seyfert I

Seyfert II

•

Elliptical galaxy

Black holejet

Radio lobeYour eye

Line of sight

Seyfert II

Seyfert I

Molecular Torus

Hidden central engine

Narrow line region - NLR

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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 radiolobe = 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

Mbh = 3.2 x 109 M☉

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(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 Variabilitysee 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 Fig. III-54: The rotation curve

Mbh = 3 x 107 M☉. of the core of M31

FK

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Quasars are the ultraluminous centers of distant galaxies

Examples:

Seyfert galaxies seem to be nearby, low-luminosity, radio-quiet quasars

Seyfert galaxies are spiral galaxies with bright nuclei that are strong sources of radiation

Fig. III-55: A Seyfert Galaxy NGC 7742

Fig. III-56: Quasars and Their Host Galaxies

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Fig. III-57: BL Lac

Fig. III-58: The Inner Edge of an Accretion Disk

Fig. III-59: A Dusty Torus Around a SMBH Fig. III-60: Quasar 3C 273 and Jets

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Fig. III-61: A Quasar Jet

Fig. III-62: M 87

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Fig. III-63: Central engine of AGN, showing BLR

(broad line region), innermost accretion disk-

corona emitting UV bump and X-rays, shocks and jets (taken from Newton, Vol.3, No.2, p.32,1983, by S. Tsuruta)

Fig. III-64: Quasar 3C345

in X-rays (taken from Newton,

Vo. 3, No.2, p.32, 1983, by S. Tsuruta)

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Fig. III-65: At the Core of an AGNFig. III-66: Jets from a SMBH

Fig. III-67 The Radio Galaxy Cen A

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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 S. Tsuruta)

(a) (b)

(c)

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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 of collision.

32Fig. III-70: Providing Fresh “Fuel” for a SMBH

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Fig. III-71: A Radio GalaxyFig. III-72: The Head-Tail Radio Galaxy NGC

1265