An Introduction to Galaxies and Cosmology

3.4 ~ 3.72005-07-

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3.4 The central engineActive galaxies

all active galaxies have the AGN.

AGN propertiessmall sizehigh luminosity

What is their energy source? (central engine)

3.4.1 The size of AGNs

l=d×

For a small angle Size estimation for a distant object

STScI-PRC1993-19

NGC 4395

d=4.3 Mpc

HST resolution = 0.05 arcsec

less than 1.0 pcFig. 3.26

3.4.1 The size of AGNs

t= Rc

Fig. 3.28 spherical lampshade

If R is the size of Earth's orbit

t~500 s

If the bulb fricker several times a secondthe frickering have no effect on the observed brightness of the lampshade.

There is a limit variability frequency which can be observed.

3.4.1 The size of AGNs

Fig. 3.28 spherical lampshade

t= Rc

R~c tmin

We can estimate the object size from its light variability.

3.4.1 The size of AGNs

Fig. 3.27 MCG-6-30-15 light curve

t~104s

MCG-6-30-15X-ray variability observed by XMM-Newton

R~c tFrom

R~10−4 pc

By optical estimationAGN size < 1 pc

Size estimation from X-ray variability gives a higher constraint about the AGN size.

3.4.1 The size of AGNsAnother way of the AGN size estimation

http://www.drao-ofr.hia-iha.nrc-cnrc.gc.ca/science/vlbi/vsop/index.shtml

Radio (VLBI)

highly angular resolution

Even so, AGNs remain unresolved.

3.4.2 The luminoisty of AGNsOur Galaxy L~2×1010 L⊙

normal galaxy - mainly radiate at optical wave lengthseyfert galaxy - infrared, optical, ultraviolet, ...

AGN radiates at least three times its optical luminosity.

quasar's AGN must be far more luminous than the normal galaxies and AGNs of seyfert galaxies.

quasar host galaxies are not less luminous than normal galaxies.

3.4.2 The luminoisty of AGNs

radio galaxy

Fig. Cygnus A

the power input into the lobes must exceed the luminosity of a normal galaxy by a a large factor.

Only AGN is a plausible candidate.

Not much luminous in optical wavelength.

3.4.2 The luminoisty of AGNsAn AGN outputs ~ 1038 W from a tiny region.

What is the AGN energy source ?

3.4.3 A super massive black holeBlack hole

small size and very strong gravitational fieldevent horizon - escape speed > c

Rs=2 GMc2

Rs : Schwarzschild radius

Estimate maximum BH mass inside an AGN.

BH

RsRs=3.0×1011 m

M = Rs×c2

2G= 2×1038 kg = 1×108 M ⊙

variability < 1 day RAGN≤ 3×1012 m

Super Massive Black Hole (SMBH)

3.4.4 An accretion discOne cloud

far nothing new

A number of gas clouds

they will collide with each others.

kinetic energy thermal energy

viscosity heating the gas, it will fall inwards

This process will continue until a complete accretion disc is formed around the black hole.

Fig. 3.30

Fig. 3.29

3.4.4 An accretion disc

heating effect ∝ gravitational field

accretion discs around black holeshigh temperaturesluminous sources of electromagnetic radiation

Credit: Jörn Wilms (Tübingen) et al.; ESA

3.4.5 Accretion powerIf a mass m falls into the black hole

radiation - 0.1 mc2 10 % of rest energy

high efficiency (star nuclear fusion ~ 0.7%)

The luminosity will depend on the rate at which matter falls in.

mass accretion rate Q

orL=0.1Q c2 Q= L0.1c2

About AGN L = 1038 W, c = 3 x 108 m/s

Q = 1022 kg s−1 ≃ 0.2 M ⊙ yr−1

3.4.5 Accretion power

Eddington limit

The previous result doesn't depend on BH mass.Why AGNs need super massive black hole?

L=0.1Q c2

BH

Rs gravitational force

radiation pressure

LE

W= 1.3×1031 M

M ⊙

This is an upper limit of the luminosity of a BH mass M

L = 1038 W M = 7.7×106 M ⊙

We need a supermassive black hole to accound for the engine in an AGN.

3.4.6 Jets

Fig. 3.31 A scenario for the formation of jets

Active galaxies often have jets.jets projecting up to several hundreds kiloparsecs.

Why jets are beaming?- due to magnetic fields ?

quasars and the more powerful radio galaxies appears to have a single jet.

effects of a relativistic beaming

3.5 Models of active galaxies

Fig. 3.32 A generic model for an active galaxy.

3.5.1 The obscuring torus

If AGN consisted solely of the central engine, we observe X-rays and ultraviolet radiation from the hot accretion disc.

AGN also emits optical and infrared wavelength radiation.

To account for the strong infrared emission from many AGNs, the model includes a torus of gas and dust that surrounds the central engine.

the cloud temperature < 2000 K (dust vaporize or sublimate)

3.5.1 The obscuring torusSublimation radius

power absorbed = a2× L4 r2 = La2

4 r2

cross section flux

power emitted = 4a2T 4 Stefan-Boltzmann law

L a2

4 r2 = 4a2T 4 r = L16T 4

12

absorbed = emitted

For AGN L = 1038 W, T = 2000 K r ~ 0.1 pc

r

3.5.1 The obscuring torus

AGN

extensive gas and dust (not proven) Fig. 3.34(a) radio image

jet

The disc plane is parpendicular to the axis of the radio jets.

3.5.2 The broad- and narrow-line regions

Our model of an AGN

gas clouds

They emit characteristic lines of the gases making up the clouds.

two kinds of line-emitting regionsbroad line region

dense fast moving cloudnarrow line region

low-density more slowly clouds

3.5.2 The broad- and narrow-line regionsBroad-line region

The radius of BLR ~ 1014 morbital speed ~ several thousand km/s(consistent with Doppler broadening)

temperature ~ 104 K

broad lines are not seen in every AGN.

3.5.2 The broad- and narrow-line regionsNarrow-line region

orbital speeds ~ 200-900 km/salways in viewSeveral NLRs have been imaged by the HST.

Fig. 3.35 NGC 5252

3.5.3 Unified modelsall AGNs are essentially the same

mass of the black holethe mass accretion rate

observed radiation will depend on the direction from which the AGN is viewed

Fig. 3.36Two tentative unified models for AGNs

no type 2 quasar

dust torus diminished by the intense radiation?

ULIRG?or

3.5.3 Unified modelsRadio-loud AGNs

Fig. 3.36 (b) Radio-loud AGNs unified model

3.5.3 Unified models

Why some AGNs are radio-loud while most are radio quiet?

Current thinking relates the presence of jets to the angular momentum of the black hole.

Only faster-spinning black holes able to produce jets.

radio-loud sources tend to be found in ellipticals.

high spin rate could be achieved by a merger of two massive black hole.

giant elliptical galaxies formed from mergers

3.6.1 Do supermassive black holes really exiest?

NGC 4151 (Sy1)line variations ~ 10 days

central engine take 10 days to 'light up' the broad line region

Gas moving speed ~ 7000 km/s

M = r v2

G M =108 M ⊙

This is consistent with its value calculated from consideration of the Eddington limit.

M 87 (radio galaxy) M =3×109 M ⊙

NGC 4258 (weak seyfert) M =4×107 M ⊙

3.6.2 Where are they now?

Where AGNs come from?This question relates with the origins of galaxies.

In past, close interactions and collisions between galaxies were much more common than they are now.

Even today, active galaxies are more likely than normal galaxies to be within the gravitational influence of a companion galaxy.

15 % seyferts have companion.normal galaxies - 3 %

3.6.2 Where are they now?How long AGNs live?

nearby AGNs high redshift AGNscompare

It should be possible to see if they have changed over the lifetime of the Universe.

z

n(z)

2~3 (~ 10 billion years ago)

quasar number density

This indicates that quasar phenomenon is short-lived.

3.6.2 Where are they now?

The modern view is that most galaxies contain supermassive black holes.

Normal galaxies we observe at present might have gone through an active stage in the past.

Why quasars die (or perhaps dormant)?

As time passes the AGN may 'sweep clean' the gas from its immediate environment.

However, this is not the end of the story.

galactic collision or merger will supply the gas to the black hole, and restore the activity.