# Active Galactic Nuclei

(1)

## Active Galactic Nuclei

(2)

(3)

(4)

(5)

### Farthest QSO known to date

Mortlock et al. (2011)

(6)

(7)

### (outshine host galaxy, so they look point-like)

Credit: A. Simonnet

(8)

6 8

### Gas being accreted forms a disk which is heated by frictionUV, optical and X-ray

14.3. MAXIMUM ENERGY RELEASE IN SPHERICAL ACCRETION 337

Table 14.1: Energy released by accretion onto various objects Accretion onto Max energy released (erg g−1) Ratio to fusion

Black hole 4.5 × 1020 75

Neutron star 1.3 × 1020 20

White dwarf 1.3 × 1017 0.02

Normal star 1.9 × 1015 10−4

14.3 Maximum Energy Release in Spherical Accretion

The most spectacular consequence of accretion is that it is an efficient mechanism for extracting gravitational energy.

• The energy released by accretion is approximately

∆Eacc = G Mm R ,

where M is the mass of the object, R is its radius, and m is the mass accreted.

• In Table 14.1 the amount of energy released per gram of hydrogen accreted onto the surface of various ob- jects is summarized (see Exercise).

• From Table 14.1, we see that accretion onto very compact objects is a much more efficient source of energy than is hydrogen fusion.

• But accretion onto normal stars or even white dwarfs is much less efficient than converting the equivalent amount of mass to energy by fusion.

14.3. MAXIMUM ENERGY RELEASE IN SPHERICAL ACCRETION 337

Table 14.1: Energy released by accretion onto various objects Accretion onto Max energy released (erg g−1) Ratio to fusion

Black hole 4.5 × 1020 75

Neutron star 1.3 × 1020 20

White dwarf 1.3 × 1017 0.02

Normal star 1.9 × 1015 10−4

acc

−1

20

20

17

15

−4

acc

### • But accretion onto normal stars or even white dwarfs is much less efficient than converting the equivalent amount of mass to energy by fusion.

Credit: M. Guidry

338 CHAPTER 14. BLACK HOLES AS CENTRAL ENGINES

Let us assume for the moment, unrealistically, that all ki- netic energy generated by conversion of gravitational en- ergy in accretion is radiated from the system (we address the issue of efficiency for realistic accretion shortly). Then the accretion luminosity is

Lacc = GM ˙M

R ≃ 1.3 × 1021! M/M R/km

" ! M˙ g s−1

"

erg s−1,

if we assume a steady accretion rate M.˙

(9)

### The central engine (cont.)

14.3. MAXIMUM ENERGY RELEASE IN SPHERICAL ACCRETION 339

Table 14.2: Some Eddington-limited accretion rates Compact object Radius (km) Max accretion rate (g s−1)

White dwarf ∼ 104 1021

Neutron star ∼ 10 1018

14.3.1 Limits on Accretion Rates

Ledd = 4

GMmpc

,

### • For fully ionized hydrogen, we may approximate σ by the Thomson cross section to give

Ledd ≃ 1.3 × 1038! M M

"

erg s−1.

### • If the Eddington luminosity is exceeded (in which case we say that the luminosity is

super-Eddington),

Lacc

Ledd

### gives

M˙max ≃ 1017! R km

"

g s−1

### Eddington-limited accretion rates based on this formula are given in Table 14.2.

14.3. MAXIMUM ENERGY RELEASE IN SPHERICAL ACCRETION 339

Table 14.2: Some Eddington-limited accretion rates Compact object Radius (km) Max accretion rate (g s−1)

White dwarf ∼ 104 1021

Neutron star ∼ 10 1018

14.3.1 Limits on Accretion Rates

Ledd = 4πGMmpc

σ ,

σ

σ

### by the Thomson cross section to give

Ledd ≃ 1.3 × 1038! M M

"

erg s−1.

### • If the Eddington luminosity is exceeded (in which case we say that the luminosity is

super-Eddington),

Lacc

Ledd

### gives

M˙max ≃ 1017! R km

"

g s−1

### Eddington-limited accretion rates based on this formula are given in Table 14.2.

14.3. MAXIMUM ENERGY RELEASE IN SPHERICAL ACCRETION 339

Table 14.2: Some Eddington-limited accretion rates Compact object Radius (km) Max accretion rate (g s−1)

White dwarf ∼ 104 1021

Neutron star ∼ 10 1018

edd

p

edd

38

−1

acc

edd

max

17

−1

### Eddington-limited accretion rates based on this formula are given in Table 14.2.

14.3. MAXIMUM ENERGY RELEASE IN SPHERICAL ACCRETION 339

Table 14.2: Some Eddington-limited accretion rates Compact object Radius (km) Max accretion rate (g s−1)

White dwarf ∼ 104 1021

Neutron star ∼ 10 1018

edd

p

edd

38

−1

acc

edd

max

17

−1

### Eddington-limited accretion rates based on this formula are given in Table 14.2.

Credit: M. Guidry

### Accretion efficiencies:

340 CHAPTER 14. BLACK HOLES AS CENTRAL ENGINES

14.3.2 Accretion Efficiencies

• For the gravitational energy released by accretion to be extracted, it must be radiated or matter must be ejected at high kinetic en- ergy (for example, in AGN jets).

• Generally, we expect that such processes are inefficient and that only a fraction of the potential energy available from accretion can be extracted to do external work.

• This issue is particularly critical when black holes are the cen- tral accreting object, since they have no “surface” onto which accretion may take place and the event horizon makes energy extraction acutely problematic.

• Let us modify our previous equation for accretion power by in- troducing an efficiency factor η that ranges from 0 to 1:

Lacc = 2ηGM ˙M R .

• Specializing for the black hole case, it is logical to take the Schwarzschild radius (the radius of the event horizon for a spher- ical black hole), which is given by

Rsc = 2GM

c2 = 2.95! M M

"

km,

to define the “accretion radius”, since any energy to be extracted from accretion must be emitted from outside that radius.

340 CHAPTER 14. BLACK HOLES AS CENTRAL ENGINES

14.3.2 Accretion Efficiencies

• For the gravitational energy released by accretion to be extracted, it must be radiated or matter must be ejected at high kinetic en- ergy (for example, in AGN jets).

• Generally, we expect that such processes are inefficient and that only a fraction of the potential energy available from accretion can be extracted to do external work.

• This issue is particularly critical when black holes are the cen- tral accreting object, since they have no “surface” onto which accretion may take place and the event horizon makes energy extraction acutely problematic.

• Let us modify our previous equation for accretion power by in- troducing an efficiency factor η that ranges from 0 to 1:

Lacc = 2ηGM ˙M R .

• Specializing for the black hole case, it is logical to take the Schwarzschild radius (the radius of the event horizon for a spher- ical black hole), which is given by

Rsc = 2GM

c2 = 2.95! M M

"

km,

to define the “accretion radius”, since any energy to be extracted from accretion must be emitted from outside that radius.

(10)

(11)

Tristram et al.

Circinus

(12)

(13)

(14)

(15)

– 9 –

### (more recent classif.)

Direct AGN light

Type 2 Type 1

Light dominated by host galaxy

Edd L/L > 0.01Edd

* Weak (or absent) narrow, low

* Old stellar population; little SF

* FR1 or FR2 radio morphology

* Very massive early−type galaxy

* Very massive black hole

* Massive early−type galaxy

* Massive black hole

* Old stellar population with some on−going star formation

* Mostly FR2 morphology

* Strong high−ionisation narrow lines

* Direct AGN light

Host galaxy properties like Type−2

* Direct AGN light

* Bias towards face−on orientation

AGN LINER

* Old stellar population; little SF

* Massive early−type galaxy

* Massive black hole

* Moderate strength, low−ionisation narrow emission lines

* Moderate mass black hole

* Weak or no radio jets

* Strong high−ionisation narrow lines galaxy with pseudo−bulge

ionisation emission lines

Host galaxy properties like high−

* Significant central star−formation

L/L < 0.01~ ~

Radio Quiet QSO / Seyfert 1 Type 2 QSO / Seyfert 2

* QSOs more luminous than Seyferts

QSO and Seyfert 2, respectively, but with addition of:

* Moderately massive early−type disk

Fig. 4.— The categorisation of the local AGN population adopted throughout this review. The blue text describes typical properties of each AGN class. These, together with the spread of properties for each class, will be justified throughout the review.

2.2. Finding AGN

This review is focused on insights into the co-evolution of SMBHs and galaxies that have been derived from large surveys of the local universe. For such investigations of the radiative-mode AGN it is the obscured (Type 2) AGN that are far and away the more valuable. In these objects the blinding glare of the UV and optical continuum emission from the central accretion disk has been blocked by the natural coronagraph created by the dusty obscuring structure. The remaining UV and optical continuum is generally dominated by the galaxy’s stellar component (Kauﬀmann et al. 2003a) which can then be readily characterized. In the sections to follow we will therefore restrict our discussion of radiative-mode AGN to techniques that can recognize Type 2 AGN. For the jet-mode AGN the intrinsic UV and optical emission from the AGN is generally weak or absent unless the observer is looking directly down the jet axis (e.g. Urry & Padovani 1995). Thus, the host galaxy properties can be easily studied without contamination.

Heckman & Best (2013)

(16)

(17)

### The X-ray spectrum

Credit: G. Risaliti

(18)

(19)

### The infrared spectrum

Quasar 3C249.1

Siebenmorgen et al. (2005) dusty torus

(power law at 1-5 um)

Silicate absorption

(20)

(21)

(22)

Credit: J. Silk

### positive feedback

Cresci et al. (2015)

(23)

(24)

Updating...

## References

Related subjects :