Active Galactic Nuclei

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Active Galactic Nuclei

Karina Caputi

Physics of Galaxies 2016-2017 Q4

Rijksuniversiteit Groningen

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A bit of history…

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Seyfert galaxies

Broad-line emission from galactic nuclei are know since early 1900’s

The displayed broad lines could only be excited by photons more energetic than those from young stars

Carl Seyfert

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QSO first discovery

Boom of radioastronomy in 1950s: Third Cambridge (3C) Catalogue

Most 3C sources were identified with elliptical galaxies

…but a few looked point-like (like stars)

They indicated redshifts unusually high for such bright objects

Maarten Schmidt 3C 273 has B,V < 13 mag and z=0.158

And contemporary works by Sandage, Matthews, etc.

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Farthest QSO known to date

Mortlock et al. (2011)

z=7.085

Peak of QSO activity at z~2-3 (age of Universe ~ 2-3 Gyr)

More recent source discovered at z>8, but it is still unclear whether AGN

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The AGN components

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QSO and AGN

AGN/QSO classification is complex - QSO are the the most luminous AGN

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

Credit: A. Simonnet

✦ blue light excess

✦ light variability in some cases

✦ optical light polarisation

✦ X-ray emission due to accretion

✦ radio quiet or loud (some with jets)

✦ some have broad (> 1000 km/s) line emission (permitted lines) - due e 1

✦ Others only narrow lines - AGN type 2

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The central engine

The central engine is a supermassive black hole accreting gas

Black hole mass ~ 10 - 10 Msun - event horizon size of solar system

⁶ ⁸

Gas supplied at a rate of ~ 1 Msun/yr

Gas being accreted forms a disk which is heated by friction UV, 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 objects 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.

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

∆E

acc

= 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 × 10

²⁰

75 Neutron star 1.3 × 10

²⁰

20 White dwarf 1.3 × 10

¹⁷

0.02 Normal star 1.9 × 10

¹⁵

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

∆E

acc

= 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.

Credit: M. Guidry

338 CHAPTER 14. BLACK HOLES AS CENTRAL ENGINES

Let us assume for the moment, unrealistically, that all kinetic energy generated by conversion of gravitational energy 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.˙

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The central engine (cont.)

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

The Eddington luminosity is

Ledd = 4

π

GMmpc

σ

^,

with σ ithe effective cross section for photon scattering.

• 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),

accretion will be blocked by the radiation pressure, implying that there is a maximum accretion rate on compact objects.

• Equating

Lacc

and

Ledd

gives

M˙_max ≃ 10¹⁷! R km

"

g s⁻¹

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

14.3.1 Limits on Accretion Rates

The Eddington luminosity is

Ledd = 4πGMmpc

σ ,

with

σ

ithe effective cross section for photon scattering.

• 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),

accretion will be blocked by the radiation pressure, implying that there is a maximum accretion rate on compact objects.

• Equating

Lacc

and

Ledd

gives

M˙max ≃ 10¹⁷! R km

"

g s⁻¹

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

14.3.1 Limits on Accretion Rates

The Eddington luminosity is

L

edd

= 4πGMm

p

c

σ ,

with σ ithe effective cross section for photon scattering.

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

L

edd

≃ 1.3 × 10

³⁸

! M M

_⊙

"

erg s

⁻¹

.

• If the Eddington luminosity is exceeded (in which case we say that the luminosity is super-Eddington), accretion will be blocked by the radiation pressure, implying that there is a maximum accretion rate on compact objects.

• Equating L

acc

and L

edd

gives

M ˙

max

≃ 10

¹⁷

! R km

"

g s

⁻¹

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

14.3.1 Limits on Accretion Rates The Eddington luminosity is

L

edd

= 4πGMm

p

c

σ ,

with σ ithe effective cross section for photon scattering.

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

L

edd

≃ 1.3 × 10

³⁸

! M M

_⊙

"

erg s

⁻¹

.

• If the Eddington luminosity is exceeded (in which case we say that the luminosity is super-Eddington), accretion will be blocked by the radiation pressure, implying that there is a maximum accretion rate on compact objects.

• Equating L

acc

and L

edd

gives

M ˙

max

≃ 10

¹⁷

! R km

"

g s

⁻¹

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

Credit: M. Guidry

Accretion efficiencies:

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 energy (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 central 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 spherical 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.

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 energy (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 central 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 = 2η^{GM ˙}^M R .

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

R_sc = 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.

η=0.1 - typical value

(up to 0.3-0.4 for rotating black holes)

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The broad-line region

Urry & Padovani (1995)

Broad-line region extends 0.01-0.1 pc around central engine

Very hot gas clouds w/ v ~1000-10,000 km/s

Although different components are present (scaled) in both stellar and supermassive black holes, broad-line regions are exclusive to supermassive black holes

Direct visibility is extremely difficult

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The dusty torus

Current evidence suggests that dusty torus is clumpy rather than homogenous

Tristram et al.

Circinus

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The narrow-line region

Narrow-line region extends 100-1000 pc out of central engine

Well resolved for nearby AGN with HST Gas clouds w/ v ~100-500 km/s

Overlaps host galaxy (distinction unclear)

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AGN Classification

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The Unification Scheme

AGN type 1-2 classification depends only on the viewing angle

Key: polarised light

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Radiative versus jet mode

^{– 9 –}

(more recent classif.)

Direct AGN light

Jet mode Radiative mode

Low−excitation radio source

Type 2 Type 1

High−excitation radio source

Light dominated by host galaxy

Edd L/L > 0.01Edd

Radio LoudRadio Quiet

* Weak (or absent) narrow, low

* Old stellar population; little SF

* FR1 or FR2 radio morphology

* Moderate radio luminosity

* 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

* High radio luminosity

* Mostly FR2 morphology

* Strong high−ionisation narrow lines

excitation radio source, but with addition of:

* Direct AGN light

* Broad permitted emission lines

Host galaxy properties like Type−2

* Direct AGN light

* Broad permitted emission lines

* Bias towards face−on orientation

* Sometimes, beamed radio emission

AGN LINER

* Old stellar population; little SF

* Weak, small−scale radio jets

* 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−loud QSO

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)

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Spectral Properties

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The X-ray spectrum

Credit: G. Risaliti

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The optical spectrum

Broad lines

Narrow lines

BPT

diagram

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The infrared spectrum

Quasar 3C249.1

Siebenmorgen et al. (2005) dusty torus

(power law at 1-5 um)

Silicate absorption

The importance of silicate absorption and PAH emission varies among AGN

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AGN host galaxies

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How to study AGN host galaxies

PSF subtraction is critical

point-like source

(AGN) can outshine

host in some cases

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AGN feedback: positive or negative?

Negative feedback (i.e., which suppresses star formation) is

necessary to explain SF quenching of massive galaxies

Credit: J. Silk