On the nature of AGN in hierarchical galaxy formation models

Nikos Fanidakisand

C.M. Baugh, R.G. Bower, S. Cole, C. Done, C. S. Frenk

Leicester, March 6, 2009

Outline

• Hierarchical galaxy formation• Supermassive black hole (SMBH) growth in

hierarchical cosmologies • Cosmological black hole (BH) spin

evolution• Predicting the radio loudness of active

galactic nuclei (AGN) • Conclusions

The idea of hierarchical galaxy formation

t=0 Gyr

t=4.7 Gyr

t=13.7 Gyr

z

Hierarchical cosmologyHierarchical cosmology

Large scale structures

Large scale structures

merg

er

sg

ravit

y

Small-scale structure formation

Small-scale structure formation

Initial quantum fluctuations

Initial quantum fluctuations

The GALFORM semi-analytical model

• Direct N-body simulations have limited dynamical range.

• Semi-analytical approach: use of analytical prescriptions for modeling different physical processes.

Why semi-analytics?

Mass, SpinMass, Spin

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Dimensionless spin parameter : a = cJBH /GMBH2

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a = 0 →Schwarzschild BH

a = 0.998 → Maximally rotating Kerr BH

BHs are the simplest objects in the Universe. They can be completely described by just two parameters, the mass and the spin. Each galaxy is believed to host a BH at its centre with a mass of 106-109M.

BHs are the simplest objects in the Universe. They can be completely described by just two parameters, the mass and the spin. Each galaxy is believed to host a BH at its centre with a mass of 106-109M.

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0 ≤ a ≤ 0.998 : ⎧ ⎨ ⎩

Facts

Rotating SMBHs in astrophysics

Rotating SMBHs in astrophysics

MCG -6 – 30 – 15

NF

, m

ast

ers

th

esi

s (2

00

7)

What do we observe?MCG -6 – 30 – 15 is a nearby Seyfert galaxy whose X-ray spectrum shows a prominent iron line at 6.4KeV.

The emission is believed to originate at ~2rschw, indicating:

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a ≥ 0.94

The gravitational wave detector LISA (to be launched in 2018) is expected to directly observe mergers of SMBH binaries and precisely measure the spin of the final BH remnant.

What will LISA observe?

Energy (keV)

- Progenitor haloes

Hot gas

Disk

- Satellite galaxy sinkstowards the central galaxythrough dynamical friction- Major merger

- Starburst (quasar mode) and spheroid formation

- Accretion of hot gas (radio mode) forms new disk

- DM haloes merge

- Each disk hosts a SMBH

- Satellite SMBH sinkstowards the central SMBHthrough dynamical friction-BH binary forms – emission of GW- Binary merger

- Vast amounts of gas are beingaccreted onto the SMBH

- Quiescent accretion of gas fromthe hot halo onto the SMBH

- DM haloes merge

Host galaxies SMBHs

How do we model SMBHs in hierarchical cosmologies?

• Co-evolution of SMBHs and host spheroids leads to a BH mass-bulge relation.

• Channels of SMBH growth: Cold gas accretion:

accretion of gas during galaxy mergers and disk instabilities.

Hot gas accretion: quiescent accretion from hot halo.

BH binary mergers.• The growth of mass

affects the evolution of the BH spin.

The growth of SMBHs mass

The growth of SMBHs mass

• Co-evolution of SMBHs and host spheroids leads to a BH mass-bulge relation.

• Channels of SMBH growth: Cold gas accretion:

accretion of gas during galaxy mergers and disk instabilities.

Hot gas accretion: quiescent accretion from hot halo.

BH binary mergers.• The growth of mass

affects the evolution of the BH spin.

BH spin change due to accretion

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a f =rlso

1/ 2

3

MBHin

MBHfin

4 − 3rlso1/ 2 MBH

in

MBHfin

− 2 ⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 2 ⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

Bardeen (1970)

Gas accreted via an accretion disk transfers its angular momentum at the last stable orbit to the BH:• Co-rotating gas – spin up• Counter-rotating gas – spin

down

last stable orbit (lso)

accretion disk

BH

The size of the disk is limited by its self gravity.

If the disk does not lie on the equatorial plane of the BH Lense-Thirring precession will (anti-) align the disk.

Alignment occurs if:

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Jd > 2Jh and cosθ > −Jd

2Jbh King etal. (2005)

BH spin change due to binary mergers

M1, S1M2, S2

L2 Binary BHs form during galaxy

mergers. The system hardens due to the

emission of gravitational waves. During the merger the satellite BH

transfers its angular momentum and spin to the central BH.

The final remnant is always a rotating BH.

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ii. Random Spins : a fin = a fin (a1,a2,cosa,cosβ,cosγ)

where cosa = S1 ⋅S2, cosβ = S1 ⋅L, cosγ = S2 ⋅L€

i. Non - spinning BH's : a fin = 2 3q /(1+ q)2 − 2.029q2 /(1+ q)4

where q = M2 / M1

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iii. Special case : M1 = M2 and S1 = S2 = 0 ⇒ a fin ≈ 0.69

Rezzolla et al. (2007)

BH binary

evolution in

numerical

relativity

Evolution of SMBH spins in hierarchical cosmologies

Evolution of spin with redshift Distributions in different mass bins

AGN Jets

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L jet ∝ (H /R)2 Bφ2MBH ˙ m a2

Imag

e s

ou

rce:

Nara

yan

+0

5

M87

• Jet formation:• Twisted magnetic

lines collimate outflows of plasma.

• The jet removes energy from the disk/BH.

• The plasma trapped in the lines accelerates and produces large-scale flows.

• The jet power increases proportionally to the BH spin:

Blandford & Znajek 1977

AGN radio loudness dichotomy

AGN radio loudness: Observational factsAGN radio loudness: Observational facts

• AGN can be divided into two classes:– Radio-loud objects (strong jets)– Radio-quiet objects (no jets)

• AGN form two distinct sequences on the Lbol –Lradio plane:– Upper sequence: objects hosted

by giant ellipticals with MSMBH>108M

– Lower sequence: objects hosted by ellipticals and spirals with MSMBH<108M

• Spin paradigm: the BH spin is believed to determine the radio loudness of an AGN

Data: Sikora et al. 2007

AGN in GALFORM: modelling the disk/jet

Accretion in GALFORM spans a wide range of accretion rates: Note: The geometry of the disk depends on the accretion rate!

Accretion in GALFORM spans a wide range of accretion rates: Note: The geometry of the disk depends on the accretion rate!

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˙ m Thin disk

ADAF

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10−6 ≤ ˙ m ≤102, ˙ m = ˙ M / MEdd*

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i) ˙ m ≥ 0.01: Thin Accretion Disks

Disk → Lbol,TD = εMBH ˙ m c 2

Jet → L jet,TD = 4 ×1036 MBH1.1 ˙ m 1.2a2

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ii) ˙ m ≤ 0.01: Advection Dominated

Accretion Flows (ADAFs)

Disk → Lbol,ADAF ∝ MBH ˙ m 2

Jet → L jet ,ADAF = 2 ×1038 MBH ˙ m a2

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* MEdd ∝ MBH

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H ~ R

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H << R

Meie

r 19

99

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L jet ∝ (H /R)2 Bφ2MBH ˙ m a2Remem

ber:

AGN radio loudness: theoretical predictions

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log10(LB )[erg/s]

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log10(LB )[erg/s]

AGN radio loudness: theoretical predictions

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log10(LB )[erg/s]

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log10(LB )[erg/s]

A unified models for the radio loudness of AGN?

Conclusions

• We have developed a model using GALFORM for explaining the radio loudness of AGN in hierarchical cosmological models.

• We find that in the present universe SMBHs have a bimodal distribution of spins.

• Giant ellipticals are found to host massive SMBHs (MBH>108Msun) that rotate rapidly.

• Using the Blandford-Znajek mechanism we model the radio emission from AGN. Our results are in good agreement with the observations.

• In our model the radio properties of an AGN seem to be determined by the spin and accretion rate characterising the central SMBH.

Quasar LF