Growth of massive black holes during radiatively inefficient accretion phases

Xinwu Cao

Shanghai Astronomical Observatory, CAS

Growth of massive black holes is mainly through accretion.

The black hole mass density accreted in bright AGN phases is

which can match to the measured local black hole mass density, if a suitable radiative efficiency is adopted.

Observations show nuclear activities in many low-luminosity AGNs/normal galaxies.

Motivation

1. How were the bright AGNs switched off? 2. How much local massive black hole mass was accreted in lo

w-luminosity AGNs/normal galaxies?

,),()1(

)(2

bolaccbh dLzL

c

Ldz

dz

dtz

Lz

Different accretion modes

Slim disk: optically thick, disk-thickness: Standard thin disk: optically thick, disk-thickness:

Radiatively inefficient accretion flow (RIAF): optically thin, hot, disk-thickness: faint due to low radiative efficiency

Accretion mode transition occurs while

,1~/ RH 2.0m

1/ RH

.01.0~,2.0 critcrit mmm

.01.0~, critcrit mmm 1~/ RH

.01.0~,~ critcrit mmm

Spectra of slim disks (Wang et al. 1999)

Spectra of RIAFs (Manmoto 2000)

Spectra of thin disks (Laor & Netzer1989)

Constraints from the X-ray background(XRB)

Observed X-ray background (Comastri 2004)

Slim/thin disks: bright AGNsRIAFs: low-luminosity AGNs/normal galaxies

Difficulty: only local faint AGNs/normal galaxies can be well observed.

How were the bright AGNs switched off? (Cao, 2005, ApJ, 631, L101)

1. estimate number density of faint AGNs from the hard XLF(Ueda et al. 2003).

the duration of RIAFs in faint AGN accreting at a critical rate,

2. calculate the spectra of RIAFs

3. calculate the contribution to the XRB from RIAFs in faint AGNs

4. comparison with the observed XRB provides a constraint on the timescale

)()( bb

RIAFf zN

t

tzN

RIAFt.01.0~critm

.RIAFt

Blue: 0.05Red: 0.01Green: 0.005

bRIAF / tt

Conclusion: bright AGNs are switched off in less than ~1-5% of bright AGN lifetime. Most LLAGNs/normal galaxies are accreting at

So, how low is the accretion rate for LLAGNs/normal galaxies?

or ?510~ m

.01.0m

410

The contribution from the RIAFs in all inactive galaxies to the cosmological XRB can be calculated by

where is the template spectrum of a RIAF surrounding a black hole ( solar masses) accreting at

)(x El810 .aver

inactm

active galaxies: described by the Ueda’s X-ray LF, with

inactive galaxies:

.10 5.4110keV-2X L

.10 5.4110keV-2X L

maxzz

z

Black hole density in inactive galaxies The black hole mass density at redshift accreted during active galaxy phases from to is

where

given by Marconi et al. (2004).

The black hole mass density at redshift accreted during inactive galaxy

phases from to is

z

zmaxz

The total accreted black hole mass density is

The total black hole mass density

Thus,

The black hole mass density in active galaxies is

The initial condition:

If the number ratio of Compton-thick to Compton-thin AGNs is 0.6, the radiative efficiencies are ~0.1-0.17.

Black hole spin and radiative efficiency

The evolution of black hole mass and spin through accretion is described by

The evolution of black hole mass as a function of spin parameter is

Black hole spin and radiative efficiency

Corresponding to the radiative efficiencies derived, the spin parameter is required.

9.0~a

XRB synthesis for inactive black hole mass density derivedfor local black hole mass density .105.2 5local

bh

.9.0a4averinact 100.3 m for

XRB synthesis for inactive black hole mass density derivedfor local black hole mass density .100.4 5local

bh

.9.0a4averinact 105.2 m for

The number of Compton-thick AGNs with is assumed to be half of those with

2524log H N.2423log H N

.105.2 5localbh 4aver

inact 105.2 m for .100.4 5localbh 4aver

inact 100.2 m for

The constraints on accretion rates as functions of the number ratio of Compton-thick AGNs with to those with .2423log H N.2524log H N

3. Conclusions

1. Bright AGNs are switched off in less than ~1-5% of bright AGN lifetime, which may be the evidence that the gas near the black hole is blown away by AGN radiation.

2. Black hole growth is not important during inactive galaxy phases (<2-5% local black hole mass density was accreted in inactive galaxies). The average accretion rate of inactive galaxies should be

5

.100.30.1 4

Template spectra for RIAFs

a=0 a=0.9

The scenario for AGN formation and evolution 1. Birth of AGNs

Mergers between galaxies trigger nuclear gas flows to feed the black hole, and trigger nuclear starbursts. The nucleus is obscured by dense gas.

2. Bright AGN phase

accreting at around or slightly less than the Eddington rate. The gas becomes transparent and the nucleus can be seen.

3. Death of bright AGNs

radiation of AGNs expels gases to quench both accretion and star formation.

4. Faint AGN phases

accreting at very low rates

./100.30.1 Edd4 MM

years10~ 8

years1005.001.0~ 8