Download 2. Bright AGN phase

Black hole accretion history of active galactic nuclei
曹新伍
中国科学院上海天文台
Outline
1. Introduction to accretion history of bright AGNs
2. Inefficient accretion history of faint AGNs
2.1. Different accretion modes
2.2. Constraints from the hard X-ray background
2.3. Constraints from the Eddington ratio distribution
2.4. Confront with the numerical simulation
3. Summary
1. Introduction to accretion history of bright AGNs
Quasar space density as a function of redshift.
Luminosity function  ( L, z ) describes the space number density
of AGNs with luminosity L at redshift z.
The black hole mass density accreted in bright AGN phases is

dt
(1   ) Lbol
 ( z )   dz 
( L, z )dL,
2
dz L
c
z
acc
bh
where  ( L, z ) is the AGN luminosity function,
 is radiation efficiency.
The local black hole mass density can be estimated from the SDSS data by
using M bh   relation (Yu & Tremaine 2002):
Local black hole mass density mainly comes from the accretion during
bright AGN phases.
Results for hard X-ray LF.
Lifetime of bright AGNs:
different investigations gives different values:
t b ~ 107  108
years.
The Hubble timescale t H ~ 1010 years,
t b  t H .
The Salpeter timescale: t S ~ 4.5  108  /(1   ) years.
(mass-doubling timescale for accreting at Eddington rate)
Observations show an upper limit on massive back hole mass
 1010 solar masses.
Why the lifetime t b is so short compared with t H ?
what halted the black hole accretion?
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  0.01M / M Edd .
2. Inefficient accretion history of faint AGNs
2.1. Different accretion modes
Slim disk: optically thick, disk-thickness: H / R ~ 1, m  0.2
Standard thin disk: optically thick, disk-thickness: H / R  1
m crit  m  0.2, m crit ~ 0.01.
Radiatively inefficient accretion flow (RIAF): m  m crit , m crit ~ 0.01.
optically thin, hot, disk-thickness: H / R ~ 1
faint due to low radiative efficiency
Accretion mode transition occurs while m ~ m crit , m crit ~ 0.01.
Spectra of slim disks (Wang et al. 1999)
Spectra of thin disks (Laor & Netzer1989)
Spectra of RIAFs (Manmoto 2000)
2.2. Constraints from the hard X-ray background
Observed X-ray background
(Comastri 2004)
Synthesis models for X-ray background
The cosmological XRB is mostly contributed by AGNs.
A typical synthesis model consists of
1. A template X-ray spectrum for AGNs:
a power-law spectrum + an exponential cutoff
at several hundred keV;
for example,
2. A soft/hard X-ray luminosity function for AGNs
Synthesis model based on hard X-ray luminosity function given by Ueda et al.
Blue dashed line: observed XRB
Thick solid black line: synthesis model (taken from Ueda et al. 2003)
bright AGNs+Compton-thick type 2 AGNs
X-ray spectra of RIAFs
The contribution to HXRB is dominantly from bright AGNs, but HXRB
(especially in ~ 10  1000keV) can constrain accretion history of faint
AGNs
The total monochromatic X-ray luminosity of faint AGNs in the co-moving
volume (Mpc 3 ) is ( t f is the faint AGN lifetime)
where nf(Mbh, z) is the black hole mass function for faint AGNs and Lx (Mbh,
E) is the X-ray luminosity of nan
the critical rate.
( LRIAF
)  [accreting
dL / dt ]at
.
1
The source number density
where lX(E, t) is the faint-AGN light curve for a black hole with 108 solar mass.
In order to calculate the contribution to XRB from RIAFs in faint AGNs,
one need to know the space number density of faint AGNs, most of which
have not been observed in any waveband except those in the local universe.
Every bright AGN will finally be switched to a faint AGN, so the density of
faint AGNs is ( t b is the bright AGN lifetime)
where N b ( z )(bright AGNs) described by X-ray LF, N f ( z ) is for faint AGNs).
The black hole mass density of faint AGNs is
where fbh > 1 is the ratio of average black hole masses of faint AGNs to
bright AGNs.
The RIAF timescale t RIAF is defined by
The contribution from the RIAFs in all faint AGNs to the cosmological XRB
can be calculated as
Comparison with observed XRB can set constraints on t RIAF / t b .
t RIAF /t b
Blue:
0.05
Red:
0.01
Green: 0.005
Cao, 2005, ApJ, 631, L101
2.3. Constraints from the Eddington ratio distributions
The number counts in unit logrithm of Eddington ratio is
 [d log( L / LEdd ) / dt ]1
where L(t ) is mainly determined by time-dependent accretion rate m (t ).
In principle, we can derive the accretion history m (t ) from the
comparison with the observed Eddington ratio distributions.
Eddington ratio distributions
Blue: Ho 2002; red: corrected distribution by Hopkins et al. 2005
Cartoon cross-section of a RIAF+standard thin disk system
 / m crit )
Standard disk+RIAF, rtr  rtr,0 (m
Solid: p  1 ; dashed: p  2 .
p
Blue lines: for Ho (2002)’s sample; red lines: for the sample
corrected by Hopkins et al (2005).
Solid lines: p  1; dotted lines: p  2.
2.4. Confront with the numerical simulation
(Taken from Di Matteo et al., 2005, Nature, 433, 604)
T~1.7Gyr , AGN radiation is peaked at around Eddington luminosity.
Taken from Di Matteo et al., 2005, Nature, 433, 604
3. Summary
1. Bright AGN lifetime is ~ 108 years, comparable with the
Salpeter timescale, which may be governed by the feedback
from AGNs.
2. Accretion rate declines rapidly (compared with bright
AGN lifetime) from m crit ~ 0.01 to m  0.01, which provides
evidence that the gas near the black hole is blown away by
AGN radiation.
3. Black hole growth is not important in faint AGN phases.
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
~ 108 years
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
~ 0.01  0.05  108 years
radiation of AGNs expels gases to quench both accretion and star
formation.
4. Faint AGN phases
accreting at very low rates
 0.01M / M Edd .