Transcript Document
AGN Feedback at the Parsec Scale
Feng Yuan
Shanghai Astronomical Observatory, CAS
with:
F. G. Xie (SHAO)
J. P. Ostriker (Princeton University)
M. Li (SHAO)
OUTLINE
Intermittent activity of compact radio sources
Outburst: 10^4 years
Quiescent: 10^5 years
previous interpretation & its problem
thermal instability of radiation-dominated thin disk
Explaining the intermittent activity with Global Compton
scattering feedback mechanism in hot accretion flows
What is global Compton scattering ?
When L > 0.02 L_Edd: no steady solutions; BH activity oscillates
Estimations of durations of active and inactive phases
AGN feedback: an important role in
galaxy formation & evolution
M BH
correlation
suppression of star formation in elliptical galaxies
Great progress made; still many details need further
exploration (Ostriker 2010)
seeking direct observational evidence
Feedback often causes intermittent activity of AGNs
Investigating feedback at various scales
Observational evidence (I):
Relics and new jets
Observational evidence (II):
double-double radio sources
Courtesy: A. Siemiginowska
Population problem of compact
young radio sources
Many compact young (10^3 year) radio sources found
If the total activity lasts for 10^8 yr, the number of
sources with the ages < 10^3 yr should be ~ 10^5
times lower than the number of sources older than
10^3 yr
But the population studies show far too many compact
young sources: what’s the reason?
Interpretation: intermittent activity
Courtesy: A. Siemiginowska
Compact radio sources: Age
1. Kinematic age
2. Synchrotron age
Typical age: <10^4 yr
Czerny et al. 2009
Compact radio sources: Luminosity
Typical bolometric L:
0.1L_Edd or
0.02 L_Edd (preferred)
Czerny et al. 2009
Existing models for intermittent
activity
Galaxy merger: 10^8 year
Ionization instability: 10^8 year
Thermal instability of radiation-pressure
dominated thin disk (Czerny et al. 2009)
Limit-cycle behavior intermittent activity
But two questions:
Can jets be formed in standard thin disk?
Is the radiation-dominated thin disk unstable?
Jet can only be formed in hard states
(hot accretion flows)
soft/high state:
Standard thin disk
No radio emission
without jets
Low/hard state:
Hot accretion flow
Strong radio emission
with jets
Thermal stability of Radiationdominated standard thin disks
It has been thought radiationdominated thin disk (L>0.2) is
thermally unstable (e.g., Piran 1978;
M
Janiuk et al. 2002)
However:
Observations:
Gierlinski & Done (2004): a sample
of soft state BHXBs; 0.01<
L/L_Edd<0.5;
no variability quite stable
Possible exception: GRS1915+105:
L too high?
Confirmed by 3D MHD
Numerical Simulations
(Hirose, Krolik & Blaes 2009)
Stable or not??
Two interpretations for the stability
“Time-lag” model
(Hirose, Krolik & Blaes 2009, ApJ)
r P
Fluctuations in thermal energy are correlated
to fluctuations in turbulent magnetic and
kinetic energies, but with a time lag
causality
“Magnetic pressure” model
const. ,
(Zheng, Yuan, Gu & Lu 2011, ApJ)
Assume: B H
then we have:
BH
Result: The critical Mdot of instability increases!
Advantage: can explain why GRS 1915+105 is unstable
R
We propose:
Global Compton heating feedback
as an interpretation
Hot Accretion (ADAF&LHAF)
Hot ( virial) & Geometrically thick
“Optically thin” in radial & vertical
directions: photons will freely escape with
little collisions with electrons
Convectively unstable outflow
(no radiation: Stone, Pringle & Begelman 1999;
strong radiation: Yuan & Bu 2010)
\dot{M} low: ADAF;
\dot{M} high: LHAF
Radiative efficiency: a function of
\dot{M}; can reach 10%L_Edd!
Yuan 2003
Two effects of Compton scattering
in accretion flows
Consider collision between photons and electrons in hot
accretion flow, two effects:
Momentum
U
c T
Radiation force:
c
Balance with grav. force Eddington luminosity
Energy
For photons: Compton up-scattering or Comptonization, which is
the mechanism of producing X-ray emission in BH systems
For electrons: they can obtain or loss energy due to the scattering
with photons (e.g., Compton radiative cooling)
We will focus on electrons and “non-local” scattering
(because hot accretion flow is optically thin in radial direction)
Assume the electrons have Te and the photon energy is Є, after each
scattering on average the electron will obtain energy:
Thompson limit:
The spectrum received at radius r
It is difficult to directly calculate
the radiative transfer when scattering
is important.
So we use two-stream approximation,
calculate the vertical radiative
transfer in a zone around r’.
The spectrum before Comptonization is:
The spectrum after Comptonization is calculated based on
Coppi & Blandford (1990)
The spectrum received at radius r
When calculating the radiative
transfer from dr’ to r, we neglect
for simplicity the scattering.
Then from the region inside of r:
From the region outside of r:
The Compton heating/cooling rate
The number of scattering at
radius r with unit length and
optical depth es is :
unit length in r
So the heating/cooling rate (per unit volume of the accretion flow)
at radius r is:
When Compton heating/cooling important?
We compare Compton heating/cooling with viscous heating
Yuan, Xie & Ostriker 2009
Result: Cooling is important when Mdot>0.01
Heating is important when Mdot>0.2 (function of r!)
Getting the self-consistent solutions
R
M 4RH v M out
Rout
dv
1 dp
2
v
k 2 r
dr
dr
p
2
v(r j ) r
.
.
d e p e
v
2
dr
d i pi
v
2
dr
s
δ~0.5 (from the modeling to Sgr A*)
The new Compton heating/cooling term
d
q qie q q c om p
dr
d
(1 )q qie
dr
Get the self-consistent solutions
using the iteration method
procedure:
guess the value of Compton heating/cooling at each
radius,
solve the global solution,
compare the obtained Compton heating/cooling
with the guessed value to see whether they are
identical.
If not, use the new value of Compton heating and
get the new solution until they are identical.
When Mdot is large: oscillation
When L >~0.02 L_Edd, Compton heating is so strong
that electrons at r_virial~10^5r_s will be heated above
T_virial
Thus gas will not be captured by BH, no steady hot
solution exists!
5
rvirial ~ 10 rs (L / 2% LEdd )
1/ 2
Accretion resumes after cooled down “oscillation”
of the activity of BH
Oscillation scenario: general picture
Active phase
Inactive phase
Active phase
Duration of active phase:
accretion timescale at r_virial
for L ~ 2% LEdd , rvirial ~ 105 rs
So:
why more luminous sources tend to be younger:
rvirial ~ 105 rs (L / 2%LEdd )1/ 2
Inactive phase
What is the spatial range of heated gas during the active phase?
The energy equation of electrons:
The solution is:
From:
We get the range of heated gas:
Inactive phase
Properties of heated gas:
temperature: T= T_x ~ 10^9K
Density=?
From pressure balance with ISM:
n_inact T_x = n_ISM T_ISM
(T_ISM~10^7 K)
(how to know n_ISM? L ~ 2%L_Edd Mdot n_ISM)
Duration of inactive phase
Cooling timescale:
for T_x & n_ISM, t_cool~10^5 yr
accretion time at 10^6r_s: >> 10^5 yr
We should choose the shorter one
Summary
The global Compton scattering feedback can explain:
L~0.02 L_Edd
More luminous sources are younger
Duration of active phase: 3 10^4 yr
Duration of inactive phase: 10^5 yr
Thank you!