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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

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AGN Feedback at the Parsec Scale

Feng Yuan

Shanghai Astronomical Observatory, CAS


F. G. Xie (SHAO)

J. P. Ostriker (Princeton University)

M. Li (SHAO)

  • 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
AGN feedback: an important role in galaxy formation & evolution
  • 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
population problem of compact young radio sources
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
Interpretation: intermittent activity

Courtesy: A. Siemiginowska

compact radio sources age
Compact radio sources: Age

1. Kinematic age

2. Synchrotron age

Typical age: <10^4 yr

Czerny et al. 2009

compact radio sources luminosity
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
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

Jet can only be formed in hard states (hot accretion flows)
thermal stability of radiation dominated standard thin disks
Thermal stability of Radiation-dominated standard thin disks
  • It has been thought radiation-dominated thin disk (L>0.2) is thermally unstable (e.g., Piran 1978; Janiuk et al. 2002)


  • 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



Two interpretations for the stability
  • “Time-lag” model

(Hirose, Krolik & Blaes 2009, ApJ)

Fluctuations in thermal energy are correlated to fluctuations in turbulent magnetic and kinetic energies, but with a time lag

  • “Magnetic pressure” model

(Zheng, Yuan, Gu & Lu 2011, ApJ)

Assume: ,

then we have:


Result: The critical Mdot of instability increases!

Advantage: can explain why GRS 1915+105 is unstable

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!

Hot Accretion (ADAF&LHAF)

Yuan 2003

two effects of compton scattering in accretion flows
Two effects of Compton scattering in accretion flows
  • Consider collision between photons and electrons in hot accretion flow, two effects:
  • Momentum
    • Radiation force:
    • 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
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 r1
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 Compton heating/cooling rate
  • The number of scattering at

radius r with unit length and

optical depth is :

  • So the heating/cooling rate (per unit volume of the accretion flow) at radius r is:

unit length in r

when compton heating cooling important
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
Getting the self-consistent solutions

δ~0.5 (from the modeling to Sgr A*)

The new Compton heating/cooling term

get the self consistent solutions using the iteration method
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 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!
  • Accretion resumes after cooled down “oscillation” of the activity of BH
oscillation scenario general picture
Oscillation scenario: general picture

Active phase

Inactive phase

active phase
Active phase
  • Duration of active phase:

accretion timescale at r_virial

  • why more luminous sources tend to be younger:


inactive phase
Inactive phase

What is the spatial range of heated gas during the active phase?

The energy equation of electrons:

The solution is:


We get the range of heated gas:

inactive phase1
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

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