The%20Regulation%20of%20Star%20Formation%20by%20AGN%20Feedback

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The Regulation of Star Formation by AGN Feedback

• DAVID RAFFERTY
• (Penn State / Ohio U.)

Collaborators Brian McNamara (Waterloo) and
Paul Nulsen (CfA)
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Star Formation the ICM

• Indirect evidence links the ICM to star formation
  in the central galaxy. For example
• Indicators of star formation correlate with
  properties of the cooling flow (e.g., Heckman et
  al. 1981, McNamara OConnell 1989, Cardiel et
  al. 1995)
• Optical line emission seen only in BCGs at the
  cores of cooling flows (e.g., Edwards et al.
  2007)
• Cooling and star formation rates are in rough
  agreement
• If star formation is fueled by the cooling ICM,
  there should be some relation between the
  presence of SF and the central cooling
  time/entropy of the ICM

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Identifying Star Formation

• Indicators of star formation
• Optical line emission from ionized gas
• Far-IR emission from heated dust
• Excess blue/UV emission, beyond that expected
  from the underlying population

A1068
A2597
McNamara et al. (2004)
Koekemoer et al. (1999)
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Optical Data

• Sample 46 systems in the Chandra archive with a
  wide range of central cooling time
• U, R, and I imaging
• Search for excess blue emission in color profiles

U-I
UI images of A2390 taken at the MDM observatory
Radius (arcsec)
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Results
The Cooling-time / Entropy Threshold

• Star formation (indicated by positive gradients)
  occurs only where cooling times are short (t
  7-8108 yr), whereas
• Red systems have a wide range of cooling times
• This threshold may correspond to onset of thermal
  instabilities in the ICM (see Voit et al. 2008,
  also Soker 2008)

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Results
CDG Location and Star Formation

• Star formation seen only in systems with small
  separations between X-ray and CDG cores
• However, small separations and short cooling
  times are necessary, but not sufficient,
  conditions
• Why do some systems lack star formation?

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AGN Feedback

• Systems with excess AGN heating
• Cooling is quenched
• Little active star formation
• Systems that are underheated
• Some cooling proceeds
• Active star formation

MS 0735.67421
Chandra X-ray (blue) B. R. McNamara VLA Radio
(red) L. Bîrzan HST Optical B. R. McNamara
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Results
Feedback and Star Formation

• Systems in which the AGN quenches cooling
• Generally, no recent star formation
• Systems in which the AGN does not quench cooling
  
• Tendency for recent star formation

Net cooling Star formation
Quenched
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Summary

• Many cooling flows have central galaxies with
  active star formation
• Star formation found only in systems where
• Central cooling times are short (tcool lt 5 ? 108
  yr) or entropies are low (S lt 30 keV cm2)
• The galaxy is very near the cluster core (?r lt 20
  kpc)
• The ratio of AGN heating rate to cooling
  luminosity is approximately less than unity
• Cooling, regulated by AGN heating, leads to star
  formation in the central galaxy

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Thermal Instability

• Cooling and star formation may be driven by
  thermal instabilities in the hot gas
• A blob of cooling gas becomes unstable to cooling
  when growth rate of instabilities exceeds damping
  rate from conduction

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Radio Luminosity

• Galaxies with active star formation have larger
  radio luminosities
• Evidence that star formation and AGN activity
  both fueled by the cooling ICM?