The Regulation of Star Formation by AGN Feedback

DAVID RAFFERTY

(Penn State / Ohio U.)

Collaborators: Brian McNamara (Waterloo) and Paul Nulsen (CfA)

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 & O’Connell 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

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

McNamara et al. (2004)

A2597

Koekemoer et al. (1999)

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:

Radius (arcsec)

U-I

U+I images of A2390 taken at the MDM observatory

• Star formation (indicated by positive gradients) occurs only where cooling times are short (t ≤ 7-8×108 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)

Results – The Cooling-time / Entropy Threshold

≈ 8108 yr ≈ 30 keV cm2

• 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?

Results – CDG Location and Star Formation

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 0735.6+7421 Chandra X-ray (blue): B. R. McNamaraVLA Radio (red): L. Bîrzan

HST Optical: B. R. McNamara

• 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

Results – Feedback and Star Formation

€

Pcav

LICM

≥1

€

Pcav

LICM

<1 Quenched

Net cooling +Star formation

• Many cooling flows have central galaxies with active star formation

• Star formation found only in systems where: 1. Central cooling times are short (tcool < 5 108 yr) or

entropies are low (S < 30 keV cm2)

2. The galaxy is very near the cluster core (r < 20 kpc)

3. The ratio of AGN heating rate to cooling luminosity is approximately less than unity

1. Cooling, regulated by AGN heating, leads to star formation in the central galaxy

Summary

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

Radio Luminosity• Galaxies with active

star formation have larger radio luminosities:Evidence that star

formation and AGN activity both fueled by the cooling ICM?