The Regulation of Star Formation by AGN Feedback

1. The Regulation of Star Formation by AGN Feedback DAVID RAFFERTY (Penn State / Ohio U.) Collaborators: Brian McNamara (Waterloo) and Paul Nulsen (CfA)

2. 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

3. 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: A2597 A1068 Koekemoer et al. (1999) McNamara et al. (2004)

4. 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 U+I images of A2390 taken at the MDM observatory Radius (arcsec)

5. Results ≈ 30 keV cm2 ≈ 8108 yr – The Cooling-time / Entropy Threshold • 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)

6. 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?

7. 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. McNamara VLA Radio (red): L. Bîrzan HST Optical: B. R. McNamara

8. 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

9. Summary • Many cooling flows have central galaxies with active star formation • Star formation found only in systems where: • Central cooling times are short (tcool < 5  108 yr) or entropies are low (S < 30 keV cm2) • The galaxy is very near the cluster core (r < 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

11. 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

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