1. The Regulation of Star Formation by AGN Feedback D AVID R AFFERTY (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: A1068 McNamara et al. (2004) A2597 Koekemoer et al. (1999)

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: Radius (arcsec) U-I U+I images of A2390 taken at the MDM observatory

5. Star formation (indicated by positive gradients) occurs only where cooling times are short (t ≤ 7-8×10 8 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 ≈ 8  10 8 yr ≈ 30 keV cm 2

6. 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

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. 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 Quenched Net cooling + Star formation

9. Many cooling flows have central galaxies with active star formation Star formation found only in systems where: 1.Central cooling times are short (t cool < 5  10 8 yr) or entropies are low (S < 30 keV cm 2 ) 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  Cooling, regulated by AGN heating, leads to star formation in the central galaxy Summary

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?