Spitzer Observations of Star Formation in Brightest Cluster Galaxies

We summarize the results of Spitzer IRAC and MIPS observations of 62 brightest cluster galaxies (BCGs) with optical line emission located in the cores of X-ray luminous clusters. We find that at least half of these sources have signs of excess infrared emission. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding several systems dominated by an AGN, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely powered by star formation. The IR luminosity (and thus star formation) is higher in BCGs with shorter cooling times in the central hot ICM suggesting that the gas which cools from the ICM ultimately forms stars. We find a correlation between mass deposition rates estimated from the X-ray emission and the star formation rate estimated from the infrared luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates suggesting that the re-heating of the ICM is generally very effective in reducing the amount of mass cooling from the hot phase but does not eliminate it completely.