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Quillen et al.(2007) presented an imaging survey with the {\it Spitzer SpaceTelescope} of 62 brightest cluster galaxies with optical line emission locatedin the cores of X-ray luminous clusters. They found that at least half of thesesources have signs of excess infrared emission. Here we discuss the nature ofthe IR emission and its implications for cool core clusters. The strength ofthe mid-IR excess emission correlates with the luminosity of the opticalemission lines. Excluding the four systems dominated by an AGN, the excessmid-infrared emission in the remaining brightest cluster galaxies is likelyrelated to star formation. The mass of molecular gas (estimated from COobservations) is correlated with the IR luminosity as found for normal starforming galaxies. The gas depletion time scale is about 1 Gyr. The physicalextent of the infrared excess is consistent with that of the optical emissionline nebulae. This supports the hypothesis that the star formation occurs inmolecular gas associated with the emission line nebulae and with evidence thatthe emission line nebulae are mainly powered by ongoing star formation. We finda correlation between mass deposition rates (${\dot M}_X$) estimated from theX-ray emission and the star formation rate estimated from the infraredluminosity. The star formation rates are 1/10 to 1/100 of the mass depositionrates suggesting that the re-heating of the ICM is generally very effective inreducing the amount of mass cooling from the hot phase but not eliminating itcompletely.

An Infrared Survey of Brightest Cluster Galaxies. II: Why are Some Brightest Cluster Galaxies Forming Stars?