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Star Formation Efficiency in the Cool Cores of Galaxy Clusters

We have assembled a sample of high spatial resolution far-UV (Hubble Space Telescope Advanced Camera for Surveys Solar Blind Channel) and Halpha (Maryland-Magellan Tunable Filter) imaging for 15 cool core galaxy clusters. These data provide a detailed view of the thin, extended filaments in the cores of these clusters. Based on the ratio of the far-UV to Halpha luminosity, the UV spectral energy distribution, and the far-UV and Halpha morphology, we conclude that the warm, ionized gas in the cluster cores is photoionized by massive, young stars in all but a few (Abell 1991, Abell 2052, Abell 2580) systems. We show that the extended filaments, when considered separately, appear to be star-forming in the majority of cases, while the nuclei tend to have slightly lower far-UV luminosity for a given Halpha luminosity, suggesting a harder ionization source or higher extinction. We observe a slight offset in the UV/Halpha ratio from the expected value for continuous star formation which can be modeled by assuming intrinsic extinction by modest amounts of dust (E(B-V) ~ 0.2), or a top-heavy IMF in the extended filaments. The measured star formation rates vary from ~ 0.05 Msun/yr in the nuclei of non-cooling systems, consistent with passive, red ellipticals, to ~ 5 Msun/yr in systems with complex, extended, optical filaments. Comparing the estimates of the star formation rate based on UV, Halpha and infrared luminosities to the spectroscopically-determined X-ray cooling rate suggests a star formation efficiency of 14(+18)(-8)%. This value represents the time-averaged fraction, by mass, of gas cooling out of the intracluster medium which turns into stars, and agrees well with the global fraction of baryons in stars required by simulations to reproduce the stellar mass function for galaxies. This result provides a new constraint on the efficiency of star formation in accreting systems.