  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.
