Star Formation Activity in CLASH Brightest Cluster Galaxies
  The CLASH X-ray selected sample of 20 galaxy clusters contains ten brightest
cluster galaxies (BCGs) that exhibit significant ($>$5 $\sigma$)
extinction-corrected star formation rates (SFRs). Star formation activity is
inferred from photometric estimates of UV and H$\alpha$+[\ion{N}{2}] emission
in knots and filaments detected in CLASH HST ACS and WFC3 observations.
UV-derived SFRs in these BCGs span two orders of magnitude, including two with
a SFR $\gtrsim$ 100 M$_{\odot}$ yr$^{-1}$. These measurements are supplemented
with [\ion{O}{2}], [\ion{O}{3}], and H$\beta$ fluxes measured from spectra
obtained with the SOAR telescope. We confirm that photoionization from ongoing
star formation powers the line emission nebulae in these BCGs, although in many
BCGs there is also evidence of a LINER-like contribution to the line emission.
Coupling these data with Chandra X-ray measurements, we infer that the star
formation occurs exclusively in low-entropy cluster cores and exhibits a
correlation with gas properties related to cooling. We also perform an in-depth
study of the starburst history of the BCG in the cluster RXJ1532.9+3021, and
create 2D maps of stellar properties on scales down to $\sim$350 pc. These maps
reveal evidence for an ongoing burst occurring in elongated filaments,
generally on $\sim$ 0.5-1.0 Gyr timescales, although some filaments are
consistent with much younger ($\lesssim$ 100 Myr) burst timescales and may be
correlated with recent activity from the AGN. The relationship between BCG SFRs
and the surrounding ICM gas properties provide new support for the process of
feedback-regulated cooling in galaxy clusters and is consistent with recent
theoretical predictions.
