An HST/WFC3-UVIS View of the Starburst in the Cool Core of the Phoenix
  Cluster
  We present Hubble Space Telescope Wide Field Camera 3 observations of the
core of the Phoenix Cluster SPT-CLJ2344-4243 in five broadband filters spanning
rest-frame 1000--5500A. These observations reveal complex, filamentary blue
emission, extending for >40kpc from the brightest cluster galaxy. We observe an
underlying, diffuse population of old stars, following an r^1/4 distribution,
confirming that this system is somewhat relaxed. The spectral energy
distribution in the inner part of the galaxy, as well as along the extended
filaments, is a smooth continuum and is consistent with that of a star-forming
galaxy, suggesting that the extended, filamentary emission is not due to the
central AGN, either from a large-scale ionized outflow or scattered polarized
UV emission, but rather a massive population of young stars. We estimate an
extinction-corrected star formation rate of 798 +/- 42 Msun/yr, consistent with
our earlier work based on low spatial resolution ultraviolet, optical, and
infrared imaging. The lack of tidal features and multiple bulges, combine with
the need for an exceptionally massive (>10^11 Msun) cold gas reservoir, suggest
that this star formation is not the result of a merger of gas-rich galaxies.
Instead, we propose that the high X-ray cooling rate of ~2700 Msun/yr is the
origin of the cold gas reservoir. The combination of such a high cooling rate
and the relatively weak radio source in the cluster core suggests that feedback
has been unable to halt cooling in this system, leading to this tremendous
burst of star formation.
