Far Ultraviolet Morphology of Star Forming Filaments in Cool Core
  Brightest Cluster Galaxies
  We present a multiwavelength morphological analysis of star forming clouds
and filaments in the central ($< 50$ kpc) regions of 16 low redshift ($z<0.3$)
cool core brightest cluster galaxies (BCGs). New Hubble Space Telescope (HST)
imaging of far ultraviolet continuum emission from young ($\sim 10$ Myr),
massive ($> 5$ \Msol) stars reveals filamentary and clumpy morphologies, which
we quantify by means of structural indices. The FUV data are compared with
X-ray, Ly$\alpha$, narrowband H$\alpha$, broadband optical/IR, and radio maps,
providing a high spatial resolution atlas of star formation locales relative to
the ambient hot ($\sim10^{7-8}$ K) and warm ionised ($\sim 10^4$ K) gas phases,
as well as the old stellar population and radio-bright AGN outflows. Nearly
half of the sample possesses kpc-scale filaments that, in projection, extend
toward and around radio lobes and/or X-ray cavities. These filaments may have
been uplifted by the propagating jet or buoyant X-ray bubble, or may have
formed {\it in situ} by cloud collapse at the interface of a radio lobe or
rapid cooling in a cavity's compressed shell. The morphological diversity of
nearly the entire FUV sample is reproduced by recent hydrodynamical simulations
in which the AGN powers a self-regulating rain of thermally unstable star
forming clouds that precipitate from the hot atmosphere. In this model,
precipitation triggers where the cooling-to- freefall time ratio is
$t_{\mathrm{cool}}/t_{\mathrm{ff}}\sim 10$. This condition is roughly met at
the maxmial projected FUV radius for more than half of our sample, and
clustering about this ratio is stronger for sources with higher star formation
rates.
