Hubble Space Telescope Observations of Dusty Filaments in Hercules A:
  Evidence for Entrainment
  We present U, V, and I-band images of the host galaxy of Hercules A (3C 348)
obtained with HST/WFC3/UVIS. We find a network of dusty filaments which are
more complex and extended than seen in earlier HST observations. The filaments
are associated with a faint blue continuum light (possibly from young stars)
and faint H-alpha emission. It seems likely that the cold gas and dust has been
stripped from a companion galaxy now seen as a secondary nucleus. There are
dusty filaments aligned with the base of the jets on both eastern and western
sides of the galaxy. The morphology of the filaments is different on the two
sides - the western filaments are fairly straight, while the eastern filaments
are mainly in two loop-like structures. We suggest that despite the difference
in morphologies, both sets of filaments have been entrained in a slow moving
boundary layer outside the relativistic flow. As suggested by Fabian et al.
(2008), magnetic fields in the filaments may stabilize them against disruption.
We consider a speculative scenario to explain the relation between the radio
source and the shock and cavities in the hot ICM seen in the Chandra data
(Nulsen et al. 2005). We suggest the radio source originally (~60 Myr ago)
propagated along a position angle of ~35 degrees where it created the shock and
cavities. The radio source axis changed to its current orientation (~100
degrees) possibly due to a supermassive black hole merger and began its current
epoch of activity about 20 Myr ago.
