A High-Resolution View of Fast Radio Burst Host Environments
  We present Hubble Space Telescope (HST/WFC3) ultraviolet and infrared
observations of eight fast radio burst (FRB) host galaxies with sub-arcsecond
localizations, including the hosts of three known repeating FRBs. We quantify
their spatial distributions and locations with respect to their host galaxy
light distributions, finding that they occur at moderate host
normalized-offsets of 1.4 $r_e$ ([0.6,2.1] $r_e$; 68% interval), occur on
fainter regions of their hosts in terms of IR light, but overall trace the
radial distribution of IR light in their galaxies. The FRBs in our tested
distribution do not clearly trace the distributions of any other transient
population with known progenitors, and are statistically distinct from the
locations of LGRBs, H-poor SLSNe, SGRBs, and Ca-rich transients. We further
find that most FRBs are not in regions of elevated local star formation rate
and stellar mass surface densities in comparison to the mean global values of
their hosts. We also place upper limits to the IR flux at the FRB positions of
$m_{\rm IR}\gtrsim\!24.8-27.6$~AB~mag, constraining both satellite and
background galaxies to luminosities well below the host luminosity of
FRB121102. We find that 5/8 FRB hosts exhibit clear spiral arm features in IR
light, and that the positions of all well-localized FRBs located in such hosts
are consistent with their spiral arms, although not on their brightest regions.
Our results do not strongly support the primary progenitor channel of FRBs
being connected either with the most massive (stripped-envelope) stars, or with
events which require kicks and long delay times (neutron star mergers).
