Interstellar-Medium Mapping in M82 Through Light Echoes Around Supernova
  2014J
  We present multiple-epoch measurements of the size and surface brightness of
the light echoes from supernova (SN) 2014J in the nearby starburst galaxy M82.
Hubble Space Telescope (HST) ACS/WFC images were taken ~277 and ~416 days after
B-band maximum in the filters F475W, F606W, and F775W. Observations with HST
WFC3/UVIS images at epochs ~216 and ~365 days (Crotts 2015) are included for a
more complete analysis. The images reveal the temporal evolution of at least
two major light-echo components. The first one exhibits a filled ring structure
with position-angle-dependent intensity. This radially extended, diffuse echo
indicates the presence of an inhomogeneous interstellar dust cloud ranging from
~100 pc to ~500 pc in the foreground of the SN. The second echo component
appears as an unresolved luminous quarter-circle arc centered on the SN. The
wavelength dependence of scattering measured in different dust components
suggests that the dust producing the luminous arc favors smaller grain sizes,
while that causing the diffuse light echo may have sizes similar to those of
the Milky Way dust. Smaller grains can produce an optical depth consistent with
that along the supernova-Earth line of sight measured by previous studies
around maximum light. Therefore, it is possible that the dust slab, from which
the luminous arc arises, is also responsible for most of the extinction towards
SN 2014J. The optical depths determined from the Milky Way-like dust in the
scattering matters are lower than that produced by the dust slab.
