New Observations of the Very Luminous Supernova 2006gy: Evidence for
  Echoes
  Supernova (SN) 2006gy was a hydrogen-rich core-collapse SN that remains one
of the most luminous optical supernovae ever observed. The total energy budget
(> 2 x 10^51 erg radiated in the optical alone) poses many challenges for
standard SN theory. We present new ground-based near-infrared (NIR)
observations of SN 2006gy, as well as a single epoch of Hubble Space Telescope
(HST) imaging obtained more than two years after the explosion. Our NIR data
taken around peak optical emission show an evolution that is largely consistent
with a cooling blackbody, with tentative evidence for a growing NIR excess
starting at day ~100. Our late-time Keck adaptive optics (AO) NIR image, taken
on day 723, shows little change from previous NIR observations taken around day
400. Furthermore, the optical HST observations show a reduced decline rate
after day 400, and the SN is bluer on day 810 than it was at peak. This
late-time decline is inconsistent with Co56 decay, and thus is problematic for
the various pair-instability SN models used to explain the nature of SN 2006gy.
The slow decline of the NIR emission can be explained with a light echo, and we
confirm that the late-time NIR excess is the result of a massive (>10 Msun)
dusty shell heated by the SN peak luminosity. The late-time optical
observations require the existence of a scattered light echo, which may be
generated by the same dust that contributes to the NIR echo. Both the NIR and
optical echoes originate in the proximity of the progenitor, ~10^18 cm for the
NIR echo and <~10-40 pc for the optical echo, which provides further evidence
that the progenitor of SN 2006gy was a very massive star.
