  Owing to its extremely high luminosity and long duration, SN2006gy radiated
more energy in visual light than any other known SN. Two hypotheses to explain
its high luminosity -- that it was powered by shock interaction with CSM as
implied by its Type IIn spectrum, or that it was fueled by radioactive decay
from a large mass of 56Ni synthesized in a pair-instability SN -- predicted
different late-time properties. Here we present observations of SN2006gy
obtained more than a year after discovery. We were unable to detect it at
visual wavelengths, but clear near-IR K and H-band detections show that it is
still at least as luminous as the peak of a normal SN II. We also present
spectra giving an upper limit to the late-time Ha luminosity of about 1e39
erg/s. Based on the weak late-time Ha, X-ray, and radio emission, combined with
the difficulty of explaining the shift to IR wavelengths, we can rule out
ongoing CSM interaction as the primary late-time power source. Instead, we
propose that the evolution of SN2006gy is consistent with one of two possible
scenarios: (1) A pair-instability SN plus modest CSM interaction, where the
radioactive decay luminosity shifts to the IR because of dust formation. (2) An
IR echo, where radiation emitted during peak luminosity heats a pre-existing
dust shell at radii near 1 light year, requiring the progenitor star to have
ejected another 10 Msun shell about 1500 yr before the SN.
