  Recently, a few peculiar Type Ia supernovae (SNe) that show exceptionally
large peak luminosity have been discovered. Their luminosity requires more than
1 Msun of 56Ni ejected during the explosion, suggesting that they might have
originated from super-Chandrasekhar mass white dwarfs. However, the nature of
these objects is not yet well understood. In particular, no data have been
taken at late phases, about one year after the explosion. We report on Subaru
and Keck optical spectroscopic and photometric observations of the SN Ia
2006gz, which had been classified as being one of these "overluminous" SNe Ia.
The late-time behavior is distinctly different from that of normal SNe Ia,
reinforcing the argument that SN 2006gz belongs to a different subclass than
normal SNe Ia. However, the peculiar features found at late times are not
readily connected to a large amount of 56Ni; the SN is faint, and it lacks [Fe
II] and [Fe III] emission. If the bulk of the radioactive energy escapes the SN
ejecta as visual light, as is the case in normal SNe Ia, the mass of 56Ni does
not exceed ~ 0.3 Msun. We discuss several possibilities to remedy the problem.
With the limited observations, however, we are unable to conclusively identify
which process is responsible. An interesting possibility is that the bulk of
the emission might be shifted to longer wavelengths, unlike the case in other
SNe Ia, which might be related to dense C-rich regions as indicated by the
early-phase data. Alternatively, it might be the case that SN 2006gz, though
peculiar, was actually not substantially overluminous at early times.
