SUBARU AND KECK OBSERVATIONS OF THE PECULIAR TYPE IA SUPERNOVA 2006GZ AT LATE PHASES

Recently, a few peculiar Type Ia supernovae (SNe) that show exceptionally\nlarge peak luminosity have been discovered. Their luminosity requires more than\n1 Msun of 56Ni ejected during the explosion, suggesting that they might have\noriginated from super-Chandrasekhar mass white dwarfs. However, the nature of\nthese objects is not yet well understood. In particular, no data have been\ntaken at late phases, about one year after the explosion. We report on Subaru\nand Keck optical spectroscopic and photometric observations of the SN Ia\n2006gz, which had been classified as being one of these "overluminous" SNe Ia.\nThe late-time behavior is distinctly different from that of normal SNe Ia,\nreinforcing the argument that SN 2006gz belongs to a different subclass than\nnormal SNe Ia. However, the peculiar features found at late times are not\nreadily connected to a large amount of 56Ni; the SN is faint, and it lacks [Fe\nII] and [Fe III] emission. If the bulk of the radioactive energy escapes the SN\nejecta as visual light, as is the case in normal SNe Ia, the mass of 56Ni does\nnot exceed ~ 0.3 Msun. We discuss several possibilities to remedy the problem.\nWith the limited observations, however, we are unable to conclusively identify\nwhich process is responsible. An interesting possibility is that the bulk of\nthe emission might be shifted to longer wavelengths, unlike the case in other\nSNe Ia, which might be related to dense C-rich regions as indicated by the\nearly-phase data. Alternatively, it might be the case that SN 2006gz, though\npeculiar, was actually not substantially overluminous at early times.\n