Supernovae in the Subaru Deep Field: An initial sample and Type Ia rate out to redshift 1.6

Large samples of high-redshift supernovae (SNe) are potentially powerful probes of cosmic star formation, metal enrichment and SN physics. We present initial results from a new deep SN survey, based on re-imaging in the R, i′, z′ bands, of the 0.25 deg² Subaru Deep Field (SDF), with the 8.2-m Subaru telescope and Suprime-Cam. In a single new epoch consisting of two nights of observations, we have discovered 33 candidate SNe, down to a z′-band magnitude of 26.3 (AB). We have measured the photometric redshifts of the SN host galaxies, obtained Keck spectroscopic redshifts for 17 of the host galaxies and classified the SNe using the Bayesian photometric algorithm of Poznanski et al. that relies on template matching. After correcting for biases in the classification, 55 per cent of our sample consists of Type Ia SNe and 45 per cent of core-collapse SNe. The redshift distribution of the SNe Ia reaches z ≈ 1.6, with a median of z ≈ 1.2. The core-collapse SNe reach z ≈ 1.0, with a median of z ≈ 0.5. Our SN sample is comparable to the Hubble Space Telescope/Great Observatories Origins Deep Survey (GOODS) sample in both size and redshift range. The redshift distributions of the SNe in the SDF and in GOODS are consistent, but there is a trend (which requires confirmation using a larger sample) for more high-z SNe Ia in the SDF. This trend is also apparent when comparing the SN Ia rates we derive to those based on GOODS data. Our results suggest a fairly constant rate at high redshift that could be tracking the star formation rate. Additional epochs on this field, already being obtained, will enlarge our SN sample to the hundreds, and determine whether or not there is a decline in the SN Ia rate at z ≳ 1.