Host Galaxy Spectra and Consequences for SN Typing From The SDSS SN
  Survey
  We present the spectroscopy from 5254 galaxies that hosted supernovae (SNe)
or other transient events in the Sloan Digital Sky Survey II (SDSS-II).
Obtained during SDSS-I, SDSS-II, and the Baryon Oscillation Spectroscopic
Survey (BOSS), this sample represents the largest systematic, unbiased,
magnitude limited spectroscopic survey of supernova (SN) host galaxies. Using
the host galaxy redshifts, we test the impact of photometric SN classification
based on SDSS imaging data with and without using spectroscopic redshifts of
the host galaxies. Following our suggested scheme, there are a total of 1166
photometrically classified SNe Ia when using a flat redshift prior and 1126 SNe
Ia when the host spectroscopic redshift is assumed. For 1024 (87.8%) candidates
classified as likely SNe Ia without redshift information, we find that the
classification is unchanged when adding the host galaxy redshift. Using
photometry from SDSS imaging data and the host galaxy spectra, we also report
host galaxy properties for use in future nalysis of SN astrophysics. Finally,
we investigate the differences in the interpretation of the light curve
properties with and without knowledge of the redshift. When using the SALT2
light curve fitter, we find a 21% increase in the number of fits that converge
when using the spectroscopic redshift. Without host galaxy redshifts, we find
that SALT2 light curve fits are systematically biased towards lower photometric
redshift estimates and redder colors in the limit of low signal-to-noise data.
The general improvements in performance of the light curve fitter and the
increased diversity of the host galaxy sample highlights the importance of host
galaxy spectroscopy for current photometric SN surveys such as the Dark Energy
Survey and future surveys such as the Large Synoptic Survey Telescope.
