Host Galaxy Spectra and Consequences for SN Typing From The SDSS SN Survey

TL;DR

This study analyzes a large sample of SDSS supernova host galaxy spectra to evaluate how spectroscopic redshifts influence supernova classification accuracy and light curve analysis, emphasizing the importance of host spectroscopy for current and future surveys.

Contribution

It provides the largest unbiased spectroscopic dataset of SN host galaxies and demonstrates the impact of host galaxy redshifts on supernova typing and light curve fitting accuracy.

Findings

87.8% of SN Ia candidates remain classified the same with host redshift.

Using host spectroscopic redshifts increases SALT2 fit convergence by 21%.

Without host redshifts, SALT2 fits are biased towards lower redshifts and redder colors.

Abstract

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.