SPECTROSCOPY OF HIGH-REDSHIFT SUPERNOVAE FROM THE ESSENCEPROJECT: THE FIRST FOUR YEARS

摘要

We present the results of spectroscopic observations from the ESSENCE high-redshift supernova (SN) surveyduring its first four years of operation. This sample includes spectra of all SNe Ia whose light curves were pre-sented by Miknaitis et al. and used in the cosmological analyses of Davis et al. and Wood-Vasey et al. The samplerepresents 273 hr of spectroscopic observations with 6.5-10 m class telescopes of objects detected and selected forspectroscopy by the ESSENCE team. We present 184 spectra of 156 objects. Combining this sample with that ofMatheson et al., we have a total sample of 329 spectra of 274 objects. From this, we are able to spectroscopicallyclassify 118 Type Ia SNe. As the survey has matured, the efficiency of classifying SNe Ia has remained constantwhile we have observed both higher-redshift SNe la and SNe Ia farther from maximum brightness. Examiningthe subsample of SNe Ia with host-galaxy redshifts shows that redshifts derived from only the SN Ia spectraare consistent with redshifts found from host-galaxy spectra. Moreover, the phases derived from only the SN Iaspectra are consistent with those derived from light-curve fits. By comparing our spectra to local templates, wefind that the rate of objects similar to the overluminous SN 1991T and the underluminous SN 1991bg in oursample are consistent with that of the local sample. We do note, however, that we detect no object spectroscop-ically or photometrically similar to SN 199 lbg. Although systematic effects could reduce the high-redshift ratewe expect based on the low-redshift surveys, it is possible that SN 199 lbg-like SNe Ia are less prevalent at highredshift.