Preparing for cosmology with photometric type Ia supernovae in the Dark Energy survey year five sample

Abstract

Type Ia supernovae (SNe Ia) are the massive explosions of stars that are said to occur in a binary system consisting of a white dwarf and a companion star. As standard candles that exhibit consistent absolute peak luminosities of around MB = −19.4, they are excellent candidates for investigating and measuring cosmological distances and probing the nature of dark energy. The Dark Energy Survey (DES) was a six-year survey that was designed to constrain cosmological measurements including dark energy using different astrophysical probes like SNe Ia. The DES Supernovae Programme (DES-SN) was a transient survey that observed and measured the light curves of thousands of SNe Ia for use in cosmology and distance estimates. As current and future supernova surveys like the Legacy Survey of Space and Time (LSST), Euclid, and Nancy Grace Roman Space Telescope continue to increase the number of SNe Ia being observed, the necessity to classify and analyse SNe Ia using only photometry is becoming more imperative in order to remove the limitations imposed by spectroscopy – i.e. time constraints and observing costs. The DES Year Five (DES5YR) sample used photometric classification for SNe Ia selection and photometric redshifts (photo-z) and spectroscopic redshifts (spec-z) of their host galaxies for 1,410 SNe Ia for analysis. A sub-sample of DES5YR includes the X3 field which includes solely photometric information for the host galaxies of 282 SNe Ia for analysis. In evaluating the reliability of using solely photometric classification and redshifts for large-scale cosmological analyses of SNe Ia, Hubble diagram comparisons between the DES5YR spec-z and the X3 photo-z samples indicate an initial difference in the cosmological distance measurements when using different redshift information. A sample of 1,332 simulated photo-z SNe Ia is subsequently generated to test the efficiency of using a photo-z only sample like X3 and further investigate the comparisons between distance estimates and light curve parameters including stretch and colour dependent on redshift. The simulated photo-z sample reveals a bias in the colour evolution of SNe Ia fits that is attributed to the technique used to generate the sample as well as SALT2 fits and implementation of SALT2’s SED fitting method for the light curves. This bias can be modelled and corrected for during analysis, and, therefore, does not prevent accurate cosmology where a spec-z is not provided. Two additional DES fields - C3 and E2 - are also evaluated to expand the number of SNe Ia currently available in the DES5YR photo-z only sample from 282 to 893 SNe Ia in preparation for further cosmological analysis.

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Identifiers

ORCID for Mark Sullivan: orcid.org/0000-0001-9053-4820

ORCID for Philip Wiseman: orcid.org/0000-0002-3073-1512

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