The effect of environment on the properties of type Ia supernovae

Abstract

Since the discovery that the expansion of the Universe is accelerating, the field of modern observational cosmology with type Ia supernovae has strived to ratify and confirm this result. While one way to do so is to reduce uncertainties on measured cosmological parameters via the increase in sample size, recent years have seen an increased focus on understanding the underlying astrophysical phenomena that alter the measurable properties of type Ia supernovae. Many alterations currently are simply corrected for, without a complete understanding of the underlying cause. In this Thesis, we use the Dark Energy Survey’s 5 year, photometrically confirmed type Ia supernovae (SNe Ia) sample, and investigate the effects of large and small scale environment on many properties of type Ia supernovae. We aim to investigate how these properties affect the supernovae themselves, and probe the underlying causes of some of these changes. We identify that 66 of these supernovae have occurred within red-sequence selected galaxy clusters from the redMaPPer SVA1 catalogue. We compare light-curve and host galaxy properties of the cluster SNe to 1024 SNe Ia located in field galaxies, the largest comparison of two such samples at high redshift (z > 0.1) to date. We find that cluster SN light curves decline faster than those in the field at 97.7 per cent confidence. However, when limiting these samples to host galaxies of similar colour and mass, there is no significant difference in the SN light curve properties. Additionally, we measure the intrinsic rate of SNe Ia in cluster and field environments. We find the average ratio of the SN Ia rate per galaxy between high mass (10 ≤ log (M∗/M⊙) ≤ 11.25) cluster and field galaxies to be 0.594 ± 0.068. This difference is mass-dependent, with the ratio declining with increasing mass, which suggests that the stellar populations in cluster hosts are older than those in field hosts. We show that the mass-normalised rate (or SNe per unit mass) in massive-passive galaxies is consistent between cluster and field environments. Additionally, both of these rates are consistent with rates previously measured in clusters at similar redshifts. We conclude that in massive-passive galaxies, which are the dominant hosts of cluster SNe, the cluster delay time distribution, determining the expected rate of supernovae as a function of the time passed since a sample of white dwarfs is formed, is comparable to the field. Motivated by our detection of a declining light curve width as a function of normalised galactic separation, we also investigate the effects of this separation on SNe Ia light curves and standardisation. We use 1533 SNe Ia and show, for the first time, that the difference in SN Ia post-standardisation brightnesses between high and low-mass hosts reduces from 0.078 ± 0.011 mag in the full sample to 0.036 ± 0.018 mag for SNe Ia located in the outer regions of their host galaxies, while increasing to 0.100 ± 0.014 mag for SNe in the inner regions. In these inner regions, the difference in post-standardisation brightness between high and low mass hosts between can be reduced (but not removed) using a model where the

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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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