Quantatitive analysis of type Ia supernova spectra and implications for cosmology

Thesis

This thesis presents the spectroscopic observations of the Supernova Legacy Survey (SNLS) from the Gemini-North telescope from May 2006 – May 2008. During this time 68 of 95 targets were confirmed as Type Ia supernovae (SNe Ia) or classified as probable SNe Ia events. We use these objects and reanalyse other high-z spectra obtained as part of SNLS to search for signs of evolution in SNe Ia properties. To do this we use a multicolour method to remove host galaxy contamination from the high-z spectra and measure the equivalent widths of three spectral features in the rest-frame blue part of the spectrum: CaII H&K, SiII and Mg II. These are compared to a sample of low-z objects drawn from the literature. No evidence for evolution is found, but a change in the properties of the SiII feature is observed. This can be explained by changing supernova demographics, an outcome of the two-component progenitor model for SNe Ia.

The utility of SNe Ia supernovae for cosmology is confirmed and the possible use of spectral features as alternative calibrators is investigated. The equivalenth width of the SiII feature is anticorrelated with light curve stretch and this is used to define a "spectral stretch" from the low-z sample, which is used to reduce the scatter in the low-z sample. This holds promise for improving the calibration of high-z SNe Ia, particularly if new facilities are used to provide higher-quality spectra.

Authors: Walker, ES

• Publication date: 2009
• DOI: 10.5287/ora-pz9zq98xz
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: cosmology; supernovae
• Subjects: Astrophysics; Physics