Abstract

This thesis is devoted to the study of optical spectra of thermonuclear supernovae, known as ``Type Ia'' supernovae (SN Ia). These violent stellar explosions, visible across a large fraction of the observable universe, are used to measure distances on cosmological scales. By directly probing the expansion dynamics of the universe, measurements of relative luminosity distances to SN Ia have shown the universal expansion to be accelerating. Such an acceleration can only be explained if the universe is pervaded by a new form of energy with negative pressure -- or ``Dark Energy'', such as Einstein's cosmological constant, Lambda. The use of SN Ia as distance indicators requires the use of a purely empirical calibration scheme relating the shape of the SN~Ia light curve with its peak luminosity. Although this relation is well verified for SN Ia in the local universe, it lacks convincing theoretical basis. Moreover, in view of the current uncertainties in modeling the explosion mechanisms and inferring the progenitor systems of SN Ia, its extrapolation to higher redshifts could be systematically affected by evolutionary effects, thereby biasing the cosmological results. Spectroscopy is better suited than photometry to make quantitative comparisons between SN Ia at different redshifts. Large amounts of information are conveyed by spectra on the properties of the ejecta (chemical composition, velocity/density gradients, excitation level); subtle differences, blurred together in photometric measurements, will show up in the spectra. However, comparisons of SN Ia at different redshifts have so far only been qualitative in nature. This thesis presents original results on a quantitative comparison, based on several analysis tools developed and/or tested during the course of the past three years. The thesis is structured as follows: the first two chapters serve as an introduction to the reader on the cosmological use of SN Ia, and on their optical spectra (theory and observation). Chap. 3 (Blondin et al. 2005a) presents a two-dimensional deconvolution method to separate a supernova spectrum from the contaminating background of its host galaxy. This algorithm was used to reduce the SN~Ia spectra, taken with the ESO Very Large Telescope, presented in Matheson et al. 2005 (see Appendix A). In Chap. 4, we discuss the use of a cross-correlation tool to determine the redshift of a SN Ia based on its spectrum alone -- i.e., not relying on narrow lines in the spectrum of the host galaxy. The main focus of this thesis is Chap. 5 (Blondin et al. 2005b): using characteristics of line-profile shapes in SN Ia, we provide the first clear quantitative evidence that the high-redshift SN~Ia are indeed similar to their local counterparts, providing a confirmation of their use as reliable cosmological distance indicators. Finally, in Chap. 6 we present preliminary results on cosmological time-dilation effects in high-redshift SN Ia spectra.