Optical Spectra of Thermonuclear Supernovae in the Local and Distant Universe

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.