Thesis defense [before December 2013]
Ph.D. Thesis: Extragalactic extinction and spectral properties of Type Ia supernovae
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Europe/Stockholm
FA32
FA32
Description
The use of Type Ia supernovae as standardisable candles for probing cosmological parameters with
high precision requires good knowledge about how the supernova light is affected along the line
of sight and how the intrinsic brightness varies between objects. The work in this thesis addresses
these topics.
One potential problem is if there is an evolution of the intrinsic brightness of Type Ia supernovae
with redshift. To investigate this we have compared spectral features of intermediate-redshift
supernovae with local ones. No redshift evolution could be detected up to z = 0.3. Correlations
of the strength of some of the spectral features with supernova colour and lightcurve shape were
found, in particular for the feature which primarily is a product of SiII 4130 absorption.
Another difficulty concerns dust present in the line of sight which could both dim and redden
the supernovae. We investigated dust extinction in distant galaxies by comparing the colours of
background quasars with what is expected for different types of dust. A wide range of fitted values
of Rv, the total-to-selective extinction ratio, was found, indicating that the dust properties in other
galaxies could potentially be different from the Milky Way value, Rv ~ 3.1.
The light could also be affected by an intergalactic dust population with an almost flat extinction
curve. We found, using quasar observations, that any dimming larger than 0.2 magnitudes in the
rest-frame B-band for a Type Ia supernova at z = 1 is ruled out. If the intergalactic dust has
an extinction law similar to the one in the Milky Way, the corresponding limit would be 0.03
magnitudes.
Another problem is that if axions exist, photons could be converted to axions over large
cosmological distances, in the presence of magnetic fields, leading to a dimming of distant objects.
For some model parameters, a dimming as large as 0.6 magnitudes for a Type Ia supernova at z =
1 would be allowed from quasar observations.