Analyze Si??6355 Velocity Of Type Ia Supernovae With SNfactory Data

The Si??6355 absorption line is one of the distinct features in Type la supernovae(SNe la)near-max spectra.In this thesis,we focus on the blueshift velocity of the Si??6355 absorption line measured at B-band maximum brightness.It has been suggested that a group of SNe la as those with Si??6355 velocity higher than 12000 km s-1 constitute a unique subclass(Wang et al.2009;Foley&Kasen 2011;Wang et al.2013),denoted as high-velocity(HV)SNe la,which may have colors and standardized brightnesses discrepant from those of other normal-velocity(NV)SNe la and thus might potentially impact the determination of cosmological parameters.Concerning this,we study Si??6355 velocity and HV SNe la using use a rich,high-quality and unbiased set of low-redshift spectral data from the Nearby Supernova Factory(SNfactory).We perform statistical analyses of Si??6355 velocity distributions and identify HV SNe la within samples.Our discovery is that the fraction of HV SNe la in the SNfactory-discovered sample,the one obtained from the SNfactory blind search,is only 9.4+1.52.6%,lower than the fraction of 30.6-3.6+4.7%in the sample selected by Wang et al.(2013).We also use other SNe la datasets to examine Si?A6355 velocity distributions and HV fractions,making comparisons among different samples and investigating potential sources of biases.Due to its untargeted nature,no strong bias has been found in the SNfactory-discovered sample.We then measure the properties of SNe la host galaxy environments and find that the HV tail of a Si??6355 velocity distribution shows dependences on host properties.This is in broad agreement with the conclusions by Wang et al.(2013),but it is not evident that NV and HV SNe la are distinct populations based on host analysis.We have also discovered that a portion of discrepancies among Si??6355 velocity distributions for different samples might be attributed to host absolute magnitude,which is the main selection difference between blind and galaxy-targeted searches.Furthermore,we include high-redshift spectral data from the Sloan Digital Sky Survey-? SNe survey(SDSS-?)to study the Si?A6355 velocity dependence on host absolute magnitude and possible evolution with redshift.Although a strong decline of average Si??6355 velocity with redshift(>5?)has been detected,we are cautious against over-interpretation on the evolution of Si??6355 velocity,given large velocity uncertainties for the SDSS-? sample and selection-dependent offsets among surveys.Finally,we discuss the requirements for conducting further studies of Si??6355 velocity and HV SNe la at high redshifts in future SNe la surveys.