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Type Ia supernova (SN Ia) cosmology provides the most direct evidence for thepresence of dark energy. This result is based on the assumption that thelook-back time evolution of SN Ia luminosity, after light-curve corrections,would be negligible. Recent studies show, however, that the Hubble residual(HR) of SN Ia is correlated with the mass and morphology of host galaxies,implying the possible dependence of SN Ia luminosity on host galaxy properties.In order to investigate this more directly, we have initiated spectroscopicsurvey for the early-type host galaxies, for which population age andmetallicity can be more reliably determined from the absorption lines. As thefirst paper of the series, here we present the results from highsignal-to-noise ratio (>100 per pixel) spectra for 27 nearby host galaxies inthe southern hemisphere. For the first time in host galaxy studies, we find asignificant (~3.9sigma) correlation between host galaxy mass (velocitydispersion) and population age, which is consistent with the "downsizing" trendamong non-host early-type galaxies. This result is rather insensitive to thechoice of population synthesis models. Since we find no correlation withmetallicity, our result suggests that stellar population age is mainlyresponsible for the relation between host mass and HR. If confirmed, this wouldimply that the luminosity evolution plays a major role in the systematicuncertainties of SN Ia cosmology.

Early-type Host Galaxies of Type Ia Supernovae. I. Evidence for Downsizing