The dependence of Type Ia Supernovae luminosities on their host galaxies

Abstract

(Abridged) Precision cosmology with Type Ia supernovae (SNe Ia) makes use of\nthe fact that SN Ia luminosities depend on their light-curve shapes and\ncolours. Using Supernova Legacy Survey (SNLS) and other data, we show that\nthere is an additional dependence on the global characteristics of their host\ngalaxies: events of the same light-curve shape and colour are, on average,\n0.08mag (~4.0sigma) brighter in massive host galaxies (presumably metal-rich)\nand galaxies with low specific star-formation rates (sSFR). SNe Ia in galaxies\nwith a low sSFR also have a smaller slope ("beta") between their luminosities\nand colours with ~2.7sigma significance, and a smaller scatter on SN Ia Hubble\ndiagrams (at 95% confidence), though the significance of these effects is\ndependent on the reddest SNe. SN Ia colours are similar between low-mass and\nhigh-mass hosts, leading us to interpret their luminosity differences as an\nintrinsic property of the SNe and not of some external factor such as dust. If\nthe host stellar mass is interpreted as a metallicity indicator, the luminosity\ntrends are in qualitative agreement with theoretical predictions. We show that\nthe average stellar mass, and therefore the average metallicity, of our SN Ia\nhost galaxies decreases with redshift. The SN Ia luminosity differences\nconsequently introduce a systematic error in cosmological analyses, comparable\nto the current statistical uncertainties on parameters such as w. We show that\nthe use of two SN Ia absolute magnitudes, one for events in high-mass\n(metal-rich) galaxies, and one for events in low-mass (metal-poor) galaxies,\nadequately corrects for the differences. Cosmological fits incorporating these\nterms give a significant reduction in chi^2 (3.8-4.5sigma). We conclude that\nfuture SN Ia cosmological analyses should use a correction of this (or similar)\nform to control demographic shifts in the galaxy population.\n