UC Berkeley

Context. Use of Type Ia supernovae (SNe Ia) as distance indicators has proven to be a powerful technique for measuring the dark-energy equation of state. However, recent studies have highlighted potential biases correlated with the global properties of their host galaxies, large enough to induce systematic errors into such cosmological measurements if not properly treated. Aims. We study the host galaxy regions in close proximity to SNe Ia in order to analyze relations between the properties of SN Ia events and environments where their progenitors most likely formed. In this paper we focus on local Hα emission as an indicator of young progenitor environments. Methods. The Nearby Supernova Factory has obtained flux-calibrated spectral timeseries for SNe Ia using integral field spectroscopy. These observations enabled the simultaneous measurement of the SN and its immediate vicinity. For 89 SNe Ia we measured or set limits on Hα emission, used as a tracer of ongoing star formation, within a 1 kpc radius around each SN. This constitutes the first direct study of the local environment for a large sample of SNe Ia with accurate luminosity, color, and stretch measurements. Results. Our local star formation measurements provide several critical new insights. We find that SNe Ia with local Hα emission are redder by 0.036 ± 0.017 mag, and that the previously noted correlation between stretch and host mass is driven entirely by the SNe Ia coming from locally passive environments, in particular at the low-stretch end. There is no such trend for SNe Ia in locally star-forming environments. Our most important finding is that the mean standardized brightness for SNe Ia with local Hα emission is 0.094 ± 0.031 mag fainter on average than for those without. This offset arises from a bimodal structure in the Hubble residuals, with one mode being shared by SNe Ia in all environments and the other one exclusive to SNe Ia in locally passive environments. This structure also explains the previously known host-mass bias. We combine the star formation dependence of this bimodality with the cosmic star formation rate to predict changes with redshift in the mean SN Ia brightness and the host-mass bias. The strong change predicted is confirmed using high-redshift SNe Ia from the literature. Conclusions. The environmental dependences in SN Ia Hubble residuals and color found here point to remaining systematic errors in the standardization of SNe Ia. In particular, the observed brightness offset associated with local Hα emission is predicted to cause a significant bias in current measurements of the dark energy equation of state. Recognition of these effects offers new opportunities to improve SNe Ia as cosmological probes. For instance, we note that the SNe Ia associated with local Hα emission are more homogeneous, resulting in a brightness dispersion of only 0.105 ± 0.012 mag. © ESO, 2013.