Beyond the Mass Step: Improving the Hubble Diagram with an [O II] Correction to SN Ia Light Curves

For the last decade, SN Ia light-curves have been corrected for the mass step, a phenomenon by which more massive galaxies host SNe Ia that are systematically brighter after standard light-curve corrections. We have obtained WiFeS IFU spectra of over 100 SN Ia host galaxies, and found that there is an interconnected system of correlations between SN Ia luminosities and host properties that is not accounted for by a mass-step correction. Instead, we find that correcting for the [O II] equivalent width - a tracer of sSFR - accounts for the system of correlations we see, and reduces the scatter in the Hubble diagram to a greater extent than a mass step correction. The availability of the [O II] feature in optical spectra out to z~1.5 makes it an excellent option for the correction of SN Ia light-curves.