Dependence of Type Ia supernova luminosities on their local environment^★

¹ Sorbonne Université, Université Paris Diderot, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 Place Jussieu, 75252 Paris, France
e-mail: mroman@lpnhe.in2p3.fr
² European Southern Observatory (ESO), Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ NRC Herzberg Institute for Astrophysics, 5071 West Saanich Road, Victoria V9E 2E7, British Columbia, Canada
⁵ Department of Physics, Lancaster University, Lancaster, Lancs, LA1 4YB, UK
⁶ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr., Suite 102, Goleta, CA 93117, USA
⁷ Physics Department, University of California, Santa Barbara, CA 93106, USA
⁸ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
⁹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australia
¹⁰ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Canberra, Australia
¹¹ CENTRA e Departamento de Física, Instituto Superior Técnico, ULisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal
¹² LAL, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay, France
¹³ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁴ Department of Physics and Astronomy, University of Victoria, Victoria BC V8T 1M8, Canada
¹⁵ Sorbonne Université, Institut Lagrange de Paris (ILP), 98 bis Boulevard Arago, 75014 Paris, France
¹⁶ Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Received: 23 June 2017
Accepted: 30 January 2018

Abstract

We present a fully consistent catalog of local and global properties of host galaxies of 882 Type Ia supernovæ (SNIa) that were selected based on their light-curve properties, spanning the redshift range 0.01 < z < 1. This catalog corresponds to a preliminary version of the compilation sample and includes Supernova Legacy Survey (SNLS) 5-year data, Sloan Digital Sky Survey (SDSS), and low-redshift surveys. We measured low- and moderate-redshift host galaxy photometry in SDSS stacked and single-epoch images and used spectral energy distribution fitting techniques to derive host properties such as stellar mass and U − V rest-frame colors; the latter are an indicator of the luminosity-weighted age of the stellar population in a galaxy. We combined these results with high-redshift host photometry from the SNLS survey and thus obtained a consistent catalog of host stellar masses and colors across a wide redshift range. We also estimated the local observed fluxes at the supernova location within a proper distance radius of 3 kpc, corresponding to the SNLS imaging resolution, and transposed them into local U − V rest-frame colors. This is the first time that local environments surrounding SNIa have been measured at redshifts spanning the entire Hubble diagram. Selecting SNIa based on host photometry quality, we then performed cosmological fits using local color as a third standardization variable, for which we split the sample at the median value. We find a local color step significance of − 0.091 ± 0.013 mag (7σ), which effect is as significant as the maximum mass step effect. This indicates that the remaining luminosity variations in SNIa samples can be reduced with a third standardization variable that takes the environment into account. Correcting for the maximum mass step correction of − 0.094 ± 0.013 mag, we find a local color effect of − 0.057 ± 0.012 mag (5σ), which shows that additional information is provided by the close environment of SNIa. Departures from the initial choices were investigated and showed that the local color effect is still present, although less pronounced. We discuss the possible implications for cosmology and find that using the local color in place of the stellar mass results in a change in the measured value of the dark energy equation-of-state parameter of 0.6%. Standardization using local U − V color in addition to stretch and color reduces the total dispersion in the Hubble diagram from 0.15 to 0.14 mag. This will be of tremendous importance for the forthcoming SNIa surveys, and in particular for the Large Synoptic Survey Telescope (LSST), for which uncertainties on the dark energy equation of state will be comparable to the effects reported here.