Supernovae in Early‐Type Galaxies: Directly Connecting Age and Metallicity with Type Ia Luminosity

Joseph S Gallagher,Saurabh W Jha,Robert P Kirshner,Peter M Garnavich,Alexei V Filippenko,Weidong Li,Mohan Ganeshalingam,Nelson Caldwell

doi:10.1086/590659

Abstract

We have obtained optical spectra of 29 early-type (E/S0) galaxies that hosted type Ia supernovae (SNe Ia). We have measured absorption-line strengths and compared them to a grid of models to extract the relations between the supernova properties and the luminosity-weighted age/composition of the host galaxies. The same analysis was applied to a large number of early-type field galaxies selected from the SDSS spectroscopic survey. We find no difference in the age and abundance distributions between the field galaxies and the SN Ia host galaxies. We do find a strong correlation suggesting that SNe Ia in galaxies whose populations have a characteristic age greater than 5 Gyr are ~ 1 mag fainter at V(max) than those found in galaxies with younger populations. However, the data cannot discriminate between a smooth relation connecting age and supernova luminosity or two populations of SN Ia progenitors. We find that SN Ia distance residuals in the Hubble diagram are correlated with host-galaxy metal abundance, consistent with the predictions of Timmes, Brown & Truran (2003). The data show that high iron abundance galaxies host less-luminous supernovae. We thus conclude that the time since progenitor formation primarily determines the radioactive Ni production while progenitor metal abundance has a weaker influence on peak luminosity, but one not fully corrected by light-curve shape and color fitters. Assuming no selection effects in discovering SNe Ia in local early-type galaxies, we find a higher specific SN Ia rate in E/S0 galaxies with ages below 3 Gyr than in older hosts. The higher rate and brighter luminosities seen in the youngest E/S0 hosts may be a result of recent star formation and represents a tail of the "prompt" SN Ia progenitors.

Highlights

Type Ia supernovae (SNe Ia) have proven to be effective standardizable candles, and their high peak luminosity has made them excellent cosmological probes
The code measures the indices of a onedimensional input spectrum and calculates an error estimate based on photon statistics (Cardiel et al 1998; Vazdekis & Arimoto 1999; Cenarro et al 2001)
In each case the nonYemission-corrected host-galaxy indices are plotted in the upper frames, and the emission-corrected indices are plotted in the lower frames

Summary

Introduction

Type Ia supernovae (SNe Ia) have proven to be effective standardizable candles, and their high peak luminosity has made them excellent cosmological probes. Subsequent publications by Riess et al (1998) and Perlmutter et al (1999) came to the surprising conclusion that the universe is expanding at an accelerating rate, driven by a mysterious dark energy; see Kirshner (2002), Filippenko (2005b), or Frieman et al (2008) for reviews This surprising result has been confirmed by more recent supernova observations ( Knop et al 2003; Tonry et al 2003; Barris et al 2004; Riess et al 2004) and by complementary measurements of the cosmic microwave background (CMB) anisotropies (e.g., Bennett et al 2003) and large-scale structure (e.g., Peacock et al 2001). The simplified goal of stellar population synthesis modeling is to find a combination of stars for which the integrated spectrum of the stars matches the observed spectrum of the population under study. The final model is obtained by converting isochrone parameters to observed spectra and integrating along the isochrone

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