The Astrophysical Journal, 648:868–883, 2006 September 10 A # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A. RATES AND PROPERTIES OF TYPE Ia SUPERNOVAE AS A FUNCTION OF MASS AND STAR FORMATION IN THEIR HOST GALAXIES M. Sullivan,1 D. Le Borgne,2 C. J. Pritchet,3 A. Hodsman,1 J. D. Neill,3 D. A. Howell,1 R. G. Carlberg,1 P. Astier,4 E. Aubourg,2,5 D. Balam,3 S. Basa,6 A. Conley,1 S. Fabbro,7 D. Fouchez,8 J. Guy,4 I. Hook,9 R. Pain,4 N. Palanque-Delabrouille,2 K. Perrett,1 N. Regnault,4 J. Rich,2 R. Taillet,4,10 S. Baumont,4 J. Bronder,9 R. S. Ellis,11 M. Filiol,6 V. Lusset,2 S. Perlmutter,12 P. Ripoche,8 and C. Tao8 Received 2006 February 6; accepted 2006 May 17 ABSTRACT We show that Type Ia supernovae (SNe Ia) are formed within both very young and old stellar populations, with observed rates that depend on the stellar mass and mean star formation rates (SFRs) of their host galaxies. Models in which the SN Ia rate depends solely on host galaxy stellar mass are ruled out with >99% confidence. Our analysis is based on 100 spectroscopically confirmed SNe Ia, plus 24 photometrically classified events, all from the Supernova Legacy Survey (SNLS) and distributed over 0:2 < z < 0:75. We estimate stellar masses and SFRs for the SN Ia host galaxies by fitting their broadband spectral energy distributions with the galaxy spectral synthesis code PE´ GASE.2. We show that the SN Ia rate per unit mass is proportional to the specific SFR of the parent galaxies—more vigorously star-forming galaxies host more SNe Ia per unit stellar mass, broadly equivalent to the trend of increasing SN Ia rate in later type galaxies seen in the local universe. Following earlier suggestions for a simple ‘‘two-component’’ model approximating the SN Ia rate, we find bivariate linear dependencies of the SN Ia rate on both the stellar masses and the mean SFRs of the host systems. We find that the SN Ia rate can be well represented as the sum of 5:3 Æ 1:1 ; 10À14 SNe À1 À1 À4 À1 À1 À1 yr M and 3:9 Æ 0:7 ; 10 SNe yr (M yr ) of star formation. We also demonstrate a dependence of distant SN Ia light-curve shapes on star formation in the host galaxy, similar to trends observed locally. Passive galaxies, with no star formation, preferentially host faster declining/dimmer SNe Ia, while brighter events are found in systems with ongoing star formation. Subject headinggs: distance scale — galaxies: evolution — supernovae: general — surveys Online material: color figures 1. INTRODUCTION SN explosion following accretion of material from a secondary companion (e.g., Madau et al. 1998a; see Greggio [2005] for a Type Ia supernovae (SNe Ia) represent cosmologists’ most di- comprehensive analysis of rate model formalizations). rect probe of the cosmic expansion history, yet an understanding Various pieces of observational evidence have been used to of the composition of their progenitor systems has not yet been place different constraints on the value of this delay time. The prin- achieved (e.g., Hillebrandt & Niemeyer 2000). In principle, this cipal approach compares observed SN Ia rates with a predicted uncertainty can be reduced and constraints placed on the nature rate generated by convolving a delay function with an assumed of the progenitor if the typical explosion timescale of SNe Ia can cosmic star formation history (e.g., Madau et al. 1998a; Gal-Yam be determined. This ‘‘delay time’’ parameterizes the distribution of times between the binary system formation and subsequent & Maoz 2004). Studies using this approach have determined a wide range of delay times: ’2–4 Gyr (Strolger et al. 2004, 2005), 2 Gyr (Gal-Yam & Maoz 2004), and 1 Gyr (Barris & Tonry 1 Department of Astronomy and Astrophysics, University of Toronto, 60 St. 2006). George Street, Toronto, ON M5S 3H8, Canada. A second technique uses comparisons of the SN Ia rate in z < 1 2 DAPNIA/Service d’Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France. galaxy clusters with the observed cluster iron content. If SNe Ia 3 Department of Physics and Astronomy, University of Victoria, P.O. Box are assumed to represent the dominant source of iron in clusters, 3055, Victoria, BC V8W 3P6, Canada. the low cluster SN Ia rate at low redshift implies much of the iron 4 Laboratoire de Physique Nucleaire et de Haute Energies de Paris (LPNHE), must have been produced from events at higher redshift—and CNRS-IN2P3; and University of Paris VI and VII, 75005 Paris, France. 5 APC, 11 Place Marcelin Berthelot, 75231 Paris Cedex 5, France. hence suggests delay times of <2Gyr(Maoz&Gal-Yam2004). 6 Laboratoire Astrophysique de Marseille (LAM), CNRS, BP8, Traverse du The third piece of evidence follows from the observation that Siphon, 13376 Marseille Cedex 12, France. SNe Ia are substantially more common in star-forming later type 7 Centro Multidisciplinar de Astrofı´sica (CENTRA), Instituto Superior galaxies than in early-type systems (e.g., Oemler & Tinsley 1979). Te´cnico, Avenida Rovisco Pais, 1049 Lisbon, Portugal. The rate per unit mass is significantly higher both in later type 8 Centre de Physique des Particules de Marseille (CPPM), CNRS-IN2P3; and University Aix Marseille II, Case 907, 13288 Marseille Cedex 9, France. galaxies than in E/S0 systems (van den Bergh 1990; Della Valle & 9 Department of Astrophysics, University of Oxford, Denys Wilkinson Build- Livio 1994; Mannucci et al. 2005) and in bluer galaxies than in red ing, Keble Road, Oxford OX1 3RH, UK. galaxies (Mannucci et al. 2005), with an enhancement of SNe Ia 10 Universite´ de Savoie, 73000 Chambe´ry, France. in early-type galaxies that are radio loud (Della Valle et al. 2005). 11 California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125. Furthermore, within star-forming galaxies, SNe Ia are rarer in gal- 12 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA axy bulges than in their disks (Wang et al. 1997). This suggests 94720. some dependence of the SN Ia rate on recent star formation (and 868 RATES AND PROPERTIES OF TYPE Ia SUPERNOVAE 869 hence very short delay times). Finally, there is the observation that axy identification and flux measurement. Section 3 describes the SNe Ia are common in old evolved systems with little recent star galaxy SED fitting, and x 4 details the various incompleteness formation activity (Cappellaro et al. 1999; Mannucci et al. 2005), corrections we apply to our galaxy and SN Ia samples. Our anal- suggesting some progenitors have delay times of at least several ysis is contained in xx 5 and 6. In x 5 we examine the rate of gigayears; other studies of the star formation histories of local SNe Ia as a function of various host galaxy parameters, and in x 6 SN Ia host galaxies claim a delay time lower limit of 2Gyr we examine the impact of environment on SN Ia light-curve (Gallagher et al. 2005). shape parameters. We summarize in x 7. These contradictions can be resolved by removing the con- Throughout the paper, we assume a cosmology of M ¼ 0:3, À1 À1 straint of a single delay time parameterizing all SN Ia explosions à ¼ 0:7, H0 ¼ 70 km s Mpc , and use the AB photomet- and instead using a ‘‘two-component’’ distribution (or even a more ric system (Oke & Gunn 1983). general function), similar to the models proposed by Mannucci et al. (2005, 2006) and Scannapieco & Bildsten (2005, hereafter 2. THE SNLS DATA SET SB05). Such models comprise a ‘‘prompt’’ (or small delay time) SN Ia component, essentially dependent on recent star formation, This section describes the observational data set that we use and an ‘‘old’’ (larger delay time) component, dependent on the for our analysis in this paper. We first describe the data set of number of lower mass stars. The total rate of SNe Ia is then a SNe Ia, and then the data sets of both the SN Ia host galaxies and combination of these different functions. These scenarios are able the general galaxy population. to resolve many of the observational contradictions described 2.1. Supernova Data above (see discussion in SB05). These two-component models have implicit but important im- Our SN Ia data come from the SNLS. The SNLS is a rolling plications for the progenitors of SNe Ia that might impact their search for distant SNe with a primary science goal of using 700 use as calibrated standard candles to derive cosmological param- high-redshift SNe Ia to determine the average equation-of-state eters (Riess et al. 1998; Perlmutter et al. 1999; Astier et al. 2006). parameter of dark energy, hiw (see Astier et al. 2006). SNLS ex- The possibility of subtle differences between SNe Ia from the ploits the square degree MegaCam camera (Boulade et al. 2003) two components, or a change in the relative fraction of the two on the Canada-France-Hawaii Telescope (CFHT) to conduct re- types with redshift, are of potential concern. As such, it is vital to peat g0r0i0z0 imaging offour low Galactic extinction fields (named test and parameterize the SN Ia models in as many ways as pos- D1–D4; see Sullivan et al. [2006] for the field coordinates), im- sible. One way to do this is to look at the rates and properties of aged as part of the ‘‘deep’’ component of the 5 year CFHT Legacy SNe Ia in relation to the environments or galaxies in which they Survey (CFHTLS).