The supernova rate and delay time distribution in the Magellanic Clouds

Abstract

We use the supernova remnants (SNRs) in the Magellanic Clouds (MCs) as a supernova (SN) survey, "conducted" over tens of kyr, from which we derive the current SN rate, and the SN delay time distribution (DTD), i.e., the SN rate vs. time that would follow a hypothetical brief burst of a star formation. In Badenes, Maoz, & Draine (2010) we have compiled a list of 77 SNRs in the MCS, and argued it is a fairly complete record of the SNRs in the Sedov phase of their expansions. We recover the DTD by comparing the numbers of SNRs observed in small individual "cells" in these galaxies to the star-formation histories of each cell, as calculated from resolved stellar populations by Harris & Zaritsky. The visibility times of SNRs are the Sedov-phase lifetimes, which depend on the local ambient densities. The local densities are estimated from HI emission, from an inverse Schmidt law based on either Halpha flux or on the resolved star-formation rate, and from combinations of these tracers. In the DTD, we detect "prompt" type-Ia SNe (that explode within 330 Myr of star formation) at >99% confidence level (c.l.). The best fit for the number of prompt SNe-Ia per stellar mass formed is (2.7-11.0) x 10^{-3} /Msun, depending on the density tracer used. The 95% c.l. range for a "delayed" SN Ia component (from 330 Myr to a Hubble time) is < 1.6 x 10^{-13} SN/yr/Msun, consistent with rate measurements in old populations. The current total (core-collapse+Ia) SN rate in the MCs is 2.5-4.6 SNe per millenium (68% c.l.+systematics), or 1.7-3.1 SNuM [SNe/100 yr/10^{10}Msun], in agreement with the historical record and with rates measured in other dwarf irregulars. Conversely, assuming the SNRs are in free expansion, rather than in their Sedov phase, would impose on the SNRs a maximum age of 6 kyr, and would imply a MC SN rate per unit mass that is 5 times higher than in any type of galaxy.