The Star Formation Rates of Elliptical Galaxies from Core-Collapse
  Supernovae
  The level of star formation in elliptical galaxies is poorly constrained, due
to difficulties in quantifying the contamination of flux-based estimates of
star formation from unrelated phenomena, such as AGN and old stellar
populations. We here utilise core-collapse supernovae (CCSNe) as unambiguous
tracers of recent star formation in ellipticals within a cosmic volume. We
firstly isolate a sample of 421 z < 0.2, r < 21.8 mag CCSNe from the SDSS-II
Supernova Survey. We then introduce a Bayesian method of identifying
ellipticals via their colours and morphologies in a manner unbiased by redshift
and yet consistent with manual classification from Galaxy Zoo 1. We find ~ 25 %
of z < 0.2 r < 20 mag galaxies in the Stripe 82 region are ellipticals (~ 28000
galaxies). In total, 36 CCSNe are found to reside in ellipticals. We
demonstrate that such early-types contribute a non-negligible fraction of star
formation to the present-day cosmic budget, at 11.2 $\pm$ 3.1 (stat)
$^{+3.0}_{-4.2}$ (sys) %. Coupling this result with the galaxy stellar mass
function of ellipticals, the mean specific star formation rate (SSFR;
$\overline{S}$) of these systems is derived. The best-fit slope is given by log
($\overline{S}(M)$/yr) = - (0.80 $\pm$ 0.59) log ($M/10^{10.5}\rm{M}_{\odot}$)
- 10.83 $\pm$ 0.18. The mean SSFR for all log ($M/\rm{M}_{\odot}$) > 10.0
ellipticals is found to be $\overline{S} = 9.2 \pm 2.4$ (stat) $^{+2.7}_{-2.3}$
(sys) $\times 10^{-12}$ yr$^{-1}$, which is consistent with recent estimates
via SED-fitting, and is 11.8 $\pm$ 3.7 (stat) $^{+3.5}_{-2.9}$ (sys) % of the
mean SSFR level on the main sequence as also derived from CCSNe. We find the
median optical spectrum of elliptical CCSN hosts is statistically consistent
with that of a control sample of ellipticals that do not host CCSNe, implying
that these SN-derived results are well-representative of the total low-z
elliptical population.
