Type Ia Supernova Rate Measurements to Redshift 2.5 from CANDELS :
  Searching for Prompt Explosions in the Early Universe
  The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)
was a multi-cycle treasury program on the Hubble Space Telescope (HST) that
surveyed a total area of ~0.25 deg^2 with ~900 HST orbits spread across 5
fields over 3 years. Within these survey images we discovered 65 supernovae
(SN) of all types, out to z~2.5. We classify ~24 of these as Type Ia SN (SN Ia)
based on host-galaxy redshifts and SN photometry (supplemented by grism
spectroscopy of 6 SN). Here we present a measurement of the volumetric SN Ia
rate as a function of redshift, reaching for the first time beyond z=2 and
putting new constraints on SN Ia progenitor models. Our highest redshift bin
includes detections of SN that exploded when the universe was only ~3 Gyr old
and near the peak of the cosmic star-formation history. This gives the CANDELS
high-redshift sample unique leverage for evaluating the fraction of SN Ia that
explode promptly after formation (<500 Myr). Combining the CANDELS rates with
all available SN Ia rate measurements in the literature we find that this
prompt SN Ia fraction is fP=0.53 +0.09 -0.10 (stat) +0.10 -0.26 (sys),
consistent with a delay time distribution that follows a simple t^{-1} power
law for all times t>40 Myr. However, a mild tension is apparent between
ground-based low-z surveys and space-based high-z surveys. In both CANDELS and
the sister HST program CLASH, we find a low rate of SN Ia at z>1. This could be
a hint that prompt progenitors are in fact relatively rare, accounting for only
~20% of all SN Ia explosions -- though further analysis and larger samples will
be needed to examine that suggestion.
