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 \sim 0.25 deg ^ { 2 } with \sim 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 \approx 2.5 .
We classify \sim 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 \sim 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 f _ { P } =0.53 \substack { \pm \ \mbox { \scalebox { 0.45 } { stat } } } ^ { 0.09 } _ { 0.10 } \substack { \pm \ \mbox { \scalebox { 0.45 } { sys } } } ^ { 0.10 } _ { 0.26 } , 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 \sim 20 % of all SN Ia explosions – though further analysis and larger samples will be needed to examine that suggestion .