How old are SN Ia Progenitor Systems? New Observational Constraints on
  the Distribution of Time Delays from GALEX
  The time delay between the formation of the progenitor systems of Type Ia
supernovae (SNe Ia) and their detonation is a vital discriminant between the
various progenitor scenarios that have been proposed for them. We use SDSS
optical and GALEX ultraviolet observations of the early-type host galaxies of
21 nearby SNe Ia and quantify the presence or absence of any young stellar
population to constrain the minimum time delay for each supernova. We find that
early-type host galaxies lack `prompt' SNe Ia with time delays of <100 Myr and
that ~70% SNe Ia have minimum time delays of 275 Myr -- 1.25 Gyr, with a median
of 650 Myr, while at least 20% SNe Ia have minimum time delays of at least 1
Gyr at 95% confidence and two of these four SNe Ia are are likely older than 2
Gyr. The distribution of minimum time delays observed matches most closely the
expectation for the single-degenerate channel with a main sequence donor.
Furthermore, we do not find any evidence that sub-luminous SNe Ia are
associated with long time delays.
