Supernovae and their host galaxies -- VI. Normal Type Ia and 91bg-like
  supernovae in ellipticals
  We present an analysis of the galactocentric distributions of the "normal"
and peculiar "91bg-like" subclasses of 109 supernovae (SNe) Ia, and study the
global parameters of their elliptical hosts. The galactocentric distributions
of the SN subclasses are consistent with each other, and with the radial light
distribution of host stellar populations, when excluding bias against central
SNe. Among the global parameters, only the distributions of u-r colours and
ages are inconsistent significantly between the ellipticals of different SN Ia
subclasses: the normal SN hosts are on average bluer/younger than those of
91bg-like SNe. In the colour-mass diagram, the tail of colour distribution of
normal SN hosts stretches into the Green Valley - transitional state of galaxy
evolution, while the same tail of 91bg-like SN hosts barely reaches that
region. Therefore, the bluer/younger ellipticals might have more residual star
formation that gives rise to younger "prompt" progenitors, resulting in normal
SNe Ia with shorter delay times. The redder and older ellipticals that already
exhausted their gas for star formation may produce significantly less normal
SNe with shorter delay times, outnumbered by "delayed" 91bg-like events. The
host ages (lower age limit of the delay times) of 91bg-like SNe does not extend
down to the stellar ages that produce significant u-band fluxes - the 91bg-like
events have no prompt progenitors. Our results favor SN Ia progenitor models
such as He-ignited violent mergers that have the potential to explain the
observed SN/host properties.
