Supernova Host Galaxies in the Dark Energy Survey: I. Deep Coadds,
  Photometry, and Stellar Masses
  The five-year Dark Energy Survey supernova programme (DES-SN) is one of the
largest and deepest transient surveys to date in terms of volume and number of
supernovae. Identifying and characterising the host galaxies of transients
plays a key role in their classification, the study of their formation
mechanisms, and the cosmological analyses. To derive accurate host galaxy
properties, we create depth-optimised coadds using single-epoch DES-SN images
that are selected based on sky and atmospheric conditions. For each of the five
DES-SN seasons, a separate coadd is made from the other 4 seasons such that
each SN has a corresponding deep coadd with no contaminating SN emission. The
coadds reach limiting magnitudes of order $\sim 27$ in $g$-band, and have a
much smaller magnitude uncertainty than the previous DES-SN host templates,
particularly for faint objects. We present the resulting multi-band photometry
of host galaxies for samples of spectroscopically confirmed type Ia (SNe Ia),
core-collapse (CCSNe), and superluminous (SLSNe) as well as rapidly evolving
transients (RETs) discovered by DES-SN. We derive host galaxy stellar masses
and probabilistically compare stellar-mass distributions to samples from other
surveys. We find that the DES spectroscopically confirmed sample of SNe Ia
selects preferentially fewer high mass hosts at high redshift compared to other
surveys, while at low redshift the distributions are consistent. DES CCSNe and
SLSNe hosts are similar to other samples, while RET hosts are unlike the hosts
of any other transients, although these differences have not been disentangled
from selection effects.
