The Redshift Completeness of Local Galaxy Catalogs
  There is considerable interest in understanding the demographics of galaxies
within the local universe (defined, for our purposes, as the volume within a
radius of 200 Mpc or z<0.05). In this pilot paper, using supernovae (SNe) as
signposts to galaxies, we investigate the redshift completeness of catalogs of
nearby galaxies. In particular, type Ia SNe are bright and are good tracers of
the bulk of the galaxy population, since they arise in both old and young
stellar populations. Our input sample consists of SNe with redshift < 0.05,
discovered by the flux-limited ASAS-SN survey. We define the redshift
completeness fraction (RCF) as the number of SN host galaxies with known
redshift prior to SN discovery, determined, in this case, via the NASA
Extragalactic Database (NED), divided by the total number of newly discovered
SNe. Using SNe Ia, we find RCF = 78 (+6, -7, 90% confidence interval) for z <
0.03. We examine the distribution of host galaxies with and without cataloged
redshifts as a function of absolute magnitude and redshift, and,
unsurprisingly, find that higher-z and fainter hosts are less likely to have a
known redshift prior to the detection of the SN. However, surprisingly, some L*
galaxies are also missing. We conclude with thoughts on the future improvement
of RCF measurements that will be made possible from large SN samples resulting
from ongoing and especially upcoming time-domain surveys.
