Galactic Supernova Remnant Candidates Discovered by THOR
  There is a considerable deficiency in the number of known supernova remnants
(SNRs) in the Galaxy compared to that expected. Searches for extended
low-surface brightness radio sources may find new Galactic SNRs, but confusion
with the much larger population of HII regions makes identifying such features
challenging. SNRs can, however, be separated from HII regions using their
significantly lower mid-infrared (MIR) to radio continuum intensity ratios. We
use the combination of high-resolution 1-2 GHz continuum data from The HI, OH,
Recombination line survey of the Milky Way (THOR) and lower-resolution VLA 1.4
GHz Galactic Plane Survey (VGPS) continuum data, together with MIR data from
the Spitzer GLIMPSE, Spitzer MIPSGAL, and WISE surveys to identify SNR
candidates. To ensure that the candidates are not being confused with HII
regions, we exclude radio continuum sources from the WISE Catalog of Galactic
HII Regions, which contains all known and candidate H II regions in the Galaxy.
We locate 76 new Galactic SNR candidates in the THOR and VGPS combined survey
area of 67.4deg>l>17.5deg, |b|<1.25deg and measure the radio flux density for
52 previously-known SNRs. The candidate SNRs have a similar spatial
distribution to the known SNRs, although we note a large number of new
candidates near l=30deg, the tangent point of the Scutum spiral arm. The
candidates are on average smaller in angle compared to the known regions,
6.4'+/-4.7' versus 11.0'+/-7.8', and have lower integrated flux densities. If
the 76 candidates are confirmed as true SNRs, for example using radio
polarization measurements or by deriving radio spectral indices, this would
more than double the number of known Galactic SNRs in the survey area. This
large increase would still, however, leave a discrepancy between the known and
expected SNR populations of about a factor of two.
