G51.04+0.07 and its environment: Identification of a new Galactic
  supernova remnant at low radio frequencies
  We have identified a new supernova remnant (SNR), G51.04+0.07 (G51.04), using
observations at 74 MHz from the VLA Low-Frequency Sky Survey Redux. Earlier,
higher frequency radio continuum, recombination line, and infrared data had
correctly inferred the presence of nonthermal radio emission within a large
complex including ionised nebulae and active star formation. However, our
observations have allowed us to redefine at least one SNR as a relatively small
source (7'.5 x 3' in size) located at the southern periphery of the originally
defined SNR candidate. The integrated flux density of G51.04 at 74 MHz is 6.1
+/- 0.8 Jy, while its radio continuum spectrum has a slope alpha=-0.52 +/-
0.05, typical of a shell-type remnant. We also measured spatial variations in
the spectral index between 74 and 1400~MHz across the source, ranging from a
steeper spectrum (alpha~-0.50) coincident with the brightest emission to a
flatter component (alpha~-0.30) in the surrounding fainter region. To probe the
interstellar medium into which the redefined SNR is likely evolving, we have
analysed the surrounding atomic and molecular gas using the 21~cm neutral
hydrogen (HI) and 13CO (J=1-0) emissions. We found that G51.04 is confined
within an HI cavity and that its radio emission is consistent with the remains
of a stellar explosion that occurred ~6300 yr ago at a distance of ~7.7 kpc.
Kinematic data suggest that the newly discovered SNR lies in front of HII
regions in the complex, consistent with the lack of a low frequency turnover in
the continuum spectrum. The CO observations revealed molecular material that
traces the central and northern parts of G51.04. The interaction between the
cloud and the radio source is not conclusive. The relatively low flux density
(~1.5 Jy at 1400 MHz) of G51.04 is consistent with many similar SNRs hidden
along complex lines of sight towards inner Galactic emission complexes.
