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We have identified a new supernova remnant (SNR), G51.04+0.07 (G51.04), usingobservations at 74 MHz from the VLA Low-Frequency Sky Survey Redux. Earlier,higher frequency radio continuum, recombination line, and infrared data hadcorrectly inferred the presence of nonthermal radio emission within a largecomplex including ionised nebulae and active star formation. However, ourobservations have allowed us to redefine at least one SNR as a relatively smallsource (7'.5 x 3' in size) located at the southern periphery of the originallydefined 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 inthe spectral index between 74 and 1400~MHz across the source, ranging from asteeper spectrum (alpha~-0.50) coincident with the brightest emission to aflatter component (alpha~-0.30) in the surrounding fainter region. To probe theinterstellar medium into which the redefined SNR is likely evolving, we haveanalysed the surrounding atomic and molecular gas using the 21~cm neutralhydrogen (HI) and 13CO (J=1-0) emissions. We found that G51.04 is confinedwithin an HI cavity and that its radio emission is consistent with the remainsof 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 HIIregions in the complex, consistent with the lack of a low frequency turnover inthe continuum spectrum. The CO observations revealed molecular material thattraces the central and northern parts of G51.04. The interaction between thecloud 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 hiddenalong complex lines of sight towards inner Galactic emission complexes.

G51.04+0.07 and its environment: Identification of a new Galactic supernova remnant at low radio frequencies