Visibility of Old Supernova Remnants in Hi 21 - cm

We estimate the number of old, radiative supernova remnants (SNRs) detectable in Hi 21-cm emission line in the Galaxy. We assume that old SNRs consist of expanding Hi shells and that they are visible if the line-of-sight velocities are sufficiently outside the velocity range of the Galactic background Hi emission. This criterion of visibility makes it possible to calculate the background contamination and to make a comparison with observation. The Galactic disk in our model is filled with atomic gas of moderate (∼ 0.1 cm) density representing the warm neutral interstellar medium. We assume that only Type Ia supernovae produce isolated SNRs with expanding Hi shells, or “Hi SNRs”. According to our result, the contamination due to the Galactic background Hi emission limits the number of visible SNRs to ≃ 270, or ≃ 9% of the total Hi SNRs. They are concentrated along the loci of tangential points. The telescope sensitivity further limits the number. We compare the result with observations to find that the observed number (6 25) of Hi SNRs is much less than the expected. A plausible explanation is that previous observational studies, which were made toward the SNRs identified mostly in radio continuum, missed most of the Hi SNRs because they are too faint to be visible in radio continuum. We propose that the faint, extended Hi 21-cm emission line wings protruding from the Galactic background Hi emission in large-scale (l, v) diagrams could be possible candidates for Hi SNRs, although our preliminary result shows that their number is considerably less than the expected in the inner Galaxy. We conclude that a possible explanation for the small number of Hi SNRs in the inner Galaxy is that the interstellar space there is largely filled with a very tenuous gas as in the three-phase ISM model, not with the warm neutral medium of moderate density.