Visibility of old supernova remnants in H i 21-cm emission line

Abstract

We estimate the number of old, radiative supernova remnants (SNRs) detectable in H i 21-cm emission line in the Galaxy. We assume that old SNRs consist of expanding H i shells and that they are visible if the line-of-sight velocities are sufficiently outside the velocity range of the Galactic background H i emission. This criterion of visibility makes it possible to calculate the background contamination and to make a comparison with observation. The Galactic disc in our model is filled with atomic gas of moderate (∼0.1 cm−3) density representing the warm neutral interstellar medium. We assume that only Type Ia supernovae produce isolated SNRs with expanding H i shells, or ‘H i SNRs’. According to our result, the contamination due to the Galactic background H i emission limits the number of visible SNRs to ≃270, or ≃9 per cent of the total H i 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 (≤25) of H i SNRs is much less than expected. A plausible explanation is that previous observational studies, which were made towards the SNRs identified mostly in radio continuum, missed most of the H i SNRs because they are too faint to be visible in radio continuum. We propose that the faint, extended H i 21-cm emission line wings protruding from the Galactic background H i emission in large-scale (l, v) diagrams could be possible candidates for H i SNRs, although our preliminary result shows that their number is considerably less than expected in the inner Galaxy. We conclude that a possible explanation for the small number of H i SNRs in the inner Galaxy is that the interstellar space there is largely filled with a very tenuous gas as in the three-phase interstellar medium (ISM) model, not with the warm neutral medium of moderate density.