Visibility of old supernova remnants in HI 21-cm emission line

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 (similar to0.1 cm(-3)) density representing the warm neutral interstellar medium. We assume that only Type la 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 similar or equal to270, or similar or equal to9 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 (less than or equal to25) 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.