Visibility of old supernova remnants in H i 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 (∼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.