The neutral hydrogen distribution in merging galaxies: Differences between stellar and gaseous tidal morphologies

As part of several H I synthesis-mapping studies of merging galaxies, we have mapped the tidal gas in the three disk-disk merger systems Arp 157 (NGC 520), Arp 220, and Arp 299 (NGC 3690). These systems differ from the majority of the mergers mapped in H I in that their stellar and gaseous tidal features do not coincide. In particular, they exhibit large stellar tidal features with little if any accompanying neutral gas and large gas-rich tidal features with little if any accompanying starlight. On smaller scales, there are striking anticorrelations in which the gaseous and stellar tidal features appear to cross. We explore several possible causes for these differences, including dust obscuration, ram pressure stripping, and ionization effects. No single explanation can account for all of the observed differences. The fact that each of these systems shows evidence for a starburst-driven superwind expanding in the direction of the most striking anticorrelations leads us to suggest that the superwind is primarily responsible for the observed differences, either by sweeping the features clear of gas via ram pressure or by excavating a clear sightline toward the starburst and allowing UV photons to ionize regions of the tails. If this suggestion is correct, only systems hosting a galactic superwind and experiencing a high-inclination encounter geometry (such that tidal gas is lifted high above the starburst regions) should exhibit such extreme differences between their H I and optical tidal morphologies.