HI, HII, and R-Band Observations of a Galactic Merger Sequence

Abstract

bWe present high-quality aperture synthesis observations of the neutral hydrogen distribution in a sample of five galactic systems believed to be involved in progressive stages of merging: Arp 295, NGC 4676, NGC 520, NGC 3921, and NGC 7252. This is the first time that the atomic hydrogen in such a broad range of disk mergers has been imaged. These data are supplemented by wide-field images taken through a narrowband Hα filter, and by deep (μ_R_ > 26.5 mag arcsec^-2^) R-band surface photometry. We identify several trends along the merging sequence. In the early stages, large amounts of HI still exist within the galactic disks and star formation is widespread. The ionized gas emission often takes on the appearance of plumes and arcs emanating from the nuclear regions, which are presumably the sites of interaction-induced starbursts. In the final stages there is little if any H I within the remnant bodies, and tidal material is seen moving inward. We conclude that as the merger rearranges the light profiles of the progenitor disk galaxies into r^1/4^ profiles, it leads to an efficient conversion of the atomic gas into other forms within the main bodies of the merger remnants. This suggests that these remnants will evolve into elliptical galaxies in their atomic gas contents as well as their photometric properties. However, the observations of NGC 520 reveal an extensive rotating gaseous disk, suggesting that perhaps some mergers will not destroy the atomic gas disks of the progenitors. The morphological similarity between the gaseous and stellar tails and the smooth gas kinematics confirm that gravity plays the dominant role in producing them. There are, however, some striking differences between the faint gas and light distributions. H I mapping often reveals gaseous extension not at all apparent optically, and tidal features of different optical morphologies have different gas characteristics: The edge-brightened tails are gas rich, while the featureless plumes and halos are gas poor. Some of these features may be explained by the different velocity dispersions of the gas and stars, some by different gas contents in the progenitors, and some remain unexplained. Overall, large quantities of both gas and starlight (M_H I_ > 10^9^h^-2^ M_sun_, L_R_ > 10^9^h^-2^ L_sun_) are seen at large radii (r > 50 h^-1^ kpc). Since this material evolves on very long time scales, it may leave observable signatures for many Gyr.