Extended Ionized Gas Emission and Kinematics of the Compact Group Galaxies in Hickson Compact Group 16: Signatures of Mergers

Abstract

We report on kinematic observations of Hα emission from four late-type galaxies of Hickson Compact Group 16 (H16a, b, c, d) obtained with a scanning Fabry-Perot interferometer and samplings of 16 km s-1 and 1''. The velocity fields show kinematic peculiarities for three of the four galaxies: H16b, H16c, and H16d. Misalignments between the kinematic and photometric axes of gas and stellar components (H16b-H16d), double gas systems (H16c) and severe warping of the kinematic major axis (H16b and H16c) were some of the peculiarities detected. We conclude that major merger events have taken place in at least two of the galaxies of the group, H16c and H16d, based on their significant kinematic peculiarities, their double nuclei, and their high infrared luminosities. Their Hα gas content is strongly spatially concentrated; H16d contains a peculiar barlike structure confined to the inner ~1 h-1 kpc region. These observations are in agreement with predictions of simulations, namely, that the gas flows toward the galaxy nucleus during mergers, forms bars, and fuels the central activity. Galaxy H16b, an Sb galaxy, also presents some of the kinematic evidence for past accretion events. Its gas content, however, is very sparse, limiting our ability to find other kinematic merging indicators, if they are present. We find that the merger remnants in the compact group HCG 16 have significantly smoother optical profiles than isolated mergers, i.e., they show an amorphous morphology and no signs of tidal tails. Tidal arms and tails formed during the mergers may have been stripped by the group potential, or, alternatively, they may have never been formed. The velocity field of the galaxy H16a shows grand-design isovelocity lines with no signs of disturbances inside a radius of ~R25. This result is contrary to expectations given that the galaxy has a high infrared luminosity, central activity, tidal tails at large radii, and is embedded in a common group envelope observed in H I and X-rays. The normality of the velocity field suggests that this galaxy may be a fairly recent acquisition of the compact group. Our observations suggest that HCG 16 may be a young compact group in formation through the merging of close-by objects in a dense environment.