Evolution of compact groups from intermediate to final stages
Authors: Jones M. G. , Verdes-Montenegro L. , Damas-Segovia A. , Borthakur S. , Yun M. , del Olmo A. , Perea J. , Román J. , Luna S. , Lopez Gutierrez D. , Williams B. , Vogt F. P. A. , Garrido J. , Sanchez S. , Cannon J. , Ramírez-Moreta P. 
Published: October 2019

Context. Hickson Compact Group (HCG) 16 is a prototypical compact group of galaxies in an intermediate stage of the previously proposed evolutionary sequence, where its galaxies are losing gas to the intra-group medium (IGrM). The group hosts galaxies that are H I-normal, H I-poor, and centrally active with both AGNs and starbursts, in addition to a likely new member and a H I tidal feature of ∼160 kpc in length. Despite being a well-studied group at all wavelengths, no previous study of HCG 16 has focused on its extraordinary H I component.

Aims. The characteristics of HCG 16 make it an ideal case study for exploring which processes are likely to dominate the late stages of evolution in compact groups, and ultimately determine their end states. In order to build a coherent picture of the evolution of this group we make use of the multi-wavelength data available, but focus particularly on H I as a tracer of interactions and evolutionary phase.

Methods. We reprocess archival VLA L-band observations of HCG 16 using the multi-scale CLEAN algorithm to accurately recover diffuse features. Tidal features and galaxies are separated in three dimensions using the SlicerAstro package. The H I deficiency of the separated galaxies is assessed against the benchmark of recent scaling relations of isolated galaxies. This work has been performed with particular attention to reproducibility and is accompanied by a complete workflow to reproduce all the final data products, figures, and results.

Results. Despite the clear disruption of the H I component of HCG 16 we find that it is not globally H I deficient, even though HCG 16a and b have lost the majority of their H I and almost 50% of the group’s H I is in the IGrM. The H I content of HCG 16d shows highly disturbed kinematics, with only a marginal velocity gradient that is almost perpendicular to its optical major axis. The tail of ∼160 kpc in length extending towards the southeast appears to be part of an even larger structure which spatially and kinematically connects NGC 848 to the northwest corner of the group.

Conclusions. This study indicates that in the recent past (∼1 Gyr) galaxies HCG 16a and b likely underwent major interactions that unbound gas without triggering significant star formation. This gas was then swept away by a close, high-speed encounter with NGC 848. The starburst events HCG 16c and d, likely initiated by their mutual interaction, triggered galactic winds which, in the case of HCG 16d, appear to have disrupted its H I reservoir. The tidal features still connected to all these galaxies indicate that more H I will soon be lost to the IGrM, while that which remains in the discs will likely be consumed by star-formation episodes triggered by their ongoing interaction. This is expected to result in a collection of gas-poor galaxies embedded in a diffuse H I structure, which will gradually (over several Gyr) be evaporated by the UV background, resembling the final stage of the evolutionary model of compact groups.