Distribution of Faint Atomic Gas in Hickson Compact Groups

TL;DR

This study uses 21cm HI observations to map faint atomic gas in Hickson Compact Groups, revealing its distribution, origin, and survival timescales, thus enhancing understanding of intragroup medium properties.

Contribution

It provides new estimates of HI masses in HCGs using GBT, detecting more diffuse gas than previous VLA imaging and analyzing its spatial distribution and origin.

Findings

Detected 65% more HI with GBT than VLA in HCG92.

Faint gas in some groups shares spatial distribution with high-surface brightness HI.

Faint gas is likely of tidal origin and can remain neutral for at least 500 Myrs.

Abstract

We present 21cm HI observations of four Hickson Compact Groups with evidence for a substantial intragroup medium using the Robert C. Byrd Green Bank Telescope (GBT). By mapping H I emission in a region of 25$^{\prime}\times$25$^{\prime}$ (140-650 kpc) surrounding each HCG, these observations provide better estimates of HI masses. In particular, we detected 65% more \HI than that detected in the Karl G. Jansky Very Large Array (VLA) imaging of HCG92. We also identify if the diffuse gas has the same spatial distribution as the high-surface brightness (HSB) HI features detected in the VLA maps of these groups by comparing the HI strengths between the observed and modeled masses based on VLA maps. We found that the HI observed with the GBT to have a similar spatial distribution as the HSB structures in HCGs 31 and 68. Conversely, the observed HI distributions in HCGs44 and 92 were extended and showed significant offsets from the modeled masses. Most of the faint gas in HCG44 lies to the Northeast-Southwest region and in HCG 92 lies in the Northwest region of their respective groups. The spatial and dynamical similarities between the total (faint+HSB) and the HSB HI indicate that the faint gas is of tidal origin. We found that the gas will survive ionization by the cosmic UV background and the escaping ionizing photons from the star forming regions and stay primarily neutral for at least 500 Myrs.