Galaxy Interactions in Compact Groups II: abundance and kinematic anomalies in HCG 91c

TL;DR

This study reveals localized abundance and kinematic anomalies in HCG 91c, likely caused by infalling gas clouds due to early-stage gravitational interactions within the compact group, highlighting early galaxy evolution effects.

Contribution

First detailed integral field spectroscopic analysis showing localized chemical and kinematic anomalies in a compact group galaxy, suggesting early interaction effects.

Findings

HII regions with ~0.15 dex lower oxygen abundance

Small kinematic offsets of 5-10 km/s in star forming regions

Evidence of infalling, collapsing gas clouds at the disk-halo interface

Abstract

Galaxies in Hickson Compact Group 91 (HCG 91) were observed with the WiFeS integral field spectrograph as part of our ongoing campaign targeting the ionized gas physics and kinematics inside star forming members of compact groups. Here, we report the discovery of HII regions with abundance and kinematic offsets in the otherwise unremarkable star forming spiral HCG 91c. The optical emission line analysis of this galaxy reveals that at least three HII regions harbor an oxygen abundance ~0.15 dex lower than expected from their immediate surroundings and from the abundance gradient present in the inner regions of HCG 91c. The same star forming regions are also associated with a small kinematic offset in the form of a lag of 5-10 km/s with respect to the local circular rotation of the gas. HI observations of HCG 91 from the Very Large Array and broadband optical images from Pan-STARRS suggest that HCG 91c is caught early in its interaction with the other members of HCG 91. We discuss different scenarios to explain the origin of the peculiar star forming regions detected with WiFeS, and show that evidence point towards infalling and collapsing extra-planar gas clouds at the disk-halo interface, possibly as a consequence of long-range gravitational perturbations of HCG 91c from the other group members. As such, HCG 91c provides evidence that some of the perturbations possibly associated with the early phase of galaxy evolution in compact groups impact the star forming disk locally, and on sub-kpc scales.