The Kinematics of the Warm Gas in the Interacting Hickson Compact Group of Galaxies HCG 90

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Abstract
We present kinematic observations of Hα emission for two early-type galaxies and one disk system, members of the Hickson compact group 90 (HCG 90) obtained with a scanning Fabry-Perot interferometer and samplings of 16 km s-1 and 1''. Mapping of the gas kinematics was possible to ~2reff for the disk galaxy NGC 7174 and to ~1.3reff and ~1.7reff for the early-type galaxies NGC 7176 and NGC 7173, respectively. Evidence for ongoing interaction was found in the properties of the warm gas of the three galaxies, some of which do not have stellar counterparts. The system H90bd (NGC 7176–NGC 7174), which was previously suspected to be an optical double, may in reality be a system in interaction. In the region where the galaxies spatially overlap in projection (their continuum centers are only 25'' apart), the gas profiles are separated in velocity space by ~50–100 km s-1. The gas component of the early-type galaxy is highly concentrated in the region closest to the irregular galaxy, where the interaction between the two galaxies may be taking place. The velocity fields of these galaxies are disturbed, most probably because of the ongoing interaction; they indicate that the galaxies are in prograde orbits, which is a favorable condition for merging. H90c, morphologically classified as an elliptical galaxy, has a disk of ionized gas that rotates around an axis oriented 60° with respect to the stellar rotation axis. This is strong evidence that the gas has an external origin. As is also the case for H90b, this galaxy may be a true S0 that was misclassified as an elliptical galaxy. We suggest the following evolutionary scenario for the system: H90d is the warm-gas reservoir of the group, in the process of fueling H90b with gas. H90c and H90d have experienced past interaction with gas exchange. The gas acquired by H90c has already settled and relaxed, but the effects of the interaction can still be seen in the morphology of the two galaxies and their stellar kinematics. This process will possibly result in a major merger.
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