The Hi Environment of Counter-rotating Gas Hosts: Gas Accretion from Cold Gas Blobs

TL;DR

This study investigates the HI properties and environments of three early-type barred galaxies with counter-rotating gas, providing evidence of gas accretion from neighboring dwarf galaxies through tidal interactions and HI morphology analysis.

Contribution

It presents new HI observations of three galaxies, demonstrating gas accretion from companions as a source of counter-rotating gas, highlighting the role of cold gas blobs in galaxy evolution.

Findings

NGC 128 and NGC 3203 show HI connected to dwarf companions.

NGC 7332's counter-rotating gas likely acquired from NGC 7339.

HI morphology suggests tidal interactions as the accretion mechanism.

Abstract

We probe the HI properties and the gas environments of three early-type barred galaxies harbouring counter-rotating ionized gas, NGC 128, NGC 3203 and NGC 7332. Each system has one or more optically-identified galaxy, at a similar or as yet unknown redshift within a 50 kpc projected radius. Using HI synthesis imaging data, we investigate the hypothesis that the counter-rotating gas in these galaxies has been accreted from their neighbours. In NGC 128 and NGC 3203, we find 9.6e7 and 2.3e8 Msun of HI, respectively, covering almost the entire stellar bodies of dwarf companions that appear physically connected. Both the HI morphology and kinematics are suggestive of tidal interactions. In NGC 7332, we do not find any directly-associated HI. Instead, NGC 7339, a neighbour of a comparable size at about 10 kpc, is found with 8.9e8 Msun of HI gas. More recently in a single dish observation, however, another group discovered a large HI structure which seems to be an extension of NGC 7339's HI disc and also covers NGC 7332. All these observations thus suggest that HI gas is being accreted in these three galaxies from their companions, which is likely responsible for the kinematically-decoupled gas component present in their central region. Considering the incompleteness of existing studies of the faint dwarf galaxy population both in the optical and in HI, accretion from cold gas blobs, presumably gas-rich dwarfs, is expected to occur even more frequently than what is inferred from such cases that have been observed to date.