The H i environment of counter-rotating gas hosts: gas accretion from cold gas blobs

Abstract

We probe the H i 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 H i 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 9.6 × 107 and 2.3 × 108 M⊙ of H i, respectively, covering almost the entire stellar bodies of dwarf companions that appear physically connected. Both the H i morphology and kinematics are suggestive of tidal interactions. In NGC 7332, we do not find any directly associated H i. Instead, NGC 7339, a neighbour of a comparable size at about 10 kpc, is found with 8.9 × 108 M⊙ of H i gas. More recently in a single dish observation, however, another group discovered a large H i structure which seems to be an extension of NGC 7339’s H i disc and also covers NGC 7332. All these observations thus suggest that H i 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. In particular, the dynamical friction time-scales of the nearest neighbours with H i gas of NGC 128 and NGC 3203 are comparable to their orbital time-scales around the counter-rotators, several ∼108 yr, implying that those neighbours will likely soon merge with the primary galaxies, fuelling them with gas. NGC 7332 also appears to be in the merging process with its neighbour through the common H i envelope. Besides, we find some other potential gas donors around NGC 128 and NGC 7332: two H i-rich galaxies with and 2.5 × 109 M⊙ at a distance of ≈67 kpc from NGC 128 and two dwarf systems with MHI= 3.9 × 107 and 7.4 × 107 M⊙ at ≲100 kpc from NGC 7332. Among the seven H i features identified in this study, three of them are associated with dwarf galaxies, two of which have only been recently identified in a blind survey, while the third one is still not catalogued at optical wavelengths. Considering the incompleteness of existing studies of the faint dwarf galaxy population both in the optical and in H i, 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.