A study of neutral hydrogen in five small galaxy groups

Abstract

High-resolution H I imaging observations of a heterogeneous sample of small galaxy groups are presented. The five galaxy groups studied show a broad range of individual H I properties: e.g. loose groups surrounding LGG 138 and the genuinely compact LGG 455 are identified; a massive ring of neutral gas is discovered encircling two luminous galaxies in the LGG 138 group; a galaxy-sized mass of H I is found in LGG 455 confined to an extragalactic cloud which exceeds the threshold density for star formation, yet is optically invisible; and the CCG 1 group is argued to be a chance alignment of Centaurus cluster galaxies. Global results of the study are that the deficit of H I flux in synthesis imaging data compared with single-dish data is put forward as a quantitative measure of the diffuseness of neutral gas in galaxy groups; several groups contain gas-poor galaxies that ordinarily would contain detectable quantities of H I - this is interpreted as being caused by an increased chance of gas-sweeping collisions in the group environment; and some evidence is found to support previous findings that compact groups preferentially occur in loose systems.