A study of neutral hydrogen in five small galaxy groups

High-resolution H I imaging observations of a heterogeneous sample of small galaxy groups are presented. The five galaxy groups studied show a broad r ange of individual H I properties: e.g. loose groups surrounding LGG 138 an d the genuinely compact LGG 455 are identified; a massive ring of neutral g as is discovered encircling two luminous galaxies in the LGG 138 group; a g alaxy-sized mass of H I is found in LGG 455 confined to an extragalactic cl oud which exceeds the threshold density for star formation, yet is opticall y invisible; and the CCG 1 group is argued to be a chance alignment of Cent aurus 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 f orward as a quantitative measure of the diffuseness of neutral gas in galax y groups; several groups contain gas-poor galaxies that ordinarily would co ntain detectable quantities of H I - this is interpreted as being caused by an increased chance of gas-sweeping collisions in the group environment; a nd some evidence is found to support previous findings that compact groups preferentially occur in loose systems.