The neutral hydrogen content of the M81/M82 group of galaxies – I. The observations

Observations have been made with high sensitivity of the neutral hydrogen emission from the nearby M81 group of galaxies. The galaxies M81, M82, NGC 3077 and NGC 2976 are shown to be embedded in a large dynamically complex cloud of hydrogen. Bridges of emission appear to connect the three companion galaxies to M81. In the case of NGC 2976 the situation is complicated by the presence of a large cloud of gas (the South-West Cloud) associated with a low-luminosity bridge. It is suggested that tidal interaction between the group members is responsible for many of the observed emission features. For the brighter features, a comparison is made with recent aperture synthesis observations of the area. The mass of neutral hydrogen observed in the group was found to be <f>$5.4\\times 10^9 M_\\odot$</f> which corresponds to approximately 3 per cent of the total mass of the individual group members. Approximately 25 per cent of the H <scp>I</scp> lies in the intracluster medium. A most interesting feature of the survey is the South-West Cloud of neutral hydrogen lying 50 arcmin south-west of the centre of M81. Its H <scp>I</scp> mass of <f>$3.2\\times 10^8 M_\\odot$</f> is only capable of binding it gravitationally if it is seen edge-on. The question of its separate existence as a bound system is discussed. Two galaxies of low surface brightness, A 0951+68 and DDO66, lie in the survey area. Integrated parameters are derived for each. Observations were also made of IC 2574, another possible member of the M81 group. Although this galaxy is at a projected distance of only three degrees (180 kpc) from M81, it does not show the optical or neutral hydrogen characteristics of a tidally distorted galaxy.