Dwarf galaxies provide important clues to the origin and evolution of structure in the Universe. Being low-mass objects, dwarfs are most vulnerable to interactions with the environment. They are hence ideal test particles to study evolutionary processes in different galaxy environments and, furthermore, to map the gravitational potential of galactic halos, groups and clusters.
Studies of dwarf galaxies have concentrated on the Local Group, and on clusters such as Virgo and Fornax. Local Group dwarfs can be studied in great detail, but there are only a few of them. The clusters, albeit rich in dwarfs, are relatively distant. Here, the M 81 group provides the ideal probe: it has about three times the dwarf content of the Local Group but is at only about a quarter of the Virgo cluster distance.
An extensive survey of M 81 group dwarfs has been carried out by Börngen et al. (1982), resulting in a list of dwarf members to a limiting absolute magnitude of , for the adopted distance of 4 Mpc to the group. They furthermore report the clustering of a number of unusual, very low-surface brightness about 7\ southeast of M 81. Although classified as dwarf members of the M 81 group (Börngen et al. 1984), the nature of these objects is uncertain. Are these remarkable objects dwarfs at the extreme faint end of the luminosity function? And if true, do they contain any gas and might thus be the lowest HI-mass objects yet observed?
The goal of the survey presented here is to determine the HI properties of the dwarfs of the M 81 group. Dwarf galaxies in groups and in the field generally are irregulars (Binggeli et al. 1990) - hence gas-rich. We therefore searched at Nançay for 21cm HI line emission in 23 candidate dwarf galaxy members of the M 81 group with considerably lower rms noise than previously obtained.