1998A&AS..127..397V

Sensitive 21cm HI line observations were performed for 23 dwarf members and possible members of the nearby M 81 group of galaxies, including five objects of a clustering of extremely low-surface brightness objects of unknown nature. With the Nancay decimetric radio telescope the radial velocity range of -529 to 1826 km/s was searched to an rms noise of ∼2-4mJy. Only three objects were detected. However, their high radial velocities (between 600 and 1150km/s) show them to lie behind the M 81 group. These three objects, classified as dS0: (UGC 4998) and Im (Kar 1N and UGC 5658), have HI masses of 0.4, 1.6 and 2.0x10⁸M_☉, for the assumed distance of 4Mpc, and HI mass-to-blue light ratios of 0.04, 0.73 and 0.18M_☉/L_☉,B, respectively. Considering that half of the observed objects are classified as irregular dwarfs, hence expected to be relatively gas-rich, the resulting detection rate of about 1/3 is quite low. However, the mean redshift and velocity dispersion of the M 81 group (<V≥101km/s, σ=114km/s) suggest that the HI emission of low velocity HI-rich members of the M 81 group may still remain hidden within the strong Galactic HI emission (typically -150≲V≲115km/s) or, for the 6 dwarf candidates in the immediate vicinity of M 81, overshadowed by the very extended HI envelope encompassing M 81, M 82, NGC 3077, and NGC 2976 (-280≲V≲355km/s).