| Abstract | 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 \mboxkms(-1) was searched to an rms noise of ~ 2-4\ mJy. Only three objects were detected. However, their high radial velocities (between 600 and 1150 \mboxkms(-1) ) 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 2.0\ 10(8) {cal M_sun}, for the assumed distance of 4 Mpc, and Hi mass-to-blue light ratios of 0.04, 0.73 and 0.18 {cal M_sun}LBsun, 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 ( = 101 \mboxkms(-1) , sigma =114 \mboxkms(-1) ) 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 < 115 \mboxkms(-1) ) 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 < 355 \mboxkms(-1) ). |
