IAU Symposium Abstract S186-019P

An HI line search for optically selected dwarf galaxy candidates in the M81 group

W.~van Driel1, R.C.~Kraan-Korteweg2, B.~Binggeli3, W.K.~Huchtmeier4
1 USN, Observatoire de Paris, France (vandriel@mesioq.obspm.fr)
2 DAEC, Observatoire de Paris, France
3 Astronomical Institute, University of Basel,Switzerland
4 Max-Planck-Institut f\"ur Radioastronomie, Bonn, Germany

Sensitive 21cm HI line observations were performed for 23 dwarf members and possible members of the nearby M81 group of galaxies. With the Nan\c{c}ay decimetric radio telescope the radial velocity range of -529 to 1826 kms was searched to an rms noise of ~ 2-4 mJy. Only three objects were detected. However, their high radial velocities (between 600 and 1150 \kms) show them to lie behind the M81 group. These three objects, classified as dS0: and Im, have HI masses of 0.4, 1.6 and 2.0$\cdot10^8$\Msun, for the assumed distance of 4 Mpc, and HI mass-to-blue light ratios of 0.04, 0.73 and 0.18 Msun/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 M81 group (<V> = 95 kms, sigma=123 kms) suggest that the \HI emission of low velocity HI-rich members of the M81 group may still remain hidden within the strong Galactic HI emission (typically -150 < V < 115 kms) or, for the 6 dwarf candidates in the immediate vicinity of M81, overshadowed by the very extended \HI envelope encompassing M81, M82, NGC 3077, and NGC 2976 (-280 < V < 355 kms).