A&A Supplement series, Vol. 127, February I 1998, 397-408
Received October 14, 1996; accepted May 22, 1997
W. van Driel, R.C. Kraan-Korteweg, B. Binggeli, - W.K. Huchtmeier
Send offprint request: W. van Driel, e-mail: email@example.com
Nançay Radio Observatory, USN, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Cedex, France
DAEC, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Cedex, France
Astronomical Institute, University of Basel, Venusstrasse 7, CH-4102 Binningen, Switzerland
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
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 Nançay decimetric radio telescope the radial velocity range of -529 to 1826 was searched to an rms noise of . Only three objects were detected. However, their high radial velocities (between 600 and 1150 ) 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 , for the assumed distance of 4 Mpc, and HI mass-to-blue light ratios of 0.04, 0.73 and 0.18 , 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> = 101 , ) 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 115 ) 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 ( ).
keywords: galaxies: distances and redshifts -- galaxies: irregular -- galaxies: ISM -- radio lines: galaxies -- galaxies: clusters of: M 81 group