An H  i line search for optically identified dwarf galaxy candidates in the M 81 group

¹ Nançay Radio Observatory, USN, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Cedex, France
² DAEC, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Cedex, France
³ Astronomical Institute, University of Basel, Venusstrasse 7, CH-4102 Binningen, Switzerland
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Send offprint request to: W. van Driel

Received: 14 October 1996
Accepted: 22 May 1997

Abstract

Sensitive 21 cm H I  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 H I   masses of 0.4, 1.6 and , for the assumed distance of 4 Mpc, and H I   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 is quite low. However, the mean redshift and velocity dispersion of the M 81 group ( , ) suggest that the H I  emission of low velocity H I -rich members of the M 81 group may still remain hidden within the strong Galactic H I  emission (typically 115 ) or, for the 6 dwarf candidates in the immediate vicinity of M 81, overshadowed by the very extended H I  envelope encompassing M 81, M 82, NGC 3077, and NGC 2976 ( ).