A search for intergalactic neutral hydrogen in three nearby groups of galaxies.

Results are reported for a systematic unbiased search with a 40-m telescope for 21-cm H I emission from intergalactic space in three nearby groups of galaxies, as well as for a higher-sensitivity search with a 100-m telescope in selected areas of the same groups. The search covered over 300 sq deg of arc in the M81, CVn I, and NGC 1023 groups, but no hydrogen clouds were detected. Upper limits are placed on the space density of clouds with masses of at least 40 million, 80 million, and 460 million solar masses in the three groups. The higher-sensitivity survey detected four previously uncataloged dwarf irregular galaxies of very low surface brightness, in which the mass appears to be predominantly in the form of H I. The combined results indicate that: (1) the galactic high-velocity clouds are unlikely to be intergalactic gas in the Local Group; (2) the extended H I clouds around NGC 55 and NGC 300 in the Sculptor group and the H I companion to M33 are likely to be foreground gas clouds in the Galaxy; and (3) the Magellanic Stream is most probably not primordial gas but is due to the interaction of the Magellanic Clouds with the Galaxy.