A high-resolution image of atomic hydrogen in the M81 group of galaxies.

It has long been recognized that interactions between galaxies are important in determining their evolution. The distribution of gas--out of which new stars are formed--is strongly affected; in particular, gas may be concentrated near the nucleus, leading to a burst of star formation. Here we present a map of atomic hydrogen (H I) in the nearest interacting group of galaxies (that dominated by M81), obtained by combining 12 separate fields observed with the Very Large Array. The H I that surrounds M81, M82 and NGC3077 (the most prominent galaxies in the group) is dominated by filamentary structures, clearly demonstrating the violent disruption of this system by tidal interactions. These observations should have detected all H I complexes more massive than 10(6) solar masses, meaning that our map contains all structures that might evolve into new dwarf galaxies.