Leo Triplet spiral galaxy NGC 3628. I. VLA observations of the neutral hydrogen absorption

Hydrogen absorption is observed against the nuclear region of NGC 3628, an edge-on spiral galaxy in the Leo Triplet. VLA observations against the extended continuum reveal approximately 10 individual H I features which are resolved both spatially and in velocity space. A circular, rotating disk is identified as well as several lower velocity features which seem to form a second structure. This may be an expanding spiral arm, similar to the 3 kpc arm in the Galaxy, or a second disk which is either expanding with respect to the primary disk or elliptical in shape. All of the features are optically thick with tau roughly 1 in many cases.Estimated densities are high with n(HI+) greater than 100/cu cm and M roughly 100,000 solar masses for a typical feature. The high degree of activity in this galaxy, which includes complex hydroxyl absorption and H I extensions in the form of plumes and bridges, may have been triggered by a tidal encounter with NGC 3627, a second spiral in the triplet. 21 references