An H I Study of the Galaxy NGC 4945 with a Two-Element Synthesis Telescope

TEST, a two-element synthesis telescope operating at 1.4 GHz, consists of the Parkes 64-m radio telescope and a movable 18-m antenna. The system has several unconventional features, which are described. TEST has been used for an H I study of the edge-on spiral galaxy NGC 4945; the angular resolution was 47×73 arcsec2. H I absorption and emission are both present in the galaxy. The absorption occurs against the small-diameter radio nucleus and its velocity range (⁠|$380-740\,\text{km}\,\text{s}^{-1}$|⁠) is almost as large as the range observed across the entire galaxy. The absorbing clouds are probably located within a few hundred parsecs of the nucleus. The H I emission is extended along the optical major axis of the galaxy. Structure symmetrical about the nucleus in the H I distribution may be related to spiral features or rings of gas. A rotation curve fitted to the velocity field yields a total mass of |$8.8\,\times\,{10}^{10}\,{M}_{\odot}$|⁠. The velocity field shows departures from uniform circular motion which have been interpreted in terms of radial motions of |$20-30\,\text{km}\,\text{s}^{-1}$| towards the nucleus.