Atomic and molecular gas in the starburst galaxy NGC 4945

Abstract

Spatial and kinematical correlations between the H I and 12CO(2-1) emission of the southern spiral galaxy NGC 4945 are studied with a common angular resolution of ∼23″ (corresponding to 750 pc at D = 6.7 Mpc) and a velocity resolution of ∼7 km s-1. The 1.4 GHz continuum emission is also observed. The H I kinematics yield a galaxy mass of ∼1.4 × 1011 M⊙ within radius R = 380″, with molecular and neutral atomic gas each contributing ∼2%, respectively. A central continuum source of size 7″.6 × 3″.4 (250 × 110 pc) is enveloped by a molecular cloud of mass 1.5 × 109 M⊙ for R ≤ 7″.5, and is rapidly rotating with Vrot ∼ 160kms-1. H I emission from the central region at velocities |V-Vsys| > 200km s-1 may be related to optically detected gas that is believed to trace an outflow directed towards the halo. Nuclear H I absorption at V - Vsys ∼ +80 km s-1 suggests inflow towards the centre, that was so far only seen in molecular lines. H I features at each end of the major axis (|R| ∼ 600″) are interpreted as spiral arms that are viewed tangentially and that also cause prominent emission features in the radio continuum, H I, and CO further inside the galaxy. A central elongated region showing non-circular motions is interpreted as a bar which fuels the nuclear starburst. The H I and CO position-velocity data have been analysed using linear resonance theory, and possible locations of resonances are identified.