Atomic and molecular gas in the starburst galaxy NGC 4945

Abstract

Spatial and kinematical correlations between the Hi and 12 CO(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 ${\rm km s}^{-1}$. The 1.4 GHz continuum emission is also observed. The Hi kinematics yield a galaxy mass of ~$1.4\times10^{11}$ $M_{\odot}$ within radius $R=380''$, with molecular and neutral atomic gas each contributing ~2% , respectively. A central continuum source of size 7$ .\!\!^{\prime\prime}$6 $\times$ 3$ .\!\!^{\prime\prime}$4 (250 $\times$ 110 pc) is enveloped by a molecular cloud of mass $1.5\times10^9$ $M_{\odot}$ for R ≤ 7$ .\!\!^{\prime\prime}$5, and is rapidly rotating with $V_{\rm rot} \sim 160$ ${\rm km s}^{-1}$. Hi emission from the central region at velocities $|V-$ V sys $|$ > 200 ${\rm km s}^{-1}$ may be related to optically detected gas that is believed to trace an outflow directed towards the halo. Nuclear Hi absorption at V - V sys ~ +80 ${\rm km s}^{-1}$ suggests inflow towards the centre, that was so far only seen in molecular lines. Hi 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, Hi, 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 Hi and CO position-velocity data have been analysed using linear resonance theory, and possible locations of resonances are identified.