The nuclear molecular clouds of NGC 4945

Abstract

The 3-mm transitions of HCN, HCO+ and HNC in the dense molecular clouds near the nucleus of the southern starburst galaxy NGC 4945 have been observed with three antennas of the Australia Telescope Compact Array (ATCA). Molecular-line emission was detected within the velocity range (300–800 km s−1) seen in single-dish spectra, although emission with extent greater than about 15 arcsec would not have been detected because of the lack of sufficiently small Compact Array (CA) baselines. Imaging the results at velocity intervals of 22.5 km s−1 and with a restoring beam of dimension 5.6 × 3.5 arcsec2 yielded images of typical deconvolved dimensions 7 × 3 arcsec2 (130 × 60 pc for an assumed galaxy distance of 4 Mpc). The positions of the images vary systematically with velocity, consistent with the result expected for a rotating molecular cloud ensemble viewed edge-on and inclined at a position angle (PA) of 45°. Although the brightest images occurred at velocities of the two major features (~430 and ~710 km s−1) in Swedish-ESO Submillimetre Telescope (SEST) spectra, the images at intervening velocities were unexpectedly faint or even in absorption. The results have been interpreted in terms of an extended cloud component not detected by the CA in conjunction with absorption against the nuclear continuum emission. Some of the HCO+ images are dominated by absorption and the overall results do not truly represent the cloud structure. The HNC results appear to contain little absorption, and the emission integrated over velocity yielded an elliptical image centred on an H2O ‘megamaser’ located at the galaxy's nucleus. The HNC position—velocity distribution along the major axis is consistent with an edge-on circumnuclear molecular ring with a rotational velocity of 135 km s−1. However, the ring radius of ~60 km s−1 is significantly less than previously derived from carbon monoxide (CO) observations. It can be reconciled with the CO results if the HNC ring traces high-density gas concentrated around the inside of the molecular ring, whereas the CO traces low-density gas that extends to higher radii. The HNC feature appears to be associated with features revealed in published infrared (IR) studies of the distribution of dust and star formation regions in the nuclear region of NGC 4945. Limited results on the variation of HNC/HCN abundance ratios within the nuclear molecular clouds support previous conclusions that the clouds are at an advanced evolutionary stage within a starburst period.