Dense gas in nearby galaxies^*,**

XVI. The nuclear starburst environment in NGC 4945

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: mwang@pmo.ac.cn
² Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
³ Department of Physics, Tamkang University, 251-37 Tamsui, Taipei County, Taiwan
⁴ Australia Telescope National Facility, CSIRO Radiophysics Labs., PO Box 76, Epping, NSW 2121, Australia
⁵ School of Physics, UNSW, 2052 Sydney, Australia
⁶ IRAM, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain

Received: 21 November 2003
Accepted: 7 April 2004

Abstract

A multi-line millimeter-wave study of the nearby starburst galaxy NGC 4945 has been carried out using the Swedish-ESO Submillimeter Telescope (SEST). The study covers the frequency range from 82 GHz to 354 GHz and includes 80 transitions of 19 molecules. 1.3 mm continuum data of the nuclear source are also presented. An analysis of CO and 1.3 mm continuum fluxes indicates that the conversion factor between H₂ column density and CO  integrated intensity is smaller than in the galactic disk by factors of 5-10. A large number of molecular species indicate the presence of a prominent high density interstellar gas component characterized by  cm^-3. Some spectra show Gaussian profiles. Others exhibit two main velocity components, one at ~450 km s^-1, the other at ~710 km s^-1. While the gas in the former component has a higher linewidth, the latter component arises from gas that is more highly excited as is indicated by HCN, HCO⁺ and CN spectra. Abundances of molecular species are calculated and compared with abundances observed toward the starburst galaxies NGC 253 and M 82 and galactic sources. Apparent is an “overabundance” of HNC in the nuclear environment of NGC 4945. While the HNC/HCN  line intensity ratio is ~0.5, the HNC/HCN abundance ratio is ~1. From a comparison of  and 1 HNCO line intensities, an upper limit to the background radiation of 30 K is derived. While HCN is subthermally excited ( K), CN is even less excited ( K), indicating that it arises from a less dense gas component and that its line can be optically thin even though its emission is moderately optically thick. Overall, fractional abundances of NGC 4945 suggest that the starburst has reached a stage of evolution that is intermediate between those observed in NGC 253 and M 82. Carbon, nitrogen, oxygen and sulfur isotope ratios are also determined. Within the limits of uncertainty, carbon and oxygen isotope ratios appear to be the same in the nuclear regions of NGC 4945 and NGC 253. High ¹⁸O/¹⁷O, low ¹⁶O/¹⁸O and ¹⁴N/¹⁵N and perhaps also low ³²S/³⁴S ratios (, , and  in NGC 4945, respectively) appear to be characteristic properties of a starburst environment in which massive stars have had sufficient time to affect the isotopic composition of the surrounding interstellar medium.