Dense gas in nearby galaxies - XVI. The nuclear starburst environment in NGC 4945

作者：Wang M^*; Henkel C; Chin YN; Whiteoak JB; Cunningham MH; Mauer**erger R; Muders D

来源：Astronomy & Astrophysics, 2004, 422(3): 883-905.

DOI：10.1051/0004-6361:20035722

• galaxies : abundances
• ISM : molecules
• galaxies : individual : NGC 4945
• galaxies : starburst
• galaxies : ISM
• radio lines : galaxies

A multi-line millimeter-wave study of the nearby starburst galaxy NGC4945 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 H-2 column density and CO J = 1-0 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 n(H2) similar to 10(5) cm(-3). Some spectra show Gaussian profiles. Others exhibit two main velocity components, one at similar to450 km s(-1), the other at similar to710 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 NGC4945. While the HNC/HCN J = 1-0 line intensity ratio is similar to0.5, the HNC/HCN abundance ratio is similar to1. From a comparison of K-a = 0 and 1 HNCO line intensities, an upper limit to the background radiation of 30 K is derived. While HCN is subthermally excited (T-ex similar to 8 K), CN is even less excited (Tex similar to 3-4 K), indicating that it arises from a less dense gas component and that its N = 2-1 line can be optically thin even though its N = 1-0 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 O-18/O-17, low O-16/O-18 and N-14/N-15 and perhaps also low S-32/S-34 ratios (6.4 +/- 0.3, 195 +/- 45, 105 +/- 25 and 13.5 +/- 2.5 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.

• 出版日期2004-8
• 单位CSIRO; 中国科学院紫金山天文台