Issue |
A&A
Volume 422, Number 3, August II 2004
|
|
---|---|---|
Page(s) | 883 - 905 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20035722 | |
Published online | 16 July 2004 |
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
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.
Key words: galaxies: abundances / ISM: molecules / galaxies: individual: NGC 4945 / galaxies: starburst / galaxies: ISM / radio lines: galaxies
© ESO, 2004
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