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 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 H 2 ∼ 10 5 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 J = 1−0 line intensity ratio is ∼0.5, the HNC/HCN abundance ratio is ∼1. 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 ∼ 8 K), CN is even less excited (T ex ∼ 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. S 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.

With the goal of deriving the physical and chemical conditions of star forming regions in the Large Magellanic Cloud (LMC), a spectral line survey of the prominent star forming region N113 is presented. The observations cover parts of the frequency range from 85 GHz to 357 GHz and include 63 molecular transitions from a total of 16 species, among them spectra of rare isotopologues. Maps of selected molecular lines as well as the 1.2 mm continuum distribution are also presented. Molecular abundances in the core of the complex are consistent with a photon dominated region (PDR) in a nitrogen deficient environment. While CO shows optical depths of order τ ∼10, 13 CO is optically thin. The most prominent lines of CS, HCN, and HCO + show signs of weak saturation (τ ∼0.5). Densities range from 5×10 3 cm −3 for CO to almost 10 6 for CS, HCN, and a few other species, indicating that only the densest regions provide sufficient shielding even for some of the most common species. An ortho-to para-H 2 CO ratio of ∼3 hints at H 2 CO formation in a warm ( > ∼ 40 K) environment. Isotope ratios are 12 C/ 13 C ∼ 49±5, 16 O/ 18 O ∼ 2000±250, 18 O/ 17 O ∼ 1.7±0.2 and 32 S/ 34 S ∼ 15. Agreement with data from other star forming clouds shows that the gas is well mixed in the LMC. The isotope ratios do not only differ from those seen in the Galaxy. They also do not form a continuation of the trends observed with decreasing metallicity 6 Based on observations with the Swedish/ESO Submillimeter Telescope (SEST) at the European Southern Observatory (ESO, La Silla, Chile) and the Atacama Pathfinder EXperiment (APEX, Chajnantor, Chile) of the Max-Planck-Institut für Radioastronomie (MPIfR), ESO, and Onsala Space Observatory (OSO) from the inner to the outer Galaxy. This implies that the outer Galaxy, even though showing an intermediate metallicity, is not providing a transition zone between the inner Galaxy and the metal poor environment of the Magellanic Clouds. A part of this discrepancy is likely caused by differences in the age of the stellar populations in the outer Galaxy and the LMC. While, however, this scenario readily explains measured carbon and oxygen isotope ratios, nitrogen and sulfur still lack a self-consistent interpretation.

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We present aperture synthesis imaging of dense molecular gas in the Large Magellanic Cloud, taken with the prototype millimeter receivers of the Australia Telescope Compact Array (ATCA). Our observations of the N113 H II region reveal a condensation with a size of ∼6 ′′ (1.5 pc) FWHM, detected strongly in the 1-0 lines of HCO + , HCN and HNC, and weakly in C 2 H. Comparison of the ATCA observations with singledish maps from the Mopra Telescope and sensitive spectra from the Swedish-ESO Submillimetre Telescope indicates that the condensation is a massive clump of ∼10 4 M ⊙ within a larger ∼10 5 M ⊙ molecular cloud. The clump is centered adjacent to a compact, obscured H II region which is part of a linear structure of radio continuum sources extending across the molecular cloud. We suggest that the clump represents a possible site for triggered star formation. Examining the integrated line intensities as a function of interferometer baseline length, we find evidence for decreasing HCO + /HCN and HCN/HNC ratios on longer baselines. These trends are consistent with a significant component of the HCO + emission arising in an extended clump envelope and a lower HCN/HNC abundance ratio in dense cores.

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