A SEARCH FOR NH IN THE LARGE MAGELLANIC CLOUD

Introduction

Observations of the state of the interstellar medium (ISM) in nearby galaxies allow us to probe a much greater range in physical conditions (e.g. radiation field, mass density, gas density, abundance) than is possible in our own Galaxy. The molecular phase in particular is relevant to our knowledge of processes leading to the formation of stars and stellar clusters. The most abundant observable tracer of molecular gas is . However in case of LMC, the nearest galaxy to us which is internally bound, the emission is generally weak when compared with similar Milky Way regions (Cohen et al. 1988). The likely reason is that the strong ultraviolet radiation field in the LMC leads to greater photodissociation and lower ratios of /\ (Israel & de Graauw 1991), being less affected by photodissociation. The actual sites of young star formation are better traced by molecules such as CS and HCO which have both been detected in the LMC. However, Booth & de Graauw (1991) remark that, because of their large dipole moment, these molecules may suffer excitation by electrons at low molecular densities when the ionization fraction is high. Hence, it may be difficult to derive overall molecular abundances using those species. , on the other hand, has a dipole moment which is much lower than that of CS and HCO, so should not suffer abnormal excitation. In our own Galaxy, observations have proven to be an excellent probe of dense and cold molecular cloud cores (see the review article by Ho & Townes 1983, or observational results in Ho et al. 1978 and Benson & Myers 1989). The richness of the spectrum, especially of the (J,K)=(1,1) transition and its 18 hyperfine components, allows the observer to determine numerous conditions of the interstellar gas such as optical depth, excitation temperature and gas kinetic temperature. If detectable in the LMC, would therefore prove to be a useful tool for the estimation of physical conditions in the LMC.

We have used the Parkes 64-m telescope in an attempt to search for in a few chosen positions in the LMC. We concentrated our search near 30 Doradus, which is the largest HII region in the LMC and the most luminous and active star forming region in the Local Group. In this region, we can observe all phases of the interstellar gas: the warm diffuse gas; the cool atomic gas; the molecular clouds; and the ionized gas around newly formed stars and supernova remnants expanding into the ISM. Located south of 30 Doradus we find N159, a bright HII region with many characteristics of the typical star-forming region.

We have also recently mapped the 21cm HI emission with high spatial and velocity resolution around these regions with the Australia Telescope Compact Array (ATCA) and the Parkes telescope. This, for the first time, gives us sufficient spatial resolution ( pc) to probe dense clumps of cool neutral gas which may indicate sites of current or future star formation. The interstellar absorption survey of Dickey et al. (1994) has already shown that the cool atomic phase of the ISM near 30 Doradus is abundant in the LMC, in contrast to the relatively small number of molecular clouds detected by Cohen et al. (1988). In addition, the ESO-SEST key programme on CO in the Magellanic Clouds (Israel et al. 1993) is beginning to publish data which more directly pinpoint likely sites of dense molecular material.

We describe the observations and data reduction in Sect. 2; the selection of candidate positions in Sect. 3; summarise the results in Sect. 4; and discuss the implications and future work in Sect. 5.

© Copyright Astronomical Society of Australia 1997