A SEARCH FOR NH IN THE LARGE MAGELLANIC CLOUD

Discussion

The inversion lines of are generally weak due to subthermal excitation or to clumping of the molecules. We know from our own galaxy that cloudlets can be quite small, in the order of 0.1 pc (C.M. Walmsley, private communication). Thus, even if the molecular cloud (as detected in ) is extended, the effective filling fraction might be much smaller, as only a tiny part of the cloud surface area is producing emission. Consequently, with the Parkes beam of 1'.4 (20 pc) at 23 GHz, we may face severe beam dilution leading to even lower temperatures of the line. For an extragalactic object, it is even more crucial to know exactly where the density peaks of the molecular material are. For defining candidate positions, we used data which traces gas with a density n( ) 10 . Therefore, we may have missed the dense gas clumps. A tracer different from might supply more successful candidates for a future search for emission. The (1-0) transitions of HCN or have proved to be better extragalactic tracer for dense gas (Nguyen-Q-Rieu et al. 1992) as one needs densities of n( ) 10 for their excitation. Both molecules have now been detected in the LMC (Booth and Johansson 1991) and the coming millimetre data from SEST will pinpoint the density peaks in their observed fields.

© Copyright Astronomical Society of Australia 1997