A SEARCH FOR NH IN THE LARGE MAGELLANIC CLOUD

The Target Positions

  
Figure 1: A comparison of our Parkes spectra with the Compact Array HI spectra ( a. - e.) and with a CO spectrum (f.) observed with the CTIO 1.2m telescope. The numbers refer to Table 1. No emission is seen. In all of the HI\ spectra, features occur at typical LMC velocities, showing the two major gas components of the LMC at 260 km and 280 km . Note the absorption in some of the HI spectra. The velocity scale is heliocentric.

CO

Searching for good candidates for high-density molecular clouds is not an easy task in the LMC. The overall CO map of Cohen et al. (1988) has a spatial resolution of 6' which is too coarse to find high density clumps (nevertheless, we observed towards Molecular Cloud Complex 35, no. 7 in Table 1). Results from the ESO-SEST key project are still being published. The only maps published at the time of the observations were those in Booth & de Graauw (1991). Based on these maps, Hunt & Whiteoak (1994) cite coordinates of three major CO condensations in N159, which we include in Table 1 (nos. 6, 9 and 10 in Table 1). Also included is a feature which Hunt & Whiteoak discovered in their compact 4.8 GHz continuum image and the IRAS point source LI-LMC 1518. Israel et al. (1993) give coordinates and spectra of observations towards IRAS point sources, observed with SEST. Three further positions were chosen from this list: LI-LMC 1501 in N159 which has the highest brightness temperature; LI-LMC 1609 which has a complex CO spectrum; and LI-LMC 1541 which exhibits a small line width, thus presumably tracing a very cold, dense molecular feature. In our Galaxy, the relationship between molecular material and infrared sources is striking. IRAS sources are often associated with high density clumps and thus trace the highly clumped molecular material.

HI

Existing surveys of the atomic hydrogen in the LMC were all done with the Parkes telescope and have an angular resolution of 14.'8 (e.g. Luks and Rohlfs, 1992) which is far too coarse to pick out compact condensations. However, the Compact Array observations described in Sect. 2.2 are ideal for this purpose. To select candidates for observations, we produced a map of total hydrogen column density and chose one position showing strong absorption (no. 3 in Table 1) and another position with a high column density (no. 4 in Table 1). We also produced column density maps covering more restricted (13 {kms) velocity ranges. In the first of these maps which has a mean heliocentric velocity of 212 kms we extracted one position (no. 8 in Table 1). At a mean velocity of 278 kms we extracted a further point (no. 2 in Table 1). Finally, one position (no. 1 in Table 1) was extracted from a map centered at 313 kms.

© Copyright Astronomical Society of Australia 1997