1. Introduction

The Magellanic Clouds provide unique opportunities for studying molecular clouds and star formation in galaxies whose environment is very different from that of the Milky Way. At the 50 kpc distance of the Large Magellanic Cloud (LMC), 1 arc minute covers 15 pc. This means that we can make CO maps of individual molecular clouds with even modest single dishes. For example, the 15-m Swedish-ESO Submillimetre Telescope (SEST) provides an angular resolution of 43'' at 2.6 mm, corresponding to a linear resolution of 10 pc. This is well suited to measure the properties of clouds that are a few tens of pc in extent. In addition, the line can provide 5 pc resolution where necessary. Prior to the availability of the SEST, information was obtained on the extent of CO emission in the LMC (Cohen et al. 1988) and in the SMC (Rubio et al. 1991) using a 1 m telescope.

The problem of star formation in irregular galaxies is very different from that in spirals. In irregulars there is no organized pattern of star formation. Rather, there appear to be isolated bursts. It is interesting to know what triggers those bursts, and whether there is some propagation of the star formation activity. Also, the interstellar medium in irregulars is dominated by atomic hydrogen, rather than molecular hydrogen. However, in the Milky Way (and other spirals) we have come to accept the idea that molecular clouds are necessary for star formation. Therefore, the role of HI in star formation could be potentially more important in irregulars than in the Milky Way. Also, understanding star formation in the HI dominated, low metallicity, environment of these galaxies, might provide clues on the formation of the first stellar generations in all galaxies.

To study molecular clouds, we rely on millimeter wave observations of carbon monoxide (CO), a trace constituent. In the Milky Way, the issue of how we convert from the CO luminosities of molecular clouds to their masses has been a topic of considerable debate. (See e.g. Combes 1991, and references therein for some discussion of this problem). Even though there is a growing consensus among observers on how to treat this problem in the Milky Way, the question of how to approach it in other galaxies is still unresolved. A major source of uncertainty is the effect of metallicity and gas-to-dust ratio on the conversion factor. In the LMC, the metallicity is a factor of four lower than in our Galaxy (Dufour 1984) and the gas to dust ratio is four times the Galactic value (Koornneef 1982). Moreover, in the Milky Way, the consensus has emerged after studies that included full maps of a statistically significant sample of clouds. In the Magellanic Clouds, we have the opportunity to make such maps, even with the modest angular resolution of single dishes.

The study of CO emission has been designated as a Key Programme on the SEST. As such, approximately 40 half days per year have been devoted to observations since 1988 May . The goal of the Key Programme is to produce fully sampled maps of CO emission from several representative areas in the LMC and SMC. With resolution and sensitivity to map many types of regions, we can address a number of important questions:

1. What is the nature of the molecular emission from the Magellanic Clouds? Does the emission come mostly from well defined clouds, as in the Milky Way, or is there extensive low level emission?
2. If there are well defined clouds, what are their size distributions? Is most of emission coming from a relatively small number of giant molecular clouds (GMCs), with extents of tens of parsecs, as in the Milky Way, or is there a higher proportion of smaller clouds? Different theories of cloud formation, evolution and destruction predict different size distributions.
3. What are the cloud masses? We can determine virial masses for a statistically significant sample of clouds. There is growing evidence that the virial mass is a good indicator of cloud mass (to within a factor of two) for large ensembles of clouds.
4. We can address the question of the conversion from CO luminosity, to . In particular by comparing that conversion from the Milky Way to the LMC to the SMC, we can see how changing metallicity affects that conversion.
5. Once we have a good idea of the distribution of molecular material we can also address the questions of star formation activity and morphology. Of course, these require infrared and radio observations, that are beyond the immediate goals of the Key Programme.

Some of the initial results are presented in Johansson (1991) and Rubio (1991). The results of selected pointed observations in the LMC are presented by Israel et al. (1990; hereafter Paper I). Observations of the SMC are reported by Rubio et al. (1993a,b, hereafter Paper II and Paper III, Lequeux et al. 1994 Paper IV, and Rubio et al. 1996 Paper V).

The regions chosen for extensive mapping are shown in Fig. 1 (click here), superimposed on the contours of CO emission mapped by Cohen et al. (1988) with an 8.8 arcminute beam. Three regions have been singled out, (1) the immediate vicinity of the HII region associated with 30 Dor, (2) the clouds in the vicinity of the isolated HII region N11, (3) the dark cloud complex extending some 2 kpc south of 30 Dor. It is this third region that is the subject of this Paper. Extensive CO emission associated with this feature was noted by Cohen et al. (1988). The SEST CO(1 0)  maps are presented in this Paper, and the individual cloud properties are analyzed in Paper VII.

These regions were selected as result of a fully sampled unbiased single strip extending for 3.5 N-S through the center of the 30 Doradus Complex. Part of that strip is shown in Fig. 1 (click here). The purpose of observing this strip was in recognition of the fact that there is a large jump in resolution from the Cohen et al. data to the SEST data. If the emission is patchy, then regions inside the lowest contour in the Cohen et al. map might have no emission as seen from the SEST, and regions outside the lowest contour might have detectable emission. After the full strip was made, some regions where emission was detected were chosen for full mapping. In this sense, the data in this paper are different than for Paper I, in which IRAS peaks were selected for study. The data presented in this Paper fall into two distinct regions in Fig. 1 (click here), and we designate them as the Central and Southern regions of the 30 Dor Complex.

  
Figure 1: Locations of regions observed with the SEST, superimposed on the CO contours of the data taken from Cohen et al. (1988). The regions studied with the SEST are shown as rectangular outlines and part of the north-south strip, observed at 20'' intervals, is marked with a vertical line