1 Introduction

Figure 1: Left: N 11 sketch from Meaburn et al. (1989). H$\alpha$ emission is marked by dark zones; OB associations are indicated by their LH numbers (see text). Right: Overall distribution of integrated J=1-0 $~{\rm ^{12}CO}$ emission in N 11 on the same scale.

The ESO SEST Key Programme was established to investigate the molecular gas in the nearest neighbours to the Milky Way, the Magellanic Clouds. Considerations pertinent to this programme were given by Israel et al. (1993; Paper I). Following ESO's discontinuation of the concept of Key Programmes, the observational programme was ended in 1995, although the processing of data obtained has continued. In this paper we present the results of observations of the HII region complex N 11 (Henize 1956), located in the northwestern corner of the Large Magellanic Cloud (LMC). After 30 Doradus with its retinue of HII regions, supernova remnants and dark clouds, this complex is the second-brightest in the LMC. CO observations of the former, also made within the context of the Key Programme, have been published by Johansson et al. (1998) and Kutner et al. (1997).

The N 11 complex is also known as DEM 34 (Davies et al. 1976), and has an overall diameter of about 45', corresponding to a linear extent of 705 pc for an assumed LMC distance of 54 kpc (Westerlund 1990, but see Walker 1999). In Fig. 1 we present a sketch map of the optical nebulosity. In the west, N 11 contains the small supernova remnant N 11L (= DEM 34a). From the main body of the N 11 complex, a loop of HII regions and more diffuse H$\alpha$ emission extends to the northeast. This loop delineates the eastern half of LMC supergiant shell SGS-1 (Meaburn 1980) which has a diameter of about a kiloparsec and is centered on OB association LH 15 (Lucke & Hodge 1970 - not marked in Fig. 1).

N 11 is prominent not only at optical wavelengths, but also in the infrared and radio continua (Schwering & Israel 1990; Haynes et al. 1991) and in CO line emission (Cohen et al. 1988). It has a complex structure (see Fig. 1). The southern part of N11 appears to be a filamentary shell of diameter 200 pc enclosing the OB association LH 9 (Lucke & Hodge 1970) also known as NGC 1760. In the center of this shell, we find the relatively inconspicuous HII-region N 11F. At the northern rim of the shell, another OB association, LH 10 (a.k.a. NGC 1763, IC 2115, IC 2116) is associated with the very bright HII region N 11B and the bright, compact object N 11A (Heydari-Malayeri & Testor 1983). The eastern rim of the shell is likewise marked by the OB association LH 13 (NGC 1769) exciting the bright HII regions N 11C and N 11D. Finally, OB association LH 14 (NGC 1773), coincident with HII region N 11E, marks the point where the northeastern loop SGS-1 meets the filamentary shell around LH 9. The HII regions N 11B, N 11CD, N 11E and N 11F are all identified with thermal radio sources in the catalog published by Filipovic et al. (1996). The far-infrared emission from warm dust does not show the same spatial distribution as the radio continuum and H$\alpha$ line emission from ionized hydrogen gas (see Fig. 3 in Xu et al. 1992). The latter fills the whole shell region, whereas the former is clearly enhanced at the shell edges. The radio HII regions have typical rms electron-densities of 15  $~{\rm cm^{-3}}$, masses of 10⁴- $10^{5}~M_{\odot}$, emission measures of 10⁴ pc cm^-6 and appear to be well-evolved (Israel 1980). The OB associations powering the complex are all rich associations. For instance, LH 9 contains 28 O stars, and LH 10 contains 24 O stars (Parker et al. 1992). Likewise, LH 13 contains some 20 O stars, and LH 14 about a dozen (Heydari-Malayeri et al. 1987). It is possible that star formation in the N 11 complex is at least partly triggered by the expanding shell surrounding LH 9 (Rosado et al. 1996).

The low-resolution (12') CO observations by Cohen et al. (1988) showed that the N 11 group of HII regions is associated with an extended molecular cloud complex. It is the third brightest CO source in their survey, after the very extended 30 Doradus complex, and the more modest N 44 complex. Cohen et al. estimated for the N 11 molecular complex a mass of about $M(~{\rm H_{2}}) = 5 \times 10^{6}~M_{\odot}$, although the comparison of these data with IRAS results by Israel (1997) suggests about half this value. A higher-resolution (2.6') CO survey, carried out by Mizuno et al. (2001) had insufficient sensitivity to reproduce the actual CO structure; the low (virial) mass estimate given appears to be rather uncertain. Using the same instrument, Yamaguchi et al. (2001) conducted a more sensitive survey in which extended emission from a CO cloud complex is seen to follow the outline of the ionized gas making up the HII region complex.

Because of its prominence and its interesting optical structure, we have mapped N 11 and its surroundings in the J = 1-0  $~{\rm ^{12}CO}$ transition within the framework of the ESO-SEST Key Programme. Preliminary results have been presented by Israel & de Graauw (1991) and Caldwell & Kutner (1996). We have also mapped parts of the complex in the J = 2-1 $~{\rm ^{12}CO}$ transition, and in the corresponding transitions of  $~{\rm ^{13}CO}$.