These single-dish observations of the neutral gas content in the LMC have low-spatial resolution, typically around 220 pc for 21 cm surveys made with the Parkes telescope, corresponding to 14-15' on the sky. Our H I synthesis observations undertaken with the Australia Telescope Compact Array (ATCA) provide a mosaiced map of the LMC with much higher spatial resolution, 1' on the sky. This resolution corresponds to 15 pc scale in the LMC (at a distance to the LMC of 50 kpc), therefore, this H I map of the LMC can trace the detailed H I structure of the LMC. Furthermore, this high-resolution H I map of the LMC provide a unique laboratory for studying the small-scale structure of the ISM together with the Halpha image of the LMC. Since the LMC is the closest extragalactic neighbour and has low internal reddening (typically EB-V = 0.13) and has less confusion along the line of sight owing to the small inclination unlike the Milky Way. In order to match the H I survey, we undertook new Halpha observations of the LMC with a camera lens mounted on a 16 inch telescope at Siding Spring Observatory.
The full mosaic of the LMC consists of 1344 pointing centres covering 11.1° × 12.4° on the sky. The size of the H I cube is 1998 × 2230 × 120 pixels and the pixel size is 20". Superuniform weighting (Sramek and Schwab 1989) was applied to the uv data with an additional Gaussian taper. The final data cube used here has a velocity coverage of 190 to 387 km s-1, and a velocity resolution of 1.65 km s-1. All H I velocities reported in this paper are heliocentric.
The main objective of the Halpha survey of the LMC was to obtain an Halpha image covering the same region of sky as our H I survey. We therefore mounted a 153 mm f/5.0 camera lens on the mount of the 16 inch Telescope at Siding Spring with a near focal plane filter assembly. The images were recorded with a thick Loral (2048 × 2048) CCD cooled. With this set up, each 15 µm pixel corresponds to 20.63" on the sky, giving a total field size of 11.7° square. The Halpha filter was centred at 6569 Å and had a FWHM of 15 Å. In addition to the Halpha images, continuum images were taken through a filter centred at 6620 Å and subtracted from the final Halpha image.
It is difficult to distinguish a bar structure in the global H I emission comparable to the optical bar. The largest H I gas concentration is known to lie in the 30 Dor cloud complex and extends about 2° further south. This region also contains the most prominent of the giant molecular cloud complexes in the LMC (Cohen et al. 1988). In our map this region displays the most complex small scale structure, resulting from the high density, the presence of many clouds, and the complex pattern of star formation in this region.
The most prominent and overriding feature of the map presented in Fig. 1 on both the small and the medium spatial scales is the complex system of filamentary structures, combined with numerous H I holes and shells. These emphasise the small-scale flocculent structure of the ISM in the LMC. Many of these structural features in the surface density distribution of H I are correlated with giant and supergiant shells (Meaburn 1980) identified through the Halpha emission, of which the Shapley Constellation III (Dopita et al. 1985) is a particularly fine example.
Another feature of Fig. 1 is the existence of clear spiral features, particularly in the outer regions. The arms in the south are the strongest and appear to emanate from close to the optical bar, following the 'B3' stub identified by de Vaucouleurs & Freeman (1972). They bifurcate at α = 05h 36m, δ = -71° 46' (J2000), a position which lies some 2.8° south of 30 Dor. The southern-most arm appears to stretch towards the SMC and merges into the eastern part of the H I bridge which exists between the LMC and the SMC (McGee & Newton 1986). It appears therefore that this particular arm might be tidal in origin, and that, correspondingly, some of the gas in the bridge region is also tidal.
A full resolution H I velocity field of the LMC (Fig. 6 in Kim et al. 1998) shows the overall rotation pattern. Noticingly, the iso-velocity contour across the minor axis has the distorted S-shape and there is a deviation from circular rotation. This might be caused by the kinematical response of the H I to the gravitational field of the stellar bar.
The kinematic centre was found to lie at α = 05h 17.6m, δ = -69° 02' (J2000), which is about 0.5° W and 0.7° N of the nominal centroid of the optical bar. This is in reasonable agreement with Luks & Rohlfs (1992) who quote offsets of 1.0° W and 0.7° N. The kinematic inclination and position angle are derived as i∼33° and θ∼168°. The rotation curve rises rapidly in the inner 1.5 kpc to Vrot∼55 km s-1 and then rises smoothly to a peak Vrot∼63 km s-1 at R∼2.4 kpc. After that it declines rather steeply. The rotation curves predicted by the stellar and neutral mass distributions are shown in Fig. 2 by the dashed and dotted lines, respectively. The best fit was obtained using a mass-to-light ratio of M/LR = 1.8. Estimates based on the rotation curve sets a upper bound of ∼3.5·109 Msun for the mass of the LMC out to a radius of 4 kpc, assuming a disk inclination of 33°. The RMS velocity dispersion along the line of sight, <Δ v2>1/2, is 15.8±0.2 km s-1.
| First version: | 13th | July, | 1998 |
| Last update: | 08th | October, | 1998 |