Structures of the Interstellar Medium in the LMC

The structure of the ISM is determined by the interplay between massive stars and the ISM: molecular clouds form massive stars; massive stars dissociate, ionize, and dynamically structure the ambient medium; the ionized medium eventually cools, recombines, contracts under gravity, and forms molecular clouds. This simplistic picture can qualitatively explain the dramatically different structures of the ionized and neutral interstellar components in the LMC.

The structure of the molecular ISM in the LMC can be seen in the low-resolution CO map presented by Cohen et al. (1988). The large molecular concentrations are usually the sites of star formation activities. The most intense star formation region, the 30 Dor complex, sits on the northern tip of the largest and most massive concentration of molecular gas. In high-resolution maps of star forming regions, such as N11 (Caldwell & Kutner 1996), remnant molecular gas is present and associated with the densest compact HII regions.

The structure of the neutral atomic ISM in the LMC is revealed in detail in the ATCA HI mosaic (Kim et al. 1997a). The differences between this ATCA mosaic and the Parkes 64m map (Rohlfs et al. 1984) are clearly caused by the difference in resolution; however, the absence of a prominent HI ridge to the south of 30 Dor in the ATCA map might be indicative of a ``missing short baseline" problem, which prohibits the detection of wide features. The ATCA image, reproduced in the left panel of Figure 1, is dominated by HI filaments with interspersed holes across the entire galaxy. Hardly any correspondence between the HI features and distributions of stars or other components of the ISM are obvious, besides the HI cavities in the supergiant shells (SGSs) LMC4 and LMC5 (Kim et al. 1997a).

 
Figure 1: HI (left) and H (right) images of the LMC.

It is conceivable that the HI map should show a complex filamentary structure because it represents the cumulative effects of dynamic interactions of massive stars with the ISM in the past. If the HI maps are displayed with velocity resolution (in channel maps), correspondence between HI holes and ionized gas shells may become clear (Kim et al. 1997b).

The structure of the 10 K ionized interstellar gas is well depicted by H images, which can be obtained with high resolution () easily. The H image of the LMC displayed in the right panel of Figure 1 shows prominent HII regions, SGSs with sizes up to 1400 pc, and diffuse gas that contributes up to 30-40% of the total H emission of the LMC (Kennicutt et al. 1995). The 10 K ionized gas is closely associated with massive stars. Massive stars are responsible for not only the ionization of the medium, but also the dynamic structuring of the medium via fast stellar winds and supernova ejecta. Massive stars have short lifetimes, hence the structure of the 10 K ionized ISM is freshly minted within the last 10 yrs. It is thus not surprising that the H and the HI images of the LMC (Figure 1) look so different.

Examined closely, the 10 K ionized gas often show shell structures in supernova remnants (SNRs) and shell HII regions around OB associations (superbubbles). The top panel of Figure 2 shows an H image of a portion of the LMC centered near the 30 Dor complex; the SGSs LMC2 and LMC3, the superbubbles in the HII regions N158 and N160, and the large shells of SNRs DEM299 and DEM316 are clearly visible.

The 10 K ionized gas is produced dynamically by fast stellar winds from massive stars or supernova ejecta. It is thus expected that some correspondence between the 10 K ionized gas and the 10 K ionized gas exists. Indeed, Figure 2 shows that the 10 K ionized gas, as traced by soft X-ray emission, is detected in the interiors of SGSs, superbubbles, and SNRs, which have been identified by their HII shells. The X-ray emission from the SGS LMC4 has been analyzed by Bomans et al. (1994); X-ray emission mechanisms for superbubbles have been summarized by Chu & Mac Low (1996); a multi-wavelength study of the SNR DEM316 has been reported by Williams et al. (1997).

The relationship between the 10 K ionized gas and the 10 K ionized gas and the production of the 10 K gas by shocks are demonstrated in Figure 3, the central 9 of 30 Doradus. It is evident that the bright diffuse X-ray emission is closely correlated with the presence of high-velocity, shocked material detected in the H echellograms.

On large scales, the distribution of the 10 K gas show features whose origin is not very clear. For example, the LMC bar contains only a few scattered HII regions, but it displays bright diffuse X-ray emission. Another example is the bright X-ray spur extending to the south of the supergiant shell LMC2; no optical counterpart is known or detected. The on-going ROSAT HRI survey of the LMC will provide high-resolution X-ray images and may help clarify the origin of this hot gas.

Finally, a 10 K gas halo of the LMC has been unambiguously detected by Wakker et al. (1997) using HST GHRS observations of five probe stars. They find the hot gas halo to be patchy and expanding outward relative to the warm ionized gas (seen in H) in the disk. Observations along more sightlines are needed to confirm these results and to determine the relationship between the 10 K halo gas and the 10 K hot gas readily visible in soft X-ray images.

 
Figure 2: H (top) and X-ray (bottom) images of a portion of the LMC centered near the 30 Dor complex. Bright HII regions, supernova remnants (SNRs), and supergiant shells (SGSs) are labeled. The HII region and SNR names are from Henize (1956, ``N"), Davies, Elliott, & Meaburn (1976, ``DEM"), and Mathewson et al. (1983).

 
Figure 3: H (top left) and X-ray (top right) images of the central of 30 Doradus. The X-ray image is extracted from a ROSAT PSPC observation in the 0.1-2.4 keV band. The bottom panel contains long-slit H echellograms of the same region, reproduced from Chu & Kennicutt (1994). The wavelength increases upwards. The orientation is the same as the images above. The separation between adjacent slits corresponds to 45'' in the sky or 600 km in velocity.

© Copyright Astronomical Society of Australia 1997