AN H-I APERTURE SYNTHESIS MOSAIC OF THE SMALL-MAGELLANIC-CLOUD

We present the results of a survey of neutral hydrogen emission in the Small Magellanic Cloud (SMC) with the Australia Telescope Compact Arr ay (ATCA). The survey consists of a mosaic of 320 separate pointings o f the 375-m array, resulting in a resolution of 1.6 arcmin (28 pc, for a distance of 60 kpc) over a held of 20 deg(2). The rms brightness te mperature sensitivity is 1.4 K, corresponding to an H I column density sensitivity of 4 x 10(18) cm(-2) for each velocity channel of width 1 .6 km s(-1). The H I distribution is complex and, on scales less than or similar to 1 kpc, appears to be dominated by the effects of expandi ng H I shells, which are probably driven by the combined effects of su pernovae and stellar winds from massive stars. The picture of the SMC that arises from the current data seems to challenge the earlier belie f that the SMC consists of two or more spatially separate structures w ith different systemic velocities. We find that the observed multiple components are, in many cases, caused by the combined effects of the n umerous shells and supershells. Altogether, we identify six supershell s (defined here as those with radii greater than 300 pc) and 495 giant shells. For each of these, we measure positions, radii, velocities an d expansion rates, and derive ages and kinetic energy requirements; Th e apparent age distribution of shells is remarkably narrow, with a mea n age of 5.4 Myr and an intrinsic dispersion of 2 Myr. Southern shells appear to be older, on average, by 2.5 Myr. The kinetic energy of the shells is a large fraction of the gravitational binding energy of the SMC, implying that further disintegration of the SMC will occur with time, and especially at the next close passage with the Large Magellan ic Cloud (LMC) or the Galaxy, unless the SMC possesses a massive halo. Because of their interferometric nature, the images presented here ar e insensitive to structures of size greater than or similar to 0 degre es.6, and should not be used for deriving total H I column densities.