The large-scale H I structure of the Small Magellanic Cloud

We combine new Parkes telescope observations of neutral hydrogen (H I) in t he Small Magellanic Cloud (SMC) with an Australia Telescope Compact Array ( ATCA) aperture synthesis mosaic to obtain a set of images sensitive to all angular (spatial) scales between 98 arcsec (30 pc) and 4 degrees (4 kpc), T he new data are used to study the H I spatial power spectrum over a range o f contiguous scale sizes wider than those previously achieved in any other galaxy, including our own, The spatial power spectrum closely obeys the rel ation P(k) proportional to k(gamma), with gamma = -3.04 +/- 0.02, similar t o values obtained by other authors for our own Galaxy which are in the rang e gamma = -3.0 to -2.8. This is surprising given the very different morphol ogy, gas-richness, star-formation rate and evolution of the two systems, an d may imply similar mechanisms for structure formation, One interpretation of the P(k) power-law is that the interstellar medium (ISM) of the SMC is f ractal in nature, consisting of a hierarchy of H I cloud structures created , for example, by homogeneous turbulence. The projected fractal dimension o f D-p = 1.5 is similar to values obtained by other authors for molecular cl ouds in the Galaxy in the size range similar to 0.05 to 100 pc. Such a mode l is consistent with a low space-filling factor for the neutral gas. A kinematic study of the H I data reveals the existence of three supergiant shells which were previously undetectable in the ATCA data alone, These sh ells have diameters up to 1.8 kpc and require energies (in the standard sup ernova-driven models) up to 2 x 10(54) erg. The structure and evolution of the ISM in the SMC are heavily influenced by the formation of these supergi ant shells.