Cool dust and gas in the Small Magellanic Cloud

Abstract

We present high-angular-resolution, far-infrared images of the Small Magellanic Cloud (SMC). The images were reconstructed from data obtained with the Infrared Astronomical Satellite (IRAS), using the Pyramid Maximum Entropy algorithm. The angular resolutions of the images are 1 arcmin at 12, 25 and 60 mu m, and 1.7 arcmin at 100 mu m, which provides an excellent complement to the recent neutral hydrogen (H I) survey from the Australia Telescope Compact Array, which has a resolution of 1.6 arcmin. We use the 60- and 100-mu m data, together with a standard dust model, to derive a temperature map and a dust column density distribution. Dust temperatures range from 23 to 45 K, which is, on average, warmer than for the Galaxy, although there is a deficit of warm, very small dust grains in the SMC. The total amount of cool dust in the SMC is 1.8(-0.2)(+1.3) x 10(4) M.. There is a significant variation of the observed dust-to-gas ratio across the SMC. This may be caused by corresponding variations in metallicity, but this explanation is not supported by the limited available data in the literature. Alternatively, there may be a large amount of molecular hydrogen (H(2)) in the SMC, similar to 8 x 10(8) M., and/or a large amount (similar to 10(6) M.) of cold dust (less than or equal to 15 K) not seen by IRAS or COBE/DIRBE. The average dust-to-atomic gas ratio is, by mass, 8.2 x 10(-5), which is a factor of similar to 30 below the Galactic value. The spatial power spectrum of the dust column density image can be approximated by P(d)(k) proportional to k(-3.1), which shows that, as with the H I power spectrum, there is no preferred scale size for dust clouds. A spatial coherence analysis shows that, over a wide range of spatial scales (greater than or similar to 50 pc), H I and dust are well correlated (> 60 per cent).