The Small Magellanic Cloud (SMC) provides a unique environment for studying interstellar matter. The abundance of C, N, O, and heavier elements has been found to be an order of magnitude lower in the SMC than in the disk of the Milky Way (Dufour et al. 1982), so heating, cooling, and chemical processes dependent upon metal abundances can be expected to be different from our local interstellar medium (ISM). In particular, the properties of dust in the SMC could be quite different from dust in the Milky Way, because dust is composed almost entirely (by mass) of elements that are substantially less abundant in the SMC. The visible extinction per unit H column density is about an order of magnitude lower in the SMC than in the Milky Way, suggesting that the total dust abundance may scale with the total metal abundance (Bouchet et al. 1985). The spectrum of SMC extinction (or the SMC `extinction curve') is significantly different from that of our Galaxy: the SMC has relatively stronger ultraviolet extinction, and it lacks the 2175 Å bump that is so prominent in the Milky Way extinction curve (Savage & Mathis 1979). Infrared emission from the SMC was detected by the Infrared Astronomical Satellite (IRAS ), with bright and extended 60 and 100 µm emission and weaker emission at 12 and 25 µm (Schwering & Israel 1991, Sauvage et al. 1990, Okumura 1993). The 12 µm IRAS image shows very little diffuse emission, being dominated by a few compact regions. The weak 12 µm emission from the SMC suggested low-metallicity galaxies may be lacking the aromatic hydrocarbons that dominate the 12µm emission from the Milky Way. In this paper, we describe one observation that is part of a larger Infrared Space Observatory (ISO ; Kessler et al. 1996) Guaranteed Time program designed to characterize the mid-infrared emission from interstellar dust in a wide variety of environments. The cloud SMC B1#1 was selected as an extragalactic target to characterize a quiescent, low-metallicity environment. The cloud was discovered serendipitously during observations as part of the Swedish-ESO Submillimeter Telescope Key Program to map CO-line emission from some star forming regions in the SMC (Rubio et al. 1993; Lequeux et al. 1994). The cloud is far from any trace of O stars in existing H and radio continuum maps (see Fig. 5 of Rubio et al. 1993), and its molecular-line emission is narrow, indicating that the gas is dynamically quiescent. Therefore the dust emission from the cloud is likely excited by a combination of the average SMC radiation field and, possibly, some associated stars. As such, the cloud represents a relatively simple laboratory for studying the properties of SMC dust using its infrared spectrum, away from sites of high-mass star formation.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000