First Detection of Submillimeter [C I] Emission in the Small Magellanic Cloud

Abstract

We report the first detection of [C I] (³P₁-->³P₀) emission at 609 μm in a region of the Small Magellanic Cloud (N27). Environments poor in heavy elements and dust such as the SMC are thought to be dominated by photodissociation regions (PDRs). This is the lowest metallicity source where submillimeter neutral carbon emission has been detected. Studying the I_[CI]/I_CO ratio in several sources spanning more than an order of magnitude in metallicity Z, we find that the I_[CI]/I_CO ratio increases for decreasing Z. The existence of such a trend points to a photodissociation origin for most of the neutral carbon in molecular clouds, in agreement with standard PDR models. We also report Infrared Space Observatory far-infrared (FIR) spectroscopic observations of N27 and use them to derive its physical properties. Comparison between the density and radiation field revealed by FIR diagnostics (n~300-1000 cm^-3, χ_uv~30χ₀-100χ₀) and those derived from millimeter and submillimeter data (n~10⁵ cm^-3, χ_uv<~30χ₀) suggests that the FIR lines originate in more diffuse gas and are perhaps dominated by the interclump medium. Regardless of the cause, analysis of the FIR and millimeter-submillimeter data produces a discrepancy of 2 orders of magnitude for the density of this source.