Infrared Dark Clouds in the Small Magellanic Cloud?

Abstract

We have applied the unsharp-masking technique to the 24 $\mu$m image of the Small Magellanic Cloud (SMC), obtained with the Spitzer Space Telescope, to search for high-extinction regions. This technique has been used to locate very dense and cold interstellar clouds in the Galaxy, particularly infrared dark clouds (IRDCs). Fifty five candidate regions of high-extinction, namely high-contrast regions (HCRs), have been identified from the generated decremental contrast image of the SMC. Most HCRs are located in the southern bar region and mainly distributed in the outskirts of CO clouds, but most likely contain a significant amount of H2. HCRs have a peak-contrast at 24 $\mu$m of 2 - 2.5 % and a size of 8 - 14 pc. This corresponds to the size of typical and large Galactic IRDCs, but Galactic IRDCs are 2 - 3 times darker at 24 $\mu$m than our HCRs. To constrain the physical properties of the HCRs, we have performed NH3, N2H+, HNC, HCO+, and HCN observations toward one of the HCRs, HCR LIRS36-EAST, using the Australia Telescope Compact Array and the Mopra single-dish radio telescope. We did not detect any molecular line emission, however, our upper limits to the column densities of molecular species suggest that HCRs are most likely moderately dense with n ~ 10^{3} cm-3. This volume density is in agreement with predictions for the cool atomic phase in low metallicity environments. We suggest that HCRs may be tracing clouds at the transition from atomic to molecule-dominated medium, and could be a powerful way to study early stages of gas condensation in low metallicity galaxies. Alternatively, if made up of dense molecular clumps < 0.5 pc in size, HCRs could be counterparts of Galactic IRDCs, and/or regions with highly unusual abundance of very small dust grains.

accepted for publication in the Astronomical Journal

References in corpus (6)

• The Spitzer Survey of the Small Magellanic Cloud: FIR Emission and Cold Gas in the SMC
• The Structure of a Low-Metallicity Giant Molecular Cloud Complex
• NH3 Observations of the Infrared Dark Cloud G28.34+0.06
• Abundances and Isotope Ratios in the Magellanic Clouds: The Star Forming Environment of N113
• Massive Young Stellar Objects in the Large Magellanic Cloud: water masers and ESO-VLT 3-4 micron spectroscopy
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• A Spitzer Space Telescope far-infrared spectral atlas of compact sources in the Magellanic Clouds. II. The Small Magellanic Cloud
• Spitzer and HHT observations of starless cores: masses and environments
• Organic species in Infrared Dark Clouds
• Sulfur-Bearing Molecules In Massive Star-Forming Regions: Observations Of OCS, CS, H2S and SO
• Measurement of the core-collapse progenitor mass distribution of the Small Magellanic Cloud
• Detection of a Nearby Halo Debris Stream in the WISE and 2MASS Surveys
• An interesting candidate for isolated massive star formation in the Small Magellanic Cloud