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 H _ { 2 } .
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 NH _ { 3 } , N _ { 2 } H ^ { + } , 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 \sim 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 .