Infrared Dark Clouds ( IRDCs ) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background .
Millimeter continuum maps toward 38 IRDCs reveal extended cold dust emission to be associated with each of the IRDCs .
IRDCs range in morphology from filamentary to compact and have masses of 120 to 16,000 M _ { \odot } , with a median mass of \sim 940 M _ { \odot } .
Each IRDC contains at least one compact ( \leq 0.5 pc ) dust core and most show multiple cores .
We find 140 cold millimeter cores unassociated with MSX 8 \mu m emission .
The core masses range from 10 to 2,100 M _ { \odot } , with a median mass of \sim 120 M _ { \odot } .
The slope of the IRDC core mass spectrum ( \alpha \sim 2.1 \pm 0.4 ) is similar to that of the stellar IMF .
Assuming that each core will form a single star , the majority of the cores will form OB stars .
IRDC cores have similar sizes , masses , and densities as hot cores associated with individual , young high-mass stars , but they are much colder .
We therefore suggest that IRDC represent an earlier evolutionary phase in high-mass star formation .
In addition , because IRDCs contain many compact cores , and have the same sizes and masses as molecular clumps associated with young clusters , we suggest that IRDCs are the cold precursors to star clusters .
Indeed , an estimate of the star formation rate within molecular clumps with similar properties to IRDCs ( \sim 2 M _ { \odot } yr ^ { -1 } ) is comparable to the global star formation rate in the Galaxy , supporting the idea that all stars may form in such clumps .