We have conducted a survey of a sample of infrared-dark clouds ( IRDCs ) with the Spitzer Space Telescope in order to explore their mass distribution .
We present a method for tracing mass using dust absorption against the bright Galactic background at 8 \mu m. The IRDCs in this sample are comprised of tens of clumps , ranging in sizes from 0.02 to 0.3 pc in diameter and masses from 0.5 to a few 10 ^ { 3 } M _ { \sun } , the broadest dynamic range in any clump mass spectrum study to date .
Structure with this range in scales confirms that IRDCs are the the precursors to stellar clusters in an early phase of fragmentation .
Young stars are distributed in the vicinity of the IRDCs , but the clumps are typically not associated with stars and appear pre-stellar in nature .
We find an IRDC clump mass spectrum with a slope of \alpha =1.76 \pm 0.05 for masses from 30 M _ { \sun } to 3000 M _ { \sun } .
This slope is consistent with numerous studies , culled from a variety of observational techniques , of massive star formation regions and is close to the mass function of Galactic stellar clusters and star clusters in other galaxies .
We assert that the shape of the mass function is an intrinsic and universal feature of massive star formation regions , that are the birth sites of stellar clusters .
As these clouds evolve and their constituent clumps fragment , the mass spectrum will steepen and eventually assume the form of the core mass function that is observed locally .