Detection of Structure in Infrared-Dark Clouds with Spitzer: Characterizing Star Formation in the Molecular Ring

TL;DR

This study uses Spitzer observations to analyze infrared-dark clouds, revealing their mass distribution, structure, and potential as precursors to stellar clusters, with a focus on the universality of the mass function in star formation regions.

Contribution

The paper introduces a novel method for tracing mass in IRDCs using dust absorption at 8 microns and provides the broadest dynamic range of clump masses studied to date.

Findings

IRDCs contain tens of clumps with sizes 0.02-0.3 pc and masses 0.5-30 Msun.

Clump mass spectrum has a slope of 1.76, consistent with other star formation regions.

IRDCs are likely precursors to stellar clusters, with a universal mass function shape.

Abstract

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 microns. 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 Msun, 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 1.76 +/- 0.05 for masses from 30 to 3000 Msun. 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.