Mid-infrared Extinction Mapping of Infrared Dark Clouds: Probing the Initial Conditions for Massive Stars and Star Clusters

Infrared Dark Clouds (IRDCs) are cold, dense regions of giant molecular clouds that are opaque at wavelengths ∼ 10μm or more and thus appear dark against the diffuse Galactic background emission. They are thought to be the progenitors of massive stars and star clusters. We use 8 μm imaging data from Spitzer GLIMPSE to make extinction maps of 10 IRDCs, selected to be relatively nearby and massive. The extinction mapping technique requires construction of a model of the Galactic IR background intensity behind the cloud, which is achieved by interpolation from the surrounding regions. We investigate three different methods for this interpolation, finding that systematic differences are at about the 10% level. For our adopted dust opacities and dust-to-gas ratio, such an uncertainty corresponds to a mass surface density of Σ = 0.013 g cm, above which we conclude this extinction mapping technique attains validity. We examine the probability distribution function of mass surface densities in IRDCs. From a qualitative comparison with numerical simulations of astrophysical turbulence, many clouds appear to have relatively narrow distributions suggesting relatively low (< 5) Mach numbers and/or dynamically strong magnetic fields. Given cloud kinematic distances, we derive cloud masses. Within the clouds, cores have been identified and had their masses measured via mm dust emission by Rathborne, Jackson & Simon. For 43 cores, we compare these mass estimates with those derived from our extinction mapping, finding good agreement: typically factors of . 2 difference for individual cores and an average systematic offset of . 10% for the adopted fiducial assumptions of each method. We find tentative evidence for a systematic variation of these mass ratios as a function of core density, which is consistent with models of ice mantle formation on dust grains and subsequent grain growth by coagulation, and/or with a temperature decrease in the densest cores. Subject headings: ISM: clouds, dust, extinction — stars: formation