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 ~10 mm 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 mm imaging data from Spitzer Galactic Legacy Mid-Plane Survey Extraordinaire 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 correcting for foreground emission and then interpolating from the surrounding regions. The correction for foreground emission can be quite large, up to ~50% for clouds at ~5 kpc distance, thus restricting the utility of this technique to relatively nearby clouds. We investigate three methods for the interpolation, finding systematic differences at about the 10% level, which, for fiducial dust models, corresponds to a mass surface density = 0.013 g cm-2, above which we conclude that this extinction mapping technique attains validity. We examine the probability distribution function of in IRDCs. From a qualitative comparison with numerical simulations of astrophysical turbulence, many clouds appear to have relatively narrow distributions suggesting relatively low (less than five) Mach numbers and/or dynamically strong magnetic fields. Given cloud kinematic distances, we derive cloud masses. Rathborne, Jackson, and Simon identified cores within the clouds and measured their masses via millimeter dust emission. 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.