The darkest shadows: Deep mid-infrared extinction mapping of a massive protocluster

Abstract

We use deep 8 μm Spitzer-IRAC imaging of massive Infrared Dark Cloud (IRDC) G028.37+00.07 to construct a mid-infrared (MIR) extinction map that probes mass surface densities up to Σ1 g cm-2 (AV200 mag), amongst the highest values yet probed by extinction mapping. Merging with an NIR extinction map of the region creates a high dynamic range map that reveals structures down to AV1 mag. We utilize the map to: (1) measure a cloud mass 7 × 104 M within a radius of 8 pc. 13CO kinematics indicate that the cloud is gravitationally bound. It thus has the potential to form one of the most massive young star clusters known in the Galaxy. (2) Characterize the structures of 16 massive cores within the IRDC, finding they can be fit by singular polytropic spheres with and k ρ = 1.3 ± 0.3. They have - relatively low values that, along with their measured cold temperatures, suggest that magnetic fields, rather than accretion-powered radiative heating, are important for controlling fragmentation of these cores. (3) Determine the Σ (equivalently column density or AV ) probability distribution function (PDF) for a region that is nearly complete for AV> 3 mag. The PDF is well fit by a single log-normal with mean mag, high compared to other known clouds. It does not exhibit a separate high-end power law tail, which has been claimed to indicate the importance of self-gravity. However, we suggest that the PDF does result from a self-similar, self-gravitating hierarchy of structures present over a wide range of scales in the cloud. © 2014. The American Astronomical Society. All rights reserved..