The Darkest Shadows: Deep Mid-Infrared Extinction Mapping of a Massive
  Protocluster
  We use deep $8\:\mu m$ Spitzer-IRAC imaging of a massive Infrared Dark Cloud
(IRDC) G028.37+00.07 to construct a Mid-Infrared (MIR) extinction map that
probes mass surface densities up to $\Sigma\:\sim 1\:\rm{g~cm^{-2}}$
($A_V\sim200\:$mag), amongst the highest values yet probed by extinction
mapping. Merging with a NIR extinction map of the region, creates a high
dynamic range map that reveals structures down to $A_V\sim1\:$mag. We utilize
the map to: (1) Measure a cloud mass $\sim7\times10^4\:M_\odot$ within a radius
of $\sim8\:$pc. $^{13}$CO 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 $\rho\propto{r}^{-k_\rho}$ and $k_\rho=1.3\pm0.3$. They have
$\overline{\Sigma}\simeq0.1-0.4\:\rm{g~cm^{-2}}$ --- relatively low values
that, along with their measured cold temperatures, suggest magnetic fields,
rather than accretion-powered radiative heating, are important for controlling
fragmentation of these cores. (3) Determine the $\Sigma$ (equivalently column
density or $A_V$) probability distribution function (PDF) for a region that is
near complete for $A_V>3\:$mag. The PDF is well fit by a single log-normal with
mean $\overline{A}_V\simeq9\:$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 structure being
present over a wide range of scales in the cloud.
