The Distribution of Mass Surface Densities in a High-Mass Protocluster

TL;DR

This study analyzes the probability distribution of mass surface densities in a massive infrared dark cloud to understand the physical processes influencing cloud structure and star formation, revealing a mostly log-normal distribution with hints of a high-density tail.

Contribution

It provides the first detailed analysis of the mass surface density PDF in a high-mass protocluster using combined extinction and dust emission data, highlighting the distribution shape and potential high-density tail.

Findings

PDF is predominantly log-normal with a peak at 0.021 g/cm².

Tentative evidence for a high-density power-law tail containing 3-8% of the mass.

The distribution shape offers insights into turbulence, gravity, and magnetic fields in the cloud.

Abstract

We study the probability distribution function (PDF) of mass surface densities, $\Sigma$, of infrared dark cloud (IRDC) G028.37+00.07 and its surrounding giant molecular cloud. This PDF constrains the physical processes, such as turbulence, magnetic fields and self-gravity, that are expected to be controlling cloud structure and star formation activity. The chosen IRDC is of particular interest since it has almost 100,000 solar masses within a radius of 8 parsecs, making it one of the most massive, dense molecular structures known and is thus a potential site for the formation of a "super star cluster." We study $\Sigma$ in two ways. First, we use a combination of NIR and MIR extinction maps that are able to probe the bulk of the cloud structure up to $\Sigma\sim1\:{\rm{g\:cm}^{-2}}\:$($A_V\simeq200$ mag). Second, we study the FIR and sub-mm dust continuum emission from the cloud utilizing Herschel PACS and SPIRE images and paying careful attention to the effects of foreground and background contamination. We find that the PDFs from both methods, applied over a $\sim20^\prime$(30 pc)-sized region that contains $\simeq1.5\times10^5\:M_\odot$ and encloses a minimum closed contour with $\Sigma\simeq0.013\:{\rm{g\:cm}^{-2}}\:$($A_V\simeq3$ mag), shows a log-normal shape with the peak measured at $\Sigma\simeq0.021\:{\rm{g\:cm}^{-2}}\:$($A_V\simeq4.7$ mag). There is tentative evidence for the presence of a high-$\Sigma$ power law tail that contains from $\sim3\%$ to 8\% of the mass of the cloud material. We discuss the implications of these results for the physical processes occurring in this cloud.