Core Emergence in a Massive Infrared Dark Cloud: A Comparison Between Mid-IR Extinction and 1.3 mm Emission

Abstract

Stars are born from dense cores in molecular clouds. Observationally, it is crucial to capture the formation of cores in order to understand the necessary conditions and rate of the star formation process. The {\it Atacama Large Mm/sub-mm Array} (ALMA) is extremely powerful for identifying dense gas structures, including cores, at mm wavelengths via their dust continuum emission. Here we use ALMA to carry out a survey of dense gas and cores in the central region of the massive ($\sim 10^5{M}_{\odot }$) Infrared Dark Cloud (IRDC) G28.37+0.07. The observation consists of a mosaic of 86 pointings of the 12m-array and produces an unprecedented view of the densest structures of this IRDC. In this first paper about this data set, we focus on a comparison between the 1.3 mm continuum emission and a mid-infrared (MIR) extinction map of the IRDC. This allows estimation of the "dense gas" detection probability function (DPF), i.e., as a function of the local mass surface density, $\Sigma$, for various choices of thresholds of mm continuum emission to define "dense gas". We then estimate the dense gas mass fraction, $f_{\mathrm{dg}}$, in the central region of the IRDC and, via extrapolation with the DPF and the known $\Sigma$ probability distribution function, to the larger-scale surrounding regions, finding values of about 5\% to 15\% for the fiducial choice of threshold. We argue that this observed dense gas is a good tracer of the protostellar core population and, in this context, estimate a star formation efficiency per free-fall time in the central IRDC region of ${\epsilon }_{\mathrm{ff}}\sim$10\%, with approximately a factor of two systematic uncertainties.

arXiv comment11 pages, 4 figures, 1 table, accepted by ApJL, comments welcome

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• The ALMA Survey of 70 $\mu$m dark High-mass clumps in Early Stages (ASHES). I. Pilot Survey: Clump Fragmentation
• Widespread Molecular Outflows in the Infrared Dark Cloud G28.37+0.07: Indications of Orthogonal Outflow-Filament Alignment
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• Filament Formation via Collision-induced Magnetic Reconnection -- Formation of a Star Cluster