Core emergence in a massive infrared dark cloud: a comparison between mid-IR extinction and 1.3 mm emission

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 Atacama Large Millimeter/submillimeter Array (ALMA) is extremely powerful for identifying dense gas structures, including cores, at millimeter 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 (similar to 10(5) M-circle dot) infrared dark cloud (IRDC) G28.37+ 0.07. The observation consists of a mosaic of 86 pointings of the 12 m array and produces an unprecedented view of the densest structures of this IRDC. In this first Letter 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 millimeter continuum emission to define "dense gas." We then estimate the dense gas mass fraction, f(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(ff) similar to 10%, with approximately a factor of two systematic uncertainties.