ALMA-IRDC: Dense gas mass distribution from cloud to core scales

Infrared dark clouds (IRDCs) are potential hosts of the elusive early phases of high-mass star formation (HMSF). Here we conduct an in-depth analysis of the fragmentation properties of a sample of 10 IRDCs, which have been highlighted as some of the best candidates to study HMSF within the Milky Way. To do so, we have obtained a set of large mosaics covering these IRDCs with ALMA at band 3 (or 3 mm). These observations have a high angular resolution (∼ 3″; ∼ 0.05 pc), and high continuum and spectral line sensitivity (∼ 0.15 mJy beam−1 and ∼ 0.2 K per 0.1 km s−1 channel at the N2H+ (1 − 0) transition). From the dust continuum emission, we identify 96 cores ranging from low- to high-mass (M = 3.4 − 50.9M⊙) that are gravitationally bound (αvir = 0.3 − 1.3) and which would require magnetic field strengths of B = 0.3 − 1.0 mG to be in virial equilibrium. We combine these results with a homogenised catalogue of literature cores to recover the hierarchical structure within these clouds over four orders of magnitude in spatial scale (0.01 pc – 10 pc). Using supplementary observations at an even higher angular resolution, we find that the smallest fragments (< 0.02 pc) within this hierarchy do not currently have the mass and/or the density required to form high-mass stars. Nonetheless, the new ALMA observations presented in this paper have facilitated the identification of 19 (6 quiescent and 13 star-forming) cores that retain >16 M⊙ without further fragmentation. These high-mass cores contain trans-sonic non-thermal motions, are kinematically sub-virial, and require moderate magnetic field strengths for support against collapse. The identification of these potential sites of high-mass star formation represents a key step in allowing us to test the predictions from high-mass star and cluster formation theories.