ALMA survey of massive cluster progenitors from ATLASGAL: Limited fragmentation at the early evolutionary stage of massive clumps

The early evolution of massive cluster progenitors is poorly understood. We investigate the fragmentation properties from 0.3 pc to 0.06 pc scalesof a homogenous sample of infrared-quiet massive clumps within 4.5 kpc selected from the ATLASGAL survey. Using the ALMA 7 m array wedetect compact dust continuum emission towards all targets and find that fragmentation, at these scales, is limited. The mass distribution of thefragments uncovers a large fraction of cores above 40M, corresponding to massive dense cores (MDCs) with masses up to∼400M. Seventy-seven percent of the clumps contain at most 3 MDCs per clump, and we also reveal single clumps/MDCs. The most massive cores are formedwithin the more massive clumps and a high concentration of mass on small scales reveals a high core formation efficiency. The mass of MDCshighly exceeds the local thermal Jeans mass, and we lack the observational evidence of a sufficiently high level of turbulence or strong enoughmagnetic fields to keep the most massive MDCs in equilibrium. If already collapsing, the observed fragmentation properties with a high coreformation efficiency are consistent with the collapse setting in at parsec scales