Institution(s): 1. MPIfR, Bonn, 2. Univ. Stuttgart
SOFIA high-resolution spectroscopic far-IR observations with the GREAT heterodyne instrument at 1.81 THz (NH3 redshifted absorption) have demonstrated high infall rates (molecular gas accretion rates of the order of 10-3 Msun/yr) towards several strong continuum sources selected from the ATLASGAL submm Galactic Plane Survey. We will describe these new and unique observations and discuss the vast potential of SOFIA/GREAT absorption spectroscopy for dynamical studies of proto-cluster cloud collapse, tracing the velocity field of infalling molecular gas during the earliest phases of (massive) star cluster formation.