G0.253+0.016: A centrally condensed, high-mass protocluster
  Despite their importance as stellar nurseries and the building blocks of
galaxies, very little is known about the formation of the highest mass
clusters. The dense clump G0.253+0.016 represents an example of a clump that
may form an Arches-like, high-mass cluster. Here we present molecular line maps
toward G0.253+0.016 taken as part of the MALT90 molecular line survey,
complemented with APEX observations. Combined, these data reveal the global
physical properties and kinematics of G0.253+0.016. Recent Herschel data show
that while the dust temperature is low (~19 K) toward its centre, the dust
temperature on the exterior is higher (~27 K) due to external heating. Our new
molecular line data reveal that, overall, the morphology of dense gas detected
toward G0.253+0.016 matches very well its IR extinction and dust continuum
emission. An anti-correlation between the dust and gas column densities toward
its centre indicates that the clump is centrally condensed with a cold, dense
interior in which the molecular gas is chemically depleted. The velocity field
shows a strong gradient along the clump's major axis, with the blue-shifted
side at higher Galactic longitude. The optically thick gas tracers are
systematically red-shifted with respect to the optically thin and hot gas
tracers, indicating radial motions. The gas kinematics and line ratios support
the recently proposed scenario in which G0.253+0.016 results from a tidal
compression during a recent pericentre passage near SgrA*. Because G0.253+0.016
represents an excellent example of a clump that may form a high-mass cluster,
its detailed study should reveal a wealth of knowledge about the early stages
of cluster formation.
