2012ApJ...746..117L

Young massive clusters (YMCs) with stellar masses of 10⁴-10⁵ M_☉ and core stellar densities of 10⁴-10⁵ stars per cubic pc are thought to be the "missing link" between open clusters and extreme extragalactic super star clusters and globular clusters. As such, studying the initial conditions of YMCs offers an opportunity to test cluster formation models across the full cluster mass range. G0.253 + 0.016 is an excellent candidate YMC progenitor. We make use of existing multi-wavelength data including recently available far-IR continuum (Herschel/Herschel Infrared Galactic Plane Survey) and mm spectral line (H₂ O Southern Galactic Plane Survey and Millimetre Astronomy Legacy Team 90 GHz Survey) data and present new, deep, multiple-filter, near-IR (Very Large Telescope/NACO) observations to study G0.253 + 0.016. These data show that G0.253 + 0.016 is a high-mass (1.3x10⁵ M_☉), low-temperature (T_dust∼ 20 K), high-volume, and column density (n ∼ 8 x10⁴/cm3; N_H2∼10²³/cm2) molecular clump which is close to virial equilibrium (M_dust∼ M_virial) so is likely to be gravitationally bound. It is almost devoid of star formation and, thus, has exactly the properties expected for the initial conditions of a clump that may form an Arches-like massive cluster. We compare the properties of G0.253 + 0.016 to typical Galactic cluster-forming molecular clumps and find it is extreme, and possibly unique in the Galaxy. This uniqueness makes detailed studies of G0.253 + 0.016 extremely important for testing massive cluster formation models.