New Insights into the Physical Conditions and Internal Structure of a Candidate Proto-globular Cluster

We present ∼0.″1 resolution (∼10 pc) Atacama Large Millimeter/submillimeter Array observations of a molecular cloud identified in the merging Antennae galaxies with the potential to form a globular cluster, nicknamed the “Firecracker.” Since star formation has not yet begun at an appreciable level in this region, this cloud provides an example of what the birth environment of a globular cluster may have looked like before stars formed and disrupted the natal physical conditions. Using emission from 12CO(2–1), 12CO(3–2), 13CO(2–1), HCN(4–3), and HCO+(4–3) molecular lines, we are able to resolve the cloud’s structure and find that it has a characteristic radius of 22 pc and a mass of (1–9) × 106 M⊙. We also put constraints on the abundance ratios 12CO/13CO and H2/12CO. Based on the calculation of the mass, we determine that the commonly used CO-to-H2 conversion factor in this region varies spatially, with average values in the range cm−2 (K km s−1)−1. We demonstrate that if the cloud is bound (as is circumstantially suggested by its bright, compact morphology), an external pressure in excess of P/k > 108 K cm−3 is required. This would be consistent with theoretical expectations that globular cluster formation requires high-pressure environments, much higher than typical values found in the Milky Way. The position–velocity diagram of the cloud and its surrounding material suggests that this high pressure may be produced by ram pressure from the collision of filaments. The radial profile of the column density can be fit with both a Gaussian and a Bonnor–Ebert profile. If the Bonnor–Ebert fit is taken to be indicative of the cloud’s physical structure, it would imply the cloud is gravitationally stable and pressure-confined. The relative line strengths of HCN and HCO+ in this region also suggest that these molecular lines can be used as a tracer for the evolutionary stage of a cluster.