We present \sim 0.1 '' resolution ( \sim 10 pc ) ALMA 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 form and disrupt the natal physical conditions .
Using emission from ^ { 12 } CO ( 2-1 ) , ^ { 12 } CO ( 3-2 ) , ^ { 13 } CO ( 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 \times 10 ^ { 6 } M _ { \odot } .
We also put constraints on the abundance ratios of ^ { 12 } CO/ ^ { 13 } CO and H _ { 2 } / ^ { 12 } CO. Based on the calculation of the mass , we determine that the commonly used CO-to-H _ { 2 } conversion factor in this region varies spatially , with average values in the range X _ { CO } = ( 0.12 - 1.1 ) \times 10 ^ { 20 } 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 > 10 ^ { 8 } 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 .