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

Abstract

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, 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 conditions. Using emission from ${}^{12}$CO(2-1), ${}^{12}$CO(3-2), ${}^{13}$CO(2-1), HCN(4-3), and HCO${}^{+}$(4-3), 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 constrain the abundance ratios of ${}^{12}$CO/${}^{13}$CO and H${}_2$/\twelveCO. Based on the calculated mass, we determine that the commonly used CO-to-H${}_2$ conversion factor 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 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. The position-velocity diagram of the cloud and its surrounding material suggests that this pressure may be produced by the collision of filaments. The radial profile of the column density can be fit with both a Gaussian and Bonnor-Ebert profile. The relative line strengths of HCN and HCO${}^{+}$ in this region suggest that these molecular lines can be used as tracers for the evolutionary stage of a cluster.

18 pages, 14 figures, Accepted to ApJ 2019 March 4

References in corpus (8)

• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10
• Luminous Infrared Galaxies with the Submillimeter Array: I. Survey Overview and the Central Gas to Dust Ratio
• Isolating signatures of major cloud-cloud collisions using position-velocity diagrams
• ALMA Observations of the Antennae Galaxies: I. A New Window on a Prototypical Merger
• The Physical Conditions in a Pre Super Star Cluster Molecular Cloud in the Antennae Galaxies
• Star formation in a high-pressure environment: An SMA view of the Galactic centre dust ridge
• Dense CO in Mrk 71-A: Superwind Suppressed in a Young Super Star Cluster

Cited by in corpus (4)

• Cloud-cloud collisions in the common foot point of molecular loops 1 and 2 in the Galactic Center
• The formation of young massive clusters by colliding flows
• The role of collision speed, cloud density, and turbulence in the formation of young massive clusters via cloud-cloud collisions
• Connection among environment, cloud-cloud collision speed, and star formation activity in the strongly barred galaxy NGC1300