Quantifying the decline in CO(2-1) intensity around PHANGS-HST star clusters: Constraints on molecular gas dispersal, stellar feedback, and cluster formation efficiency

The study combines the largest survey of star clusters in nearby galaxies to date, PHANGS-HST, with molecular gas maps from PHANGS-ALMA to constrain the efficiency of cluster formation and the typical timescales over which clusters become dissociated from their natal molecular gas.

It analyzes 29,233 clusters in 36 galaxies using CO maps at 150 pc resolution. For each cluster with stellar mass >= 10^3.5 solar masses, the authors measure radial profiles of CO(2-1) integrated intensity and extract local background-subtracted CO intensity at the cluster center.

They group clusters by age and mass to quantify how background-subtracted CO intensity and the corresponding molecular gas mass depend on these properties. To account for cluster mass, they also compute the normalized mass ratio M_mol/M_SC.

The CO intensity varies systematically with cluster age and mass. Clusters spatially associated with H-alpha emission have the highest background-subtracted CO intensity, 7.4 K km/s, corresponding to M_mol/M_SC = 82 and an implied observed cluster formation efficiency of ~1.2%.

Young clusters without associated H-alpha show lower, age-dependent values: background-subtracted CO intensity and M_mol/M_SC decrease from 1.3 K km/s and 15 at ages 1-3 Myr to approximately zero for clusters older than 9-10 Myr. Clusters older than 10 Myr show no statistical excess of CO intensity.

Using the H-alpha-associated clusters as initial conditions, the authors model the impact of cluster drift and find it unlikely to drive the observed cluster-gas spatial disassociation on short 4-6 Myr timescales. Instead, early stellar feedback appears to be the main driver.