Resolved Star Formation Efficiency in the Antennae Galaxies

TL;DR

This study measures the star formation efficiency in the Antennae Galaxies using ALMA and Hubble data, revealing how clusters evolve and lose gas over time, with implications for their bound status.

Contribution

It introduces a novel approach to determine the instantaneous mass ratio in merging galaxies, linking molecular gas, stellar mass, and cluster evolution.

Findings

Few clusters have high initial star formation efficiency (>0.2).

Most clusters lose their molecular gas within 10^7.5 years.

IMR(t) correlates with extinction, indicating evolutionary stages.

Abstract

We use Atacama Large Millimeter Array CO(3-2) observations in conjunction with optical observations from the Hubble Space Telescope to determine the ratio of stellar to gas mass for regions in the Antennae Galaxies. We adopt the term "instantaneous mass ratio" IMR(t) = M$_{stars}$/(M$_{gas}$ +M$_{stars}$), that is equivalent to the star formation efficiency for an idealized system at t = 0. We use two complementary approaches to determining the IMR(t) based on 1) the enclosed stellar and molecular mass within circular apertures centered on optically-identified clusters, and 2) a tessellation algorithm that defines regions based on CO emission. We find that only a small number of clusters appear to have IMR(0) = SFE > 0.2, which suggests that only a small fraction of these clusters will remain bound. The results suggest that by ages of $10^{6.7}$ years, some clusters will have lost all of their associated molecular gas, and by $10^{7.5}$ years this is true for the majority of clusters. There appears to be slight dependence of the IMR(t) on the CO surface brightness, which could support the idea that dense molecular environments are more likely to form bound clusters. However, the IMR(t) appears to have a strong dependence on extinction, which likely traces the evolutionary state of clusters.