Heart of Darkness: dust obscuration of the central stellar component in globular clusters younger than ~100Myr in multiple stellar population models

TL;DR

This paper proposes observational signatures, such as dust obscuration and gas emission, to test theories that globular clusters younger than 100 million years contain large gas reservoirs necessary for multiple star formation episodes.

Contribution

It identifies specific observable indicators to validate or challenge models of globular cluster formation involving multiple star formation events.

Findings

Young clusters should show dust obscuration and gas emission if they have large gas reservoirs.

No observational evidence of such signatures exists in nearby young massive clusters.

This challenges the idea that large gas reservoirs are common in young globular clusters.

Abstract

To explain the observed anomalies in stellar populations within globular clusters, many globular cluster formation theories require two independent episodes of star formation. A fundamental prediction of these models is that the clusters must accumulate large gas reservoirs as the raw material to form the second stellar generation. We show that young clusters containing the required gas reservoir should exhibit the following observational signatures: (i) a dip in the measured luminosity profile or an increase in measured reddening towards the cluster centre, with Av >10mag within a radius of a few pc; (ii) bright (sub)mm emission from dust grains; (iii) bright molecular line emission once the gas is dense enough to begin forming stars. Unless the IMF is anomalously skewed towards low-mass stars, the clusters should also show obvious signs of star formation via optical emission lines (e.g. H_alpha) after the stars have formed. These observational signatures should be readily observable towards any compact clusters (radii of a few pc) in the nearby Universe with masses > 10^6 Msun and ages <100Myr. This provides a straightforward way to directly test globular cluster formation models which predict large gas reservoirs are required to form the second stellar generation. The fact that no such observational evidence exists calls into question whether such a mechanism happens regularly for YMCs in galaxies within a few tens of Mpc.