Comparing Young Massive Clusters and their Progenitor Clouds in the Milky Way

TL;DR

This study compares the gas mass distributions of potential YMC progenitor clouds with the stellar distributions of existing YMCs, revealing evolutionary trends and supporting a conveyor-belt formation scenario.

Contribution

It provides the first detailed comparison between gas cloud profiles and YMC stellar profiles, highlighting differences and suggesting ongoing mass accumulation during star formation.

Findings

Gas in progenitor clouds is less centrally concentrated than in YMCs.

Models with very dense initial conditions are inconsistent with observed clouds.

Clouds show evidence of mass accumulation towards centers after star formation begins.

Abstract

Young massive clusters (YMCs) have central stellar mass surface densities exceeding $10^{4} M_{\odot} pc^{-2}$. It is currently unknown whether the stars formed at such high (proto)stellar densities. We compile a sample of gas clouds in the Galaxy which have sufficient gas mass within a radius of a few parsecs to form a YMC, and compare their radial gas mass distributions to the stellar mass distribution of Galactic YMCs. We find that the gas in the progenitor clouds is distributed differently than the stars in YMCs. The mass surface density profiles of the gas clouds are generally shallower than the stellar mass surface density profiles of the YMCs, which are characterised by prominent dense core regions with radii ~ 0.1 pc, followed by a power-law tail. On the scale of YMC core radii, we find that there are no known clouds with significantly more mass in their central regions when compared to Galactic YMCs. Additionally, we find that models in which stars form from very dense initial conditions require surface densities that are generally higher than those seen in the known candidate YMC progenitor clouds. Our results show that the quiescent, less evolved clouds contain less mass in their central regions than in the highly star-forming clouds. This suggests an evolutionary trend in which clouds continue to accumulate mass towards their centres after the onset of star formation. We conclude that a conveyor-belt scenario for YMC formation is consistent with the current sample of Galactic YMCs and their progenitor clouds.