The formation of massive stellar clusters in converging galactic flows with photoionisation

TL;DR

This study uses simulations to show how converging galactic flows and photoionisation feedback influence the formation, mass, and structure of massive stellar clusters in spiral arms.

Contribution

It demonstrates that converging flows accelerate cluster formation and mergers, and quantifies how photoionisation limits cluster mass and affects their gas environment.

Findings

Massive clusters form faster in strongly converging flows.

Cluster mergers are frequent in regions with converging flows.

Photoionisation reduces cluster mass by about 20% on average.

Abstract

We have performed simulations of cluster formation along two regions of a spiral arm taken from a global Milky Way simulation, including photoionising feedback. One region is characterised by strongly converging flows, the other represents a more typical spiral arm region. We find that more massive clusters are able to form on shorter timescales for the region with strongly converging flows. Mergers between clusters are frequent in the case of the strongly converging flows and enable the formation of massive clusters. We compare equivalent clusters formed in simulations with and without ionisation. Photoionisation does not prevent massive cluster formation, but can be seen to limit the masses of the clusters. On average the mass is reduced by around 20%, but we see a large spread from ionisation having minimal difference to leading to a 50% reduction in mass. Photoionisation is also able to clear out the gas in the vicinity of the clusters on Myr timescales, which can produce clusters with larger radii that are surrounded by more massive stellar halos. We find that the ionising feedback has more impact in our second region which is less dense and has less strongly converging flows.