Super-star clusters versus OB associations

TL;DR

This paper explores how shear in spiral galaxies inhibits super star cluster formation, favoring OB associations, while in interacting and dwarf galaxies, lack of shear allows dense, bound clusters to form.

Contribution

It proposes a shear-based explanation for the differing prevalence of super star clusters versus OB associations across galaxy types.

Findings

Shear impedes super star cluster formation in spiral galaxies.

Interacting and dwarf galaxies lack shear, enabling dense cluster formation.

Different galactic environments lead to distinct star cluster types.

Abstract

Super Star Clusters (Mecl > 10^5 Msol) are the largest stellar nurseries in our local Universe, containing hundreds of thousands to millions of young stars within a few light years. Many of these systems are found in external galaxies, especially in pairs of interacting galaxies, and in some dwarf galaxies, but relatively few in disk galaxies like our own Milky Way. We show that a possible explanation for this difference is the presence of shear in normal spiral galaxies which impedes the formation of the very large and dense super star clusters but prefers the formation of loose OB associations possibly with a less massive cluster at the center. In contrast, in interacting galaxies and in dwarf galaxies, regions can collapse without having a large-scale sense of rotation. This lack of rotational support allows the giant clouds of gas and stars to concentrate into a single, dense and gravitationally bound system.