A critical look at the merger scenario to explain multiple populations and rotation in iron-complex globular clusters

TL;DR

This study uses N-body simulations to evaluate whether merging progenitors can explain the multiple populations and rotation observed in iron-complex globular clusters, highlighting the importance of initial density ratios.

Contribution

It demonstrates that merging can produce observed features in iron-complex GCs if progenitors have specific density ratios, providing a new perspective on their formation.

Findings

Merging results depend on initial mass and density ratios.

Less massive progenitors can dominate central regions if denser.

Remnants exhibit rotation patterns consistent with oblate rotators.

Abstract

Merging has been proposed to explain multiple populations in globular clusters (GCs) where there is a spread in iron abundance (hereafter, iron-complex GCs). By means of N-body simulations, we investigate if merging is consistent with the observations of sub-populations and rotation in iron-complex GCs. The key parameters are the initial mass and density ratios of the progenitors. When densities are similar, the more massive progenitor dominates the central part of the merger remnant and the less massive progenitor forms an extended rotating population. The low-mass progenitor can become the majority population in the central regions of the merger remnant only if its initial density is higher by roughly the mass ratio. To match the radial distribution of multiple populations in two iron-complex GCs ({\omega} Cen and NGC 1851), the less massive progenitor needs to be four times as dense as the larger one. Our merger remnants show solid-body rotation in the inner parts, becoming differential in the outer parts. Rotation velocity V and ellipticity {\epsilon} are in agreement with models for oblate rotators with isotropic dispersion. We discuss several kinematic signatures of a merger with a denser lower mass progenitor that can be tested with future observations.