Globular clusters with the extended horizontal-branch as remaining cores of galaxy building blocks

TL;DR

This paper proposes that approximately 25% of Milky Way globular clusters are remnants of early galaxy building blocks, distinguished by their extended horizontal-branch stars, multiple populations, and unique kinematics, supporting a three-stage galaxy formation model.

Contribution

It identifies a subset of globular clusters as surviving cores of galaxy building blocks, offering new insights into the Milky Way's formation history.

Findings

About 25% of Milky Way globular clusters have extended horizontal-branch stars.

These clusters show distinct orbital kinematics from normal globular clusters.

The results support a three-stage galaxy formation scenario.

Abstract

The relics of building blocks that made stellar halo and bulge are yet to be discovered unless they were completely disrupted throughout the history of the Galaxy. Here we suggest that about 25% of the Milky Way globular clusters have characteristics of the remaining cores of these early building blocks rather than genuine star clusters. They are clearly distinct from other normal globular clusters in the presence of extended horizontal-branch and multiple stellar populations, in mass (brightness), and most importantly in orbital kinematics. Based on this result, a three-stage formation picture of the Milky Way is suggested, which includes early mergers, collapse, and later accretion.