First phase space portrait of a hierarchical stellar structure in the Milky Way

TL;DR

This paper provides the first detailed observational phase-space portrait of a hierarchical stellar structure in the Milky Way, revealing complex cluster interactions and suggesting ongoing cluster assembly in the Galactic disk.

Contribution

It presents new observational evidence of a multi-cluster system with a massive halo, indicating an intermediate stage of cluster assembly in the Milky Way disk.

Findings

Seven co-moving clusters identified, including three new ones.

Evidence of mutual interactions and intra-cluster over-densities.

The stellar complex may evolve into a massive stellar system.

Abstract

We present the first detailed observational picture of a possible ongoing massive cluster hierarchical assembly in the Galactic disk as revealed by the analysis of the stellar full phase-space (3D positions and kinematics and spectro-photometric properties) of an extended area ($6^{\circ}$ diameter) surrounding the well-known $\it h$ and $\chi$ Persei double stellar cluster in the Perseus Arm. Gaia-EDR3 shows that the area is populated by seven co-moving clusters, three of which were previously unknown, and by an extended and quite massive ($M\sim10^5 M_{\odot}$) halo. All stars and clusters define a complex structure with evidence of possible mutual interactions in the form of intra-cluster over-densities and/or bridges. They share the same chemical abundances (half-solar metallicity) and age ($t\sim20$ Myr) within a small confidence interval and the stellar density distribution of the surrounding diffuse stellar halo resembles that of a cluster-like stellar system. The combination of these evidences suggests that stars distributed within a few degrees from $\it h$ and $\chi$ Persei are part of a common, sub-structured stellar complex that we named LISCA I. Comparison with results obtained through direct $N$-body simulations suggest that LISCA I may be at an intermediate stage of an ongoing cluster assembly that can eventually evolve in a relatively massive (a few $10^5 M_{\odot}$) stellar system. We argue that such cluster formation mechanism may be quite efficient in the Milky Way and disk-like galaxies and, as a consequence, it has a relevant impact on our understanding of cluster formation efficiency as a function of the environment and redshift.