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Only the Special Survive: Evolution of Long-Lived Star Clusters in Galaxy Simulations

Alessa I. Wiggins, Sarah Loebman, Peter Frinchaboy

TL;DR

The paper addresses the discrepancy between chemical gradient trends of field stars and open clusters in the Galactic disk. It uses Milky Way-mass zoom-in simulations from the FIRE-2 Latte suite to follow the evolution of long-lived open clusters (>1 Gyr) in two MW-like galaxies (m12i and m12f), tracking their birth locations, migration, and surrounding gas environments across cosmic time. The results show that clusters migrate away from birth sites with diverse patterns, spending most of their lives in under-dense ISM, with brief encounters with dense gas clouds that alter orbits, including inward or outward migration and changes in vertical height; representative cases include a 4.69 Gyr inward-migrating cluster in m12f and a 5.86 Gyr outward-migrating cluster in m12i. This work demonstrates that cluster migration and differential survivability can shape present-day spatial and chemical distributions, providing a framework to interpret observations such as OCCAM survey data and informing models of Galactic chemical evolution.

Abstract

In this work, we aim to answer one crucial question behind the discrepancy between chemical trends of field stars and clusters in the Galactic disk: is the chemical gradient mismatch driven by cluster migration and differential survivability as a function of galactic location? To answer this question, we explored the evolution of long-lived (> 1 Gyr) star clusters in Milky Way-galaxy simulations. In particular, we investigated why some star clusters remain bound over billions of years. We have traced the unique trajectories for a sample of open clusters around two FIRE galaxies throughout cosmic time. Additionally, we characterized the small-scale environment surrounding these clusters over their orbital history. We see that clusters across both FIRE galaxies spend the majority of their lives in under-dense regions of gas, except for brief passages where they interact with gas clouds, causing their orbits to be altered.

Only the Special Survive: Evolution of Long-Lived Star Clusters in Galaxy Simulations

TL;DR

The paper addresses the discrepancy between chemical gradient trends of field stars and open clusters in the Galactic disk. It uses Milky Way-mass zoom-in simulations from the FIRE-2 Latte suite to follow the evolution of long-lived open clusters (>1 Gyr) in two MW-like galaxies (m12i and m12f), tracking their birth locations, migration, and surrounding gas environments across cosmic time. The results show that clusters migrate away from birth sites with diverse patterns, spending most of their lives in under-dense ISM, with brief encounters with dense gas clouds that alter orbits, including inward or outward migration and changes in vertical height; representative cases include a 4.69 Gyr inward-migrating cluster in m12f and a 5.86 Gyr outward-migrating cluster in m12i. This work demonstrates that cluster migration and differential survivability can shape present-day spatial and chemical distributions, providing a framework to interpret observations such as OCCAM survey data and informing models of Galactic chemical evolution.

Abstract

In this work, we aim to answer one crucial question behind the discrepancy between chemical trends of field stars and clusters in the Galactic disk: is the chemical gradient mismatch driven by cluster migration and differential survivability as a function of galactic location? To answer this question, we explored the evolution of long-lived (> 1 Gyr) star clusters in Milky Way-galaxy simulations. In particular, we investigated why some star clusters remain bound over billions of years. We have traced the unique trajectories for a sample of open clusters around two FIRE galaxies throughout cosmic time. Additionally, we characterized the small-scale environment surrounding these clusters over their orbital history. We see that clusters across both FIRE galaxies spend the majority of their lives in under-dense regions of gas, except for brief passages where they interact with gas clouds, causing their orbits to be altered.
Paper Structure (4 sections, 2 figures)

This paper contains 4 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Results
  4. Acknowledgments

Figures (2)

  • Figure 1: Cluster migration in m12i and m12f: The difference between present-day radius and birth radius for long-lived OCs as a function of cluster age, all color-coded by metallicity. The two circled clustered are examples of inward & outward migrating clusters (explored in Figure \ref{['fig:example_clusters']}).
  • Figure 2: Both clusters undergo major orbital changes. (Left) Inward-moving (aka "migrating") 4.69 Gyr cluster selected from m12f. (Top) The local density relative to the median ISM is typically under-dense except for two brief encounters with dense clouds (the most significant denoted by the vertical line). (Middle) Cluster's vertical distance from the midplane over time. (Bottom) Cluster's galactocentric radial position over time, which shows that the dense interaction drives inward migration and decreases its maximum height above/below the midplane. (Right) Long-lived (5.86 Gyr) cluster selected from m12i demonstrates outward migration.