The origin of the Milky Way globular clusters

Abstract: We present a cosmological zoom-in simulation of a Milky Way-like galaxy used to explore the formation and evolution of star clusters. We investigate in particular the origin of the bimodality observed in the colour and metallicity of globular clusters, and the environmental evolution through cosmic times in the form of tidal tensors. Our results self-consistently confirm previous findings that the blue, metal-poor clusters form in satellite galaxies which are accreted onto the Milky Way, while the red, metal-rich clusters form mostly in situ or, to a lower extent in massive, self-enriched galaxies merging with the Milky Way. By monitoring the tidal fields these populations experience, we find that clusters formed in situ (generally centrally concentrated) feel significantly stronger tides than the accreted ones, both in the present-day, and when averaged over their entire life. Furthermore, we note that the tidal field experienced by Milky Way clusters is significantly weaker in the past than at present-day, confirming that it is unlikely that a power-law cluster initial mass function like that of young massive clusters, is transformed into the observed peaked distribution in the Milky Way with relaxation-driven evaporation in a tidal field.

The Origin of Milky Way Globular Clusters: Insights from Cosmological Simulations

The paper investigates the formation and evolution of globular clusters in the Milky Way using a cosmological zoom-in simulation. The study primarily aims to elucidate the bimodality observed in globular clusters, distinguished by variations in color and metallicity—commonly characterized as blue, metal-poor and red, metal-rich clusters.

Methodological Approach

Utilizing hydrodynamic and N-body $N$-body simulation facilitated by adaptive mesh refinement (AMR), the authors modeled a Milky Way-like galaxy's formation. The simulation incorporated star formation and feedback mechanisms, establishing a base for tracing star clusters' origins and evolutionary trajectories. By examining tidal tensors, the study assesses tidal fields and analyzes their impact on clusters from formation through cosmic epochs.

Key Findings

1. Cluster Formation Mechanisms: The simulation supports previous notions that blue, metal-poor clusters originate in satellite galaxies, which are eventually integrated into the Milky Way, whereas red, metal-rich clusters predominantly form in situ or during mergers with massive, self-enriched galaxies. This self-consistent confirmation aligns with semi-analytical models like those by Tonini (2013).
2. Bimodality and Galactic Environment: The results illustrate that the bimodal distribution of globular clusters results from variations in formation mechanisms and subsequent environmental influences. Clusters formed in situ experience stronger tidal fields than those accreted, underscoring differences dictated by their position in the galaxy and their genesis.
3. Tidal Influence on Cluster Evolution: The study posits that galactic tides have historically been weaker than present values, thereby challenging assertions that relaxation-driven evaporation under a strong tidal field alone gives rise to the Milky Way's observed peaked cluster distribution. This finding highlights that a constant tidal environment assumption misrepresents cluster evolution over extended time scales.

Implications and Future Directions

The implications of this study touch upon both practical and theoretical aspects within astrophysics, particularly in understanding galactic evolution and stellar dynamics. Evaluating globular cluster properties can provide insights into galactic histories and the hierarchical formation process. Moreover, the paper encourages a reconsideration of assumptions regarding tidal field strength in models of cluster mass-loss and disruption, advocating for time-averaged values that better reflect clusters’ historical environments.

Future investigations could explore resolving cluster formation at higher spacetemporal resolutions, exploring turbulence and feedback mechanisms influencing star formation sites. These advancements may facilitate holistic models linking cosmological dynamics to individual cluster properties, including mass and binding energy considerations.

In summary, by correlating cosmological simulation findings with observational evidence, the paper enriches our understanding of globular cluster bimodality and enhances theoretical models of galactic assembly and cluster evolution in the Milky Way.