ROLLIN': Rotating Globular Cluster Simulations: II. The complex morphology of globular clusters driven by multi-scale dynamics
Abstract
Context. Globular clusters (GCs) are inherently non-spherical systems that in many cases show internal rotation. Typically, rotation is considered as the main driver of a GC's morphology; however, the relationship between ellipticity and rotational support does not involve simple one-to-one mapping, and other multi-scale dynamical processes could contribute. Aims. We aim to provide a comprehensive interpretation of how morphology evolves in realistic models of rotating GCs and how it correlates with key physical ingredients and processes, including mass loss, stellar evolution, external tidal field, and two-body relaxation. Methods. Using the ROLLIN' suite of direct N-body simulations, we compared two classical methods for measuring a GC's shapeiso-density contours and the second-moment tensor method -and quantified their differences in measured ellipticity. We then measured the intrinsic ellipticity and triaxiality of our models using the tensor method and explored how these quantities change with time and the main physical mechanisms that drive them. Results. We find that the early evolution of GCs can be dominated by dynamical instabilities driven by internal rotation and velocity anisotropy, leading to the formation of bar-like structures that rapidly erode due to collisional effects around the time of the first core collapse. These bars are stronger and longer-lived (≲800 Myr) in strongly rotating clusters with longer relaxation times and subject to stellar evolution. In the long term, clusters evolve towards a less flattened and gradually triaxial configuration. This trend is more pronounced for clusters that (i) experience stronger mass loss, (ii) are more tidally filling and isotropic, and (iii) have lower rotational support. The inner (<r 50% ) and outer (>r 50% ) regions also show different morphologies. The inner parts of our models remain oblate and are dominated by internal rotation, while the outer region is triaxial and resembles the shape of the Roche lobe due to the external tidal field. Conclusions. Our models provide a physical explanation for the observational V/σ-ellipticity relation and demonstrate that morphology can serve as a reliable tracer of the dynamical state of the GC. Initially retrograde, dense, and inclined rotating models deviate systematically from the overall trend, providing physically motivated explanations for observational outliers. Upcoming large-scale photometric surveys will significantly benefit from comprehensive physical frameworks capable of interpreting GC evolution in terms of morphology and kinematics.
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BibTeX
@article{marklund2026rollin,
title = {{ROLLIN': Rotating Globular Cluster Simulations: II. The complex morphology of globular clusters driven by multi-scale dynamics}},
author = {Arn Marklund and Paolo Bianchini and Anna Lisa Varri and Katarina Kraljic and Giulia Pagnini},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202660954},
url = {https://hal.science/hal-05853198v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - ROLLIN': Rotating Globular Cluster Simulations: II. The complex morphology of globular clusters driven by multi-scale dynamics
AU - Arn Marklund
AU - Paolo Bianchini
AU - Anna Lisa Varri
AU - Katarina Kraljic
AU - Giulia Pagnini
PY - 2026
DA - 2026/10/12
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202660954
UR - https://hal.science/hal-05853198v1
AB - Context. Globular clusters (GCs) are inherently non-spherical systems that in many cases show internal rotation. Typically, rotation is considered as the main driver of a GC's morphology; however, the relationship between ellipticity and rotational support does not involve simple one-to-one mapping, and other multi-scale dynamical processes could contribute. Aims. We aim to provide a comprehensive interpretation of how morphology evolves in realistic models of rotating GCs and how it correlates with key physical ingredients and processes, including mass loss, stellar evolution, external tidal field, and two-body relaxation. Methods. Using the ROLLIN' suite of direct N-body simulations, we compared two classical methods for measuring a GC's shapeiso-density contours and the second-moment tensor method -and quantified their differences in measured ellipticity. We then measured the intrinsic ellipticity and triaxiality of our models using the tensor method and explored how these quantities change with time and the main physical mechanisms that drive them. Results. We find that the early evolution of GCs can be dominated by dynamical instabilities driven by internal rotation and velocity anisotropy, leading to the formation of bar-like structures that rapidly erode due to collisional effects around the time of the first core collapse. These bars are stronger and longer-lived (≲800 Myr) in strongly rotating clusters with longer relaxation times and subject to stellar evolution. In the long term, clusters evolve towards a less flattened and gradually triaxial configuration. This trend is more pronounced for clusters that (i) experience stronger mass loss, (ii) are more tidally filling and isotropic, and (iii) have lower rotational support. The inner (<r 50% ) and outer (>r 50% ) regions also show different morphologies. The inner parts of our models remain oblate and are dominated by internal rotation, while the outer region is triaxial and resembles the shape of the Roche lobe due to the external tidal field. Conclusions. Our models provide a physical explanation for the observational V/σ-ellipticity relation and demonstrate that morphology can serve as a reliable tracer of the dynamical state of the GC. Initially retrograde, dense, and inclined rotating models deviate systematically from the overall trend, providing physically motivated explanations for observational outliers. Upcoming large-scale photometric surveys will significantly benefit from comprehensive physical frameworks capable of interpreting GC evolution in terms of morphology and kinematics.
SN - 0004-6361
SN - 1432-0746
ER -