Phase-Space Crystallization in Galactic Globular Clusters: A Gaia-Based Metric and Implications for Technosignature Searches

We develop a model-independent framework to quantify phase-space “crystallization”, the degree of ordered radial and kinematic substructure, in 79 Galactic globular clusters using the Gaia EDR3-based membership catalogue of E. Vasiliev & H. Baumgardt (2021a).

We construct a scalar crystallization index, C_index, by combining a radial inhomogeneity metric (z_rad) and a local, cluster-centric tangential-velocity metric (z_vel) standardized against empirical nulls.

The population distribution is strongly non-Gaussian: most clusters are consistent with smooth, equilibrium expectations, while a small high-C tail (C_index >= 2) identifies dynamically complex systems, including NGC 5139 (omega Cen) and NGC 104 (47 Tuc). Correlation and fixed-N tests show that sample size affects detectability, but does not by itself explain all high-rank objects.

Through synthetic injection tests in dynamically “quiet” control clusters, we demonstrate sensitivity to ultra-cold, shell-confined kinematic components, ruling out single-shell structures comprising more than a few to ~ 10-20% of core stars in the best-sampled control clusters.

We find no evidence, within the sensitivity of the adopted diagnostics, for phase-space structures that require explanations beyond known dynamical processes. However, C_index provides a useful tool for ranking clusters by dynamical extremeness, serving both as a diagnostic for internal complexity and as a quantitative metric for prioritizing follow-up dynamical or technosignature-oriented observations.

Bo-Lun Huang, Zhen-Zhao Tao, Tong-Jie Zhang

Comments: 23 pages, 7 figures, 3 tables. Accepted for publication in The Astrophysical Journal (ApJ)
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2605.06072 [astro-ph.GA] (or arXiv:2605.06072v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2605.06072
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Submission history
From: Bolun Huang
[v1] Thu, 7 May 2026 11:58:24 UTC (1,220 KB)
https://arxiv.org/abs/2605.06072
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