Observational evidence of velocity anisotropy assembly bias in galaxy clusters
6 affiliations
- 1CONICET, Instituto de Astronomía Teórica y Experimental (IATE), Laprida 854
- 2Universidad Nacional de Córdoba (UNC), Observatorio Astronómico de Córdoba (OAC), Laprida 854
- 3INAF-Osservatorio Astronomico di Trieste
- 4IFPU Institute for Fundamental Physics of the Universe
- 5Centre for Particle Cosmology, University of Pennsylvania
- 6Departamento de Física, Universidad Técnica Federico Santa María
Abstract
Context. Cosmological simulations have shown that halo clustering depends on secondary internal properties beyond mass, an effect known as assembly bias. In particular, numerical models have shown that internal velocity anisotropy ( β ) is the halo property most tightly linked to secondary halo bias, as at a fixed mass, haloes with isotropic velocity profiles are more strongly clustered, whereas haloes in which radial motions dominate are more weakly clustered.
Aims. Despite this robust theoretical prediction, direct observational detection of velocity anisotropy assembly bias has remained elusive. Here we present observational evidence for this effect using a sample of massive galaxy clusters from the eROSITA and DESI surveys.
Methods. We measured the internal orbital anisotropy β for 91 clusters via Jeans dynamical modelling with MAMPOSSt, considering three anisotropy profile models to ensure robustness. We divided the sample into two subpopulations, isotropic and radially anisotropic, and we verified that they show no significant differences in halo mass. To quantify their large-scale clustering, we measured the projected cross-correlation function with central galaxies from the SDSS DR18. We then calculated the average relative bias b relative over scales of 5 to 20 h ⁻¹ Mpc.
Results. We report evidence of velocity anisotropy assembly bias. In particular, clusters with isotropic internal kinematics exhibit stronger clustering than radially dominated ones, a segregation that is unlikely to be attributable to residual differences in cluster mass. This trend shows varying significance across three anisotropy profiles, from a mild tendency to a highly pronounced signal. Furthermore, the assembly bias remains distinct when restricting the sample to clusters consistently classified across all three models.
Related
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BibTeX
@article{rodriguez2026observational,
title = {{Observational evidence of velocity anisotropy assembly bias in galaxy clusters}},
author = {Facundo Rodriguez and Andrea Biviano and Ravi K. Sheth and Antonio D. Montero-Dorta and Manuel Merchán},
journal = {Astronomy \& Astrophysics},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202662563},
url = {https://www.aanda.org/10.1051/0004-6361/202662563},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Observational evidence of velocity anisotropy assembly bias in galaxy clusters
AU - Facundo Rodriguez
AU - Andrea Biviano
AU - Ravi K. Sheth
AU - Antonio D. Montero-Dorta
AU - Manuel Merchán
PY - 2026
DA - 2026/10/14
JO - Astronomy & Astrophysics
DO - 10.1051/0004-6361/202662563
UR - https://www.aanda.org/10.1051/0004-6361/202662563
AB - Context. Cosmological simulations have shown that halo clustering depends on secondary internal properties beyond mass, an effect known as assembly bias. In particular, numerical models have shown that internal velocity anisotropy ( β ) is the halo property most tightly linked to secondary halo bias, as at a fixed mass, haloes with isotropic velocity profiles are more strongly clustered, whereas haloes in which radial motions dominate are more weakly clustered. Aims. Despite this robust theoretical prediction, direct observational detection of velocity anisotropy assembly bias has remained elusive. Here we present observational evidence for this effect using a sample of massive galaxy clusters from the eROSITA and DESI surveys. Methods. We measured the internal orbital anisotropy β for 91 clusters via Jeans dynamical modelling with MAMPOSSt, considering three anisotropy profile models to ensure robustness. We divided the sample into two subpopulations, isotropic and radially anisotropic, and we verified that they show no significant differences in halo mass. To quantify their large-scale clustering, we measured the projected cross-correlation function with central galaxies from the SDSS DR18. We then calculated the average relative bias b relative over scales of 5 to 20 h ⁻¹ Mpc. Results. We report evidence of velocity anisotropy assembly bias. In particular, clusters with isotropic internal kinematics exhibit stronger clustering than radially dominated ones, a segregation that is unlikely to be attributable to residual differences in cluster mass. This trend shows varying significance across three anisotropy profiles, from a mild tendency to a highly pronounced signal. Furthermore, the assembly bias remains distinct when restricting the sample to clusters consistently classified across all three models.
SN - 0004-6361
SN - 1432-0746
ER -