Baryonic assembly bias in X-ray-selected galaxy groups and clusters: insights from the Magneticum simulation
Abstract
Galaxy groups and clusters trace the large-scale matter distribution, with their clustering usually interpreted mainly as a function of halo mass. Yet, at fixed mass, their baryonic properties retain information about halo growth, gas accretion, and feedback. The intrinsic scatter in X-ray luminosity and gas fraction suggests that X-ray-selected systems may not be a random subset of the halo population. If these observables correlate with halo assembly, they may trace secondary variations in halo bias. We test this using the Magneticum hydrodynamical simulation, measuring the clustering of systems selected by X-ray luminosity and gas fraction at fixed halo mass. We construct mass-matched subsamples by ranking halos in percentiles of X-ray luminosity and derive the linear halo-matter bias from the halo-matter cross-power spectrum. X-ray-bright halos are more strongly clustered than X-ray-faint halos at fixed mass. For the 84th-16th percentile split, we find Δblin=0.17±0.03, corresponding to a ∼17% enhancement relative to the X-ray-faint sample. A 67th-33rd split gives a consistent signal, with Δblin=0.12±0.02 and a ∼12% enhancement. The effect is strongest at group scales and negligible for cluster-size halos. Gas fraction shows an even stronger clustering dependence, with relative enhancements of ∼39% and ∼26% for the two percentile splits. This signal is present from z≃2, whereas X-ray luminosity becomes significant only at z≃0.3, once the gas thermodynamic state is more closely coupled to baryon retention. Matching halos by both mass and formation time reduces the large-scale bias difference to below 2σ, indicating that formation time captures much of the signal. These results show that, in Magneticum, X-ray luminosity traces a baryonic manifestation of halo assembly bias beyond mass.
Versions
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BibTeX
@misc{marini2026baryonic,
title = {{Baryonic assembly bias in X-ray-selected galaxy groups and clusters: insights from the Magneticum simulation}},
author = {Ilaria Marini and Tiago Castro and Paola Popesso and Klaus Dolag and Veronica Biffi and Natanael de Isídio and Daudi Mazengo and Victoria Toptun},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2607.03746},
eprint = {2607.03746},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.03746},
}
RIS
TY - UNPB
TI - Baryonic assembly bias in X-ray-selected galaxy groups and clusters: insights from the Magneticum simulation
AU - Ilaria Marini
AU - Tiago Castro
AU - Paola Popesso
AU - Klaus Dolag
AU - Veronica Biffi
AU - Natanael de Isídio
AU - Daudi Mazengo
AU - Victoria Toptun
PY - 2026
DA - 2026/09/07
PB - arXiv
DO - 10.48550/arxiv.2607.03746
UR - https://arxiv.org/abs/2607.03746
AB - Galaxy groups and clusters trace the large-scale matter distribution, with their clustering usually interpreted mainly as a function of halo mass. Yet, at fixed mass, their baryonic properties retain information about halo growth, gas accretion, and feedback. The intrinsic scatter in X-ray luminosity and gas fraction suggests that X-ray-selected systems may not be a random subset of the halo population. If these observables correlate with halo assembly, they may trace secondary variations in halo bias. We test this using the Magneticum hydrodynamical simulation, measuring the clustering of systems selected by X-ray luminosity and gas fraction at fixed halo mass. We construct mass-matched subsamples by ranking halos in percentiles of X-ray luminosity and derive the linear halo-matter bias from the halo-matter cross-power spectrum. X-ray-bright halos are more strongly clustered than X-ray-faint halos at fixed mass. For the 84th-16th percentile split, we find Δb_lin=0.17±0.03, corresponding to a ∼17% enhancement relative to the X-ray-faint sample. A 67th-33rd split gives a consistent signal, with Δb_lin=0.12±0.02 and a ∼12% enhancement. The effect is strongest at group scales and negligible for cluster-size halos. Gas fraction shows an even stronger clustering dependence, with relative enhancements of ∼39% and ∼26% for the two percentile splits. This signal is present from z≃2, whereas X-ray luminosity becomes significant only at z≃0.3, once the gas thermodynamic state is more closely coupled to baryon retention. Matching halos by both mass and formation time reduces the large-scale bias difference to below 2σ, indicating that formation time captures much of the signal. These results show that, in Magneticum, X-ray luminosity traces a baryonic manifestation of halo assembly bias beyond mass.
ER -