Baryonic assembly bias in X-ray-selected galaxy groups and clusters: Insights from the Magneticum simulation

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 simulation, measuring the clustering of systems selected by X-ray luminosity and gas fraction at fixed halo mass. We constructed mass-matched halo subsamples by ranking systems in percentiles according to their X-ray luminosity and we derived the linear halo-matter bias. We found that halos with higher X-ray luminosity are more strongly clustered than those with lower luminosity. For the fiducial 84th-16th percentile split, the large-scale bias difference is Δ b lin = 0.17 ± 0.03, corresponding to a ∼17% enhancement relative to the X-ray-faint sample. A less extreme 67th-33rd percentile split gives a consistent signal, with Δ b lin = 0.12 ± 0.02, corresponding to a ∼12% clustering enhancement. When analysed as a function of halo mass, differences in the large-scale bias are of the order of 10–30% between the two samples. The signal is strongest at galaxy group scales (10 13 –10 14 M ⊙ ) and negligible for cluster-size halos (>10 14 M ⊙ ). The complementary analysis, based on the gas fraction, reveals an even stronger clustering dependence on baryonic content: a ∼39% (∼26%) relative clustering enhancement in the 84th-16th (67th-33rd) sample. Baryonic assembly bias is already present at high redshift (from z ≃ 2), but X-ray luminosity becomes a significant tracer of it (>5 σ ) only at lower redshift ( z ≃ 0.3), once the thermodynamic state of the gas is more closely coupled to baryon retention. Matching halos simultaneously by mass and formation time reduces the large-scale bias difference to below 2 σ . This indicates that formation time captures a substantial part of the luminosity- and gas-fraction-dependent clustering signals in Magneticum. These results show that in the Magneticum simulation suite X-ray luminosity traces a secondary dependence of halo clustering beyond mass, providing a baryonic manifestation of the halo assembly bias.