Stellar halos of bright central galaxies II: Scaling relations, colors and metallicity evolution with redshift

Abstract: We study the formation and evolution of stellar halos (SHs) around bright central galaxies (BCGs), focusing on their scaling relations, colors, and metallicities across cosmic time, and compare model predictions with ultra--deep imaging data. We use the semianalytic model \textsc{FEGA25}, applied to merger trees from high--resolution dark matter simulations, including an updated treatment of intracluster light (ICL) formation. SHs are defined as the stellar component within the transition radius, linked to halo concentration. Predictions are compared with observations from the VST Early-type GAlaxy Survey (VEGAS) and Fornax Deep Survey (FDS). The SH mass correlates strongly with both BCG and ICL masses, with tighter scatter in the SH--ICL relation. The transition radius peaks at 30--40 kpc nearly independent of redshift, but can reach $\sim400$ kpc in the most massive halos, after z=0.5. SHs and ICL show nearly identical color distributions at all epochs, both reddening toward $z=0$. At $z=2$, SHs and the ICL are $\sim0.4$ dex more metal--poor than BCGs, but the gap shrinks to $\sim0.1$ dex by the present time. Observed colors are consistent with model predictions, while observed metallicities are lower, suggesting a larger contribution from disrupted dwarfs. SHs emerge as transition regions between BCGs and the ICL, dynamically and chemically coupled to both. Their properties depend on halo concentration, ICL formation efficiency, and the progenitor mass spectrum. Upcoming wide--field photometric and spectroscopic surveys (e.g. LSST, WEAVE, 4MOST) will provide crucial tests by mapping structure, metallicity, and kinematics in large galaxy samples.