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

TL;DR

The paper uses the FEGA25 semi-analytic model to study stellar halos (SHs) around bright central galaxies (BCGs) across cosmic time, defining SHs as stars within the transition radius $R_{ m trans}$ and linking ICL concentration to halo structure. SH mass scales linearly with both BCG and ICL masses, with SH–ICL scatter much smaller, and the transition radius peaks near $30$–$40$ kpc though can grow to ~400 kpc in the most massive halos at low redshift; SHs and ICL share nearly identical colors and both redden toward $z=0$, while metallicities show a ~0.4 dex BCG–SH/ICL offset at $z=2$ shrinking to ~0.1 dex today. Observed colors align well with model predictions, but observed metallicities are lower, implying a stronger contribution from disrupted dwarfs in real halos. SHs thus represent a chemically and dynamically linked interface between BCGs and the ICL, with their properties governed by halo concentration, ICL formation efficiency, and the progenitor mass spectrum; upcoming surveys like LSST, WEAVE, and 4MOST will further test these predictions by mapping structure, metallicity, and kinematics in large samples.

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.

Figures (8)

• Figure 1: Schematic representation of central galaxies and their surrounding stellar halos and intracluster stars. The stellar halo is defined as the stellar component within the transition radius $R_{\rm{trans}}$, marking the intermediate region between the galaxy and the intracluster light.
• Figure 2: Left panel: stellar halo mass as a function of the transition radius $R_{\rm{trans}}$, at different redshifts as indicated in the legend. The two black dashed lines represent the 16th and 84th percentiles of the distribution at $z=0$. At fixed $R_{\rm{trans}}$, the stellar halo mass increases slightly with redshift, since dark matter halos with similar concentrations are more evolved at earlier epochs than at later times. Right panel: distribution of transition radii at different redshifts (different color), and as observed (VEGAS, green line). The distributions are very similar and all peak around $30$–$40$ kpc, reflecting the abundance of relatively small dark matter halos at all redshifts. Overall, and nearly independent of redshift, about $90\%$ of the transition radii lie within $100$ kpc.
• Figure 3: Scaling relations between the mass of stellar halos and that of the associated central galaxy (left panel) and intracluster light (right panel). The color bar encodes the dark matter halo concentration, with redder colors corresponding to higher concentrations. The stellar halo mass correlates well with both the BCG and the ICL mass, with a notably smaller scatter in the ICL case. In both relations, halo concentration plays a key role: stellar halos tend to be more massive in less concentrated dark matter halos, which are also the most massive systems.
• Figure 4: Similar to Figure \ref{['fig:scale1']}, the plots show the same scaling relations, but with the color bar now indicating the logarithm of the efficiency of ICL production in dark matter halos. The efficiency is defined as the ratio between the ICL mass and the halo mass. A clear trend emerges in both panels: stellar halos tend to be more massive when the efficiency of ICL production is higher. In the left panel, this appears at fixed BCG mass, where redder colors correspond to more massive stellar halos, while in the right panel the trend follows the main relation itself.
• Figure 5: Relation between g-r and r-i colors for the BCGs (red), stellar halos (black), ICL (blue) at different redshifts (separate panels), and as observed in VEGAS (green diamonds at $z=0$). Overall, the three components exhibit comparable colors, largely independent of redshift, and all progressively redden (shifting towards the right side of the diagrams) as the redshift approaches the present epoch.