Cluster properties as a function of dynamical state in the DESI Legacy x UNIONS surveys

TL;DR

This study demonstrates that the dynamical state of galaxy clusters—quantified via the BCG–second galaxy magnitude gap $M_{r,12}$ and the BCG–fourth galaxy stellar-mass ratio $M_{,14}$—strongly influences both galaxy populations and mass distributions. By cross-matching DESI Legacy Imaging Surveys clusters with UNIONS weak-lensing data, the authors split clusters into evolved and evolving subsamples and compare their stellar mass functions, BCG growth, and weak-lensing profiles, finding that evolved clusters host more massive BCGs, a bimodal SMF, and steeper inner lensing profiles, in contrast to evolving clusters which show flatter centers and more massive satellites. They validate these trends in the IllustrisTNG $TNG300-1$ simulations, where evolved halos exhibit earlier mass assembly ($z_{50}\sim1.3$) and higher central concentrations, linking observable structure to assembly history. The results highlight the role of dynamical state and formation history in shaping cluster galaxy populations and mass distributions, with implications for assembly bias, halo occupation modeling, and robust cosmological inferences from cluster surveys.

Abstract

We investigate how the dynamical state of galaxy clusters influences their galaxy populations and mass distributions. Using photometrically selected clusters from the DESI Legacy Imaging Survey cross-matched with the UNIONS galaxy shear catalogue, we classify clusters as evolved or evolving based on their rest-frame r-band magnitude gaps and stellar mass ratios between the brightest cluster galaxies (BCGs) and bright satellites. We measure the stellar mass functions, weak-lensing profiles, and radial number density and red-fraction profiles of stacked clusters in both subsamples. Evolved clusters exhibit more concentrated lensing profiles, bimodal stellar mass functions dominated by massive BCGs, and a deficit of intermediate-mass satellites, while evolving clusters show flatter central lensing signals and an excess of massive satellites. Applying the same selection to IllustrisTNG clusters reproduces these trends and links the observed differences to distinct mass accretion histories. These results demonstrate the close link between cluster galaxy populations and the overall dynamical state of their underlying dark matter halo.

Figures (26)

• Figure 1: Selection of the evolved and evolving subsamples. The $x$-axis shows the absolute $r-$band magnitude gap between the BCG and the brightest satellite galaxy and the $y$-axis shows the log of the stellar mass ratio of the BCG to the 3rd most massive satellite of each galaxy cluster. The dashed lines and shaded regions indicate our selection regions for the evolved and evolving samples.
• Figure 2: Flowchart showing our sample selection process combining multiple catalogues.
• Figure 3: Distribution of cluster properties in the evolved and evolving samples. From left to right: Halo mass (average value 13.95 and 13.99 for evolved and evolving samples), redshift (average value 0.34 and 0.33 for evolved and evolving samples), and number of member galaxies within R$_{500c}$ (average value 16.7 and 20.1 for evolved and evolving samples), respectively. The average values of each sample are shown by vertical lines of the corresponding colours in each panel.
• Figure 4: Stellar mass functions (SMFs) of cluster galaxies in two redshift bins ( $0.0 \leq z < 0.25$, blue; $0.25 \leq z < 0.5$, orange) for all (evolved+evolving) clusters (left), evolved clusters (middle), and evolving clusters (right). Solid lines include all members within $R_{500c}$, while dashed lines show SMFs of satellites only, excluding the BCG. Evolved systems display a bimodal SMF, with the high-mass peak corresponding to the BCGs reaching $\log_{10} (M_{\star}/\mathrm{M}_{\odot}) \approx 12.5$, whereas BCGs in the evolving sample extend only to $\log_{10} (M_{\star}/\mathrm{M}_{\odot}) \approx 12.0$.
• Figure 5: Stellar mass functions (SMFs) of the evolved (magenta) and evolving (teal) clusters across all redshifts. The left panel shows SMF with all cluster members, and the right panel shows satellites only. The SMF with all cluster members extend to larger stellar mass for the evolved sample, whereas the trend is opposite for the satellites. Either way, the SMFs are different for the evolved and evolving subsamples.