The Radial Distribution of Galaxies in Λ Cold Dark Matter Clusters

We study the radial distribution of subhalos and galaxies using high-resolution cosmological simulations of galaxy clusters formed in the concordance Λ cold dark matter cosmology. In agreement with previous studies, we find that the radial distribution of subhalos is significantly less concentrated than that of the dark matter, when subhalos are selected using their present-day gravitationally bound mass. We show that the difference in the radial distribution is not a numerical artifact and is due to tidal stripping. The subhalos in the cluster core lose more than 70% of their initial mass since accretion, while the average tidal mass loss for halos near the virial radius is ≈30%. This introduces a radial bias in the spatial distribution of subhalos when the subhalos are selected using their tidally truncated mass. We demonstrate that the radial bias disappears almost entirely if subhalos are selected using their mass or circular velocity at the accretion epoch. The comparisons of the results of dissipationless simulations to the observed distribution of galaxies in clusters are therefore sensitive to the selection criteria used to select subhalo samples. Using simulations that also include cooling and star formation, we show that the radial distribution of subhalos is in reasonable agreement with the observed radial distribution of galaxies in clusters over the entire radial range probed by the simulations, 0.1 < R/R₂₀₀ < 2.0, if subhalos are selected using the stellar mass of galaxies that they harbor. The radial bias is minimized in this case because the stars are located in the centers of dark matter subhalos and are tightly bound. The stellar mass of an object is therefore approximately conserved as the dark matter is stripped from the outer regions. Nevertheless, the concentration of the radial distribution of simulated galaxies is systematically lower than that of the dark matter. Finally, we find that the baryon dissipation does enhance the survival of subhalos, especially in the cluster cores. However, the effect is relatively weak because the baryon dissipation affects the density distribution only at small radii.