The stellar-to-halo mass relation of central galaxies across three orders of halo mass

Abstract

The stellar content of galaxies is tightly connected to the mass and growth of their host dark matter halos. Observational constraints on this relation remain limited, particularly for low-mass groups, leaving uncertainties in how galaxies assemble their stars across halo mass scales. Accurately measuring the brightest central galaxy (BCG) stellar-to-halo mass relation (SHMR) over a wide mass range is therefore crucial for understanding galaxy formation and the role of feedback processes. Here we present the SHMR spanning $M_{\rm halo} \sim 10^{12}$-$10^{15}\,M_\odot$, using halo masses derived from eROSITA eRASS1 X-ray data and BCG stellar masses based on SDSS photometry. By stacking X-ray spectra of optically selected groups, we recover robust average halo gas temperatures for each bin, which are then converted to halo masses via the $M$-$T_X$ relation. We find that the SHMR peaks near $M_{\rm halo} \sim 10^{12}\,M_\odot$, with a declining stellar fraction at higher masses. This trend reflects a combination of processes that reduce the efficiency of stellar mass growth in massive halos, such as AGN feedback, reduced cooling efficiency, and the increasing dominance of ex-situ assembly, while halos continue to grow through mergers and accretion. Our measurements are consistent over the full mass range with previous observational studies, including weak lensing, X-ray analyses of individual clusters, and kinematical and dynamical methods. Comparisons with hydrodynamical simulations show good agreement at low masses but reveal significant discrepancies in the normalization at cluster scales, highlighting the sensitivity of BCG stellar growth to feedback prescriptions and halo assembly history. These results provide the first X-ray-based observational SHMR covering three orders of magnitude in halo mass, establish a robust benchmark for testing galaxy formation models.

Figures (9)

• Figure 1: Stacked X-ray temperatures within $R_{500}$ as a function of BCG stellar mass. Black points correspond to bins defined by stellar mass, while grey points correspond to bins defined by optically based halo mass yang_galaxy_2007; for both binning schemes, the average BCG stellar mass of each sub-bin is plotted on the $x$-axis. Error bars on the $y$-axis indicate $1\sigma$ uncertainties derived from bootstrap resampling, while error bars on the $x$-axis represent the 1$\sigma$ interval around the mean. Best-fit relations for both binning schemes are shown as black solid and grey dashed lines. The shaded area corresponds to the 1$\sigma$ and 3$\sigma$ uncertainties of the best-fit.
• Figure 2: BCG stellar-to-halo mass ratio as a function of halo mass. The meaning of the symbols is as in Figure \ref{['fig:mstar_t']}. Best-fit relations are shown as green and red dashed lines; the shaded region indicates the $1\sigma$ uncertainty of the double power-law best-fit relation. Literature measurements are color-coded by halo mass estimation method as indicated in the legend: violet for weak lensing (WL), red for X-ray, green for kinematical and dynamical studies (K), and light green for scaling relations (SR); for the literature data, filled points indicate stacking results, while empty points show results for individual sources. Orange lines correspond to semi-analytical predictions from abundance matching (AM) and empirical modeling (EM).
• Figure 3: Upper panel: Stellar-to-halo mass ratio as a function of halo mass for different assumptions on the gas metal abundance. Semi-transparent squares show the results of the main analysis, identical to those presented in Figure \ref{['fig:shmr_obs']}. Star symbols correspond to the same stacked spectra modeled using simulated metal abundances from the lower panel. Dashed lines indicate the best-fitting double powerlaw relations for each case, following Eq. \ref{['eq:behroozi']}. Lower panel: Mean gas metal abundance as a function of halo mass derived from the Magneticum simulation and adopted for the spectral modeling shown in the upper panel.
• Figure 4: BCG stellar-to-halo mass ratio as a function of halo mass: comparison with simulations. Black points show stacked results in bins of stellar mass, while grey points show stacked results in bins of halo mass. Colored lines and symbols represent predictions from state-of-the-art hydrodynamical simulations.
• Figure 5: X-ray-derived halo mass as a function of the average optically-based halo mass from yang_galaxy_2007 for bins of optical halo mass. The color of the points indicates the average stellar mass of the BCG.