The hot gas mass fraction in halos: From Milky Way-like groups to massive clusters

By using eROSITA data in the eFEDS area, we provide a measure of the f_ gas -M_ halo relation over the largest halo mass range, from Milky Way-sized halos to massive clusters, and to the largest radii (R_200) ever probed so far in local systems at z< 0.2. To cope with incompleteness and selection biases of the X-ray selection, we applied the stacking technique in eROSITA data of a highly complete and tested sample of optically selected groups. The method has been extensively tested on mock observations. In massive clusters, the hot gas alone provides a baryon budget within R_200 consistent with Ω_ b /Ω_ m , while at the group mass scale, it accounts only for 20-40% of the cosmic value. The f_ gas -M_ halo relation is well fit by a power law with a consistent slope (within 1σ) at R_500 and R_200 and a normalization varying nearly by a factor of two. Such a relation is consistent with other works in the literature that consider X-ray survey data at the same depth as eFEDS, but it provides a lower average f_gas in the group regime in comparison to works based on X-ray bright group samples. Comparison of the observed relation with the predictions of several hydrodynamical simulations (BAHAMAS, FLAMINGO, SIMBA, Illustris, IllustrisTNG, MillenniumTNG, and Magneticum) shows that all state-of-the-art simulations except Magneticum overpredict the gas fraction, with the largest discrepancy (up to a factor three) being in the 10^13.5 M_⊙ to 10^14.5 M_⊙ halo mass range. We emphasize the need for mechanisms that can effectively expel gas to larger radii in galaxy groups without excessively quenching star formation in their member galaxies. Current hydrodynamical simulations face a significant challenge in balancing their subgrid physics, as none can sufficiently evacuate gas from the halo virial region without negatively impacting the properties of the resident galaxy population.