For the first time, CLEVeR has demonstrated that most groups identified through galaxy clustering properties in spectroscopic surveys remain undetected by eROSITA's X-ray selection, as well as in other surveys of similar depth. The stacked X-ray surface brightness profile of these groups, which lack an X-ray counterpart in surveys like eRASS (eROSITA All Sky Survey), suggests they do contain hot gas, albeit with different spatial distributions compared to X-ray-detected groups. Specifically, they exhibit a much lower central gas concentration. This finding indicates that relying solely on X-ray surveys, which often probe only shallow depths over large areas, is insufficient for capturing the full range of density, entropy, and temperature profiles in low-mass halos, leading to biased conclusions.
The population of galaxy groups undetected by eROSITA represents the majority of the underlying dark matter halo distribution. Their lower central gas concentration may be attributed to supermassive black hole feedback expelling gas outward. This observation underscores potential biases in X-ray-based group selection and highlights the importance of using alternative techniques for studying gas distribution and black hole feedback, particularly at the group mass scale.
In conjunction with our observational analysis, we also produced mock observations that accurately reflect real observational datasets. Specifically, we generated mock galaxy catalogs and eROSITA mock observations using a 30x30 deg² light cone in the local Universe (z < 0.2) and a 25 deg² light cone at 0.6 < z < 1.1 based on the Magneticum simulation. This simulation is noted for its precise reproduction of optical and X-ray observables for galaxy groups and clusters. Mock eROSITA observations were created for the eRASS4 and eRASS8 depths, incorporating all instrumental effects via the SIXTE simulator. This pioneering experiment, which integrated galaxy spatial distribution with X-ray emission data, allowed us to test selection effects and better understand the physical processes modeled in the Magneticum simulation.
The mock eROSITA observations were crucial for evaluating the eRASS cluster and group selection functions against optical selection methods. We assessed the detection algorithm and selection function for eRASS groups and clusters (eSASS), revealing eROSITA's tendency to detect groups with high central gas concentration and low central entropy. Using a GAMA-like mock galaxy catalog, we evaluated different optical selection algorithms, confirming the robustness of optical group selection, which yields clean samples with minimal contamination. Comparisons between eROSITA and optical selection functions revealed that, as with observations, many optically selected groups remain undetected in the mock eROSITA data. These groups tend to show lower gas concentrations and higher entropy, possibly due to the effects of AGN feedback.
