Detecting galaxy groups populating the local Universe in the eROSITA era

Context. eROSITA will deliver an unprecedented volume of X-ray survey observations, 20 − 30 times more sensitive than ROSAT in the soft band (0.5 − 2.0 keV) and for the first time imaging in the hard band (2 − 10 keV). The final observed catalogue of sources will include galaxy clusters and groups along with obscured and unobscured (active galactic nuclei) AGNs. This calls for a powerful theoretical effort to mitigate potential systematics and biases that may influence the data analysis.
Aims: We investigate the detection technique and selection biases in the galaxy group and AGN populations within a simulated X-ray observation conducted at the depth equivalent to a four-year eROSITA survey (eRASS:4).
Methods: We generate a mock observation spanning 30 × 30 deg² based on the cosmological hydrodynamical simulation Magneticum Pathfinder from z = 0 up to redshift z = 0.2, mirroring the depth of eRASS:4 (with an average exposure of ∼600 s). We combined a physical background from the real eFEDS background analysis with realistic simulations of X-ray emission for the hot gas, AGNs, and XRB. Using a detection method similar to that utilised for eRASS data, we assessed completeness and contamination levels to reconstruct the luminosity functions for both extended and point sources within the catalogue.
Results: We define the completeness of extended detections as a function of the input X-ray flux S​₅₀₀ and halo mass M₅₀₀ at the depth of eRASS:4. Notably, we fully recovered the brightest (most massive) galaxy clusters and AGNs. However, a significant fraction of galaxy groups (M₂₀₀ < 10¹⁴ M_⊙) remain undetected. Examining gas properties between the detected and undetected galaxy groups at a fixed halo mass, we observe that the detected population typically displays higher X-ray brightness compared to the undetected counterpart. Furthermore, we establish that X-ray luminosity primarily correlates with the hot gas fraction, rather than temperature or metallicity. Our simulation suggests a systematic selection bias in current surveys, leading to X-ray catalogues predominantly composed of the lowest-entropy, gas-richest, and highest surface brightness halos on galaxy group scales.