Modelling the selection of galaxy groups with end-to-end simulations

Abstract

Context. Feedback from supernovae and active galactic nuclei (AGN) shapes the galaxy formation and evolution, but its impact remains unclear. Galaxy groups offer a crucial probe to determine this impact because their gravitational binding energy is comparable to the energy that is available from their central AGN. The XMM-Newton Group AGN Project (X-GAP) is a sample of 49 groups that were selected in the X-ray (ROSAT) and optical (SDSS) bands and provides a benchmark for hydrodynamical simulations.Aims. For this comparison, it is essential to understand the selection effects. We model the selection function of X-GAP by forward-modelling the detection process in the X-ray and optical bands.Methods. Using the Uchuu N-body simulation, we built a dark matter halo light cone, predicted X-ray group properties with a neural network trained on hydrodynamical simulations, and assigned matching observed properties to the galaxies. We compared the selected sample to the parent population in the light cone.Results. Our method provided a sample that matched the observed distribution of the X-ray luminosity and velocity dispersion. A completeness of 50% was reached at a velocity dispersion of 450 km/s in the X-GAP redshift range. The selection is driven by X-ray flux, with a secondary dependence on the velocity dispersion and redshift. We estimated a purity level of 93% for the X-GAP parent sample. We calibrated the relation of the velocity dispersion to the halo mass. We found a normalisation and slope that agree with the literature and an intrinsic scatter of about 0.06 dex. The measured velocity dispersion is only accurate within 10% for rich systems with more than about 20 members, and the velocity dispersion for groups with fewer than 10 members is biased at more than 20%.Conclusions. The X-ray follow-up refines the optical selection and enhances the purity, but reduces completeness. In an SDSS-like set-up, measurement errors for the velocity dispersion dominate the intrinsic scatter. Our selection model enables unbiased comparisons of thermodynamic properties and gas fractions between X-GAP groups and hydrodynamical simulations.