Galaxy cluster count cosmology with simulation-based inference

Abstract: The abundance and mass distribution of galaxy clusters is a sensitive probe of cosmological parameters, through the sensitivity of the high-mass end of the halo mass function to $\Omega_m$ and $\sigma_8$. While galaxy cluster surveys have been used as cosmological probes for more than a decade, the accuracy of cluster count experiments is still hampered by systematic, such as the relation between observables and halo mass, the accuracy of the halo mass function, and the survey selection function. Here we show that these uncertainties can be alleviated by forward modeling the observed cluster population with simulation-based inference. We construct a pipeline that predicts the distribution of observables from cosmological parameters and scaling relations, and then train a neural network to learn the mapping between the input parameters and the measured distributions. We focus on fiducial X-ray surveys with available flux, temperature, and redshift measurements, although the method can be easily adapted to any available observable. We apply our method to mock samples extracted from the UNIT1i simulation and demonstrate the accuracy of our approach. We then study the impact of several systematic uncertainties on the recovered cosmological parameters. We show that sample variance and the choice of the halo mass function are subdominant sources of uncertainty. Conversely, the absolute mass scale is the leading source of systematic error and must be calibrated at the $<10\%$ level to recover accurate values of $\Omega_m$ and $\sigma_8$. However, the quantity $S_8=\sigma_8(\Omega_m/0.3)^{0.3}$ appears to be less sensitive to the accuracy of the mass calibration. We conclude that simulation-based inference is a promising avenue for future cosmological studies from galaxy cluster surveys such as eROSITA and Euclid as it allows to consider all the available observables in a straightforward manner.