Retrieving the hot circumgalactic medium physics from the X-ray radial profile from eROSITA with an IlustrisTNG-based forward model

¹ Max Planck Institute for Extraterrestrial Physics (MPE), Gießenbachstraße 1, 85748 Garching, Munich, Germany
² INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, (LC), Italy
³ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Munich, Germany
⁵ Exzellenzcluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany

^⋆ Corresponding author: shreeram@mpe.mpg.de

Received: 13 March 2025
Accepted: 8 September 2025

Abstract

Aims. Recent eROSITA measurements of the radial profiles of the hot circumgalactic medium (CGM) in the Milky Way stellar mass (MW-mass) regime provide us with a new benchmark to constrain the hot gas around MW-mass central and satellite galaxies and their halo mass distributions. Modeling this rich data set with state-of-the-art hydrodynamical simulations is required to further our understanding of the shortcomings in the current paradigm of galaxy formation and evolution models.

Methods. We developed forward models for the stacked X-ray radial surface brightness profile measured by eROSITA around MW-mass galaxies. Our model contains two emitting components: hot gas (around central galaxies and around satellite galaxies hosted by more massive halos) and X-ray point sources (X-ray binaries (XRBs) and active galactic nuclei (AGNs)). We modeled the hot gas profile using the TNG300-based products. We generated mock observations with our TNG300-based model (matching stellar mass and redshift with observations) with different underlying halo mass distributions. Therefore, we tested the CGM properties as a function of their host halo mass distribution. The point sources are described by a simple point spread function of eROSITA, and we fit their normalization in this work. In total, we fit the X-ray surface brightness profile with two free parameters: the normalization of satellites in more massive host halos and the normalization of the mean point source emission.

Results. We show that for the same mean stellar mass, a factor ∼2× increase in the mean value of the underlying halo mass distribution results in ∼4× increase in the stacked X-ray luminosity from the hot CGM. Using empirical models to derive a permissible range of AGN and XRB luminosities in the MW-mass X-ray galaxy stack, we choose our forward model that best describes the hot CGM for the eROSITA observations. Our chosen model in the MW stellar mass bin is in good agreement with previous literature results. We find that at ≲40 kpc from the galaxy center, the hot CGM from central galaxies and the X-ray point sources emission (from XRBs and AGNs) each account for 40 − 50% of the total X-ray emission budget. Beyond ∼40 kpc, we find that the hot CGM around satellites (probing their more massive host halos with mean M_200m ∼ 10¹⁴ M_⊙) dominates the stacked X-ray surface brightness profile.

Conclusions. The gas physics driving the shape of the observed hot CGM (in stellar-mass-selected X-ray stacking experiments) is tightly correlated with the underlying halo-mass distribution. This work provides a novel technique to constrain the AGN X-ray luminosity jointly with the radial hot CGM gas distribution within the halo using measurements from X-ray galaxy stacking experiments. Implementing this technique on other state-of-the-art simulations will provide a new ground for testing different galaxy formation models with observations.