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

Recent eROSITA measurements of the radial profiles of the hot 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. Modelling 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. We develop 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 satellite galaxies hosted by more massive halos) and X-ray point sources (X-ray binaries and Active Galactic Nuclei). We model the hot gas profile using the TNG300-based products. We generate mock observations with our TNG300-based model (matching stellar mass and redshift with observations) with different underlying halo mass distributions. We show that for the same mean stellar mass, a factor 2x increase in the mean value of the underlying halo mass distribution results in a ~4x increase in the stacked X-ray luminosity from the hot CGM. The point sources are described by a simple point-spread-function (PSF) of eROSITA, and we fit their normalization in this work. 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 best describing the hot CGM for the eROSITA observations. We find that at < 40 kpc from the galaxy centre, the hot CGM from central galaxies and the X-ray point sources emission each account for 40-50% of the total X-ray emission budget. In summary, we show that the gas physics driving the shape of the observed hot CGM (in stellar-mass-selected samples) is tightly correlated by the underlying halo-mass distribution (abridged).

💡 Research Summary

This paper presents a forward‑modeling framework that connects the stacked X‑ray surface‑brightness profiles measured by eROSITA around Milky‑Way‑mass (MW‑mass) galaxies to the hot circumgalactic medium (CGM) predictions of the Illustris‑TNG300 cosmological hydrodynamical simulation. The authors focus on a photometric galaxy sample drawn from the Legacy Survey DR9, selecting 415 627 galaxies with stellar masses 10^10.5–10^11 M⊙ and redshifts 0.02–0.17. The observed stacked profile, S_X,total(r), is decomposed into two physically distinct components: (i) diffuse hot gas emission from central galaxies and from satellites that reside in more massive host halos, and (ii) point‑source emission from active galactic nuclei (AGN) and X‑ray binaries (XRBs).

The point‑source term is modeled as a simple normalization (N_ps) multiplied by the survey‑averaged eROSITA point‑spread function (PSF) converted from angular to physical scales using the redshift distribution of the stacked galaxies. The authors deliberately avoid using TNG’s intrinsic AGN and XRB predictions because previous work has shown that TNG over‑produces faint AGN and under‑produces bright AGN, while its star‑formation rates (and thus XRB luminosities) are uncertain for the relevant mass range. Instead, they adopt empirical constraints on the AGN X‑ray luminosity function and on the XRB L_X–SFR relation to define a plausible range for N_ps.

The hot‑gas component is generated directly from TNG300. A light‑cone (LC‑TNG300) is built using the box‑remap technique, covering redshifts 0.03–0.3 and an area of 47.28 deg². Subfind identifies centrals and satellites; for the MW‑mass stellar bin the light‑cone contains 5 109 centrals and 2 719 satellites (7 828 galaxies total). X‑ray photons in the 0.5–2 keV band are simulated with pyXsim (based on the APEC plasma model) assuming collisional ionization equilibrium. The authors introduce a single free parameter, N_sat, to scale the contribution from satellites, recognizing that the satellite’s own CGM is subdominant compared with the hot gas of the more massive host halo in which the satellite resides.

Both N_ps and N_sat are fitted to the observed stacked profile using Bayesian inference. The key findings are:

1. Halo‑mass dependence – Holding the mean stellar mass fixed, increasing the mean halo mass by a factor of two (e.g., from ~10^12.5 M⊙ to ~10^13 M⊙) boosts the stacked X‑ray luminosity from the hot CGM by roughly a factor of four. This non‑linear scaling reflects the strong dependence of gas temperature, density, and metallicity on halo potential depth.

2. Radial contribution budget – Within ~40 kpc of the galaxy centre, the hot CGM associated with central galaxies and the point‑source emission each account for about 40–50 % of the total X‑ray surface brightness. Beyond ~40 kpc, the stacked signal is dominated by the hot gas surrounding satellites, which effectively trace the CGM of more massive host halos (mean M_200m ≈ 10^14 M⊙).

3. Consistency with empirical models – The fitted N_ps lies comfortably within the range allowed by independent AGN luminosity‑function estimates and XRB scaling relations, confirming that the point‑source normalization derived from the X‑ray stack is physically plausible.

The authors discuss several implications. First, the stacked X‑ray signal of MW‑mass galaxies is highly sensitive to the underlying halo‑mass distribution; stellar mass alone is insufficient to predict CGM X‑ray output. Second, the simultaneous modeling of diffuse hot gas and point sources provides a self‑consistent way to disentangle their contributions, a crucial step for interpreting future deep X‑ray surveys. Third, the forward‑modeling pipeline can be ported to other state‑of‑the‑art simulations (e.g., EAGLE, SIMBA) to test how different feedback prescriptions affect observable X‑ray profiles.

Limitations are acknowledged. The photometric nature of the galaxy sample prevents a direct central‑vs‑satellite classification, necessitating the use of N_sat as a statistical correction. The point‑source model assumes a static PSF and a single normalization, ignoring possible variability of AGN or XRB populations. Moreover, the TNG300 resolution, while sufficient to resolve hot gas down to ~5 kpc, may still miss sub‑kiloparsec structure that could affect the innermost X‑ray profile.

In summary, this work demonstrates that a carefully constructed forward model, anchored in a high‑resolution cosmological simulation and calibrated with empirical point‑source constraints, can successfully reproduce the eROSITA‑measured X‑ray radial profiles of MW‑mass galaxies. The strong correlation between halo mass and CGM X‑ray luminosity highlighted here provides a new avenue for using X‑ray stacking as a probe of halo‑scale physics and for benchmarking galaxy‑formation models against upcoming all‑sky X‑ray data.