Forward-modelling the hot circumgalactic medium in X-rays

Shreeram, Soumya (2025): Forward-modelling the hot circumgalactic medium in X-rays. Dissertation, LMU München: Fakultät für Physik

Abstract

The hot phase of the circumgalactic medium (CGM) offers a unique window into the gas flows that shape galaxy evolution, serving as both a reservoir and a conduit for baryons cycling in and out of galaxies. The hot CGM, as observed in X-rays, probes the diffuse baryonic gas, helping to constrain feedback mechanisms and galaxy formation models. Recent advances in observational capabilities, particularly from the X-ray telescope eROSITA, have opened new avenues to investigate the properties of the hot CGM around Milky Way (MW)-mass galaxies, yielding critical benchmarks for cosmological hydrodynamical simulations. Yet significant challenges remain in interpreting these measurements due to projection effects, contributions from unresolved sources, and the complex influence of a galaxy’s large-scale environment. In this thesis, we build a fully self-consistent forward model of the hot CGM using the TNG300 cosmological hydrodynamical simulation. We construct a novel lightcone and generate mock X-ray observations based on intrinsic gas cell properties, so-called LC-TNGX, to enable direct comparisons with observations. Our analytical modelling captures intrinsic X-ray surface brightness profiles across stellar and halo mass bins. We find that higher stellar mass bins correspond to shallower slopes of the intrinsic galactocentric profiles, quantified via decreasing values of the profile surface brightness exponent β. Critically, we quantify the effect of satellite galaxies incorrectly identified as centrals in stacking experiments, which biases the derived hot CGM X-ray surface brightness profiles. For stellar mass bins similar to MW-masses, M31-masses, and twice M31-masses, we demonstrate that even modest contamination fractions (as low as 1%) can dominate the measured X-ray surface brightness profiles at large radii. Specifically, in the MW-mass bin, misclassified centrals contributing 30%, 10%, or 1% of a stacked sample dominate the measured surface brightness profile beyond radii of 0.11 × R₅₀₀, 0.24 × R₅₀₀, and 1.04 × R₅₀₀, respectively. Building on this framework, we develop forward models tailored to the eROSITA stacked X-ray radial surface brightness profiles of MW-mass galaxies. Our model includes two key emission components: hot gas around both central and satellite galaxies, and point-source contributions from X-ray binaries (XRBs) and active galactic nuclei (AGN). We simulate mock observations using the TNG300-based gas profiles, carefully matching stellar mass and redshift distributions from the observations, and explore how variations in the underlying halo mass distribution affect the results. We show that for galaxy samples matched in stellar mass, increasing the mean halo mass by a factor of ~2 leads to a ~4× enhancement in the stacked X-ray luminosity of the hot CGM. By incorporating empirical constraints on AGN and XRB luminosities, we identify the model that best matches the eROSITA data. In the MW stellar mass bin, our model agrees well with prior literature. We find that within approximately 40 kpc from the galaxy centre, the hot CGM and point-source emission each contribute ~40–50% of the total stacked X-ray emission. Beyond ~40 kpc, the hot CGM emission from satellite galaxies, tracing more massive host halos with a mean M₂₀₀ ~ 10¹⁴ solar masses, dominates the stacked signal. This approach offers a novel method to jointly constrain the mean AGN X-ray luminosity and the radial hot CGM gas distribution in MW-mass galaxies, enabling tests of AGN feedback prescriptions in hydrodynamical simulations. In addition to observational projection effects, we investigate how the large-scale cosmic environment shapes the hot CGM X-ray properties. Using the DisPerSE filament-finding algorithm on our TNG300-based lightcone spanning 0.03 ≤ z ≤ 0.3, LC-TNGX, we classify central galaxies into five distinct large-scale environment categories: clusters and massive groups, cluster outskirts, filaments, filament-void transition regions, and voids/walls. We find that the X-ray surface brightness profiles of central galaxies in filaments with M₂₀₀ > 10¹² solar masses are 20–45 % brighter in the radial range of (0.3–0.5) × R₂₀₀ compared to those in voids and walls. This excess arises from higher average gas densities, temperatures, and metallicities in filament galaxies, revealing a clear imprint of the cosmic web on hot CGM properties. Our findings highlight the importance of accounting for the cosmic environment in interpreting X-ray CGM measurements and suggest promising avenues for future studies exploring how the assembly history, gas accretion, and connectivity in the cosmic web shape the hot gas content around galaxies. Taken together, the progress outlined in this thesis underscores that fully understanding the hot CGM requires an integrated approach, combining X-ray observations, careful modelling of projection and environmental effects, and robust comparisons across multiple hydrodynamical simulations. The methods and models developed here provide a critical framework for interpreting hot CGM observations enabled by current and future X-ray surveys, allowing us to disentangle the complex interplay of baryonic physics, AGN feedback, halo demographics, and the cosmic web in shaping the hot gas around galaxies. As new data arrives from next-generation missions like NewAthena and HUBS, alongside increasingly sophisticated simulations, we stand at the threshold of using the hot CGM as a precise cosmological and astrophysical probe. The challenges ahead in linking microphysical feedback processes to large-scale observables, quantifying environmental impacts, and constraining the diverse mechanisms governing baryon cycles offer an exciting frontier where theory and observation converge to refine our models of galaxy formation and evolution.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: circumgalactic medium, hydrodynamical simulations, modelling, X-rays, extragalactic astronomy, hot gas, eROSITA, large-scale structure, galaxy formation and evolution, galaxy statistics
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 14. November 2025
• 1. Bericht­erstatter:in: Nandra, Kirpal