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arXiv · 2610.37769

Extra! Extra! X-ray Emission!: X-rays in Romulus25, Part II - Physical Drivers of X-ray Emission in the CGM

N. Nicole Sanchez·Andrew Benson·Gwen Rudie·Ben Oppenheimer·Hannah Bish·John ZuHone·Michael Tremmel
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Abstract

Characterizing the hot (T ~ 10$^{6}$ K) circumgalactic medium (CGM) around Milky Way-mass (MW-mass) galaxies is critical to a full understanding of galaxy evolution. In this second paper of the "X-rays in Romulus25" series, we unpack the specific physical processes giving rise to the presence of hot X-ray emitting gas in the halos of galaxies from the Romulus25 (R25) simulation, focusing on L$_{*}$ galaxies. We determine that a strong correlation appears between the mass of hot CGM gas, M$_{gas, T > 10^{5.5} K}$, and the X-ray emissivity, $ε_χ$, measured in the first paper of this series, (Sanchez et al. 2026a, submitted). There is also a secondary correlation between M$_{gas, T > 10^{5.5} K}$ and $ε_χ$ with the average metallicity of the hot CGM gas, $\bar{Z}_{gas, T > 10^{5.5} K}$. We use the well-studied PIONEER simulations from Sanchez et al. (2019), a set of zoom-in MW-mass galaxies run with the genetic modification technique of Roth et al. (2016) and the same physics as R25, to interpret these correlations. We determine that the presence of hot X-ray emitting gas is determined by two key processes: 1) the quantity of hot gas mass in the CGM, M$_{gas, T > 10^{5.5} K}$, is set by the combined total energy injection of $both$ stellar and AGN feedback, and 2) together the average temperature and metallicity of the hot CGM changes the soft X-ray spectrum of the gas resulting in brighter metal-rich CGM and dimmer metal-poor CGM for the same quantity of M$_{gas, T > 10^{5.5} K}$. These results indicate, for the first time, that metal-rich gas halos may bias X-ray observations of MW-mass galaxies.

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N. Nicole Sanchez, Andrew Benson, Gwen Rudie, Ben Oppenheimer, Hannah Bish, John ZuHone, Michael Tremmel. 2026-10-27. Extra! Extra! X-ray Emission!: X-rays in Romulus25, Part II - Physical Drivers of X-ray Emission in the CGM. https://arxiv.org/abs/2610.37769

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