Hot off the Stacks: X-rays in Romulus25, Part I - Comparisons to Observations from eRosita

Abstract

Recent observations from the from Spectrum Roentgen Gamma (SRG)/eROSITA all-sky survey data have helped to illuminate the hot circumgalactic medium (CGM) around galaxies at Milky Way-masses and above. In our study, we explore the hot X-ray emitting (T \textgreater 10$^6$ K) component of the CGM in over 750 galaxies from the Romulus25 cosmological simulation (Tremmel et al. 2017) between 10.0 < log(M$_*/M_{\odot}$) < 11.5. We forward-model soft X-ray [0.5 - 2.0] keV observations of our simulated galaxies using pyXSIM and SIXTE to measure the X-ray surface brightness (SB) profiles of the hot CGM. We then compare the average X-ray surface brightness profiles of six stellar-mass selected sub-samples to compare to the observational results from Chadayammuri et al. (2022a) and Zhang et al. 2024a. Broadly, we find that our stacked SB profiles are too bright compared to observations from Chadayammuri et al. (2022a) and Zhang et al.(2024a) particularly in the inner CGM. Additionally, we calculate the X-ray luminosity (L$_χ$) of the hot CGM for each galaxy and compare the average L$_χ$ across stellar mass to observations from Zhang et al. (2024b) as well as measurements from the EAGLE, TNG, SIMBA simulations. Across stellar mass, the average L$_χ$ of galaxies from Romulus25 are brighter than observations except at the lower, Milky Way-mass end where we are in relative agreement with both observations and most other simulations. We also consider a zero-metallicity emission model, because Romulus25 does not have metal-line cooling, finding the average L$_χ$ decreases by nearly an order-of-magnitude, which better matches observations particularly in the Milky Way-mass regime.