Ultraviolet signatures of the multiphase intracluster and circumgalactic media in the ROMULUS C simulation

Quasar absorption-line studies in the ultraviolet (UV) can uniquely probe the nature of the multiphase cool-warm (10⁴ < T < 10⁶ K) gas in and around galaxy clusters, promising to provide unprecedented insights into (1) interactions between the circumgalactic medium (CGM) associated with infalling galaxies and the hot (T > 10⁶ K) X-ray emitting intracluster medium (ICM), (2) the stripping of metal-rich gas from the CGM, and (3) a multiphase structure of the ICM with a wide range of temperatures and metallicities. In this work, we present results from a high-resolution simulation of an ∼ 10¹⁴ M☉ galaxy cluster to study the physical properties and observable signatures of this cool-warm gas in galaxy clusters. We show that the ICM becomes increasingly multiphased at large radii, with the cool-warm gas becoming dominant in cluster outskirts. The diffuse cool-warm gas also exhibits a wider range of metallicity than the hot X-ray emitting gas. We make predictions for the covering fractions of key absorption-line tracers, both in the ICM and in the CGM of cluster galaxies, typically observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope (HST). We further extract synthetic spectra to demonstrate the feasibility of detecting and characterizing the thermal, kinematic, and chemical composition of the cool-warm gas using H I, O VI, and C IV lines, and we predict an enhanced population of broad Ly α absorbers tracing the warm gas. Lastly, we discuss future prospects of probing the multiphase structure of the ICM beyond HST.