Soft X-ray line emission from hot gas in intervening galaxy halos and diffuse gas in the cosmic web

Department of Astronomy, Tsinghua University, 100084 Beijing, China

^⋆ Corresponding authors: zhangyuning@mail.tsinghua.edu.cn, dandanxu@tsinghua.edu.cn

Received: 20 March 2025
Accepted: 4 August 2025

Abstract

Cosmic hot-gas emission is closely related to halo gas acquisition and galactic feedback processes. Their X-ray observations reveal important physical properties and movements of the baryonic cycle of galactic ecosystems. However, the measured emissions toward a target at a cosmological distance would always include contributions from hot gases along the entire line of sight to the target. Observationally, such contaminations are routinely subtracted via different strategies. With this work, we aim to answer an interesting theoretical question regarding the amount of soft X-ray line emissions from intervening hot gases of different origins. We tackled this problem with the aid of the TNG100 simulation. We generated typical wide-field light cones and estimated their impacts on spectral and flux measurements toward X-ray-emitting galaxy-, group- and cluster-halo targets at lower redshifts. We split the intervening hot gases into three categories; that is, the hot gas that is gravitationally bound to either star-forming or quenched galaxy halos, and the diffuse gas, which is more tenuously distributed permeating the cosmic web structures. We find that along a given line of sight, the diffuse gas that permeates the cosmic web structures produces strong oxygen and iron line emissions at different redshifts. The diffuse gas emission in the soft X-ray band can be equal to the emission from hot gases that are gravitationally bound to intervening galaxy halos. The hot-gas emission from the quiescent galaxy halos can be significantly less than that from star-forming halos along the line of sight. The fluxes from all of the line-of-sight emitters as measured in the energy band of 0.4 − 0.85 keV can reach ∼20 − 200% of the emission from the target galaxy, group, and cluster halos. The fluxes from the intervening hot gas as measured in narrow bands around the O VII (r) and O VIII (Kα) are typically only a few percent of the target emission, indicating that these line emissions (as measured within narrow bands) better present the hot-gas emission of the target sources, compared to that measured in wider energy bands.