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

TL;DR

This work uses the IllustrisTNG TNG100 simulation to theoretically quantify soft X-ray line contamination from intervening hot gas along typical lines of sight toward halo targets. By constructing wide-field light cones and applying APEC-based X-ray emission calculations to gas categorized as star-forming-halo gas, quenched-halo gas, and diffuse cosmic-web gas, the study reveals that diffuse gas can produce strong line emissions at various redshifts and that line-of-sight contamination can be substantial in broad soft X-ray bands. The findings show star-forming halos typically dominate line-of-sight emission over quenched halos, while diffuse gas can contribute comparably to target halo emission in the 0.1–2 keV band; narrow-band analyses around Ovii and Oviii lines mitigate contamination, highlighting the importance of high spectral resolution for separating target CGM emission from intervening gas. These results underscore the significance of line-of-sight effects for upcoming X-ray missions and motivate cross-simulation comparisons to understand baryonic feedback's role in shaping hot gas distributions across cosmic structures.

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.

Figures (9)

• Figure 1: Specific star formation rate (sSFR) as a function of stellar mass for all galaxies at $z=0.01$ (snapshot 98). The stellar mass here was calculated using all stellar particles within twice the stellar half-mass radius of a given subhalo. The red dash-dotted line represents the mean sSFR. The red dashed line represents $1dex$ below the mean sSFR, below which galaxies are classified as quenched galaxy populations 2021MNRAS.503..726L.
• Figure 2: X-ray emissivity maps of observation targets only; i.e., generated using all gas particles bound to the target halos and no emissions from the foreground or background added to the map. The X-ray energy range is $\SIrange{0.1}{2}{keV}$. Panels from left to right: Galaxy with a total mass of $M = 3.06e12M_{\sun}$ at $z=0.03$, galaxy group with a total mass of $M = 7.00e13M_{\sun}$ at $z=0.11$, and galaxy cluster with a total mass of $M = 3.78e14M_{\sun}$ at $z=0.11$. Each panel is $256 \times 256$ in dimension, corresponding to a sky region of $1\degr \times 1\degr$. The white dash-dot circle denotes $R_{\mathrm{200c}}$ of the target.
• Figure 3: X-ray emissivity maps of the light cone with FoV of $1\degr \times 1\degr$. In each of the three panels from left to right, we selected five subregions (indicated by white dash-dotted circles) that have radii of $R_{\mathrm{200c}}$ corresponding to the target galaxy, group, and cluster (as given in Fig. \ref{['fig:targets']}), respectively.
• Figure 4: Phase-space diagrams of gas in the light cone. Left panels: Temperature versus density. Right panels: Temperature versus metallicity. Panels from top to bottom: Star-forming galaxies, quenched galaxies, and diffuse gas. The color maps represent the mass fraction of gas with respect to the total gas mass.
• Figure 5: Two example combined spectra of target galaxy (blue) and light cone emission in two selected subregions (R2 and R4). The spectrum energy range shown here is $\SIrange{0.4}{0.85}{keV}$ and the bin width is $0.2eV$. These were calculated within a radius of $R_{\mathrm{200c}}$ of the target galaxy and centered on the galaxy. The top panel presents spectra of a subregion (R4) with abundant strong emission lines from the light cone. The bottom panel shows spectra of a subregion (R2) with less abundant emission lines from the light cone. The labeled vertical black bars denote the most significant emission lines in the light cone, with identities and redshifts tagged. SFG represents star-forming galaxies, QG represents quenched galaxies, and DG represents diffuse gas.