X-ray emission in IllustrisTNG circum-cluster environments. II -- Possible origins of the soft X-ray excess emission

Abstract

An excess of soft X-ray emission (0.2-1 keV) above the contribution from the hot intra-cluster medium (ICM) has been detected in a number of galaxy clusters, including the Coma cluster. The physical origin of this emitting medium above hot ICM has not yet been determined, especially whether it be thermal or non-thermal. We aim to investigate which gas phase and gas structure more accurately reproduce the soft excess radiation from the cluster core to the outskirts, using simulations. By using the simulation TNG300, we predict the radial profile of thermodynamic properties and the Soft-X-ray surface brightness of 138 clusters within 5 $R_{200}$. Their X-ray emission is simulated for the hot ICM gas phase, the entire Warm-Hot medium, the diffuse and low-density Warm-Hot Intergalactic Medium (WHIM). Inside clusters, the soft excess appears to be produced by substructures of the WARM gas phase which host dense warm clumps (i.e, the Warm Circum-Galactic Medium, WCGM), and in fact the inner soft excess is strongly correlated with substructure and WCGM mass fractions. Outside of the virial radius, the fraction of WHIM gas that is mostly inside filaments connected to clusters boosts the soft X-ray excess. The more diffuse the gas is, the higher the soft X-ray excess beyond the virial region. The thermal emission of WARM gas phase, in the form of WCGM clumps and WHIM diffuse filaments, reproduces well the soft excess emission that was observed up to the virial radius in Coma and in the inner regions of other massive clusters. Moreover, our analysis suggests that soft X-ray excess is a proxy of cluster dynamical state, with larger excess being observed in the most unrelaxed clusters.

Figures (11)

• Figure 1: Median radial profile of the gas properties for the two main gas phases (HOT and WARM gas, as defined in Table \ref{['tab:gas']}) as a function of the mass bins $M_1$, $M_2$, and $M_3$, plotted in blue, purple, and orange, respectively (as defined in Table \ref{['tab:sample']}). The error bars are the percentiles from 20% to 80%. The average density (top left), temperature (top right), metal abundance (bottom left), and clumpiness (bottom right) profiles are illustrated in the different panels.
• Figure 2: Top panel: Mean radial profiles of the integrated ICM (solid lines), WARM (dashed lines), and WHIM (dotted lines) gas masses for the three mass bins $M_1$(salmon lines), $M_2$ (purple lines), and $M_3$ (blue lines). Bottom panels: Radial profile of the mass fraction of WARM gas (second panel), composed of WHIM and WCGM (third panel), in the hot ICM mass. The error bars are the error on the mean.
• Figure 3: Top panel: Mean radial X--ray surface brightness profiles for the three cluster mass bins ($M_1$, $M_2$, and $M_3$) simulated in the 0.2-0.4 KeV band. Bottom panels: WARM–to-ICM (dashed lines) and WHIM–to-ICM (dotted lines) surface brightness ratios. The error bars are the error on the mean.
• Figure 4: Median X--ray surface brightness profiles from $M_1$ and $M_3$ clusters in the main--band X--ray ranges. The 0.3-2.3 keV range to match the best--fit model of the eRosita observations by lyskova2023 is shown in green, and the band of 0.7-7 keV to match the observations of Coma mirakhor2020 is shown as a blue curve. The error bars in the simulated profiles are the percentiles from 20% to 80%.
• Figure 5: Left panel: Median WARM--to--ICM surface brightness ratio in the 0.2-0.4 keV band for the three cluster mass bins and the ROSAT results of bonamente2002 for a sample of 38 clusters. Right panel: Median WARM--to--ICM surface brightness ratio in the 0.2-1 keV band for clusters in the $M_4$ mass bins and the observational results of the soft-X-ray excess from the Coma cluster bonamente2022c. For each line, the shaded area represents the 20th to 80th percentiles of the soft excess within each cluster sample.