The FLAMINGO Project: Exploring the X-ray--cosmic-shear cross-correlation as a probe of large-scale structure

TL;DR

This paper uses the FLAMINGO suite of large-volume cosmological hydrodynamical simulations to study how baryonic physics, especially AGN feedback, reshapes the distribution of hot gas in haloes and its imprint on the X-ray–cosmic-shear cross-correlation. By constructing full-sky X-ray and weak-lensing maps from on-the-fly lightcones and exploring multiple cosmological and baryonic-physics variations, the authors quantify the sensitivity of C_l^{xκ} to gas within haloes of roughly 10^{14}–10^{15} M_⊙ and to feedback strength, while highlighting a degeneracy with cosmology. They show that, in the absence of unresolved AGN, the fiducial gas-only model matches DES-Y3/ROSAT measurements, but including unresolved AGN contamination shifts the interpretation toward stronger feedback; abundance-matching-based AGN populations can reconcile some stronger-feedback models with data depending on the assumed level of contamination. The results emphasize that resolving faint AGN and incorporating external constraints are essential to exploit X-ray–lensing cross-correlations as robust probes of baryonic feedback and large-scale structure in upcoming surveys (eROSITA, Euclid). Overall, the work demonstrates the X-ray–cosmic-shear cross-correlation as a promising, but contamination-sensitive, avenue to constrain cluster gas fractions and AGN feedback within the ΛCDM framework.

Abstract

Baryonic feedback processes associated with galaxy formation directly influence the large-scale structure by redistributing gas. Recent measurements of the kinetic Sunyaev-Zel'dovich effect and stacks of X-ray emission from optically selected galaxy clusters suggest that feedback from Active Galactic Nuclei (AGN) is more efficient at expelling gas from low-mass clusters than previously thought. The measurement of the cross-correlation between cosmic shear and diffuse X-ray emission provides a new probe of the distribution of gas in groups and clusters. We use the FLAMINGO cosmological, hydrodynamical simulations to examine the X-ray--cosmic-shear cross-correlation. The cross-correlation is most sensitive to the distribution of gas in haloes with masses $10^{14}\leq M_{200\mathrm{c}}/\mathrm{M}_{\odot}\leq10^{15}$. It is sensitive to the strength of feedback, but the effects of variations in cosmology and baryonic physics are largely degenerate. We compare the FLAMINGO predictions with the cross-correlation between cosmic shear from the Dark Energy Survey and ROSAT all-sky X-ray maps. We find that, if we neglect the X-ray emission from AGN that would remain unresolved by ROSAT, then the fiducial FLAMINGO model is in excellent agreement with the data, while models with stronger or weaker feedback are ruled out. However, if we account for unresolved AGN, either using the direct FLAMINGO predictions or by abundance matching to the observed (extrapolated) AGN luminosity function, then models with stronger feedback are preferred. We conclude that to exploit the potential of the X-ray--lensing cross-correlation, it will be necessary to resolve fainter AGN, and to use external constraints to break the degeneracy between baryonic feedback and cosmology.

Figures (24)

• Figure 1: The predicted ROSAT--DES-Y3 X-ray--lensing cross-correlation from the L1$\_$m9 fiducial FLAMINGO simulation (green curves) compared to the observational data points of Ferreira_2024 (black points and error bars). The numbers in the upper left corner of each panel indicate the DES-Y3 tomographic bin of the shown cross-correlations (e.g. x-1, indicates the cross-correlation of the X-ray signal with the first tomographic bin of the lensing data). Here, the X-ray emission of the L1$\_$m9 simulation is convolved with the ROSAT response matrix (see § \ref{['sec:Xray_diffuse_gas_lightcone_methods']}). The solid curves correspond to the predicted cross-correlation if only the X-ray emission from hot gas is included, i.e. it is contamination free. The dotted curves include the X-ray emission from unresolved AGN given by the base FLAMINGO BHs. The dashed curves (AM) depict the AGN contaminated cross-correlations when the unresolved AGN are given by the AM BHs. The shaded regions ($\Delta{\mathrm{AM}}$) indicate the full range of possible cross-correlations from minimising and maximising the AGN contamination given by the $\Delta{\mathrm{AM}}$ BHs as described in the text. All sets of BHs (base, AM and $\Delta\mathrm{AM}$) are described in § \ref{['sec:BH_luminosities_and_selection']}). The inset in the top right panel depicts the normalised DES-Y3 source distribution, given by figure 2. of Doux_2022, and each tomographic bin (1 through 4, moving left to right) is individually coloured. Without the inclusion of unresolved AGN the simulation reproduces the measurements. However, AGN contamination increases the power of the cross-correlation over all angular scales considered, reducing the agreement with the data.
• Figure 2: The contribution of the diffuse X-ray emission to the X-ray--lensing cross-correlation, predicted by the L1$\_$m9 simulation, decomposed by the redshift of the X-ray emitting hot gas. It is assumed that the X-ray signal is uncontaminated by unresolved AGN. The redshift range of the hot gas is indicated by the colour bar. Additionally, the cross-correlations computed from the total diffuse X-ray signal integrated over the redshift ranges $0\lesssim z \leq 0.5$ (grey dot-dashed) and $0\lesssim z \leq 3.0$ (black dotted) are overlaid for a point of reference. Note that the lensing signal is computed for the maximum redshift depth of the L1$\_$m9 lightcone ($z\leq 3.0$). The tomographic bin number of each panel is displayed in the upper right corner. The contribution to the cross-correlation decreases with the redshift of the X-ray emitting gas and more than $90\%$ of the cross-correlation is due to gas at $z\leq 0.5$.
• Figure 3: The contribution to the X-ray--lensing cross-correlation of the L1$\_$m9 simulation decomposed by the mass ($M_{200\mathrm{c}}$) of the halo responsible for the X-ray emission. Here, it is assumed that the X-ray emission is only due to hot gas (i.e. uncontaminated by unresolved AGN). The X-ray all-sky map is decomposed into the contributions from gas within $R_{200\mathrm{c}}$ of all haloes ($r \leq R_{200\mathrm{c}},\mathrm{all~} M_{200\mathrm{c}}$, dashed black curve), haloes of a given mass, ($r>R_{200\mathrm{c}}(M_{200\mathrm{c}})$, solid curves) and outside of haloes ($r > R_{200\mathrm{c}}$, dot-dashed black curve). The colour of each solid curve corresponds to the halo mass range shown in the colour bar. Additionally the cross-correlation computed using all gas is overlaid in green. The X-ray emission from gas within haloes of masses $10^{14} \leq M_{200\mathrm{c}} / {\mathrm{M}}_\odot < 10^{15}$ dominates the predicted cross-correlation.
• Figure 4: The cosmology and baryonic feedback dependence of the predicted X-ray--lensing cross-correlation. In each main panel, the coloured curves correspond to the predicted cross-correlations in the third tomographic bin (x-3) for the different L1$\_$m9 simulations as indicated by the legend (see § \ref{['sec:flamingo_overview']} and Table \ref{['tab:cosmo_table']}). The lower sub-panels compare the variations to the fiducial L1$\_$m9 (dark green) simulation (i.e. $C_{\ell}^{\mathrm{x} \kappa }/\mathrm{L1}\_\mathrm{m9}$). Note we only consider the X-ray emission from hot gas and do not convolve the X-ray emission with the response matrix of any telescope. Left: The dependence on the cluster gas fraction ($\mathrm{fgas}$) that the model has been calibrated to. Reducing the cluster gas fraction (mainly by increasing the strength of AGN feedback) suppresses the power of the cross-correlation over all angular scales considered. Centre: The predicted cross-correlations for the remaining baryonic feedback variations. Reducing the stellar mass function suppresses the signal on all scales, however this effect is secondary to varying the cluster gas fraction. The choice of AGN feedback model (jet or thermal mode of feedback) has a very minor scale-dependent effect. Right: The dependence on cosmology and neutrino mass. The cross-correlation for the LS8 cosmology is suppressed relative to the fiducial model over all scales shown. Reducing the cluster gas fraction for the LS8 cosmology (LS8$\_\mathrm{fgas}-8\sigma$) further suppresses the signal. Increasing the summed neutrino mass relative to the fiducial value ($0.06$ eV) reduces the power over all scales shown. Comparing between the three panels, it is clear that the effects of varying the cosmology and baryonic feedback are largely degenerate.
• Figure 5: The dependence of the predicted X-ray--lensing cross-correlation on the simulation box size and numerical resolution. For clarity we show only the third tomographic bin and assume the soft X-ray emission is uncontaminated by unresolved AGN. The blue shaded region and solid curve correspond to the scatter and median for each of the 8 individual observers (lightcones) for the L2p8$\_$m9 simulation. The remaining solid curves correspond to a single lightcone for a given resolution model at a fixed box side length of $1~\mathrm{Gpc}$. The grey curves correspond to varying the cluster gas fraction by $+2\sigma$ (dashed), $-2\sigma$ (dotted) and $-8\sigma$ (dot-dashed) from the fiducial L1$\_$m9 model. In the bottom panel each simulation is shown relative to L1$\_$m9. The cross-correlation is converged with box size but not with resolution, as shown by the suppressed (elevated) signal of the high (low) resolution, L1$\_$m8 (L1$\_$m10), model relative to the intermediate resolution (L1$\_$m9 and L2p8$\_$m9) models. The dependence on resolution is likely indirect, reflecting residual differences in cluster gas profiles despite the recalibration. However, the differences between the different resolutions are smaller than the differences between the feedback variations.