The hot circumgalactic medium in stacked X-rays: observations vs simulations

TL;DR

This work probes how AGN feedback shapes the hot circumgalactic medium by forward-modeling soft X-ray emission from two major cosmological simulations, SIMBA and EAGLE, and comparing to stacked eROSITA CGM observations. The authors generate synthetic X-ray maps with pyXSIM and instrument modeling via SOXS, include X-ray binaries as contaminants, and stack galaxies by stellar and halo mass to isolate CGM signals. They find that SIMBA generally matches inner CGM emission better at lower masses, while EAGLE underpredicts in some regimes, and neither model consistently reproduces the full observational sample across all radii and mass bins; inner CGM is density-dominated while outer CGM is more sensitive to temperature and metallicity. The results highlight the power of stacked X-ray observations to constrain AGN feedback physics in simulations, while also underscoring limitations from sample sizes and box volumes, and point to future multi-wavelength probes (e.g., SZ) to break degeneracies.

Abstract

Current cosmological simulations rely on active galactic nuclei (AGN) feedback to quench star formation and match observed stellar mass distributions, but models for AGN feedback are poorly constrained. The circumgalactic medium (CGM) provides a valuable laboratory to study this process, as its metallicity, temperature, and density distributions are directly shaped by AGN activity. Recent observations from the eROSITA instrument provide constraints on the CGM through measurements of extended soft X-ray emission. In this work, we generate synthetic eROSITA observations from the EAGLE and SIMBA cosmological simulations and compare them to observations of galaxies stacked by stellar mass, halo mass, and star-formation rate. SIMBA outperforms EAGLE in matching observed surface brightness profiles, but neither simulation achieves consistent agreement with observations across the full range of galaxy properties we studied. We find that variations in CGM X-ray emission between simulations are primarily driven by density differences at $R \lesssim 0.2 R_{200c} $, and temperature and metallicity changes at larger radii. These results highlight the need for further refinement of AGN feedback models in cosmological simulations and demonstrate the power of stacked X-ray observations as a tool for constraining feedback physics.

Figures (11)

• Figure 1: Halo-stellar mass relationship for the five simulations used in this work. Here we show the average central stellar mass for each 0.1 dex halo mass bin. Shaded regions represent the one sigma spread in each halo mass bin.
• Figure 2: Galaxy sample characteristics for the SIMBA (top) and EAGLE (bottom) analysis prior to resampling. Here we see redshift against stellar mass, with galaxies color-coded by halo mass. Star-forming galaxies (those with a sSFR $>$0.01) are represented with an x, while quiescent galaxies are shown as circles. Appendix \ref{['app:counts']} gives the sample sizes for the different simulated galaxy samples.
• Figure 3: Stacked synthetic observations of hot CGM emission for galaxies with $11< \;$log$(M_*/M_\odot)<11.25$ for (left to right) EAGLE, EAGLE-AGNdT9, EAGLE-NoAGN, SIMBA, SIMBA-NoAGN. Each box is 22.5 arcminutes across ($\approx$ 2.5 Mpc at $z=0.1$).
• Figure 4: Stacked synthetic observations of XRB emission for galaxies with $11< \;$log$(M_*/M_\odot)<11.25$ in EAGLE (top), and SIMBA (bottom). Each box is 22.5 arcminutes across ($\approx$ 2.5 Mpc at $z=0.1$).
• Figure 5: Radial profiles of the average mass-weighted metallicity (top), temperature (center), and density squared (bottom) of M31-type central galaxies ($11<\text{log}(M_*/M_\odot)<11.25$, $12<\text{log}(M_{200c}/M_\odot)<14$). These results only include gas particles with T$>10^6$ K, as cooler particles will not significantly contribute to the soft X-ray emission (see Figure \ref{['fig:apec']}). Error bars represent the 95$\%$ confidence interval generated by bootstrapping the radial profiles for all galaxies in the sample. Metallicities are given as the mass fraction of elements heavier than helium normalized to a solar value of 0.02.