Strong Evidence for Cosmic Ray-Supported $\sim$L$^{\ast}$ Galaxy Halos via X-ray \& tSZ Constraints

TL;DR

This work demonstrates that joint X-ray and tSZ observations of CGM around $\sim L^{*}$ galaxies place strong, complementary constraints on the halo pressure budget. Analytic models show thermal-pressure-dominated halos struggle to reproduce both the extended X-ray emission and the low tSZ signal, while CR-pressure-dominated halos naturally reconcile these observations through CR-IC X-ray emission and reduced thermal pressure. Comparisons to simulations and mock observations indicate that halo CR pressure must be at least comparable to, and often exceeding, the thermal pressure, especially at MW-mass scales, implying significant CR transport and AGN feedback energetics. These findings provide the strongest, direct evidence to date for CR support in galaxy halos and point toward CR transport as a critical ingredient in galaxy formation across mass scales.

Abstract

Many state-of-the-art galaxy simulations featuring traditional feedback modes have significant challenges producing enough extended soft X-ray ($\sim 0.5-2$ keV) emission at R $\sim 0.5-1$ R$_{\rm vir}$ observed around galaxies with stellar masses M$_{\rm \ast} \lesssim 10^{11} \rm M_\odot$, without violating galaxy mass function constraints. Moreover, thermal Sunyaev-Zel'dovich (tSZ) measurements probing the thermal pressure of similar galaxies indicate it is orders-of-magnitude lower than predictions from simple halo hydrodynamics and many hydrodynamical simulations. We demonstrate that these constraints can be met congruously with a large non-thermal pressure contribution in the form of cosmic rays (CRs) from SNe and/or AGN, which lowers the tSZ signal while CR leptons produce plentiful soft X-rays via inverse Compton scattering of the CMB. The combination of these two observations is far more constraining on the pressure budget of galactic halos than either alone -- if these novel tSZ and X-ray observations are borne out by future studies, then taken together they reveal \textit{the strongest evidence for CR support in halos to date}. Conversely, it is very difficult to produce the extended X-rays via traditional thermal emission without increasing the overall thermal pressure and thus tSZ signal in tandem, making these tensions even worse. Finally, tSZ \& X-rays together unlock a novel observational method to constrain halo CR pressure relative to thermal pressure, with implications for CR transport parameters and AGN feedback energetics across various galaxy mass scales. Taking the currently observed constraints at M$_{\rm halo} \sim 10^{\rm 12} \rm M_\odot$ imply the halo CR pressure must at least be equal to the gas thermal pressure.

Figures (4)

• Figure 1: Radial profiles of P$_{\rm th}$ and P$_{\rm CR}$ in a simulated MW-mass halo. P$_{\rm CR}$ (green dot-dashed; m12i CR+) can replace P$_{\rm th}$ (green dashed; m12i CR+) if CRs effectively escape the disk of the galaxy, and support the halo in contrast to halos supported primarily via P$_{\rm th}$ (purple solid; m12i MHD+).
• Figure 2: Analytic and simulation predictions for $\tilde{Y}_{\rm 200}$ vs. log$_{\rm 10}$(M$_{\rm halo}$). The analytic prediction for P$_{\rm th}$-dominated halos (purple solid) lies factors of several times above the upper-limits and star-forming or quenched detections of das_thermal_2025 (black triangles, diamonds & circles), whereas predictions for P$_{\rm CR}$-dominated halos (green solid) can lie along the observations. Similarly, P$_{\rm th}$-dominated simulations like SIMBA and TNG (gold & red, dotted and dashed) generally over-predict $\tilde{Y}_{\rm 200}$ for quenched and star-forming by over a dex. Meanwhile, P$_{\rm CR}$-dominated FIRE simulations (m12i,f,m in order of M$_{\rm halo}$; green Xs) are in agreement with upper-limits, whereas the same halos without CRs, instead P$_{\rm th}$-dominated (purple pluses), lie above.
• Figure 3: Left:Analytic and Simulated XSB radial profiles in MW-mass halos. The analytic XSB profile for a P$_{\rm th}$-supported halo (purple solid) truncates steeply relative to the zhang_hot_2024 observations (black shaded and dotted), similarly to the P$_{\rm th}$-supported FIRE run (purple dot-dashed), whereas fiducial C-TNG (pink dashed) can match the outer XSB shape and C-SIMBA (gold dashed) vastly under-predicts the XSB. Right:Total outer CGM soft L$_{\rm X}$, for $\sim L^{\ast}$ galaxies, here integrated between 0.5-1 R$_{\rm vir}$. P$_{\rm CR}$-dominated MW-mass simulations (green Xs) and empirical CR-IC models with varied AGN leptonic injection (green diamonds, error-bars showing 0.3 dex scatter on $\dot{M_{\ast}}$) agree with the observed best-fit (black squares, bands showing stacked range in $M_{\ast}$), while in agreement with $\tilde{Y}_{\rm 200}$. P$_{\rm th}$-supported analytic predictions (purple circles) under-/over-predict L$_{\rm X}$ at low/high M$_{\rm \ast}$, C-TNG meets MW and M31-mass constraints but under-predicts at 2M31 mass, whereas C-SIMBA universally underpredicts L$_{\rm X}$. Unilaterally, all P$_{\rm th}$-supported models over-predict $\tilde{Y}_{\rm 200}$, irrespective of L$_{\rm X}$ predictions (c.f. Fig. \ref{['fig:Y200']}).
• Figure 4: Joint X-ray and tSZ observations constrain CGM Pressure: log$_{10}$ L$_{\rm X,\, CGM}$ vs. log$_{10}$$\tilde{Y}_{200}$ for a MW-mass halo at $z=0.08$ (M$_{\rm 200,\,12} = 1.26$), here normalized to the integral of the best-fit MW-mass XSB profile of zhang_hot_2024 between 0.5-1 R$_{\rm 200}$ ($\approx$100-200 kpc) and to our $\tilde{Y}_{200}$ predictions for a P$_{\rm CR}$-dominated halo (Eq. \ref{['eq:y200_CR_selfsim']}), respectively. The L$_{\rm X}$ prediction for a P$_{\rm CR}$-dominated halo (green diamond) with $\dot{E}_{40} = 0.66,\, v_{100} = 1$ lies close to the zhang_hot_2024 best-fit value, while in agreement with the tSZ upper-limit at M$_{\rm 200,\,12} = 1.25-2.59$ from das_thermal_2025(black triangle), while the P$_{\rm th}$-dominated halo (purple circle) is over 1 dex too high in $\tilde{Y}_{\rm 200}$. Varying $n_{\rm gas}$ or $T_{\rm gas}$ (purple dashed and solid lines) necessarily move P$_{\rm th}$-dominated in the wrong directions relative to the observational constraints, whereas changing P$_{\rm CR}$ or P$_{\rm gas}$ (green dotted and solid annotations) move P$_{\rm CR}$-dominated model predictions in orthogonal directions in this plane, allowing for models to meet observational constraints.