Shaping the X-ray spectrum of galaxy clusters with AGN feedback and turbulence

TL;DR

This paper demonstrates through 3D hydrodynamic simulations that the observed soft X-ray deficit in galaxy clusters results from self-regulated AGN feedback and turbulence, which modulate the thermal instability and X-ray spectrum.

Contribution

It introduces a model showing how AGN feedback and turbulence naturally produce the observed X-ray spectral slope in galaxy clusters, resolving the soft X-ray problem.

Findings

Self-regulated AGN feedback reproduces the observed spectral slope α≈2.

Turbulence steepens the spectrum and increases scatter, with Mach numbers becoming transonic in cooler phases.

Absence of thermal instability results in an isothermal spectrum, inconsistent with observations.

Abstract

The hot plasma filling galaxy clusters emits copious X-ray radiation. The classic unheated and unperturbed cooling flow model predicts dramatic cooling rates and an isobaric X-ray spectrum with constant differential luminosity distribution. The observed cores of clusters (and groups) show instead a strong deficit of soft X-ray emission: $dL_{\rm x}/dT \propto (T/T_{\rm hot})^{\alpha=2\pm1}$. Using 3D hydrodynamic simulations, we show that such deficit arises from the tight self-regulation between thermal instability condensation and AGN outflow injection: condensing clouds boost the AGN outflows, which quench cooling as they thermalize through the core. The resultant average distribution slope is $\alpha \simeq 2$, oscillating within the observed $1<\alpha<3$. In the absence of thermal instability, the X-ray spectrum remains isothermal ($\alpha > 8$), while unopposed cooling drives a too shallow slope, $\alpha<1$. AGN outflows deposit their energy inside-out, releasing more heat in the inner cooler phase; radially distributed heating alone induces a declining spectrum, $1<\alpha<2$. Turbulence further steepens the spectrum and increases the scatter: the turbulent Mach number in the hot phase is subsonic, while it becomes transonic in the cooler phase, making perturbations to depart from the isobaric mode. Such increase in $d\ln P/d\ln T$ leads to $\alpha\approx3$. Self-regulated AGN outflow feedback can address the soft X-ray problem through the interplay of heating and turbulence.