2015MNRAS.451L..60G

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: dLx/dT∝(T/Thot)α = 2±1. 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 α ≃ 2, oscillating within the observed 1< α <3. In the absence of thermal instability, the X-ray spectrum remains isothermal (α ≳ 8), while unopposed cooling drives a too shallow slope, α < 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 <α< 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 dlnP/dlnT leads to α ~ 3. Self-regulated AGN outflow feedback can address the soft X-ray problem through the interplay of heating and turbulence.