Shaping the X-ray spectrum of galaxy clusters with AGN feedback and turbulence
The hot plasma filling galaxy clusters emits copious X-ray ra diation. The classic unheated and unperturbed cooling flow model predicts dramatic cooling ra tes 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) �=2±1 . Using 3D hydrodynamic simulations, we show that such deficit arises from the tight selfregulation between thermal instability condensation and AGN outflow injection: condensing clouds boost the AGN outflows, which quench cooling as they th ermalize through the core. The resultant average distribution slope is �≃ 2, oscillating within the observed 1 8), while unopposed cooling drives a too shallow slope, � < 1. AGN outflows deposit their energy inside-out, releasing more heat in the inner cooler phase; r adially distributed heating alone induces a declining spectrum, 1 <�< 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 � ≈ 3. Self-regulated AGN outflow feedback can address the soft X-ray problem through the interplay of heating and turbulence.