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

M. Gaspari

doi:10.1093/mnrasl/slv067

paper

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

arXiv:1410.7769 · doi:10.1093/mnrasl/slv067

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: $d L_{x} / d T \propto (T / T_{hot})^{α = 2 \pm 1}$ . 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 $d ln P / d ln T$ leads to $α \approx 3$ . Self-regulated AGN outflow feedback can address the soft X-ray problem through the interplay of heating and turbulence.

5 pages, 2 figures, published in MNRAS Letters

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

Abstract

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