Central gas entropy excess as a direct evidence for AGN feedback in
  galaxy groups and clusters
  By analyzing Chandra X-ray data of a sample of 21 galaxy groups and 19 galaxy
clusters, we find that in 31 sample systems there exists a significant central
($R^{<}_{\sim} 10h_{71}^{-1}$ kpc) gas entropy excess ($\Delta K_{0}$), which
corresponds to $\simeq 0.1-0.5$ keV per gas particle, beyond the power-law
model that best fits the radial entropy profile of outer regions. We also find
a distinct correlation between the central entropy excess $\Delta K_{0}$ and
$K$-band luminosity $L_{K}$ of the central dominating galaxies (CDGs), which is
scaled as $\Delta K_{0} \propto L_{K}^{1.6\pm0.4}$, where $L_{K}$ is tightly
associated with the mass of the supermassive black hole hosted in the CDG. In
fact, if an effective mass-to-energy conversion-efficiency of 0.02 is assumed
for the accretion process, the cumulative AGN feedback $E^{\rm AGN}_{\rm
feedback} \simeq \eta M_{\rm BH}c^{2}$ yields an extra heating of $\simeq
0.5-17.0$ keV per particle, which is sufficient to explain the central entropy
excess. In most cases the AGN contribution can compensate the radiative loss of
the X-ray gas within the cooling radius ($\simeq 0.002-2.2$ keV per particle),
and apparently exceeds the energy required to deviate the scaling relations
from the self-similar predictions ($\simeq 0.2-1.0$ keV per particle). In
contrast to the AGN feedback, the extra heating provided by supernova
explosions accounts for $\simeq 0.01-0.08$ keV per particle in groups and is
almost negligible in clusters. Therefore, the observed correlation between
$\Delta K_{0}$ and $L_{K}$ can be considered as a direct evidence for AGN
feedback in galaxy groups and clusters.
