Extended X-ray Emission in the HI Cavity of NGC 4151: Galaxy-scale AGN
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  We present the Chandra discovery of soft diffuse X-ray emission in NGC 4151
(L[0.5-2keV]~10^{39} erg s$^{-1}$), extending ~2 kpc from the active nucleus
and filling in the cavity of the HI material. The best fit to the X-ray
spectrum requires either a kT~0.25 keV thermal plasma or a photoionized
component. In the thermal scenario, hot gas heated by the nuclear outflow would
be confined by the thermal pressure of the HI gas and the dynamic pressure of
inflowing neutral material in the galactic disk. In the case of
photoionization, the nucleus must have experienced an Eddington limit outburst.
For both scenarios, the AGN-host interaction in NGC 4151 must have occured
relatively recently (some 10^4 yr ago). This very short timescale to the last
episode of high activity phase may imply such outbursts occupy $\gtrsim$1% of
AGN lifetime.
