  Galactic-scale winds driven by active galactic nuclei (AGN) are often invoked
to suppress star formation in galaxy evolution models, but the mechanisms
driving these outflows are hotly debated. Two key AGN feedback models are (1)
radiation pressure accelerating cool gas and (2) a hot outflowing wind
entraining the ISM. Highly ionized emission-line diagnostics represent a
powerful means of differentiating these scenarios because of their sensitivity
to the expected compression of the ISM clouds by the hot wind. Here, we report
the first spatially resolved UV emission spectroscopy of a prototypical
(radio-quiet) quasar-driven superwind around the obscured quasar SDSSJ1356+1026
at z=0.123. We observe ratios of OVI/CIV, NV/CIV, and CIV/HeII that are
remarkably similar for outflowing gas clouds <100 pc and ~10 kpc from the
nucleus. Such similarity is expected for clouds with AGN radiation pressure
dominated dynamics. Comparing the observed line emission to models of clouds in
balance with radiation pressure and/or a hot wind, we rule out the presence of
a dynamically important hot wind and constrain the ratio of hot gas pressure to
radiation pressure to P_hot/P_rad<0.25 both at <100 pc and ~10 kpc from the
nucleus. Moreover, the predictions of the radiation pressure confined cloud
models that best fit observed UV line ratios are consistent with the observed
diffuse X-ray spectrum. These results indicate that this AGN superwind is
driven by radiation pressure or was driven by a hot wind that has since
dissipated despite on-going AGN activity.
