  An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009
revealed an unusually rich absorption spectrum with outflow velocities, in both
RGS and EPIC spectra, up to ~ 9000 km/s (Pounds and Vaughan 2011). Evidence was
again seen for a fast ionised wind with velocity ~ 0.12c (Tombesi 2010, Pounds
and Vaughan 2012). Detailed modelling with the XSTAR photoionisation code now
confirms the general correlation of velocity and ionisation predicted by mass
conservation in a Compton-cooled shocked wind (King 2010). We attribute the
strong column density gradient in the model to the addition of strong two-body
cooling in the later stages of the flow, causing the ionisation (and velocity)
to fall more quickly, and confining the lower ionisation gas to a narrower
region. The column density and recombination timescale of the highly ionised
flow component, seen mainly in Fe K lines, determine the primary shell
thickness which, when compared with the theoretical Compton cooling length,
determines a shock radius of ~ 10^17 cm. Variable radiative recombination
continua (RRC) provide a key to scaling the lower ionisation gas, with the RRC
flux then allowing a consistency check on the overall flow geometry. We
conclude that the 2009 observation of NGC 4051 gives strong support to the idea
that a fast, highly ionised wind, launched from the vicinity of the
supermassive black hole, will lose much of its mechanical energy after shocking
against the ISM at a sufficiently small radius for strong Compton cooling.
However, the total flow momentum will be conserved, retaining the potential for
a powerful AGN wind to support momentum-driven feedback (King 2003; 2005). We
speculate that the `warm absorber' components often seen in AGN spectra result
from accumulation of shocked wind and ejected ISM.
