A compact, metal-rich, kpc-scale outflow in FBQS J0209-0438: Detailed
  diagnostics from HST/COS extreme UV observations
  We present HST/COS observations of highly ionized absorption lines associated
with a radio-loud QSO at $z=1.1319$. The absorption system has multiple
velocity components, tracing gas that is largely outflowing from the QSO at
velocities of a few 100 km s$^{-1}$. There is an unprecedented range in
ionization, with detections of HI, NIII, NIV, NV, OIV, OIV*, OV, OVI, NeVIII,
MgX, SV and ArVIII. We estimate the total hydrogen number density from the
column density ratio N(OIV*)/N(OIV) to be
$\log(n_{\textrm{H}}/\textrm{cm}^3)\sim 3$. Assuming photoionization
equilibrium, we derive a distance to the absorbing complex of $2.3<R<6.0$ kpc
from the centre of the QSO. A range in ionization parameter, covering $\sim 2$
orders of magnitude, suggest absorption path lengths in the range
$10^{-4.5}<l_{\textrm{abs}}<1$ pc. In addition, the absorbing gas only
partially covers the background emission from the QSO continuum, which suggests
clouds with transverse sizes $l_{\textrm{trans}}<10^{-2.5}$ pc. Widely
differing absorption path lengths, combined with covering fractions less than
unity across all ions pose a challenge to models involving simple cloud
geometries. These issues may be mitigated by the presence of non-equilibrium
effects, together with the possibility of multiple gas temperatures. The
dynamics and expected lifetimes of the gas clouds suggest that they do not
originate from close to the AGN, but are instead formed close to their observed
location. Their inferred distance, outflow velocities and gas densities are
broadly consistent with scenarios involving gas entrainment or condensations in
winds driven by either supernovae, or the supermassive black hole accretion
disc. In the case of the latter, the present data most likely does not trace
the bulk of the outflow by mass, which could instead manifest itself as an
accompanying warm absorber, detectable in X-rays.
