  We present a detailed analysis of a very unusual sub-damped Lyman alpha
(sub-DLA) system at redshift z=2.304 towards the quasar Q0453-423, based on
high signal-to-noise (S/N), high-resolution spectral data obtained with
VLT/UVES. With a neutral hydrogen column density of log N(HI)=19.23 and a
metallicity of -1.61 as indicated by [OI/HI] the sub-DLA mimics the properties
of many other optically thick absorbers at this redshift. A very unusual
feature of this system is, however, the lack of any CIV absorption at the
redshift of the neutral hydrogen absorption, although the relevant spectral
region is free of line blends and has very high S/N. Instead, we find high-ion
absorption from CIV and OVI in another metal absorber at a velocity more than
220km/s redwards of the neutral gas component. We explore the physical
conditions in the two different absorption systems using Cloudy photoionisation
models. We find that the weakly ionised absorber is dense and metal-poor while
the highly ionised system is thin and more metal-rich. The absorber pair
towards Q0453-423 mimics the expected features of a galactic outflow with
highly ionised material that moves away with high radial velocities from a
(proto)galactic gas disk in which star-formation takes place. We discuss our
findings in the context of CIV absorption line statistics at high redshift and
compare our results to recent galactic-wind and outflow models.
