The quasar PDS 456 ( at redshift \sim 0.184 ) has a prototype ultra-fast outflow ( UFO ) measured in X-rays .
This outflow is highly ionized with relativistic speeds , large total column densities \log { N _ { H } ( { cm } ^ { -2 } ) } > 23 , and large kinetic energies that could be important for feedback to the host galaxy .
A UV spectrum of PDS 456 obtained with the Hubble Space Telescope in 2000 contains one well-measured broad absorption line ( BAL ) at \sim 1346 Å ( observed ) that might be Ly \alpha at v \approx { 0.06 c } or N v \lambda 1240 at v \approx { 0.08 c } .
However , we use photoionisation models and comparisons to other outflow quasars to show that these BAL identifications are problematic because other lines that should accompany them are not detected .
We argue that the UV BAL is probably C iv \lambda 1549 at v \approx { 0.30 c } .
This would be the fastest UV outflow ever reported , but its speed is similar to the X-ray outflow and its appearance overall is similar to relativistic UV BALs observed in other quasars .
The C iv BAL identification is also supported indirectly by the tentative detection of another broad C iv line at v \approx { 0.19 } c .
The high speeds suggest that the UV outflow originates with the X-ray UFO crudely 20 to 30 r _ { g } from the central black hole .
We speculate that the C iv BAL might form in dense clumps embedded in the X-ray UFO , requiring density enhancements of only \gtrsim 0.4 dex compared clumpy structures already inferred for the soft X-ray absorber in PDS 456 .
The C iv BAL might therefore be the first detection of low-ionisation clumps proposed previously to boost the opacities in UFOs for radiative driving .