The connection between ultraviolet and X-ray outflows in active galactic nuclei: the case of PDS 456

High-velocity outflows from active galactic nuclei are a well-known phenomenon in terms of the broad absorption lines seen in the ultraviolet/optical. More recently, similar, possibly related, outflows have been reported in the X-ray. The most extreme example is seen in the nearby, luminous QSO PDS 456, which displays a massive, high-velocity (50 000 km s−1), high-ionization X-ray outflow of 10 M⊙ yr−1. Here we present the ultraviolet spectrum of PDS 456 as observed by the Hubble Space Telescope. We find that the ultraviolet spectrum is also extreme, displaying very broad emission lines, with C ivλ1549 blueshifted by 5000 km s−1 and a broad Lyα absorption trough blueshifted by 14 000–24 000 km s−1. No strong, broad high-ionization absorption features are seen. We interpret the combined ultraviolet and X-ray spectrum of PDS 456 as the signature of a decelerating, cooling outflow, which may be driven by radiation and/or magnetic field. This outflow may be the source of some of the broad emission- and absorption-line gas.