An MHD - driven Disk Wind Outflow in SDSS J 0300 + 0048 ?

The outflow in SDSS J0300+0048 has the highest column density yet reported for a broad absorption line quasar. The absorption from different ions is also segregated as a function of velocity in a way that can only be explained by a disk wind outflow. Furthermore, the presence of the such large column densities of gas at the high observed outflow velocities may be incompatible with purely radiative acceleration. MHD contributions to the acceleration should be considered seriously. 1. Observations and Implications SDSS J030000.56+004828.0 (Fig. 1) is a luminous, “overlapping-trough” broad absorption line (BAL) quasar (Hall et al. 2002) and one of only a handful of Ca ii BAL quasars known. High-resolution spectroscopy has been obtained for this object using the Ultraviolet Visual Echelle Spectrograph on the ESO VLT (Hall et al. 2003). Figure 2 shows the UVES spectrum around five important transitions, with outflow velocity increasing to the left. The Ca ii, Mg ii, and Mg i column densities in this object are the largest reported to date for any BAL outflow. The column density of metals is ∼200 times higher than in the only other Ca ii BAL studied at high resolution, QSO J2359−1241 (Arav et al. 2001). Figure 3 sketches the inferred structure of the outflow in SDSS J0300+0048. The large column density of Ca ii observed in this object can only exist in gas well shielded by an H i ionization front (Ferland & Persson 1989), and Ca ii is only seen at the lowest line-of-sight velocities in the outflow. Therefore, the lowest-velocity outflowing gas is farthest from the quasar. This result is very unlikely if BAL outflows are nearly spherical “cocoons” of dusty gas. On the other hand, it is easily understood in disk-wind models where the gas originating closest to the quasar is accelerated to the highest velocities, and where we can look across the streamlines of the outflow instead of just down along them. There is considerable other evidence for a disk wind outflow in this object: • looking across the streamlines can explain why the lowest velocity gas in the outflow is “detached” (blueshifted from the systemic redshift) by ∼1650 km s — the gas undergoes acceleration before it crosses our line of sight.