Unusual high-redshift radio broad absorption-line quasar 1624+3758

We present observations of the most radio-luminous broad absorption-line (BAL) quasar known, 1624+3758, at redshift z= 3.377. The quasar has several unusual properties. (1) The Fe ii UV191 1787-Å emission line is very prominent. (2) The BAL trough (BALnicity index 2990 km s−1) is detached by 21 000 km s−1 and extends to velocity υ=−29 000 km s−1. There are additional intrinsic absorbers at −1900 and −2800 km s−1. (3) The radio rotation measure of the quasar, 18 350 rad m−2, is the second highest known. The radio luminosity is P1.4 GHz= 4.3 × 1027 W Hz−1 (H0= 50 km s−1 Mpc−1, q0= 0.5) and the radio loudness is R*= 260. The radio source is compact and the radio spectrum is GHz-peaked, consistent with it being relatively young. The width of the C iv emission line, in conjunction with the total optical luminosity, implies a black hole mass MBH~109 M⊙, L/LEddington≈ 2. The high Eddington ratio and the radio-loudness place this quasar in one corner of Boroson's two-component scheme for the classification of active galactic nuclei, implying a very high accretion rate, and this may account for some of the unusual observed properties. The υ=−1900 km s−1 absorber is a possible Lyman-limit system, with N(H i) = 4 × 1018 cm−2, and a covering factor of 0.7. A complex mini-BAL absorber at υ=−2200 to −3400 km s−1 is detected in each of C iv, N v and O vi. The blue and red components of the C iv doublet happen to be unblended, allowing both the covering factor and optical depth to be determined as a function of velocity. Variation of the covering factor with velocity dominates the form of the mini-BAL, with the absorption being saturated (e−t≈ 0) over most of the velocity range. The velocity dependence of the covering factor and the large velocity width imply that the mini-BAL is intrinsic to the quasar. There is some evidence of line-locking between velocity components in the C iv mini-BAL, suggesting that radiation pressure plays a role in accelerating the outflow.