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 Feu 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 v = -29000 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 P 1.4GHz = 4.3 × 10 27 W Hz -1(H 0 = 50 km s -1 Mpc -1, q 0 = 0.5) and the radio loudness is R* = 260. The radio source is compact (≲ 2.8 kpc) 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 M BH ∼ 10 9 M ⊙, L/L Eddington ≈ 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 v = -1900 km s -1 absorber is a possible Lyman-limit system, with N(H I) = 4 × 10 18 cm -2, and a covering factor of 0.7. A complex mini-BAL absorber at v = -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 -τ ≈ 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.