Unusual high - redshift radio BAL quasar 1624 + 3758

We present observations of the most radio-luminous BAL quasar known, 1624+3758, at redshift z = 3.377. The quasar has several unusual properties: (1) The FeII UV191 1787-˚ A emission line is very prominent. (2) The BAL trough (BALnicity index 2990 kms −1) is detached by 21000 kms −1 and extends to velocity v =-29000 kms −1. There are additional intrinsic absorbers at-1900 and-2800 kms −1. (3) The radio rotation measure of the quasar, 18350 rad m −2 , is the second-highest known. The radio luminosity is P 1.4GHz = 4.3×10 27 WHz −1 (H 0 = 50 kms −1 Mpc −1 , q 0 = 0.5), radio loudness 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 CIV emission line, in conjunction with the total optical luminosity, implies 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 (2002) 2-component scheme for the classification of AGN, implying a very high accretion rate, and this may account for some of the unusual observed properties. The v =-1900 kms −1 absorber is a possible Lyman-limit system, with N (HI) = 4× 10 18 cm −2 , and covering factor 0.7. A complex mini-BAL absorber at v =-2200 –-3400 kms −1 is detected in each of CIV, NV and OVI. The blue and red components of the CIV doublet happen to be unblended, allowing both the covering factor and optical depth to be determined as a function of velocity. Variation of 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 CIV mini-BAL, suggesting that radiation pressure plays a role in accelerating the outflow.