Abstract
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-A emission line is very prominent. (2) The BAL trough (BALnicity index 2990 km s −1 )i sdetached by 21 000 km s −1 and extends to velocity v =− 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 ,i sthe second highest known. The radio luminosity is P 1.4 GHz = 4.3 × 10 27 WH z −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. Ke yw ords: galaxies: high-redshift ‐ intergalactic medium ‐ quasars: absorption lines ‐ quasars: emission lines ‐ quasars: general ‐ early Universe.
Highlights
In 10–20 per cent of optically selected quasars, broad absorption lines (BALs) are seen in the blue wings of the ultraviolet (UV) resonance emission lines (e.g. C IV), due to gas with outflow velocities up to ∼0.2c (Hewett & Foltz 2003)
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
These results suggest that BALs are more common at high redshift, perhaps because of a higher accretion rate, which might affect the solid angle subtended at the quasar by the BAL flow, and the fraction of quasars observed to have BALs
Summary
In 10–20 per cent of optically selected quasars, broad absorption lines (BALs) are seen in the blue wings of the ultraviolet (UV) resonance emission lines (e.g. C IV), due to gas with outflow velocities up to ∼0.2c (Hewett & Foltz 2003). Maiolino et al (2004) recently found that of eight z > 4.9 quasars observed, four showed strong BALs, with two of these having unusually high BALnicity index, and two being LoBALs (which are rare at low redshift) These results suggest that BALs are more common at high redshift, perhaps because of a higher accretion rate, which might affect the solid angle subtended at the quasar by the BAL flow, and the fraction of quasars observed to have BALs. In short, the role of orientation in BALs is still not clear, and it is likely that detailed measurements of physical conditions within the outflows are required to make further progress. One possible signature of radiation pressure is absorption–absorption line-locking, and this has been observed in a few quasars (see Section 3.2.2)
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