Search

We obtain a sample of 87 radio-loud quasi-stellar objects (QSOs) in the redshift range 3.6 ≤ z ≤ 4.4 by cross-correlating sources in the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) radio survey (S1.4 GHz > 1 mJy) with star-like objects having r < 20.2 in Sloan Digital Sky Survey (SDSS) Data Release 7. Of these 87 QSOs, 80 are spectroscopically classified in previous work (mainly SDSS), and form the training set for a search for additional such sources. We apply our selection to 2916 FIRST-DR7 pairs and find 15 likely candidates. Seven of these are confirmed as high-redshift quasars, bringing the total to 87. The candidates were selected using a neural-network, which yields 97 per cent completeness (fraction of actual high-z QSOs selected as such) and an efficiency (fraction of candidates which are high-z QSOs) in the range of 47-60 per cent. We use this sample to estimate the binned optical luminosity function (LF) of radio-loud QSOs at z ~ 4, and also the LF of the total QSO population and its comoving density. Our results suggest that the radio-loud fraction at high z is similar to that at low z and that other authors may be underestimating the fraction at high z. Finally, we determine the slope of the optical LF and obtain results consistent with previous studies of radio-loud QSOs and of the whole population of QSOs. The evolution of the LF with redshift was for many years interpreted as a flattening of the bright-end slope, but has recently been re-interpreted as strong evolution of the break luminosity for high-z QSOs, and our results, for the radio-loud population, are consistent with this. [ABSTRACT FROM AUTHOR]

Context. Broad absorption lines (BALs) are present in the spectra of ∼20% of quasars (QSOs); this indicates fast outflows (up to 0.2c) that intercept the observer's line of sight. These QSOs can be distinguished again into radio-loud (RL) BAL QSOs and radio-quiet (RQ) BAL QSOs. The first are very rare, even four times less common than RQ BAL QSOs. The reason for this is still unclear and leaves open questions about the nature of the BAL-producing outflows and their connection with the radio jet. Aims. We explored the spectroscopic characteristics of RL and RQ BAL QSOs with the aim to find a possible explanation for the rarity of RL BAL QSOs. Methods. We identified two samples of genuine BAL QSOs from SDSS optical spectra, one RL and one RQ, in a suitable redshift interval (2.5 < z < 3.5) that allowed us to observe the Mg ii and Hβ emission lines in the adjacent near-infrared (NIR) band. We collected NIR spectra of the two samples using the Telescopio Nazionale Galileo (TNG, Canary Islands). By using relations known in the literature, we estimated the black-hole mass, the broad-line region radius, and the Eddington ratio of our objects and compared the two samples. Results. We found no statistically significant differences from comparing the distributions of the cited physical quantities. This indicates that they have similar geometries, accretion rates, and central black-hole masses, regardless of whether the radio-emitting jet is present or not. Conclusions. These results show that the central engine of BAL QSOs has the same physical properties with and without a radio jet. The reasons for the rarity of RL BAL QSOs must reside in different environmental or evolutionary variables. [ABSTRACT FROM AUTHOR]

We aim to obtain a complete sample of redshift radio quasi-stellar objects (QSOs) from the Faint Images of the Radio Sky at Twenty cm survey (FIRST) sources having star-like counterparts in the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) photometric survey . Our starting sample of 8665 FIRST–DR5 pairs includes 4250 objects with spectra in DR5, 52 of these being QSOs. We found that simple supervised neural networks, trained on the sources with DR5 spectra, and using optical photometry and radio data, are very effective for identifying high- z QSOs in a sample without spectra. For the sources with DR5 spectra the technique yields a completeness (fraction of actual high- z QSOs classified as such by the neural network) of 96 per cent, and an efficiency (fraction of objects selected by the neural network as high- z QSOs that actually are high- z QSOs) of 62 per cent. Applying the trained networks to the 4415 sources without DR5 spectra we found QSO candidates. We obtained spectra of 27 of them, and 17 are confirmed as high- z QSOs. Spectra of 13 additional candidates from the literature and from SDSS Data Release 6 (DR6) revealed seven more QSOs, giving an overall efficiency of 60 per cent (24/40). None of the non-candidates with spectra from NASA/IPAC Extragalactic Database (NED) or DR6 is a QSO, consistently with a high completeness. The initial sample of high- z QSOs is increased from 52 to 76 sources, i.e. by a factor of 1.46. From the new identifications and candidates we estimate an incompleteness of SDSS for the spectroscopic classification of FIRST QSOs of 15 per cent for . [ABSTRACT FROM AUTHOR]

We selected from the VLA FIRST survey a sample of 94 objects with star-like counterparts in the Sloan Digital Sky Survey (SDSS), and with APM POSS-I colour , i.e. consistent with their being high-redshift quasars. 78 of the 94 candidates can be classified spectroscopically on the basis of either published data (mainly SDSS) or the observations presented here. The fractions of QSOs (51 out of 78) and redshift QSOs (23 out of 78, 29 per cent) are comparable to those found in other photometric searches for high-redshift QSOs. We confirm that selecting colour ensures inclusion of all QSOs with . The fraction of QSOs with broad absorption lines (BALs) is 27 ± 10 per cent (7/26) and the estimated BAL fraction for radio-loud QSOs is at least as high as for optically selected QSOs (∼13 per cent). Both the high BAL fraction and the high fraction of low-ionization BALs among BALs (four to five out of seven) in our sample, compared to previous work, are likely due to the red colour selection . The space density of radio-loud QSOs in the range with and is . Adopting a radio-loud fraction of 13.4 ± 3 per cent, this corresponds to , in substantial agreement with the cumulative luminosity function of SDSS QSOs in Fan et al. We note the unusual flat-spectrum radio-luminous QSO FIRST 1413+4505 , which shows strong associated Lyα absorption (rest-frame equivalent width ∼40 Å) and an extreme observed luminosity, . [ABSTRACT FROM AUTHOR]

We present observations of the most radio-luminous broad absorption-line (BAL) quasar known, 1624+3758, at redshift . 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 . 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 and the radio loudness is . 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 . 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 absorber is a possible Lyman-limit system, with , and a covering factor of 0.7. A complex mini-BAL absorber at 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 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. [ABSTRACT FROM AUTHOR]