M. Vigotti

Radio spectra and polarization properties of radio-loud broad absorption-line quasars

We present multi-frequency observations of a sample of 15 radio-emitting Broad Absorption Line Quasars (BAL QSOs), covering a spectral range between 74 MHz and 43 GHz. They display mostly convex radio spectra which typically peak at about 1-5 GHz (in the observer’s rest-frame), flatten at MHz frequencies, probably due to synchrotron self-absorption, and become steeper at high frequencies, i.e., ν &20 GHz. VLA 22-GHz maps (HPBW � 80 mas) show unresolved or very compact sources, with linear projected sizes of 61 kpc. About 2/3 of the sample look unpolarised or weakly polarised at 8.4 GHz, frequency in which reasonable upper limits could be obtained for polarised intensity. Statistical comparisons have been made between the spectral index distributions of samples of BAL and non-BAL QSOs, both in the observed and the rest-frame, finding steeper spectra among non-BAL QSOs. However constraining this comparison to compact sources results in no significant differences between both distributions. This comparison is consistent with BAL QSOs not being oriented along a particular line of sight. In addition, our analysis of the spectral shape, variability and polarisation properties shows that radio BAL QSOs share several properties common to young radio sources like Compact Steep Spectrum (CSS) or Gigahertz-Peaked Spectrum (GPS) sources.

Red quasars not so dusty

Webster et al (1995) claimed that up to 80% of QSOs may be obscured by dust. They inferred the presence of this dust from the remarkably broad range of B-K optical-infrared colours of a sample of flat-spectrum PKS radio QSOs. If such dust is typical of QSOs, it will have rendered invisible most of those which would otherwise been have detected by optical surveys. We used the William Herschel Telescope to image 54 B3 radio QSOs in K, and we find that although several have very red optical-infrared colours, most of these can be attributed to an excess of light in K rather than a dust-induced deficit in B. We present evidence that the infrared excess comes from the light of stars in the host galaxy, or from synchrotron radiation associated with flat-spectrum radio sources. Thus, either the Webster et al QSOs are atypical in having large extinctions, or their reddening is not due to dust. Either way, there is no evidence that a large fraction of QSOs has been missed by optical surveys. This work is reported in more detail in Benn et al (1997).

DECLINE OF THE SPACE DENSITY OF QUASARS BETWEEN z = 2 AND z =4

We define a new complete sample of 13 optically luminous radio quasars [MAB(1450 A) 25.7] with redshift 3.8 < z < 4.5, obtained by cross-correlating the Faint Images of the Radio Sky at Twenty cm (FIRST) radio survey and the Automatic Plate Measuring Facility catalog of POSS I. We measure the space density to be 1.0 ± 0.3 Gpc-3, a factor of 1.9 ± 0.7 smaller than the space density of similar quasars at z ≈ 2 (FIRST Bright Quasar Survey). Using a new measurement of the radio-loud fraction of quasars, we find that at z = 4 the total space density of quasars with MAB(1450 A) < -26.9 mag is 7.4 ± 2.6 Gpc-3. This is a factor of 1.8 ± 0.8 less than the space density at z ≈ 2, found by the Two-Degree Field quasar survey. This z = 2/z = 4 ratio, consistent with that of the radio-loud quasars, is significantly different from the ratio of ~10 found for samples including lower luminosity quasars. This suggests that the decline of the space density beyond z ≈ 2 is slower for optically luminous quasars than for less luminous ones.

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-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.

High-redshift QSOs in the FIRST survey

In a pilot search for high-redshift radio quasi-stellar objects (QSOs), we have obtained spectra of 55 FIRST sources (S 1 . 4 G H z > 1 mJy) with very red (O - E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 4. The remaining 45 candidates comprise: one z = 2.6 broad-absorption-line (BAL) QSO; three low-redshift galaxies with narrow emission lines; 18 probable radio galaxies; and 23 M stars (mainly misidentifications). The success rate (high-redshift QSOs / spectroscopically-observed candidates) for this search is ½ for S 1 . 4 G H z > 10 mJy, and 1/9 for S 1 . 4 G H z > 1 mJy. With an effective search area of 4030 deg 2 , the surface density of high-redshift (z > 4) QSOs discovered with this technique is 0.0015 deg - 2 .

A sample of radio-loud QSOs at redshift ∼ 4

We obtained spectra of 60 red, star-like objects (E 1 mJy. Eight are quasi-stellar objects (QSOs) with redshift z > 3.6. Combined with our earlier pilot search, our sample of 121 candidates yields a total of 18 z > 3.6 QSOs (10 of these with z > 4.0). 8 per cent of candidates with S1.4 GHz 10 mJy are QSOs with z > 3.6. The surface density of E 1 mJy, z > 4 QSOs is 0.003 deg −2 . This is currently the only well-defined sample of radio-loud QSOs at z ≈ 4 selected independently of radio spectral index. The QSOs are highly luminous in the optical (eight have MB < −28, q0 = 0.5, H 0 = 50 km s −1 Mpc −1 ). The SEDs are as varied as those seen in optical searches for high-redshift QSOs, but the fraction of objects with weak (strongly self-absorbed) Lyα emission is marginally higher (3 out of 18) than for high-redshift QSOs from SDSS (5 out of 96).

A FIRST–APM–SDSS survey for high‐redshift radio QSOs

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 O − E 2, 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 z > 3 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 O − E 2 ensures inclusion of all QSOs with 3.7 z 4.4. The fraction of 2 z 4.4 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 O − E 2. The space density of radio-loud QSOs in the range 3.7 z 4. 4( ¯ = 4.0) with M AB(1450) −26.6 and P 10 25.7 WH z −1 is 1.7 ± 0.6 Gpc −3 . Adopting a radio-loud fraction of 13.4 ± 3 per cent, this corresponds to ρ = 12.5 ± 5.6 Gpc −3 , 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 (z = 3.11), which shows strong associated Lyα absorption (rest-frame equivalent width ∼40 A) and an extreme observed luminosity, L ∼ 2 × 10 15 L� .

Evolution of the Space Density of Quasars from Z = 2 To Z = 4

From a new sample of optically-luminous radio-loud quasars (MAB (1450 Ǻ) 25.7) with 3.8 ~ 10 for optical samples. Since the quasars in our sample correspond to the bright end of the optical luminosity function, we argue that the decline of the space density beyond z≈ 2 is much slower for optically luminous quasars than for the less-luminous ones. Constant density or a moderate redshift cutoff from z = 2 to z = 4 was also reported for radio sources at the top of the radio luminosity function (Jarvis & Rawlings 2000; Jarvis et al. 2001) and for X-ray luminous AGN (Miyaji et al. 2000).

Host Galaxies of Low Luminosity Radio QSOs

Near-infrared imaging in K-Band has been obtained for 53 quasars from the B3-VLA survey. The host galaxy properties have been obtained for 16 quasars, that in two cases have z ≥ 2. Low luminosity radio quasars inhabit luminous( ~4.8L*) and large (r 1/2 > 10Kpc) galaxies. The analysis of the restored host galaxies shows that the K — z,μ 1/2 — r 1/2 and \( \alpha _{408}^{1460} - {M_{K,gal}} \) — M K,gal distributions are similar to the same distributions for radio galaxies, in a large range of redshifts (almost to z ~2.5). These results support the unification of these two families of radio-sources.