Large Bright Quasar Survey Research Articles

The Frequency of Intrinsic X-Ray Weakness among Broad Absorption Line Quasars

Abstract We present combined ≈14–37 ks Chandra observations of seven z = 1.6–2.7 broad absorption line (BAL) quasars selected from the Large Bright Quasar Survey (LBQS). These seven objects are high-ionization BAL (HiBAL) quasars, and they were undetected in the Chandra hard band (2–8 keV) in previous observations. The stacking analyses of previous Chandra observations suggested that these seven objects likely contain some candidates for intrinsically X-ray weak BAL quasars. With the new Chandra observations, six targets are detected. We calculate their effective power-law photon indices and hard-band flux weakness, and find that two objects, LBQS 1203+1530 and LBQS 1442–0011, show soft/steep spectral shapes ( and ) and significant X-ray weakness in the hard band (by factors of ≈15 and 12). We conclude that the two HiBAL quasars are good candidates for intrinsically X-ray weak BAL quasars. The mid-infrared-to-ultraviolet spectral energy distributions of the two candidates are consistent with those of typical quasars. We constrain the fraction of intrinsically X-ray weak active galactic nuclei (AGNs) among HiBAL quasars to be ≈7%–10% (2/29–3/29), and we estimate it is ≈6%–23% (2/35–8/35) among the general BAL quasar population. Such a fraction is considerably larger than that among non-BAL quasars, and we suggest that intrinsically X-ray weak quasars are preferentially observed as BAL quasars. Intrinsically X-ray weak AGNs likely comprise a small minority of the luminous type 1 AGN population, and they should not affect significantly the completeness of these AGNs found in deep X-ray surveys.

WEAK HARD X-RAY EMISSION FROM TWO BROAD ABSORPTION LINE QUASARS OBSERVED WITHNuSTAR: COMPTON-THICK ABSORPTION OR INTRINSIC X-RAY WEAKNESS?

We present NuSTAR hard X-ray observations of two X-ray weak broad absorption line (BAL) quasars, PG 1004+130 (radio loud) and PG 1700+518 (radio quiet). Many BAL quasars appear X-ray weak, probably due to absorption by the shielding gas between the nucleus and the accretion-disk wind. The two targets are among the optically brightest BAL quasars, yet they are known to be significantly X-ray weak at rest-frame 2-10 keV (16-120 times fainter than typical quasars). We would expect to obtain ~400-600 hard X-ray (>10 keV) photons with NuSTAR, provided that these photons are not significantly absorbed (NH<1E24 cm^{-2}). However, both BAL quasars are only detected in the softer NuSTAR bands (e.g., 4-20 keV) but not in its harder bands (e.g., 20-30 keV), suggesting that either the shielding gas is highly Compton-thick or the two targets are intrinsically X-ray weak. We constrain the column densities for both to be NH~7E24 cm^{-2} if the weak hard X-ray emission is caused by obscuration from the shielding gas. We discuss a few possibilities for how PG 1004+130 could have Compton-thick shielding gas without strong Fe K? line emission; dilution from jet-linked X-ray emission is one likely explanation. We also discuss the intrinsic X-ray weakness scenario based on a coronal-quenching model relevant to the shielding gas and disk wind of BAL quasars. Motivated by our NuSTAR results, we perform a Chandra stacking analysis with the Large Bright Quasar Survey BAL quasar sample and place statistical constraints upon the fraction of intrinsically X-ray weak BAL quasars; this fraction is likely 17-40%.

MASS FUNCTIONS OF THE ACTIVE BLACK HOLES IN DISTANT QUASARS FROM THE LARGE BRIGHT QUASAR SURVEY, THE BRIGHT QUASAR SURVEY, AND THE COLOR-SELECTED SAMPLE OF THE SDSS FALL EQUATORIAL STRIPE

We present mass functions of distant actively accreting supermassive black holes residing in luminous quasars discovered in the Large Bright Quasar Survey, the Bright Quasar Survey, and the Fall Equatorial Stripe of the Sloan Digital Sky Survey (SDSS). The quasars cover a wide range of redshifts (0 <~ z <= 5) and were subject to different selection criteria and flux density limits. These samples are thus complementary and can help us gain additional insight on the true underlying black hole mass distribution, free from selection effects and mass estimation errors through future studies. We see evidence that the active z~4 black hole population is somewhat different than that at lower z. In particular, there is a sharp increase in the space density of the detected active black holes (M_BH >~ 10^8 Msun) between redshifts ~4 and ~2.5. Also, the z~4 SDSS quasar mass function has a somewhat flatter high mass-end slope, beta = -1.75 +- 0.56, compared to the mass functions based on quasars below z of 3, which display typical slopes of beta =~ -3.3; the latter are consistent with the mass functions at similar redshifts based on the SDSS Data Release 3 quasar catalog presented by Vestergaard et al. We see clear evidence of cosmic downsizing in the comoving space density distribution of active black holes in the LBQS sample alone. In forthcoming papers, further analysis, comparison, and discussion of these mass functions will be made with other existing black hole mass functions, notably that based on the SDSS DR3 quasar catalog. We present the relationships used to estimate the black hole mass based on the MgII emission line; the relations are calibrated to the Hbeta and CIV relations by means of several thousand high quality SDSS spectra. Mass estimates of the individual black holes of these samples are also presented.

Using a complete spectroscopic survey to find red quasars and test the KX method

We present an investigation of quasar colour–redshift parameter space in order to search for radio-quiet red quasars and to test the ability of a variant of the KX quasar selection method to detect quasars over a full range of colour without bias. This is achieved by combining InfraRed Imager and Spectrograph 2 imaging with the complete Fornax Cluster Spectroscopic Survey to probe parameter space unavailable to other surveys. We construct a new sample of 69 quasars with measured bJ−K colours. We show that the colour distribution of these quasars is significantly different from that of the Large Bright Quasar Survey's quasars at a 99.9 per cent confidence level. We find 11 of our sample of 69 quasars have significantly red colours (bJ−K≥ 3.5) and from this, we estimate the red quasar fraction of the K≤ 18.4 quasar population to be 31 per cent, and robustly constrain it to be at least 22 per cent. We show that the KX method variant used here is more effective than the UVX selection method, and has less colour bias than optical colour–colour selection methods.

Hubble Space TelescopeUltraviolet Spectroscopy of 14 Low-Redshift Quasars

We present low-resolution ultraviolet spectra of 14 low-redshift (zem ≲ 0.8) quasars observed with the Hubble Space Telescope STIS as part of a Snapshot project to understand the relationship between quasar outflows and luminosity. By design, all observations cover the C IV emission line. Ten of the quasars are from the Hamburg-ESO catalog, three are from the Palomar-Green catalog, and one is from the Parkes catalog. The sample contains a few interesting quasars, including two broad absorption line (BAL) quasars (HE 0143-3535 and HE 0436-2614), one quasar with a mini-BAL (HE 1105-0746), and one quasar with associated narrow absorption (HE 0409-5004). These BAL quasars are among the brightest known (although not the most luminous) since they lie at zem < 0.8. We compare the properties of these BAL quasars to the zem < 0.5 Palomar-Green and zem > 1.4 Large Bright Quasar Survey samples. By design, our objects sample luminosities in between these two surveys, and our four absorbed objects are consistent with the v ∼ L0.62 relation derived by Laor & Brandt (2002). Another quasar, HE 0441-2826, contains extremely weak emission lines, and our spectrum is consistent with a simple power-law continuum. The quasar is radio-loud but has a steep spectral index and a lobe-dominated morphology, which argues against it being a blazar. The unusual spectrum of this quasar resembles the spectra of the quasars PG 1407+265, SDSS J1136+0242, and PKS 1004+13, for which several possible explanations have been entertained.

Estimating Black Hole Masses of AGNs using Ultraviolet Emission Line Properties

Based on measuredbroad line region sizes in the reverberation-mappingAGN sam- ple, two new empirical relations are introduced to estimate the central black hole masses of radio-loud high-redshift ( z> 0.5) AGNs. First, using the archival IUE /HST spectroscopy data at UV band for the reverberation-mapping objects, we obtained two new empirical re- lations between the BLR size and Mg II/C IV emission line luminosity. Secondly, using the newly determined black hole masses of the reverberation-mappingsample as calibration, we found two new relationships for determining the black hole mass with the full width at half maximum and the luminosity of Mg II/C IV line. We then apply the relations to estimate the black hole masses of the AGNs in the Large Bright Quasar Survey and a sample of radio-loud quasars. For the objects with small radio-loudness, the black hole mass estimated using the RBLR-LMg II/ CI Vrelation is consistent with that from the RBLR-L 3000 ˚ A/1350 ˚ A relation. For radio-loud AGNs, however, the mass estimated from the R BLR-LMg II/ CI V relation is sys- tematically lower than that from the continuumluminosity L 3000 ˚ A/1350 ˚ A. Because jets could have significant contributions to the UV/optical continuum luminosity of radio-loud AGNs, we emphasize once again that for radio-loud AGNs, the emission line luminosity may be a better tracer of the ionizing luminosity than the continuum luminosity, so that the relations between the BLR size and UV emission line luminosities should be used to estimate the black hole masses of high redshift radio-loud AGNs.

An ExploratoryChandraSurvey of a Well‐defined Sample of 35 Large Bright Quasar Survey Broad Absorption Line Quasars

We present 4–7 ks Chandra observations of 35 broad absorption line (BAL) quasars from the Large Bright Quasar Survey, the largest sample of sensitive, 0.5–8.0 keV X-ray observations of this class of quasars to date. The limitedrangesinbothredshift(z ¼ 1:42 2:90)andultravioletluminosity(afactorof � 12)ofthesamplealsomake itrelativelyuniform.Of 35 targets,27 aredetected fora detectionfractionof 77%, and we confirmprevious studies that find BAL quasars to be generally X-ray-weak. Five of the eight nondetections are known low-ionization BAL quasars, confirming reports of extreme X-ray weakness in this subset (� 10% of optically selected BAL quasars). ThoseBALquasarswiththehardestX-rayspectraarealsotheX-rayweakest,consistentwiththeinterpretationthat intrinsic absorption is the primary cause of X-ray weakness in this class of quasars as a whole. Furthermore, the observed trend is not consistent with simple neutral absorption, supporting findings from spectroscopic observations of individual targets that BAL quasars typically exhibit complex X-ray absorption (e.g., partially covering orionizedabsorbers).AssumingnormalquasarX-raycontinuaandusingthehard-band(observed-frame2–8keV) X-ray flux to ‘‘correct’’ for the effects of intrinsic absorption at softer energies increases the relative X-ray to optical flux ratios to much closer to the range for normal quasars, further indicating that typically neither are highionizationBALquasarsintrinsicallyX-ray-weak,nordotheysufferfromCompton-thickabsorption.Ingeneral,we find no evidence for correlations between X-ray weakness and ultraviolet absorption-line properties, with the exception of a likely correlation between the maximum outflow velocity of C iv absorption and the magnitude of X-ray weakness. We discuss the implications of our results for disk-wind models of BAL outflows in quasars. Subject headingg galaxies: active — quasars: absorption lines — quasars: general — X-rays: galaxies

The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infrared to X‐Ray Spectral Energy Distribution

We present extinction curves derived from the broad emission lines and continua of samples of 72 radio-loud and 1018 radio-quiet active galactic nuclei (AGNs). The curves are significantly flatter in the UV than curves for the local interstellar medium. The reddening curves for the radio-quiet Large Bright Quasar Survey quasars are slightly steeper than those of the radio-loud quasars in the UV, probably because of additional reddening by dust farther out in the host galaxies of the former. The UV extinction curves for the radio-loud AGNs are very flat. This is explicable with slight modifications to standard Mathis-Rumpl-Nordsieck dust models: there is a relative lack of small grains in the nuclear dust. Our continuum and broad emission line reddening curves agree in both shape and amplitude, confirming that the continuum shape is indeed profoundly affected by reddening for all but the bluest AGNs. With correction by our generic extinction curve, all of the radio-loud AGNs have optical-UV continuous spectra consistent with a single shape. We show that radio-quiet AGNs have very similar intrinsic UV to optical shape over orders of magnitude in luminosity. We also argue that radio-loud and radio-quiet AGNs probably share the same underlying continuum shape and that most of the systematic differences between their observed continuum shapes are due to higher nuclear reddening in radio-selected AGNs and additional reddening from dust farther out in the host galaxies in radio-quiet AGNs. Our conclusions have important implications for the modeling of quasar continua and the analysis of quasar demographics.

Emission Line Properties of Active Galactic Nuclei from a Post‐COSTARHubble Space TelescopeFaint Object Spectrograph Spectral Atlas

We present consistent emission-line measurements for active galactic nuclei (AGNs), useful for reliable statistical studies of emission line properties. This paper joins a series including similar measurements of 993 spectra from the Large Bright Quasar Survey and 174 spectra of AGNs obtained from the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) prior to the installation of COSTAR. This time we concentrate on 220 spectra obtained with the FOS after the installation of COSTAR, completing the emission line analysis of all FOS archival spectra. We use the same automated technique as in previous papers, which accounts for Galactic extinction, models blended optical and UV iron emission, includes Galactic and intrinsic absorption lines, and models emission lines using multiple Gaussians. We present UV and optical emission line parameters (equivalent widths, fluxes, FWHM, and line positions) for a large number (28) of emission lines including upper limits for undetected lines. Further scientific analyses will be presented in subsequent papers.

The Frequency and Radio Properties of Broad Absorption Line Quasars

A sample of 67 broad absorption line quasars (BALQSOs) from the Large Bright Quasar Survey (LBQS) is used to estimate the observed and intrinsic fraction of BAL quasars in optically selected samples at intermediate (BJ ≃ 18.5) magnitudes. The observed BALQSO fraction in the redshift range 1.5 ≤ z ≤ 3.0 is 15% ± 3%. A well-determined empirical k-correction, to allow for the differences in the spectral energy distributions of non-BALQSOs and BALQSOs shortward of ≃2100 Å in the rest frame, is applied to the sample. The result is an estimate of the intrinsic fraction of BALQSOs of 22% ± 4% for the redshift range 1.5 ≤ z ≤ 3.0. This value is twice that commonly cited for the occurrence of BALQSOs in optically selected samples, and the figure is in reasonable agreement with that from a preliminary analysis of the SDSS Early Data Release. The fraction of BALQSOs predicted to be present in an optical survey with flux limits equivalent to that of the FIRST Bright Quasar Survey (FBQS) is shown to be ≃20%. The BALQSO fractions derived from the FBQS and the LBQS suggest that optically bright BALQSOs are half as likely as non-BALQSOs to be detectable as S1.4 GHz ≳ 1 mJy radio sources.

The Parkes quarter-Jansky flat-spectrum sample

We present optical spectra and redshift measurements for 178 flat-spectrum objects from the Parkes quarter-Jansky flat-spectrum sample. These spectra were obtained in order to compile a complete sample of quasars for use in a study of quasar evolution. We present a composite optical spectrum made from the subset of 109 quasars that have flux densities in the range , and make a comparison with a composite for radio-quiet QSOs from the Large Bright Quasar Survey. Our large sample of radio-loud quasars allows us to strengthen previous reports that the Lyα and CIV emission lines have larger equivalent width in radio-loud quasars than radio-quiet QSOs to greater than the 3σ level. However we see no significant difference in the equivalent widths of CIII] or MgII. We also show that the flux decrements across the Lyman-α line (DA) measured from these spectra show the same trend with redshift as for optically selected QSOs.

Emission Line Properties of Active Galactic Nuclei from a Pre‐COSTAR Faint Object Spectrograph Hubble Space Telescope Spectral Atlas

UV/optical emission lines offer some of the most detailed information obtainable about the intrinsic properties of quasars. Studies of the density, ionization and metal abundance of gas near the accreting black hole are probed through an intriguing but poorly understood complex of correlations between emission lines and overall quasar spectral energy distributions that has long suffered from a lack of large, consistently measured samples. As part of a broader effort to expand and systematize the data upon which these studies are built, we present measurements of the UV/optical emission line parameters in a sample of 158 active galactic nuclei observed with the Faint Object Spectrograph on the Hubble Space Telescope (HST), prior to the installation of COSTAR. We use an automated technique that accounts for galactic reddening, includes iron emission blends, galactic and intrinsic absorption lines, and performs multicomponent fits to the emission line profiles. We present measured line parameters (equivalent width and FWHM) for a large number (28) of different UV/optical lines, including upper limits for undetected lines. We also study the relations between the emission line equivalent widths and luminosity (the Baldwin effect), as well as redshift (evolution). We compare results from this HST FOS sample with our previous measurements of 993 QSOs in the Large Bright Quasar Survey using the same analysis technique and sum the samples to achieve better coverage of the luminosity-redshift plane. We confirm a significant Baldwin effect for UV iron emission from Green et al. and find that evolution dominates the effect for iron and for Si IV emission. The values of the Baldwin effect slopes for all UV emission lines and the dependence of the slopes on the sample's luminosity range point to a change of the SED as the cause of the Baldwin effect in the FOS sample.

The Twice-Overlooked, Second Fanaroff-Riley II Broad Absorption Line Quasar LBQS 1138−0126

We report the correct classification of an overlooked Fanaroff-Riley class II radio-loud quasar with broad absorption lines, only the second such object so identified. The rare properties of this quasar, LBQS 1138-0126, are twice overlooked. First, LBQS 1138-0126 was found in the Large Bright Quasar Survey but only noted as a possible broad absorption line quasar without additional follow-up. Later, LBQS 1138-0126 was rediscovered and classified as a radio-loud broad absorption line quasar but not recognized as an FR II radio source. We describe the radio, absorption line, and optical polarization properties of LBQS 1138-0126 and place it in context with respect to related quasars. In particular, spectropolarimetry shows that LBQS 1138-0126 has high continuum polarization increasing from 3% in the red (rest-frame 2400 A) to over 4% in the blue (rest-frame 1650 A), essentially confirming the intrinsic nature of the absorption. The polarization position angle rotates from ~-30° in the red to ~0° in the blue; the radio lobe position angle is ~52° for comparison. LBQS 1138-0126 is additionally notable for being one of the most radio-loud broad absorption line quasars and for having low-ionization broad absorption lines as well.

Measuring the black hole masses of high-redshift quasars

A new technique is presented for determining the black hole masses of high-redshift quasars from optical spectroscopy. The new method utilizes the full-width at half-maximum (FWHM) of the low-ionization Mg ii emission line and the correlation between the broad-line region (BLR) radius and the continuum luminosity at 3000 Å. Using archival ultraviolet (UV) spectra it is found that the correlation between BLR radius and 3000-Å luminosity is tighter than the established correlation with 5100-Å luminosity. Furthermore, it is found that the correlation between BLR radius and 3000-Å continuum luminosity is consistent with a relation of the form RBLR∝λL1/2λ, as expected for a constant ionization parameter. Using a sample of objects with broad-line radii determined from reverberation mapping it is shown that the FWHM of Mg ii and Hβ are consistent with following an exact one-to-one relation, as expected if both Hβ and Mg ii are emitted at the same radius from the central ionizing source. The resulting virial black hole mass estimator based on rest-frame UV observables is shown to reproduce black hole mass measurements based on reverberation mapping to within a factor of 2.5 (1σ). Finally, the new UV black hole mass estimator is shown to produce identical results to the established optical (Hβ) estimator when applied to 128 intermediate-redshift (0.3 < z < 0.9) quasars drawn from the Large Bright Quasar Survey and the radio-selected Molonglo quasar sample. We therefore conclude that the new UV virial black hole mass estimator can be reliably used to estimate the black hole masses of quasars from z∼ 0.25 through to the peak epoch of quasar activity at z∼ 2.5 via optical spectroscopy alone.

LBQS 0015+0239: A Binary Quasar with Small Angular Separation

We present spectroscopic observations with Keck/LRIS of LBQS 0015+0239, a pair of quasars at z = 2.45 with a separation of $\Delta\theta = 2.2$ arcsec (projected linear distance of 17.8 $h_{70}^{-1}$ kpc, for $\Omega_m = 0.3$, $\Lambda = 0.7$). Lensing is an unlikely interpretation for the images, since the spectra show significant differences in the NV and CIV emission line profiles, and there is no luminous galaxy at the anticipated lens position. Rather, we interpret this pair as the highest redshift known example of a binary quasar. The redshift difference of 661+/-173 km/s between the two components is inconsistent with the lensing expectation, but is consistent with the line of sight velocity difference of a bound pair of galaxies. The spectra show associated, narrow metal absorption in highly ionized species of C and N, with a systemic velocity difference of 492+/-6 km/s. It is likely that this absorption arises in a group or cluster of galaxies surrounding the quasar pair. After a thorough search, the Large Bright Quasar Survey (LBQS) is known to contain one gravitational lens and four probable binary pairs. The existence of close binaries is evidence that quasar activity can be triggered by galaxy interactions. We speculate that the close pair LBQS 0015+0239 is a rare example of the pre-merger state of two supermassive black holes.

The statistics of wide-separation lensed quasars

ABSTRACT The absence of any wide-separation gravitational lenses in the Large Bright QuasarSurvey is used to place limits on the population of cluster-sized halos in the universe,and hence constrain a number of cosmological parameters. The results agree withprevious investigations in strongly ruling out the standard cold dark matter modelbut they are consistent with low-density universes in which the primordial fluctuationspectrum matches both cluster abundances and cosmic microwave background mea-surements. These conclusions are essentially independent of the cosmological constant,which is in stark contrast to the statistics of galaxy lenses. The constraints presentedhere are nullified if clusters have core radii of >∼ 10 kpc, but are free of a number ofpotential systematic errors, due to the homogeneity of the data.Key words: gravitational lensing – cosmology: theory – galaxies: clusters. 1 INTRODUCTIONThe fraction of quasars which are gravitationally-lensed byintervening mass concentrations is a function of the deflectorpopulation and the underlying cosmological model, whichcan then be constrained from the observed lensing frequency(e.g. Kochanek 1993, 1996). In most cases of quasar lensingknown to date the principal deflectors are elliptical galaxies(e.g. Keeton, Kochanek & Seljak 1997), and the number ofsuch lenses, together with the distribution of image separa-tions and deflector redshifts, has been used to place quitestringent limits on the galaxy population (e.g. Kochanek1993) and the cosmological model (e.g. Kochanek 1996;Falco, Kochanek & Mun˜oz 1998). Unfortunately, the depen-dencies of the expected lensing probability on the galaxypopulation and the cosmology cannot be easily separated.For instance, cosmological constant-dominated models over-predict the number of lenses if the local and cosmologicalgalaxy populations are similar, but can be made consis-tent with observations if high-redshift galaxies are opticallythick due to dust (e.g. Kochanek 1996; Malhotra, Rhoads &Turner 1997).One way to bypass the lack of knowledge about thehigh-redshift galaxy population is to concentrate on lenseswith larger image separations (i.e. >∼ 3 arcsec). These canonly be produced by groups and clusters of galaxies, a pop-ulation which is more closely linked to the underlying cos-

WFPC2 Imaging of Quasar Environments: A Comparison of Large Bright Quasar Survey andHubble Space TelescopeArchive Quasars

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) data on the large-scale environments of 16 0.39 < z < 0.51 quasars from the Large Bright Quasar Survey (LBQS). The LBQS quasars are representative of the radio-quiet population, and this is one of the first looks at their large-scale environments. We compare the LBQS environments with the environments of 27 0.15 < z < 0.55 quasars selected from the HST archive. The majority of the Archive quasars are from the PG and PKS surveys, and these quasars are more luminous on average than the LBQS. By comparing the LBQS and Archive environments, we investigate whether previous quasar environment studies have been biased as a result of studying unusually radio or optically luminous quasars. We compare observed galaxy number counts with expected counts predicted from the CNOC2 field galaxy luminosity function in order to look for statistical excesses of galaxies around the quasars. We detect a significant excess around the Archive quasars but find no such excess around the LBQS quasars. We calculate the amplitude of the spatial correlation function and find that the LBQS environments are consistent with that of the typical galaxy while the Archive environments are slightly less rich than Abell 0 clusters. We find no difference between the environments of radio-loud and radio-quiet quasars in either sample. However, comparison with previously published work shows that the LBQS radio-loud quasars are in sparse environments when compared with other radio-loud quasars, and the Archive radio-quiet quasars are in dense environments compared to other radio-quiet quasars. The richer environments of the Archive radio-quiet quasars cannot be explained by their higher optical luminosities. We find a positive correlation (95%) between radio luminosity and environment for the radio-loud quasars. This may explain why the LBQS radio-loud quasars, which are less radio luminous, are in sparser environments.

The Large Bright Quasar Survey. VII. The LBQS and FIRST Surveys

The source catalogue for the LBQS and the FIRST Survey are compared in their regions of overlap. In the 270 deg^2 common to both surveys the LBQS contains \~100,000 stellar and ~40,000 non-stellar objects, while the FIRST catalogue contains ~25,000 sources. Cross-correlation of these lists yields 67 positional coincidences between known LBQS quasars and FIRST sources and an additional 19 stellar and 149 non-stellar positional coincidences with the radio catalogue. Spectroscopy of all the stellar matches and two-thirds of the non-stellar matches produces eight new quasars. One BL Lac object, previously misclassified during the LBQS survey is also identified. The straightforward fractional incompleteness of the LBQS determined from this sample is 13+/-4%, in good agreement with the published estimate of 10%. The distributions of the ratio of radio-to-optical power, apparent magnitude and spectroscopic properties for the new objects are consistent with those of the 67 LBQS-FIRST objects previously known. The consistency of the optical and radio properties of the new objects with those of the known quasars thus supports the conclusion that no new population of objects, constituting more than ~7% of quasars detected by FIRST, has eluded the LBQS optical selection techniques. The percentage of radio-detected quasars in the LBQS catalogue is found to be 12+/-2%, considerably smaller than the value of 25% advocated by White et al. (2000) based on the FBQS. Apparent differences in the form of the number-redshift relation for the LBQS and FBQS samples are shown to arise in large part from the very different optical passbands employed.

Quasar Evolution and the Baldwin Effect in the Large Bright Quasar Survey

From a large homogeneous sample of optical/UV emission line measurements for 993 quasars from the Large Bright Quasar Survey (LBQS), we study correlations between emission-line equivalent width and both rest-frame ultraviolet luminosity (i.e., the Baldwin effect) and redshift. Our semiautomated spectral fitting accounts for absorption lines, fits blended iron emission, and provides upper limits to weak emission lines. Use of a single large, well-defined sample and consistent emission-line measurements allows us to sensitively detect many correlations, most of which have been previously noted. A new finding is a significant Baldwin effect in UV iron emission. Further analysis reveals that the primary correlation of iron emission strength is probably with redshift, implying an evolutionary rather than a luminosity effect. We show that for most emission lines with a significant Baldwin effect, and for some without, evolution dominates over luminosity effects. This may reflect evolution in abundances, in cloud covering factors, or overall cloud conditions such as density and ionization. We find that in our sample, a putative correlation between Baldwin effect slope and the ionization potential is not significant. Uniform measurements of other large quasar samples will extend the luminosity and redshift range of such spectral studies and provide even stronger tests of spectral evolution.

Emission Line Properties of the Large Bright Quasar Survey

We present measurements of the optical/UV emission lines for a large homogeneous sample of 993 quasars from the Large Bright Quasar Survey. Our largely automated technique accounts for continuum breaks and galactic reddening, and we perform multicomponent fits to emission line profiles, including the effects of blended iron emission, and of absorption lines both galactic and intrinsic. Here we describe the fitting algorithm and present the results of line fits to the LBQS sample, including upper limits to line equivalent widths when warranted. The distribution of measured line parameters, principally equivalent width and FWHM, are detailed for a variety of lines, including upper limits. We thus initiate a large-scale investigation of correlations between the high energy continuum and emission lines in quasars, to be extended to complementary samples using similar techniques. High quality, reproducible measurements of emission lines for uniformly selected samples will advance our understanding of active galaxies, especially in a new era of large surveys selected by a variety of complementary methods.