High redshift quasars in the COSMOS survey: the space density of z>3 X-ray selected QSOs
M. Brusa, A. Comastri, R. Gilli, G. Hasinger, K. Iwasawa, V. Mainieri, M. Mignoli, M. Salvato, G. Zamorani, A. Bongiorno, N. Cappelluti, F. Civano, F. Fiore, A.… 展开作者
M. Brusa, A. Comastri, R. Gilli, G. Hasinger, K. Iwasawa, V. Mainieri, M. Mignoli, M. Salvato, G. Zamorani, A. Bongiorno, N. Cappelluti, F. Civano, F. Fiore, A. Merloni, J. Silverman, J. Trump, C. Vignali, P. Capak, M. Elvis, O. Ilbert, C. Impey, S. Lilly
备注：
36 pages, 9 figures, 4 tables. To appear on ApJ, version revised according to referee comments
We present a new measurement of the space density of high redshift (3.0<z<4.5), X-ray selected QSOs obtained by exploiting the deep and uniform multiwavelength coverage of the COSMOS survey. We have assembled a statistically large (40 objects), X-ray selected (F_{0.5-2 keV} >10^{-15} cgs), homogeneous sample of z>3 QSOs for which spectroscopic (22) or photometric (18) redshifts are available. We present the optical (color-color diagrams) and X-ray properties, the number counts and space densities of the z>3 X-ray selected quasars population and compare our findings with previous works and model predictions. We find that the optical properties of X-ray selected quasars are not significantly different from those of optically selected samples. There is evidence for substantial X-ray absorption (logN_H>23 cm^{-2}) in about 20% of the sources in the sample. The comoving space density of luminous (L_X >10^{44} erg s^-1) QSOs declines exponentially (by an e--folding per unit redshift) in the z=3.0-4.5 range, with a behavior similar to that observed for optically bright unobscured QSOs selected in large area optical surveys. Prospects for future, large and deep X-ray surveys are also discussed.
We are in the process of obtaining a complete sample of exploratory X-ray observations of broad absorption line (BAL) QSOs from the Large Bright Quasar Survey (LBQS). This survey is composed of short (5-7 ks) Chandra ACIS-S3 observations designed to significantly constrain X-ray fluxes and hardness ratios. Our sample avoids the selection biases of some previous surveys while providing X-ray data of similar quality. Of 17 BAL QSOs observed to date, 13 are detected. In general, our results are consistent with absorption as the primary cause of X-ray weakness in BAL QSOs. As found by Green et al.…
We analyse mid-infrared (MIR) spectroscopic properties for 19 ultra-luminous infrared quasars (IR QSOs) in the local universe based on the spectra from the Infrared Spectrograph on board the Spitzer Space Telescope. The MIR properties of IR QSOs are compared with those of optically-selected Palomar-Green QSOs (PG QSOs) and ultra-luminous infrared galaxies (ULIRGs). The average MIR spectral features from ~ 5 to 30um, including the spectral slopes, 6.2um PAH emission strengths and [NeII] 12.81um luminosities of IR QSOs, differ from those of PG QSOs. In contrast, IR QSOs and ULIRGs have comparabl…
If high-redshift QSOs are ejected from the nuclei of low-redshift galaxies, as some have claimed, a large portion of their redshift must be intrinsic (non-Doppler). If these intrinsic components have preferred values, redshifts will tend to cluster around these preferred values and produce peaks in the redshift distribution. Doppler ejection and Hubble flow components will broaden each peak. Because ejection velocities are randomly directed and Hubble flow components are always positive, in this model all peaks are expected to show an asymmetry, extending further out in the red wing. If peaks …
Given its extraordinary spatial resolution and sensitivity, the projected Next Generation Space Telescope (NGST) is likely to detect a large number of high-redshift QSOs lensed by spiral galaxies. Using realistic models for the QSO and spiral populations, we calculate the expected number density of detectable QSOs multiply imaged by spirals, and investigate the influence of various evolution effects on that number density. It is shown that NGST will probably find of order ten lensed QSOs per square degree at 26th magnitude in the V and L bands, and that various observable quantities like the t…
We analyze archived Chandra and XMM-Newton X-ray observations of 536 Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) quasars (QSOs) at 1.7 <= z <= 2.7 in order to characterize the relative UV and X-ray spectral properties of QSOs that do not have broad UV absorption lines (BALs). We constrain the fraction of X-ray weak, non-BAL QSOs and find that such objects are rare; for example, sources underluminous by a factor of 10 comprise $\la$2% of optically-selected SDSS QSOs. X-ray luminosities vary with respect to UV emission by a factor of $\la$2 over several years for most sources. UV contin…
We report the discovery of five broad-absorption-line (BAL) QSOs in a complete sample of 111 ultraviolet excess (UVX) QSO candidates also detected in the NRAO VLA Sky Survey. All five BAL QSOs, which include two high-ionization BAL QSOs and three low-ionization BAL QSOs, are formally radio-loud. Of QSOs with z > 0.4, 3+/-2% show low-ionization BALs, and of QSOs with z > 1.5, all radio-loud, 9+/-5% show BALs; these frequencies are consistent with those of optical surveys. While the first reported radio-loud BAL QSO, FIRST J155633.8+351758, is likely to be heavily dust reddened and thus less rad…
Moderate resolution spectra have been recorded for a sample of radio-selected QSOs (z=2.5) discovered in the ``Faint Images of the Radio Sky at Twenty centimeters'' (FIRST) VLA survey. This work is motivated by the study of a heterogeneous set of QSO absorption line spectra that showed that as many as 36% of the absorbers normally thought to be intergalactic are correlated with physical properties of the background QSO, including the radio source spectral index. Spectra were taken for 24 newly discovered quasars in order to test this finding on a more homogeneous data set. The spectra have bee…
Deep $B$ and $R$ band images (R<24 and B<25) have been obtained of a field of $\sim 6 \times 6$ arcmin around seven radio-loud quasars with $1<z<1.6$. A statistically significant excess of faint ($22.5<B\le 23.5$ and $22<R\le 23.0$) galaxies was found, on scales of $r < 160''$ around the quasars ($>3σ_{N{exp}}$). The galaxies have magnitudes compatible with being at the redshift of the QSOs.
Selective amplification of the line and continuum source by microlensing in a lensed quasar can lead to changes of continuum spectral slopes and line shapes in the spectra of the quasar components. Comparing the spectra of different components of the lensed quasar and the spectra of an image observed in different epochs one can infer the presence of millilensing, microlensing and intrinsic variability. Especially, microlensing can be used for investigation of the unresolved broad line (BLR) and continuum emitting region structure in active galactic nuclei (AGN). Therefore the spectroscopic mon…
Quasars trace the most massive structures at high redshifts and their presence may influence the evolution of the massive host galaxies. We study the extended Lyman alpha emission line regions (EELRs) around seven bright, mostly radio-quiet quasars (QSOs) at 2.7<z<4.5, and compare luminosities with EELRs around radio-loud QSOs reported in the literature. Using integral field spectroscopy, we analyse the morphology and kinematics of the quiescent Lya EELRs around the QSOs. We find evidence for the presence of EELRs around four radio-quiet and one radio-loud QSO. All EELRs appear asymmetric and …
We develop and apply a novel method of analysis to study the X-ray spectral properties of 908 QSOs in the Large Bright Quasar Survey (LBQS) that were observed during the soft X-ray ROSAT All-Sky Survey (RASS). Due to the relatively short (<600sec) RASS exposure times, only 10% of the QSOs are detected in X-rays, so X-ray spectral model fits for individual QSOs are precluded by poor photon statistics. Spectral stacking provides effectively much more sensitive X-ray observations for an average QSO in bins of redshift, and for several classes of QSOs. We model the stacked X-ray spectra in a way t…
The continuum shape of 26 broad absorption line (BAL) QSOs is shown to be virtually identical to that of non-BAL QSOs when the spectra of the former are dereddened. A procedure for dereddening of BAL QSO spectra is introduced, based on measurement of the flux in three bands and determination of the colour excess in comparison with an unreddened spectrum. This method allows us to derive the type and amount of extinction. We show that reddening in high-ionization BAL QSOs is comparatively small and the range of extinction laws varies from Milky Way, LMC to SMC type. On the other hand, low-ioniza…
We report on the results of XMM-Newton observations of two Broad Absorption Line Quasars (BAL QSOs), Q1246-057 and SBS 1542+541. Unprecedented sensitivity of XMM allows spectral analysis of these X-ray weak sources. The X-ray spectral data of these sources can be fitted by a power-law with alpha_X=1.0-1.2 and either a partially covering absorber or an ionized absorber model. Rest frame UV spectroscopy together with polarimetry favors the model with a partially covering absorber with column density a few times 10^22 cm^-2 and a covering fraction of about 0.80. After correcting for absorption, t…
We present the analysis of a new flux-limited sample of bright quasars and Seyfert 1 galaxies in an effective area of 611 square deg, drawn from the Hamburg/ESO survey. We confirm recent claims that bright quasars have a higher surface density than previously thought. Special care was taken to avoid morphological and photometric biases against low-redshift quasars, and about 50 % of the sample objects are at z < 0.3, spanning a range of three decades in luminosity. While our derived space densities for low-luminosity Seyfert 1 nuclei are consistent with those found in the literature, we find t…
We have carried out a survey with the Chandra X-ray Observatory of a sample of 10 optically bright broad absorption line (BAL) QSOs. Eight out of ten sources are detected. The 6 brightest X-ray sources have only high ionization BALs (hiBALs), while the 4 faintest all show low ionization BALs (loBALs). We perform a combined spectral fit for hiBAL QSOs to determine the mean spectral parameters of this sample. We derive an underlying best-fit power-law slope Gamma=1.8+/-0.35, consistent with the mean slope for radio-quiet quasars from ASCA, but BALQSOs require a (restframe) absorbing column of 6.…
It is widely accepted that the broad absorption line region (BALR) exists in most (if not all) quasars with a small covering factor. Recent works showed that the BALR is optically thick to soft and even medium energy X-rays, with a typical hydrogen column density of a few 10$^{23}$ to $>$ 10$^{24}$ cm$^{-2}$. The electron scattering in the thick absorber might contribute significantly to the observed continuum polarization for both BAL QSOs and non-BAL QSOs. In this paper, we present a detailed study of the electron scattering in the BALR by assuming an equatorial and axisymmetric outflow mode…
This paper addresses the origin of the far-infrared (FIR) continuum of QSOs, based on the Quasar and ULIRG Evolution Study (QUEST) of nearby QSOs and ULIRGs using observations with the Spitzer Space Telescope. For 27 Palomar-Green QSOs at z <~ 0.3, we derive luminosities of diagnostic lines ([NeII]12.8um, [NeV]14.3um, [OIV]25.9um) and emission features (PAH7.7um emission which is related to star formation), as well as continuum luminosities over a range of mid- to far-infrared wavelengths between 6 and 60um. We detect star-formation related PAH emission in 11/26 QSOs and fine-structure line em…
We detect angular galaxy-QSO cross-correlations between the APM Galaxy Catalogue and a preliminary release (consisting of roughly half of the anticipated final catalogue) of the Hamburg-ESO Catalogue of Bright QSOs as a function of source QSO redshift using multiple cross-correlation estimators. Each of the estimators yield very similar results, implying that the APM catalogue and the Hamburg-ESO survey are both fair samples of the respective true galaxy and QSO populations. Though the signal matches the expectations of gravitational lensing qualitatively, the strength of the measured cross-co…
In previous studies we have shown that the optical variability of quasars increases, on average, with redshift. We explained this dependance in terms of a hardening of the spectrum during bright phases, coupled with the increase of the rest-frame frequency for increasing redshift. We re-analize now these correlations on the basis of new light curves of PG quasars, recently published by the Wise Observatory group.
We present a catalog of 5039 broad absorption line (BAL) quasars (QSOs) in the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) QSO catalog that have absorption troughs covering a continuous velocity range >= 2000 km/s. We have fit ultraviolet (UV) continua and line emission in each case, enabling us to report common diagnostics of BAL strengths and velocities in the range -25,000 to 0 km/s for SiIV $λ$1400, CIV $λ$1549, AlIII $λ$1857, and MgII $λ$2799. We calculate these diagnostics using the spectrum listed in the DR5 QSO catalog, and also for spectra from additional SDSS observing epoch…
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 on La Palma to obtain K infrared images of 54 B3 radio quasars selected at low frequency (mainly steep-spectrum), and we find that although several have very red optical-infrared colours, most of…
We have conducted an optical study of the environments of seven radio-loud quasars at redshifts 1<z<1.6. In this paper we describe deep B and R band images obtained for fields of $\sim6\times$6 arcmin around these quasars with 3$σ$ limiting magnitudes of $B\sim26$ and $R\sim25.4$. We found an statistically significant excess of {\it faint} (B>22.5 and R>22.0) galaxies on scales of r<170'' and r<35'' around the quasars. The number of excess galaxies, their magnitudes and the angular extension of the excesses are compatible with clusters of galaxies at the redshift of the QSOs. We also found tha…
Abridged) In the SDSS, several unusual QSOs have been discovered that have very blue rest-frame UV spectra but no discernible emission lines. Their UV spectra strongly resemble that of the newly discovered quasar PHL 1811 ($z=0.192$; $\rm M_{V}=-25.9$). With magnitudes of $B = 14.4$ and $R = 14.1$, PHL 1811 is the second brightest quasar known with $z>0.1$ after 3C 273. Optically it is classified as a Narrow-line Seyfert 1 galaxy (NLS1). Objects of this class are generally strong soft X-ray emitters, but a BeppoSAX observation of PHL 1811 showed that it is anomalously X-ray weak. The inferred …
We consider a contribution of microlensing to the X-ray variability of high-redshifted QSOs. Such an effect could be caused by stellar mass objects (SMO) located in a bulge or/and in a halo of this quasar as well as at cosmological distances between an observer and a quasar. Cosmologically distributed gravitational microlenses could be localized in galaxies (or even in bulge or halo of gravitational macrolenses) or could be distributed in a uniform way. We have analyzed both cases of such distributions. As a result of our analysis, we obtained that the optical depth for microlensing caused by …
We have used the IRAM Plateau de Bure mm interferometer and the UKIRT 1-5 um Imager Spectrometer to test the connection between the major phases of spheroid growth and nuclear accretion by mapping CO emission in nine submm-detected QSOs at z=1.7-2.6 with black hole (BH) masses derived from near-infrared spectroscopy. With a previously published QSO, we present sensitive CO(3-2) observations of 10 submm-detected QSOs selected at the epoch of peak activity in both QSOs and submm galaxies (SMGs). CO is detected in 5/6 very optically luminous (M_B~-28) submm-detected QSOs with BH masses M_BH~10^9-…
We present a moderate resolution (~1.15 Angstroms/pixel) survey of 79 quasars obtained using the Kast spectrograph on the Shane 3m telescope at Lick observatory. The spectra span the wavelength range of 3175-5880 Angstroms, and have typical signal to noise of 6-20 in the regions of the spectra showing Lyman alpha forest absorption. The quasars have a mean emission redshift of z=2.17, and nearly all cover the entire Lyman alpha forest between Lyman alpha and Lyman beta. Although the quasars were selected to avoid BAL, two quasars in the survey are BAL, one of which is a new discovery. We list t…
We present results of a new, large survey for high-redshift radio-loud quasars, which targets quasars with z>4. The survey is based on the PMN and NVSS radio surveys, optically identified using digitised UKST B, R and I plates. Six new z>4 flat-spectrum QSOs have been discovered, and one previously known z>4 QSO rediscovered, based on their red optical colours. The QSOs discovered in this survey are bright in both radio and optical bands; in particular PMN J1451-1512 (z=4.763, I=17.3, R=19.1) and PMN J0324-2918 (z=4.630, R=18.7) are very luminous. PMN J1451-1512 at z=4.763 is also now the most…
The Lyman edge at 912 Åis an important diagnostic region for studying quasi-stellar objects (QSOs). In particular, it reveals a great deal about the physical conditions within the atmospheres of accretion disks, a ubiquitous component of QSO theories. A robust prediction of accretion disk models is a significant polarization due to electron scattering just longward (in wavelength) of the Lyman edge because of the wavelength dependence of the Hydrogen absorption opacity. Observations of the Lyman edge regions of QSOs have shown scant evidence for the predicted features-few QSOs show the broad, …
We consider a contribution of microlensing in X-ray variability of high-redshifted QSOs. Such an effect could be caused by stellar mass objects (SMO) located in a bulge or/and in a halo of this quasar as well as at cosmological distances between an observer and a quasar. Here, we not considerability of microlensing caused by deflectors in our Galaxy since it is well-known from recent MACHO, EROS and OGLE observations that the corresponding optical depth for the Galactic halo and the Galactic bulge is lower than $10^{-6}$. Cosmologically distributed gravitational microlenses could be localized …
The evolution of two-point space correlation function of QSOs is analyzed in the framework of the theory of the large scale structure formation. For given cosmological models the agreement between theoretical predictions and observational data on correlation function of QSOs is achieved by determination of scales of structures R(z), in which could be formed quasars at different redshifts z. It is shown that quasars could be formed in the nucleus of massive galaxies or groups of galaxies owing to merges at their central region. It depends on an amplitude of the power spectrum of the initial mat…
In Paper I (Mickaelian et al. 1999), we compared the surface density of QSOs in the Bright Quasar Survey (BQS) and in the First Byurakan Survey (FBS) and concluded that the completeness of the BQS is of the order of 70% rather than 30-50% as suggested by several authors. A number of new observations recently became available, allowing a re-evaluation of this completeness. We now obtain a surface density of QSOs brighter than B = 16.16 in a subarea of the FBS covering ~2250 deg^2, equal to 0.012 deg^-2 (26 QSOs), implying a completeness of 53+/-10%.
Anticipating new spectroscopic studies of quasar companions, we calculate expected emission line spectra for the companion galaxies. Under the assumption of isotropic photoionizing radiation, we predict equivalent widths for Halpha, Hbeta, [O III], [O II], [Ne III], and Mg II lines. We consider 6 quasars, two of which (PKS 2135-14 and PKS 2349-014) have very close companions with projected separation < 4 h^{-1} kpc. For these companions the calculated emission lines should be easily observable, with W_lambda > 20 AA for the [O III], [O II], and Halpha lines. Presence of strong emission lines f…
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., >~ 20 GHz. VLA 22-GHz maps (HPBW ~ 80 mas) show unresolved or very compact sources, with linear projected sizes of <= 1 kpc. About 2/3 of the sample look unpolarised or weakly polarised at 8.4 GHz, freq…
Recent works showed that the absorbing material in broad absorption line (BAL) quasars is optically thick to major resonant absorption lines. This material may contribute significantly to the polarization in the absorption lines. In this paper, we present a detailed study of the resonant line scattering process using Monte-Carlo method to constrain the optical depth, the geometry and the kinematics of BAL Region (BALR). By comparing our results with observed polarized spectra of BAL quasars, we find: (1) Resonant scattering can produce polarization up to 9% at the absorption trough for doublet…
XMM-Newton observations of 29 high redshift (z>2) quasars, including seven radio-quiet, 16 radio-loud and six Broad Absorption Line (BAL) objects, are presented; due to the high redshifts, the rest-frame energy bands extend up to \~30-70 keV. Over 2-10 keV, the quasars can be well fitted in each case by a simple power-law, with no strong evidence for iron emission lines. The lack of iron lines is in agreement both with dilution by the radio jet emission (for the radio-loud quasars) and the X-ray Baldwin effect. No Compton reflection humps at higher energies (i.e., above 10 keV in the rest fram…
We have conducted a NIR study of the environments of seven radio-loud quasars at redshifts 1<z<1.6. In present paper we describe deep $K$ band images obtained for the fields of ~6X6 arcmin around the quasars with 3$σ$ limiting magnitudes of K~20.5. These fields were previously studied using deep B and R band images (Sanchez & Gonzalez-Serrano 1999). Using together optical and NIR data, it has been found a significant excess of galaxies which optical-NIR colours, luminosity, spatial scale, and number of galaxies are compatible with clusters at the redshift of the quasar. We have selected a samp…
We compile a large sample of broad absorption lines (BAL) quasars with X-ray observations from the \xmm archive data and Sloan Digital Sky Survey Data Release 5. The sample consists of 41 BAL QSOs. Among 26 BAL quasars detected in X-ray, spectral analysis is possible for twelve objects. X-ray absorption is detected in all of them. Complementary to that of \citet{gall06} (thereafter G06), our sample spans wide ranges of both BALnicity Index (BI) and maximum outflow velocity (\vmax). Combining our sample with G06's, we find very significant correlations between the intrinsic X-ray weakness with …
Polarization is a useful probe to investigate the geometries and dynamics of outflow in BAL QSOs. We perform a Monte-Carlo simulation to calculate the polarization produced by resonant and electron scattering in BALR. We find: 1)A rotated and funnel-shaped outflow is preferred to explain many observed polarization features. 2)The resonant scattering can contribute a significant part of NV emission line in some QSOs.
We report the discovery of the faint (V=21.7) quasar QSO03027-0010 at z=2.808 in the vicinity of Q0302-003, one of the few quasars observed with STIS to study intergalactic HeII absorption. Together with another newly discovered QSO at z=2.29, there are now 6 QSOs known near the line of sight towards Q0302-003, of which 4 are located within the redshift region 2.76<=z<=3.28 covered by the STIS spectrum. We correlated the opacity variations in the HI and HeII Lyman forest spectra with the locations of known quasars. There is no significant proximity effect in the HI Lyman alpha forest for any o…
We present results of a systematic analysis of the XMM-Newton spectra of 40 quasars (QSOs) (z<1.72) from the Palomar-Green (PG) Bright Quasar Survey sample (M_B<-23). The sample includes 35 radio-quiet quasars (RQQs) and 5 radio-loud quasars (RLQs). The analysis of the spectra above 2 keV reveals that the hard X-ray continuum emission can be modeled with a power law component with <Gamma_(2-12)> = 1.89+/-0.11 and <Gamma_(2-12)> = 1.63^(+0.02)_(-0.01) for the RQQs and RLQs, respectively. Below 2 keV, a strong, broad excess is present in most QSO spectra. This feature has been fitted with four d…
We test the hypothesis whether high redshift QSOs would preferentially appear in small groups or pairs, and if they are associated with massive, young clusters. We carried out a photometric search for \Ly emitters on scales $\lesssim 10 h^{-1}$ Mpc, in the fields of a sample of 47 $z\approx3$ known QSOs. Wide and narrow band filter color-magnitude diagrams were generated for each of the $6'.6\times6'.6$ fields. A total of 13 non resolved objects with a significant color excess were detected as QSO candidates at a redshift similar to that of the target. All the candidates are significantly fain…
We present the results of EVN+MERLIN VLBI polarization observations of 8 Broad Absorption Line (BAL) quasars at 1.6 GHz, including 4 LoBALs and 4 HiBALs with either steep or flat spectra on VLA scales. Only one steep-spectrum source, J1122+3124, shows two-sided structure on the scale of 2 kpc. The other four steep-spectrum sources and three flat-spectrum sources display either an unresolved image or a core-jet structure on scales of less than three hundred parsecs. In all cases the marginally resolved core is the dominant radio component. Linear polarization in the cores has been detected in t…
We present a model for the cosmological evolution of quasars (QSOs) under the assumption that they are powered by massive accreting black holes. Accretion flows around massive black holes make a transition from high radiative efficiency ($\sim 10%$) to low efficiency, advection-dominated flows when ${\dot M}/{\dot M}_{Edd}$ falls below the critical rate $\sim 0.3α^2\sim 10^{-2}$ where ${\dot M}$ is the mass accretion rate, ${\dot M}_{Edd}\propto M$ is the usual Eddington rate with the nominal 10% efficiency, and $α(\le 1)$ is the dimensionless viscosity parameter. We identify this transition w…
We study the degree of chemical enrichment in the Broad Emission Line Regions (BELRs) of two QSOs with unusually strong nitrogen emission lines. The N V 1240/ C IV 1549 intensity ratio is often used as a metallicity indicator for QSOs. The validity of this approach can be tested by studying objects in which the N IV] and N III] lines, in addition to N V, are unusually strong and easily measurable. If all of these ionization states of nitrogen point to the same metallicity, it implies that the large N V strengths observed in most QSOs are not due to some peculiarity of the N V 1240 line. This t…
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 0.25Jy < S(2.7GHz)< 0.5Jy, 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 Ly-alpha and CIV emi…
We present mid/far-infrared photometry of nine FeLoBAL QSOs, taken using the Spitzer space telescope. All nine objects are extremely bright in the infrared, with rest-frame 1-1000 micron luminosities comparable to those of Ultraluminous Infrared Galaxies. Furthermore, a significant fraction of the infrared emission from many, and possibly all of the sample is likely to arise from star formation, with star formation rates of order several hundred solar masses per year. We combine these results with previous work to propose that FeLoBALs mark galaxies and QSOs in which an extremely luminous star…
We have discovered two low-ionization broad absorption line quasars in programs to obtain optical spectra for radio-selected quasar candidates from the VLA FIRST Survey (Becker, White, & Helfand 1995). Both belong to the extremely rare class of BAL QSOs that exhibit narrow absorption lines from metastable excited levels of Fe II and Fe III. Until now, there was just a single object in this class, 0059-2735 (Hazard et al. 1987). In addition, one of our new objects is the first known radio-loud BAL QSO. The properties of these three unusual objects suggest a trend of increasing radio luminosity …
We have studied the distribution of $B_J<20.5$ galaxies from the ROE/NRL COSMOS/UKST catalogue around two samples of $z>0.3$ QSOs with similar redshift distributions. The first sample is formed by 144 radio-loud QSOs from the Parkes Catalogue, and the other contains 167 optically selected QSOs extracted from the Large Bright Quasar Survey. It is found that there is a $\approx 99.0%$ significance level excess of COSMOS/UKST galaxies around the PKS QSOs, whereas there is a marginal defect of galaxies around the LBQS QSOs. When the distribution of galaxies around both samples is compared, we foun…
Using the values of the cosmological parameters obtained from Wilkinson Microwave Anisotropy Probe (WMAP) and the cosmological chemical clock observed by high-redshift quasars, it is shown that star formation should have started by the redshift $z_F > 9.5$, and that $z_F$ is sensitive only to $\omegamz h^2$ in flat $Λ$CDM cosmology.
In the unusual intrinsic QSO redshift models, QSOs are ejected by active galaxies with periodic non-cosmological reshifts, thus QSOs are generally associated with active galaxies, and certain structures will be revealed in the QSO redshift distribution. As the largest homogeneous sample of QSOs and galaxies, SDSS data provide the best opportunity to examine this issue. We review the debates on this issue, focused on those based on SDSS and 2dF data, and conclude that there is no strong connection between foreground active galaxies and high-redshift QSOs. The existence of two dips in the SDSS Q…
Gravitational lensing of background compact objects like active galactic nuclei and quasars, by extended intermediate mass lenses such as globular clusters and and dark matter clumps with masses 10^5 - 10^8 M_sun, is considered. It is shown that observational study of the galaxy-quasar's associations is a powerful direct observational test of the nature of massive galaxy haloes. Optical interferometric observations with VLTI and Keck instruments are able to constrain masses and number of substructure halo objects. Evidence of gravitational lensing by globular clusters in haloes of spiral and e…
(abridged) Steidel & Sargent (1991a) found that the spectra of the components of the close quasar pairs Q1634+267A,B and Q2345+007A,B were in each case remarkably similar but that individual broad emission lines due to Ly_alpha, Si IV, C IV, and C III] exhibited differences in profile or equivalent width between the A and B components. If the A and B components of these objects result from gravitational lensing of a single QSO, the difference in light travel time for the two images is roughly 1 year for both pairs. Accordingly, the authors suggested that observations of the spectra of single Q…
The dependence of the number density of the bright QSOs at different redshifts ($n_{QSO}(z)$) on initial power spectrum is studied. It is assumed that QSO phenomenon is an early short term stage of evolution of massive galaxies with $M\geq 2\times 10^{11}h^{-1}M_{\odot}$. The duration of such QSO stage which is passed through by fraction $α$ of galaxies is determined by means of minimization of the divergence of the theoretical number density of QSOs at different redshifts for specified initial spectrum from observable one \cite{sc91}. It is shown that the nearest number densities of QSOs at $…
In this article, the hydrogen column densities derived from X-ray observations of type II (hidden) quasars and broad absorption line quasars (BALQSOs) are compared. These column densities represent the amount of absorbing material between the X-ray source and the observer. A sample of type II QSOs with strong narrow emission lines, weak UV continuum and no broad emission lines that also have deep X-ray observations was collected from the literature for analysis. The standard model of equatorial BAL (broad absorption line) winds predicts that the column densities of this type II QSO sample shou…
Averaged spectral energy distributions (SEDs) of active and starburst galaxies from the 12 micron sample in the Local Universe and Quasars, from an optically selected sample at a mean redshift <z>=0.7, are built from optical/near-IR/far-IR (IRAS & ISO) photometric observations. These SEDs are then used to predict at various redshifts the number of Seyfert type 1 and type 2, starburst, normal galaxies, and quasars, that will be detected by future Space Missions dedicated to far-infrared and submillimeter astronomy, like SIRTF and Herschel. These predictions are then compared with the expected c…
We present a catalog of 30 QSOs and their spectra, in the square degree of sky centered on the northern Hubble Deep Field. These QSOs were selected by multicolor photometry and subsequently confirmed with spectroscopy. They range in magnitude from 17.6 < B < 21.0 and in redshift from 0.44 < z < 2.98. We also include in the catalog an AGN with redshift z=0.135. Together, these objects comprise a new grid of absorption probes which can be used to study the correlation between luminous galaxies, non-luminous halos and Lyman-alpha absorbers along the line of sight toward the Hubble Deep Field.
Observations of QSOs at z ~ 5.7 - 6.4 show the appearance of Gunn-Peterson troughs around z ~ 6, and a change in the slope of the IGM optical depth tau(z) near z ~ 5.5. These results are interpreted as a signature of the end of the reionization era, which probably started at considerably higher redshifts. However, there also appears to be a substantial cosmic variance in the transmission of the IGM, both along some lines of sight, and among different lines of sight, in this intriguing redshift regime. We suggest that this is indicative of a spatially uneven reionization, possibly caused by the…
We present the discovery of nine quasars behind the Large Magellanic Cloud, with emission redshifts ranging from 0.07 to 2.0. Six of them were identified as part of the systematic variability-based search for QSOs in the objects from the OGLE-II database. Combination of variability-based selection of candidates with the candidates' colours appears to be a powerful technique for identifying quasars, potentially reaching ca. 50% efficiency. We report an apparent correlation between variability magnitude and variability timescale, which - if confirmed - could put even more constraints on QSO cand…
Observations of quasars (QSOs) shining through or close to galaxies offer a way to probe the properties of the foreground matter through dust extinction and gravitational lensing. In this paper the feasibility of measuring the dust extinction properties is investigated using the backlitting of QSOs. We test our method to search for QSOs affected by intervening extinction, by matching the coordinates in the SDSS QSO DR3 catalogue with the New York University Value-Added Galaxy Catalog. In total, 164 QSO-galaxy pairs were found with a distance of less than 30 kpc between the galaxy centre and th…
