Large Sample Of Quasars Research Articles

The covering factor of warm dust in quasars: a view from the Wide-field Infrared Survey Explorer All-Sky Data Release

By combining the newly infrared photometric data from the All-Sky Data Release of the Wide-field Infrared Survey Explorer with the spectroscopic data from the Seventh Data Release of the Sloan Digital Sky Survey, we study the covering factor of warm dust ($\CF$) for a large quasar sample, as well as the relations between $\CF$ and other physical parameters of quasars. We find a strong correlation between the flux ratio of mid-infrared to near-ultraviolet and the slope of near-ultraviolet spectra, which is interpreted as the dust extinction effect. After correcting for the dust extinction utilizing the above correlation, we examine the relations between $\CF$ and AGN properties: bolometric luminosity ($\Lbol$), black hole mass ($\MBH$) and Eddington ratio ($L/L_{\rm Edd}$). We confirm the anti-correlation between $\CF$ and $\Lbol$. Further we find that $\CF$ is anti-correlated with $\MBH$, but is independent of $L/L_{\rm Edd}$. Radio-loud quasars are found to follow the same correlations as for radio-quiet quasars. Monte Carlo simulations show that the anisotropy of UV-optical continuum of the accretion disc can significantly affect, but is less likely to dominate the $\CF$--$\Lbol$ correlation.

FINDING MISSING QUASARS IN THE 'REDSHIFT DESERT'

Quasars at high redshift can help us to probe the nature of the early universe. Current quasar sample is incomplete in the redshift range from 2.2 to 3 because of the similar quasar colors as stars in the optical band. The main objective of the Chinese LAMOST spectroscopic quasar survey is to discover these missing quasars and provide a quasar sample consisting of 0.3million new quasars brighter than the magnitude limit i = 20.5 in the next 5 years. This will hopefully provide the largest quasar sample for the further studies of AGN physics and cosmology. The improved quasar selection criteria based on the UKIDSS near-IR and SDSS optical colors are introduced, and their advantages in uncovering the missing quasars in the quasar 'redshift desert' (z=2.2 to 3) are demonstrated. We also present some recent discoveries of new quasars from the LAMOST commissioning and other spectroscopic observations. The implications of our study to the ongoing and upcoming large optical and near-IR sky surveys are discussed.

Quasar emission lines, radio structures and radio unification

Unified schemes of radio sources, which account for different types of radio AGN in terms of anisotropic radio and optical emission, together with different orientations of the ejection axis to the line of sight, have been invoked for many years. Recently, large samples of optical quasars, mainly from the Sloan Digital Sky Survey, together with large radio samples, such as FIRST, have become available. These hold the promise of providing more stringent tests of unified schemes but, compared to previous samples, lack high resolution radio maps. Nevertheless they have been used to investigate unified schemes, in some cases yielding results which appear inconsistent with such theories. Here we investigate using simulations how the selection effects to which such investigations are subject can influence the conclusions drawn. In particular, we find that the effects of limited resolution do not allow core-dominated radio sources to be fully represented in the samples, that the effects of limited sensitivity systematically exclude some classes of sources and the lack of deep radio data make it difficult to decide to what extent closely separated radio sources are associated. Nevertheless, we conclude that relativistic unified schemes are entirely compatible with the current observational data. For a sample selected from SDSS and FIRST which includes weak-cored triples we find that the equivalent width of the [OIII] emission line decreases as core-dominance increases, as expected, and also that core-dominated quasars are optically brighter than weak-cored quasars.

IR-derived covering factors for a large sample of quasars from WISE–UKIDSS–SDSS

We investigate the range of covering factors (determined from the ratio of IR to UV/optical luminosity) seen in luminous type 1 quasars using a combination of data from the WISE, UKIDSS and SDSS surveys. Accretion disk (UV/optical) and obscuring dust (IR) luminosities are measured via the use of a simple three component SED model. We use these estimates to investigate the distribution of covering factors and its relationship to both accretion luminosity and IR SED shape. The distribution of covering factors (f_C) is observed to be log-normal, with a bias-corrected mean of <log10 f_C>=-0.41 and standard deviation of 0.2. The fraction of IR luminosity emitted in the near-IR (1--5 micron) is found to be high (~40 per cent), and strongly dependant on covering factor.

SUZAKUOBSERVATIONS OF THREE FeLoBAL QUASI-STELLAR OBJECTS: SDSS J0943+5417, J1352+4239, AND J1723+5553

We present Suzaku observations of three iron low-ionization broad absorption line quasars (FeLoBALs). We detect J1723+5553 (3\sigma) in the observed 2--10 keV band, and constrain its intrinsic nh column density to nh > 6\times10^{23} \cmsq by modeling its X-ray hardness ratio. We study the broadband spectral index, aox, between the X-ray and UV bands by combining the X-ray measurements and the UV flux extrapolated from 2MASS magnitudes, assuming a range of intrinsic column densities, and then comparing the aox values for the three FeLoBALs with those from a large sample of normal quasars. We find that the FeLoBALs are consistent with the spectral energy distribution (SED) of normal quasars if the intrinsic nh column densities are nh > 7\times10^{23} \cmsq for J0943+5417, nh > 2\times10^{24} \cmsq for J1352+4293, and 6\times10^{23} < \nh < 3\times10^{24} \cmsq for J1723+5553. At these large intrinsic column densities, the optical depth from Thomson scattering can reach \sim6, which will significantly modulate the UV flux. Our results suggest that the X-ray absorbing material is located at a different place from the UV absorbing wind, likely between the X-ray and UV emitting regions. We find a significant kinetic feedback efficiency for FeLoBALs, indicating that the outflows are an important feedback mechanism in quasars.

The signatures of large-scale temperature and intensity fluctuations in the Lyman α forest

It appears inevitable that reionization processes would have produced large-scale temperature fluctuations in the intergalactic medium. Using toy temperature models and detailed heating histories from cosmological simulations of HeII reionization, we study the consequences of inhomogeneous heating for the Ly-alpha forest. The impact of temperature fluctuations in physically well-motivated models can be surprisingly subtle. In fact, we show that temperature fluctuations at the level predicted by our reionization simulations do not give rise to detectable signatures in the types of statistics that have been employed previously. However, because of the aliasing of small-scale density power to larger scale modes in the line-of-sight Ly-alpha forest power spectrum, earlier analyses were not sensitive to 3D modes with >~ 30 comoving Mpc wavelengths -- scales where temperature fluctuations are likely to be relatively largest. The ongoing Baryon Oscillation Spectroscopic Survey (BOSS) aims to measure the 3D power spectrum of the Ly-alpha forest, P_F, from a large sample of quasars in order to avoid this aliasing. We find that physically motivated temperature models can alter P_F at an order unity level at k <~ 0.1 comoving Mpc^{-1}, a magnitude that should be easily detectable with BOSS. Fluctuations in the intensity of the ultraviolet background can also alter P_F significantly. These signatures will make it possible for BOSS to study the thermal impact of HeII reionization at 2 < z < 3 and to constrain models for the sources of the ionizing background. Future spectroscopic surveys could extend this measurement to even higher redshifts, potentially detecting the thermal imprint of hydrogen reionization.

What can we learn from quasar absorption spectra?

We analyze optical-near infrared spectra of a large sample of quasars at high redshift with the aim of investigating both the cosmic reionization history at z ~ 6 and the properties of dust extinction at z > 4. In order to constrain cosmic reionization, we study the transmitted flux in the region blueward the Lya emission line in a sample of 17 quasars spectra at 5.7 ≤ zem ≤ 6.4. By comparing the properties of the observed spectra with the results of a semi-analytical model of the Lyα forest we find that actual data favor a model in which the Universe is highly ionized at z ∼ 6, thus being consistent with an epoch of reionization at higher redshifts. For what concerns the study of the high-z dust, we focus our attention on the region redward the Lya emission line of 33 quasars at 4 ≤ zem ≤ 6.4. We compute simulated dust-absorbed quasar spectra by taking into account a large grid of extinction curves. We find that the SMC extinction curve, which has been shown to reproduce the dust reddening of most quasars at z < 4, is not a good prescription for describing dust extinction also at higher redshifts.

Using the large scale quasar clustering to constrain flat quintessential universes

We search for the most suitable set of cosmological parameters that describes the observable universe. The search includes the possibility of quintessential flat universes, i.e., the analysis is restricted to the determination of the dimensionless matter density and the quintessential parameters, $\Omega_{\rm M}$ and $w_{\rm Q}$, respectively. Our study is focused on comparing the position of features at large scales in the density fluctuation field at different redshifts by analysing the evolution of the quasar two-point correlation function. We trace the density field fluctuations at large scales using a large and homogeneous sample of quasars ($\sim$ 38000 objects with 0.3 $\lesssim$ z $\le$ 2.4 and a median $z=1.45$) drawn from the Sloan Digital Sky Survey Data Release Six. The analysis relies on the assumption that, in the linear regime, the length scale of a particular feature should remain fixed at different times of the universe for the proper cosmological model. Our study does not assume any particular comoving length scale at which a feature should be found, but intends to perform a comparison for a wide range of scales instead. This is done by quantifying the amount of overlap among the quasar correlation functions at different times using a cross-correlation technique. The most likely cosmological model is $\Omega_{\rm M}=0.21\pm 0.02$ and $w_{\rm Q}=-0.93\pm0.04$, in agreement with previous studies. These constraints are the result of a good overall agreement of the correlation function at different redshifts over scales $\sim 100-300\mpc$. Under the assumption of a flat cosmological model, our results indicate that we are living in a low density universe with a quintessential parameter greater than the one corresponding to a cosmological constant.

SDSS J131339.98+515128.3: a new gravitationally lensed quasar selected based on near-infrared excess

We report the discovery of a new gravitationally lensed quasar, SDSS J131339.98+515128.3, at a redshift of 1.875 with an image separation of 1.24 arcsec. The lensing galaxy is clearly detected in visible light follow-up observations. We also identify three absorption-line doublets in the spectra of the lensed quasar images, from which we measure the lens redshift to be 0.194. Like several other known lenses, the lensed quasar images have different continuum slopes. This difference is probably the result of reddening and microlensing in the lensing galaxy. The lensed quasar was selected by correlating Sloan Digital Sky Survey spectroscopic quasars with Two Micron All Sky Survey (2MASS) sources and choosing quasars that show near-infrared (near-IR) excess. The near-IR excess can originate, for example, from the contribution of the lensing galaxy at near-IR wavelengths. We show that the near-IR excess technique is indeed an efficient method to identify lensed systems from a large sample of quasars.

A Gaia oriented analysis of a large sample of quasars

AbstractGaia photometric capabilities should distinguish quasars to a high degree of certainty. With this, they should also be able to deliver a clean sample of quasars with a negligible trace of stellar contaminants. However, a purely photometric sample could miss a non negligible percentage of ICRF sources counterparts - and this interface is required to align with the ICRS and de-rotate the GCRF (Gaia Celestial Reference Frame), on grounds of continuity. To prepare a minimum clean sample forming the initial quasar catalogue for the Gaia mission, an all sky ensemble was formed containing 128,257 candidates. Among them there is at least one redshift determination for 98.75%, and at least one magnitude determination for 99.20% of the targets. The sources were collected from different optical and radio lists. We analyze the redshift, magnitude, and color distributions, their relationships, as well as their degree of completeness.Complementary, the candidate sources enable to form an optical representation of the ICRS from first principles, namely, kinematically non-rotating with respect to the ensemble of distant extragalactic objects, aligned to the mean equator and dynamical equinox of J2000, and realized by a list of adopted coordinates of extragalactic sources.

Clustering Analyses of 300,000 Photometrically Classified Quasars. I. Luminosity and Redshift Evolution in Quasar Bias

Using 300,000 photometrically classified quasars, by far the largest quasar sample ever used for such analyses,we study the redshift and luminosity evolution of quasar clustering on scales of 50 h-1 kpc to 20 h-1 Mpc from redshifts of z 0:75-2:28. We parameterize our clustering amplitudes using realistic dark matter models and find that a ΛCDM power spectrum provides a superb fit to our data with a redshift-averaged quasar bias of bz =1:40Q = 2:41±0:08 (P 99.6%using our data set alone, increasing to >99.9999% if stellar contamination is not explicitly parameterized. We measure the quasar classification efficiency across our full sample as a = 95:6 ± 4:41:9 %, a star-quasar separation comparable to the star-galaxy separation in many photometric studies of galaxy clustering.We derive the mean mass of the dark matter halos hosting quasars as MDMH = (5:2 ± 0:6) x 1012 h-1 M⊙. At z ∼ 1:9 we find a 1.5 σ deviation from luminosity independent quasar clustering; this suggests that increasing our sample size by a factor of ∼1.8 could begin to constrain any luminosity dependence in quasar bias at z ∼ 2. Our results agree with recent studies of quasar environments at z < 0:4, which detected little luminosity dependence to quasar clustering on proper scales ≳50 h-1 kpc. At z < 1:6,our analysis suggests that bQ is constant with luminosity to within ΔbQ ∼ 0:6, and that, for g < 21, angular quasar autocorrelation measurements are unlikely to have sufficient statistical power at z ≲ 1:6 to detect any luminosity dependence in quasars’ clustering.

On the Selection Effect of Radio Quasars in the Sloan Digital Sky Survey

We identify a large sample of radio quasars, including those with complex radio morphology, from the Sloan Digital Sky Survey and the Faint Images of the Radio Sky at Twenty cm (FIRST). Using this sample, we inspect previous radio quasar samples for selection effects resulting from complex radio morphologies and adopting a positional coincidence between radio and optical sources alone. We find that 13.0% and 8.1% of the radio quasars do not show a radio core within 1.2'' and 2'', respectively, of their optical position and thus are missed in such samples. Radio flux is underestimated by a factor of more than 2 for an additional 8.7% of the radio quasars. These missing radio-extended quasars are more radio-loud, with a typical radio-to-optical flux ratio, namely, radio loudness RL 100 and radio power P 1025 W Hz-1. They account for more than one-third of all quasars with RL > 100. The color of radio-extended quasars tends to be bluer than that of radio-compact quasars. This suggests that radio-extended quasars are more radio-powerful sources, e.g., Fanaroff-Riley type 2 sources, rather than the compact sources viewed at larger inclination angles. By comparison with the radio data from the NRAO VLA Sky Survey, we find that for sources with total radio flux less than 3 mJy, low surface brightness components tend to be underestimated by FIRST, indicating that lobes in these faint radio sources are still missed.

The DEEP2 Galaxy Redshift Survey: Clustering of Quasars and Galaxies atz= 1

We present the clustering of Deep Extragalactic Evolutionary Probe 2 (DEEP2) galaxies at 0.7 < z < 1.4 around quasars identified using both the Sloan Digital Sky Survey (SDSS) and DEEP2 surveys. We measure the two-point cross-correlation of a sample of 36 optically selected, spectroscopically identified quasars from the SDSS and 16 more found in the DEEP2 survey with the full DEEP2 galaxy sample over scales 0.1 h-1 Mpc < rp < 10 h-1 Mpc. The clustering amplitude is found to be similar to the autocorrelation function of DEEP2 galaxies, with a relative bias of b = 0.89 ± 0.24 between quasars and DEEP2 galaxies at z ~ 1. No significant dependence is found on scale, quasar luminosity, or redshift over the ranges we probe here. The clustering amplitude errors are comparable to those from significantly larger quasar samples, such as the 2dF (Two Degree Field) QSO Redshift Survey. This results from the statistical power of cross-correlation techniques, which exploit the fact that galaxies are much more numerous than quasars. We also measure the local environments of quasars using the third-nearest-neighbor surface density of surrounding DEEP2 galaxies. Quasars are found in regions of similar mean overdensity to blue DEEP2 galaxies; they differ in environment from the red DEEP2 galaxy population at 2 σ significance. Our results imply that quasars do not reside in particularly massive dark matter halos at these redshifts, with a mean dark matter halo mass of M200 ~ 3 × 1012 M☉ in a concordance ΛCDM cosmology.

Quasar evolution: black hole mass and accretion rate determination

AbstractAccurate measurements of emission line properties are crucial to understand the physics of the broad line region in quasars. This region consists of warm gas that is closest to the quasar central engine and has not been spatially resolved for almost all sources. We present here an analysis of optical and IR data for a large sample of quasars, covering the Hi Hβ spectral region in the redshift range 0 ≲ z ≲ 2.5. Spectra were interpreted within the framework of the the so-called “eigenvector 1” parameter space, which can be viewed as a tentative H-R diagram for quasars. We stress the lack of spectral evolution in the low ionization lines of quasars, with prominent Fe ii emission also at z ≳ 2. We also show how selection effects influence the ability to find quasars radiating at low Eddington ratio in flux-limited surveys. The quasar similarity at different redshift is probably due to the absence of super-Eddington radiators (at least within the caveats of black hole mass and Eddington ratio determination discussed in this paper) as well as to the limited Eddington ratio range within which quasars seem to radiate.

Evidence for Rapidly Spinning Black Holes in Quasars

It has long been believed that accretion onto supermassive black holes powers quasars, but there has been relatively few observational constraints on the spins of the black holes. We address this problem by estimating the average radiative efficiencies of a large sample of quasars selected from the Sloan Digital Sky Survey, by combining their luminosity function and their black hole mass function. Over the redshift interval $0.4<z<2.1$, we find that quasars have average radiative efficiencies of $\sim 30% - 35%$, strongly suggesting that their black holes are rotating very fast, with specific angular momentum $a \approx 1$, which stays roughly constant with redshift. The average radiative efficiency could be reduced by a factor of $\sim$2, depending on the adopted zeropoint for the black hole mass scale. The inferred large spins and their lack of significant evolution are in agreement with the predictions of recent semi-analytical models of hierarchical galaxy formation if black holes gain most of their mass through accretion.

Spectral Variability of Quasars in the Sloan Digital Sky Survey. I. Wavelength Dependence

SDSS repeat spectroscopic observations have resulted in multiepoch spectroscopy for ~2500 quasars observed more than 50 days apart. From this sample, calibrating against stars observed simultaneously, we identify 315 quasars that have varied significantly between observations (with respect to assumed nonvariable stars observed concurrently). These variable quasars range in redshift from 0.5 to 4.72. This is the first large quasar sample studied spectroscopically for variability and represents a potentially useful sample for future high-redshift reverberation mapping studies. This also marks the first time the precise wavelength dependence of quasar variability has been determined, allowing both the continuum and emission-line variability to be studied. We create an ensemble difference spectrum (bright phase minus faint phase) covering rest-frame wavelengths from 1000 to 6000 Å. This average difference spectrum is bluer than the average single-epoch quasar spectrum; a power-law fit to the difference spectrum yields a spectral index αλ = -2.00, compared to an index of αλ = -1.35 for the single-epoch spectrum. This confirms that quasar continua are bluer when brighter. The difference spectrum also exhibits very weak or absent emission-line features; the strongest emission lines vary only 30% as much as the continuum. This small emission-line variability with respect to the continuum is consistent with the intrinsic Baldwin effect. Due to the lack of variability of the lines, measured photometric color is not always bluer in brighter phases but depends on redshift and the filters used. Lastly, the difference spectrum is bluer than the ensemble quasar spectrum only for λrest < 2500 Å, indicating that the variability cannot result from a simple scaling of the average quasar spectrum.

High-redshift quasar host galaxies with adaptive optics

Quasars form one of the most energetic phenomena in the universe, and can be traced out to very large redshifts. By studying the galaxies which host the active nuclei, important insights can be gained into the processes that trigger and maintain the quasar powerhouse. The evolution rate of the quasar population is furthermore similar to that of ordinary galaxies, which implies a connection between black hole accretion and star formation in the host galaxies. While the properties of quasar host galaxies at low redshift have become better constrained in recent years, less is known about hosts at earlier cosmic epochs. In addition, though radio-quiet quasars are by far more common than their radio-loud counterparts their host galaxies have not been studied to the same extent, in particular not at higher redshifts.An imaging campaign of a large sample of quasars at intermediate redshift (0.4 0.4 considerably. The morphology and mean magnitudes are found to be similar for radio-loud and radio-quiet host galaxies. Both types of host are shown to have optical colours as blue as those of present-day late-type spirals and starburst galaxies, which is likely the result of ongoing star formation.With increasing redshift, observations of host galaxies become more difficult. High spatial resolution can be achieved with adaptive optics, but the variation of the point spread function in the near-infrared wavelength band which is most suited for detection is large and rapid. A statistical approach to the problem of characterizing the point spread function has been developed, making use of simulated objects which are matched to the different atmospheric conditions. Bright, compact host galaxies showing signs of merging and interaction were detected in this way for three quasars at z~2.2, which were observed with the ESO 3.6 m telescope. The method is not restricted to host galaxy analysis but can be utilized in other applications as well, provided that the underlying extended source can be described by an analytical model.

FeiiDiagnostic Tools for Quasars

The enrichment of Fe relative to α-elements such as O and Mgrepresents a potential means to determine the age of quasars and probethe galaxy formation epoch. To explore how Fe II emission in quasars islinked to physical conditions and abundance, we have constructed an 830level Fe II model atom and investigated through photoionizationcalculations how Fe II emission strengths depend on nonabundancefactors. We have split Fe II emission into three major wavelength bands,Fe II (UV), Fe II (Opt1), and Fe II (Opt2), and explore how the Fe II(UV)/Mg II, Fe II (UV)/Fe II (Opt1), and Fe II (UV)/Fe II (Opt2)emission ratios depend on hydrogen density and ionizing flux in thebroad-line regions (BLRs) of quasars. Our calculations show that (1)similar Fe II (UV)/Mg II ratios can exist over a wide range of physicalconditions, (2) the Fe II (UV)/Fe II (Opt1) and Fe II (UV)/Fe II (Opt2)ratios serve to constrain ionizing luminosity and hydrogen density, and(3) flux measurements of Fe II bands and knowledge of the ionizing fluxprovide tools to derive distances to BLRs in quasars. To derive all BLRphysical parameters with uncertainties, comparisons of our model withobservations of a large quasar sample at low redshift (z<1) aredesirable. The STIS and NICMOS instruments aboard the Hubble SpaceTelescope offer the best means to provide such observations. (Less)

Color-Redshift Relations and Photometric Redshift Estimations of Quasars in Large Sky Surveys

With a recently constructed composite quasar spectrum and the \chi^2 minimization technique, we demonstrated a general method to estimate the photometric redshifts of a large sample of quasars by deriving the theoretical color-redshift relations and comparing the theoretical colors with the observed ones. We estimated the photometric redshifts from the 5-band SDSS photometric data of 18678 quasars in the first major data release of SDSS and compare them with the spectroscopic redshifts. The redshift difference is smaller than 0.1 for 47% of quasars and 0.2 for 68 % of them. Based on the calculation of the theoretical color-color diagrams of stars, galaxies and quasars in both the SDSS and BATC photometric systems, we expected that with the BATC system of 15 intermediate filters we would be able to select candidates of high redshift quasars more efficiently than in the SDSS, provided the BATC survey could detect objects with magnitude fainter than 21.

Long-Term Variability of Sloan Digital Sky Survey Quasars

We use a sample of 3791 quasars from the Sloan Digital Sky Survey (SDSS) Early Data Release and compare their photometry with historic plate material for the same set of quasars in order to study their variability properties. The time baseline we attain this way ranges from a few months to up to 50 yr. In contrast to monitoring programs, where relatively few quasars are photometrically measured over shorter time periods, we use existing databases to extend this baseline as much as possible, at the cost of sampling per quasar. Our method, however, can easily be extended to much larger samples. We construct variability structure functions and compare these with the literature and model functions. From our modeling, we conclude that (1) quasars are more variable toward shorter wavelengths, (2) their variability is consistent with an exponentially decaying light curve with a typical timescale of ~2 yr, and (3) these outbursts occur on typical timescales of ~200 yr. With the upcoming first data release of the SDSS, a much larger quasar sample can be used to put these conclusions on a more secure footing.