Digital Sky Survey Quasar Lens Research Articles

CONSTRAINING COSMOLOGICAL PARAMETERS WITH IMAGE SEPARATION STATISTICS OF GRAVITATIONALLY LENSED SDSS QUASARS: MEAN IMAGE SEPARATION AND LIKELIHOOD INCORPORATING LENS GALAXY BRIGHTNESS

Recent large scale surveys such as Sloan Digital Sky Survey have produced homogeneous samples of multiple-image gravitationally lensed quasars with well-defined selection effects. Statistical analysis on these can yield independent constraints on cosmological parameters. Here we use the image separation statistics of lensed quasars from Sloan Digital Sky Survey Quasar Lens Search (SQLS) to derive constraints on cosmological parameters. Our analysis does not require knowledge of the magnification bias, which can only be estimated from the detailed knowledge on the quasar luminosity function at all redshifts, and includes the consideration for the bias against small image separation quasars due to selection against faint lens galaxy in the follow-up observations for confirmation. We first use the mean image separation of the lensed quasars as a function of redshift to find that cosmological models with extreme curvature are inconsistent with observed lensed quasars. We then apply the maximum likelihood test to the statistical sample of 16 lensed quasars that have both measured redshift and magnitude of lens galaxy. The likelihood incorporates the probability that the observed image separation is realized given the luminosity of the lens galaxy in the same manner as Im et al. (1997). We find that the 95% confidence range for the cosmological constant (i.e., the vacuum energy density) is <TEX>$0.72{\leq}{\Omega}_{\Lambda}{\leq}1.0$</TEX> for a flat universe. We also find that the equation of state parameter can be consistent with -1 as long as the matter density <TEX>${\Omega}_m{\leq}0.4$</TEX> (95% confidence range). We conclude that the image separation statistics incorporating the brightness of lens galaxies can provide robust constraints on the cosmological parameters.

OBSERVATIONAL UPPER BOUND ON THE COSMIC ABUNDANCES OF NEGATIVE-MASS COMPACT OBJECTS AND ELLIS WORMHOLES FROM THE SLOAN DIGITAL SKY SURVEY QUASAR LENS SEARCH

The latest result in the Sloan Digital Sky Survey Quasar Lens Search (SQLS) has set the first cosmological constraints on negative-mass compact objects and Ellis wormholes. There are no multiple images lensed by the above two exotic objects for $\sim 50000$ distant quasars in the SQLS data. Therefore, an upper bound is put on the cosmic abundances of these lenses. The number density of negative mass compact objects is $n<10^{-8} (10^{-4}) h^3 {\rm Mpc}^{-3}$ at the mass scale $|M| > 10^{15} (10^{12}) M_\odot$, which corresponds to the cosmological density parameter $|\Omega| < 10^{-4}$ at the galaxy-scale mass range $|M|=10^{12-15}M_\odot$. The number density of the Ellis wormhole is $n<10^{-4} h^3 {\rm Mpc}^{-3}$ for a range of the throat radius $a = 10^{1-4}$pc, which is much smaller than the Einstein ring radius.

Very small scale clustering of quasars from a complete quasar lens survey

We measure the small-scale (comoving separation 10 kpc/h < r_p < 200 kpc/h) two-point correlation function of quasars using a sample of 26 spectroscopically confirmed binary quasars at 0.6<z<2.2 from the Sloan Digital Sky Survey Quasar Lens Search (SQLS). Thanks to careful candidate selections and extensive follow-up observations of the SQLS, which is aimed at constructing a complete quasar lens sample, our sample of binary quasars is also expected to be nearly complete within a specified range of angular separations and redshifts. The measured small-scale correlation function rises steeply toward smaller scales, which is consistent with earlier studies based on incomplete or smaller binary quasar samples. We find that the quasar correlation function can be fitted by a power-law reasonably well over 4 order of magnitudes, with the best-fit slope of xi(r)\propto r^{-1.92}. We interpret the measured correlation function within the framework of the Halo Occupation Distribution (HOD). We propose a simple model which assumes a constant fraction of quasars that appear as satellites in dark matter haloes, and find that measured small-scale clustering signals constrain the satellite fraction to f_sat=0.054_{-0.016}^{+0.017} for a singular isothermal sphere number density profile of satellites. We note that the HOD modelling appears to underpredict clustering signals at the smallest separations of r_p ~ 10 kpc/h unless we assume very steep number density profiles (such as an NFW profile with the concentration parameter c_vir > 30), which may be suggestive of enhanced quasar activities by direct interactions.

THE SLOAN DIGITAL SKY SURVEY QUASAR LENS SEARCH. VI. CONSTRAINTS ON DARK ENERGY AND THE EVOLUTION OF MASSIVE GALAXIES

We present a statistical analysis of the final lens sample from the Sloan Digital Sky Survey Quasar Lens Search (SQLS). The number distribution of a complete subsample of 19 lensed quasars selected from 50,836 source quasars is compared with theoretical expectations, with particular attention to the selection function. Assuming that the velocity function of galaxies does not evolve with redshift, the SQLS sample constrains the cosmological constant to \Omega_\Lambda=0.79^{+0.06}_{-0.07}(stat.)^{+0.06}_{-0.06}(syst.) for a flat universe. The dark energy equation of state is found to be consistent with w=-1 when the SQLS is combined with constraints from baryon acoustic oscillation (BAO) measurements or results from the Wilkinson Microwave Anisotropy Probe (WMAP). We also obtain simultaneous constraints on cosmological parameters and redshift evolution of the galaxy velocity function, finding no evidence for redshift evolution at z<1 in any combinations of constraints. For instance, number density evolution quantified as \nu_n=d\ln\phi_*/d\ln(1+z) and the velocity dispersion evolution \nu_\sigma=d\ln\sigma_*/d\ln(1+z) are constrained to \nu_n=1.06^{+1.36}_{-1.39}(stat.)^{+0.33}_{-0.64}(syst.) and \nu_\sigma=-0.05^{+0.19}_{-0.16}(stat.)^{+0.03}_{-0.03}(syst.) respectively when the SQLS result is combined with BAO and WMAP for flat models with a cosmological constant. We find that a significant amount of dark energy is preferred even after fully marginalizing over the galaxy evolution parameters. Thus the statistics of lensed quasars robustly confirm the accelerated cosmic expansion.

The effects of BCGs on the statistics of large-separation lensed quasars by clusters

We study the statistics of large-separation multiply-imaged quasars lensed by clusters of galaxies. In particular, we examine how the observed brightest cluster galaxies (BCGs) affect the predicted numbers of wide-separation lenses. We model the lens as an NFW-profiled dark matter halo with a truncated singular isothermal sphere to represent the BCG in its center. We mainly make predictions for the Sloan Digital Sky Survey Quasar Lens Search (SQLS) sample from the Data Release 5 (DR5) in two standard ΛCDM cosmological models: a model with matter density ΩM = 0.3 and σ8 = 0.9, as is usually adopted in the literature (ΛCDM1), and a model suggested by the WMAP seven-year (WMAP7) data with ΩM = 0.266 and σ8 = 0.801. We also study the lensing properties for the WMAP3 cosmology in order to compare with the previous work. We find that BCGs in the centers of clusters significantly enhance the lensing efficiency by a factor of 2 ∼ 3 compared with that of NFW-profiled pure dark matter halos. In addition, the dependence of mass ratios of BCGs to their host halos on the host halo masses reduces the lensing rate by ∼ 20% from assuming a constant ratio as in previous studies, but considering the evolution of this ratio with redshift out to z ∼ 1 would reduce it by ∼ 3%. Moreover, we predict that the numbers of lensed quasars with image separations larger than 10″ in the statistical sample of SQLS from DR5 are 1.22 and 0.47, respectively for ΛCDM1 and WMAP7 and 0.73 and 0.33 for separations between 10″ and 20″, which are consistent with the only observed cluster lens with such a large separation in the complete SQLS sample.

SDSS J133401.39+331534.3: A NEW SUBARCSECOND GRAVITATIONALLY LENSED QUASAR

The quasar SDSS J133401.39+331534.3 at z = 2.426 is found to be a two-image gravitationally lensed quasar with the image separation of 0.833. The object is first identified as a lensed quasar candidate in the Sloan Digital Sky Survey Quasar Lens Search, and then confirmed as a lensed system from follow-up observations at the Subaru and University of Hawaii 2.2-meter telescopes. We estimate the redshift of the lensing galaxy to be 0.557 based on absorption lines in the quasar spectra as well as the color of the galaxy. In particular, we observe the system with the Subaru Telescope AO188 adaptive optics with laser guide star, in order to derive accurate astrometry, which well demonstrates the usefulness of the laser guide star adaptive optics imaging for studying strong lens systems. Our mass modeling with improved astrometry implies that a nearby bright galaxy $\sim 4"$ apart from the lensing galaxy is likely to affect the lens potential.

F(R) gravity theories in the Palatini formalism constrained from strong lensing

f(R) gravity, capable of driving the late-time acceleration of the universe, is emerging as a promising alternative to dark energy. Various f(R) gravity models have been intensively tested against probes of the expansion history, including Type Ia supernovae (SNIa), the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO). In this paper, we propose to use the statistical lens sample from Sloan Digital Sky Survey Quasar Lens Search Data Release 3 (SQLS DR3) to constrain f(R) gravity models. This sample can probe the expansion history up to z∼ 2.2, higher than what probed by current SNIa and BAO data. We adopt a typical parametrization of the form f(R) =R−αH20(−R/H20)β with α and β constants. For β= 0[Λ cold dark matter (ΛCDM)], we obtain the best-fitting value of the parameter α=−4.193, for which the 95 per cent confidence interval that is [−4.633, −3.754]. This best-fitting value of α corresponds to the matter density parameter Ωm0= 0.301, consistent with constraints from other probes. Allowing β to be free, the best-fitting parameters are (α, β) = (−3.777, 0.06195). Consequently, we give Ωm0= 0.285 and the deceleration parameter q0=−0.544. At the 95 per cent confidence level, α and β are constrained to [−4.67, −2.89] and [−0.078, 0.202], respectively. Clearly, given the currently limited sample size, we can only constrain β within the accuracy of Δβ∼ 0.1 and thus cannot distinguish between ΛCDM and f(R) gravity with high significance, and actually, the former lies in the 68 per cent confidence contour. We expect that the extension of the SQLS DR3 lens sample to the SDSS DR5 and SDSS-II will make constraints on the model more stringent.

Is there a quad problem among optical gravitational lenses?

Most of the optical gravitational lenses recently discovered in the Sloan Digital Sky Survey Quasar Lens Search (SQLS) have two-images rather than four-images, in marked contrast to radio lenses for which the fraction of four-image lenses (quad fraction) is quite high. We revisit the quad fraction among optical lenses by taking the selection function of the SQLS into account. We find that the current observed quad fraction in the SQLS is indeed lower than, but consistent with, the prediction of our theoretical model. The low quad fraction among optical lenses, together with the high quad fraction among radio lenses, implies that the quasar optical luminosity function has a relatively shallow faint end slope.