Showing papers by "Robert C. Nichol published in 2007"

The Fifth Data Release of the Sloan Digital Sky Survey

Abstract: This paper describes the Fifth Data Release (DR5) of the Sloan Digital Sky Survey (SDSS). DR5 includes all survey quality data taken through 2005 June and represents the completion of the SDSS-I project (whose successor, SDSS-II, will continue through mid-2008). It includes five-band photometric data for 217 million objects selected over 8000 deg^2 and 1,048,960 spectra of galaxies, quasars, and stars selected from 5713 deg^2 of that imaging data. These numbers represent a roughly 20% increment over those of the Fourth Data Release; all the data from previous data releases are included in the present release. In addition to "standard" SDSS observations, DR5 includes repeat scans of the southern equatorial stripe, imaging scans across M31 and the core of the Perseus Cluster of galaxies, and the first spectroscopic data from SEGUE, a survey to explore the kinematics and chemical evolution of the Galaxy. The catalog database incorporates several new features, including photometric redshifts of galaxies, tables of matched objects in overlap regions of the imaging survey, and tools that allow precise computations of survey geometry for statistical investigations.

Measuring the Baryon Acoustic Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey

Abstract: We introduce a method to constrain general cosmological models using Baryon Acoustic Oscillation (BAO) distance measurements from galaxy samples covering different redshift ranges, and apply this method to analyse samples drawn from the Sloan Digital Sky Survey (SDSS) and 2dF Galaxy Redshift Survey (2dFGRS). BAOs are detected in the clustering of the combined 2dFGRS and SDSS main galaxy samples, and measure the distance–redshift relation at z= 0.2. BAOs in the clustering of the SDSS luminous red galaxies measure the distance–redshift relation at z= 0.35. The observed scales of the BAOs calculated from these samples and from the combined sample are jointly analysed using estimates of the correlated errors, to constrain the form of the distance measure DV(z) ≡[(1 +z)2D2Acz/H(z)]1/3. Here DA is the angular diameter distance, and H(z) is the Hubble parameter. This gives rs/DV(0.2) = 0.1980 ± 0.0058 and rs/DV(0.35) = 0.1094 ± 0.0033 (1σ errors), with a correlation coefficient of 0.39, where rs is the comoving sound horizon scale at recombination. Matching the BAOs to have the same measured scale at all redshifts then gives DV(0.35)/DV(0.2) = 1.812 ± 0.060. The recovered ratio is roughly consistent with that predicted by the higher redshift Supernova Legacy Survey (SNLS) supernova data for Λ cold dark matter cosmologies, but does require slightly stronger cosmological acceleration at a low redshift. If we force the cosmological model to be flat with constant w, then we find Ωm= 0.249 ± 0.018 and w=−1.004 ± 0.089 after combining with the SNLS data, and including the WMAP measurement of the apparent acoustic horizon angle in the cosmic microwave background.

Measuring the Baryon Acoustic Oscillation scale using the SDSS and 2dFGRS

Abstract: We introduce a method to constrain general cosmological models using Baryon Acoustic Oscillation (BAO) distance measurements from galaxy samples covering different redshift ranges, and apply this method to analyse samples drawn from the SDSS and 2dFGRS. BAO are detected in the clustering of the combined 2dFGRS and SDSS main galaxy samples, and measure the distance--redshift relation at z=0.2. BAO in the clustering of the SDSS luminous red galaxies measure the distance--redshift relation at z=0.35. The observed scale of the BAO calculated from these samples and from the combined sample are jointly analysed using estimates of the correlated errors, to constrain the form of the distance measure D_V(z)=[(1+z)^2D_A^2cz/H(z)]^(1/3). Here D_A is the angular diameter distance, and H(z) is the Hubble parameter. This gives r_s/D_V(0.2)=0.1980+/-0.0058 and r_s/D_V(0.35)=0.1094+/-0.0033 (1sigma errors), with correlation coefficient of 0.39, where r_s is the comoving sound horizon scale at recombination. Matching the BAO to have the same measured scale at all redshifts then gives D_V(0.35)/D_V(0.2)=1.812+/-0.060. The recovered ratio is roughly consistent with that predicted by the higher redshift SNLS supernovae data for Lambda cosmologies, but does require slightly stronger cosmological acceleration at low redshift. If we force the cosmological model to be flat with constant w, then we find Om_m=0.249+/-0.018 and w=-1.004+/-0.089 after combining with the SNLS data, and including the WMAP measurement of the apparent acoustic horizon angle in the CMB.

A MaxBCG catalog of 13,823 galaxy clusters from the Sloan Digital Sky Survey

Abstract: We present a catalog of galaxy clusters selected using the maxBCG red-sequence method from Sloan Digital Sky Survey photometric data. This catalog includes 13,823 clusters with velocity dispersions greater than 400 km s-1 and is the largest galaxy cluster catalog assembled to date. They are selected in an approximately volume-limited way from a 0.5 Gpc3 region covering 7500 deg2 of sky between redshifts 0.1 and 0.3. Each cluster contains between 10 and 190 E/S0 ridgeline galaxies brighter than 0.4L* within a scaled radius R200. The tight relation between ridgeline color and redshift provides an accurate photometric redshift estimate for every cluster. Photometric redshift errors are shown by comparison to spectroscopic redshifts to be small (Δ ≃ 0:01), essentially independent of redshift, and well determined throughout the redshift range. Runs of maxBCG on realistic mock catalogs suggest that the sample is more than 90% pure and more than 85% complete for clusters with masses ≥ 1 x 1014 M⊙. Spectroscopic measurements of cluster members are used to examine line-of-sight projection as a contaminant in the identification of brightest cluster galaxies and cluster member galaxies. Spectroscopic data are also used to demonstrate the correlation between optical richness and velocity dispersion. Comparison to the combined NORAS and REFLEX X-rayYselected cluster catalogs shows that X-rayYluminous clusters are found among the optically richer maxBCG clusters. This paper is the first in a series that will consider the properties of these clusters, their galaxy populations, and their implications for cosmology.

A MaxBCG Catalog of 13,823 Galaxy Clusters from the Sloan Digital Sky Survey

Abstract: We present a catalog of galaxy clusters selected using the maxBCG redsequence method from Sloan Digital Sky Survey photometric data. This catalog includes 13,823 clusters with velocity dispersions greater than 400 km/s, and is the largest galaxy cluster catalog assembled to date. They are selected in an approximately volume-limited way from a 0.5 Gpc^3 region covering 7500 square degrees of sky between redshifts 0.1 and 0.3. (ABRIGDED)

The Sloan Digital Sky Survey-II Supernova Survey: Technical Summary

Abstract: The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5 degrees wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for discovery of new objects. Supernova imaging observations are being acquired between 1 September and 30 November of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

The clustering of luminous red galaxies in the Sloan Digital Sky Survey imaging data

Abstract: We present the 3D real-space clustering power spectrum of a sample of ∼600 000 luminous red galaxies measured by the Sloan Digital Sky Survey, using photometric redshifts. These galaxies are old, elliptical systems with strong 4000-A breaks, and have accurate photometric redshifts with an average error of Δz= 0.03. This sample of galaxies ranges from redshift z= 0.2 to 0.6 over 3528 deg2 of the sky, probing a volume of 1.5 h−3 Gpc3, making it the largest volume ever used for galaxy clustering measurements. We measure the angular clustering power spectrum in eight redshift slices and use well-calibrated redshift distributions to combine these into a high-precision 3D real-space power spectrum from k= 0.005 to k= 1 h Mpc−1. We detect power on gigaparsec scales, beyond the turnover in the matter power spectrum, at a ∼2σ significance for k < 0.01 h Mpc−1, increasing to 5.5σ for k < 0.02 h Mpc−1. This detection of power is on scales significantly larger than those accessible to current spectroscopic redshift surveys. We also find evidence for baryonic oscillations, both in the power spectrum, as well as in fits to the baryon density, at a 2.5 σ confidence level. The large volume and resulting small statistical errors on the power spectrum allow us to constrain both the amplitude and the scale dependence of the galaxy bias in cosmological fits. The statistical power of these data to constrain cosmology is ∼1.7 times better than previous clustering analyses. Varying the matter density and baryon fraction, we find ΩM= 0.30 ± 0.03, and Ωb/ΩM= 0.18 ± 0.04, for a fixed Hubble constant of 70 km s−1 Mpc−1 and a scale-invariant spectrum of initial perturbations. The detection of baryonic oscillations also allows us to measure the comoving distance to z= 0.5; we find a best-fitting distance of 1.73 ± 0.12 Gpc, corresponding to a 6.5 per cent error on the distance. These results demonstrate the ability to make precise clustering measurements with photometric surveys.

The shape of the sloan digital sky survey data release 5 galaxy power spectrum

Abstract: We present a Fourier analysis of the clustering of galaxies in the combined main galaxy and LRG SDSS DR5 sample. The aim of our analysis is to consider how well we can measure the cosmological matter density using the signature of the horizon at matter-radiation equality embedded in the large-scale power spectrum. The new data constrain the power spectrum on scales 100-600 h-1 Mpc with significantly higher precision than previous analyses of just the SDSS main galaxies, due to our larger sample and the inclusion of the LRGs. This improvement means that we can now reveal a discrepancy between the shape of the measured power and linear CDM models on scales 0.01 h Mpc-1 < k < 0.15 h Mpc-1, with linear model fits favoring a lower matter density (ΩM = 0.22 ± 0.04) on scales 0.01 h Mpc-1 < k < 0.06 h Mpc-1 and a higher matter density (ΩM = 0.32 ± 0.01) when smaller scales are included, assuming a flat ΛCDM model with h = 0.73 and ns = 0.96. This discrepancy could be explained by scale-dependent bias, and by analyzing subsamples of galaxies, we find that the ratio of small-scale to large-scale power increases with galaxy luminosity, so all of the SDSS galaxies cannot trace the same power spectrum shape over 0.01 h Mpc-1 < k < 0.2 h Mpc-1. However, the data are insufficient to clearly show a luminosity-dependent change in the largest scale at which a significant increase in clustering is observed, although they do not rule out such an effect. Significant scale-dependent galaxy bias on large scales, which changes with the r-band luminosity of the galaxies, could potentially explain differences in our ΩM estimates and differences previously observed between 2dFGRS and SDSS power spectra and the resulting parameter constraints.

Intrinsic galaxy alignments from the 2SLAQ and SDSS surveys: luminosity and redshift scalings and implications for weak lensing surveys

Abstract: Correlations between intrinsic shear and the density field on large scales, a potentially important contaminant for cosmic shear surveys, have been robustly detected at low redshifts with bright galaxies in Sloan Digital Sky Survey (SDSS) data. Here we present a more detailed characterization of this effect, which can cause anticorrelations between gravitational lensing shear and intrinsic ellipticity (GI correlations). This measurement uses 36 278 luminous red galaxies (LRGs) from the SDSS spectroscopic sample with 0.15 3σ detections of the effect on large scales (up to 60 h−1 Mpc) for all galaxy subsamples within the SDSS LRG sample; for the 2SLAQ sample, we find a 2σ detection for a bright subsample, and no detection for a fainter subsample. Fitting formulae are provided for the scaling of the GI correlations with luminosity, transverse separation and redshift (for which the 2SLAQ sample, while small, provides crucial constraints due to its longer baseline in redshift). We estimate contamination in the measurement of σ8 for future cosmic shear surveys on the basis of the fitted dependence of GI correlations on galaxy properties. We find contamination to the power spectrum ranging from −1.5 per cent (optimistic) to −33 per cent (pessimistic) for a toy cosmic shear survey using all galaxies to a depth of R = 24 using scales l ≈ 500, though the central value of predicted contamination is −6.5 per cent. This corresponds to a bias in σ8 of Δσ8 = −0.004 (optimistic), −0.02 (central) or −0.10 (pessimistic). We provide a prescription for inclusion of this error in cosmological parameter estimation codes. The principal uncertainty is in the treatment of the L ≤ L blue galaxies, for which we have no detection of the GI signal, but which could dominate the GI contamination if their GI amplitude is near our upper limits. Characterization of the tidal alignments of these galaxies, especially at redshifts relevant for cosmic shear, should be a high priority for the cosmic shear community.

The 2dF-SDSS LRG and QSO survey: QSO clustering and the L-z degeneracy

Abstract: We combine the quasi-stellar object (QSO) samples from the 2dF QSO Redshift Survey (2QZ) and the 2dF-Sloan Digital Sky Survey luminous red galaxy (LRG) and QSO Survey (2dF-SDSS LRG and QSO, hereafter 2SLAQ) in order to investigate the clustering of z∼ 1.5 QSOs and measure the correlation function (ξ). The clustering signal in redshift-space and projected along the sky direction is similar to that previously obtained from the 2QZ sample alone. By fitting functional forms for ξ(σ, π), the correlation function measured along and across the line of sight, we find, as expected, that β, the dynamical infall parameter and Ω0m , the cosmological density parameter, are degenerate. However, this degeneracy can be lifted by using linear theory predictions under different cosmological scenarios. Using the combination of the 2QZ and 2SLAQ QSO data, we obtain: βQSO(z= 1.5) = 0.60+0.14−0.11, Ω0m= 0.25+0.09−0.07 which imply a value for the QSO bias, b(z= 1.4) = 1.5 ± 0.2 . The combination of the 2QZ with the fainter 2SLAQ QSO sample further reveals that QSO clustering does not depend strongly on luminosity at fixed redshift. This result is inconsistent with the expectation of simple 'high peaks' biasing models where more luminous, rare QSOs are assumed to inhabit higher mass haloes. The data are more consistent with models which predict that QSOs of different luminosities reside in haloes of similar mass. By assuming ellipsoidal models for the collapse of density perturbations, we estimate the mass of the dark matter haloes which the QSOs inhabit as ∼3 × 1012 h−1 M⊙ . We find that this halo mass does not evolve strongly with redshift nor depend on QSO luminosity. Assuming a range of relations which relate halo to black hole mass, we investigate how black hole mass correlates with luminosity and redshift, and ascertain the relation between Eddington efficiency and black hole mass. Our results suggest that QSOs of different luminosities may contain black holes of similar mass.

The luminosities, sizes and velocity dispersions of Brightest Cluster Galaxies: Implications for formation history

Abstract: The size-luminosity relation of early-type brightest cluster galaxies (BCGs), Re ∝ L0.88, is steeper than that for the bulk of the early-type galaxy population, for which Re ∝ L0.68. This is true if quantities derived from either de Vaucouleurs or Sersic fits to the surface brightness profiles are used. Contamination from an intracluster light component centered on the BCG, with parameters similar to what has been seen in some recent studies, is not able to account for this difference. In addition, although BCGs are hardly offset from the fundamental plane defined by the bulk of the early-type population, they show considerably smaller scatter. The larger than expected sizes of BCGs, and the increased homogeneity, are qualitatively consistent with models that seek to explain the colors of the most massive galaxies by invoking dry dissipationless mergers, since dissipation tends to reduce the sizes of galaxies, and wet mergers that result in star formation would tend to increase the scatter in luminosity at a fixed size and velocity dispersion. Furthermore, BCGs define the same g - r color-magnitude relation as the bulk of the early-type population. If BCGs formed from dry mergers, then BCG progenitors must have been red for their magnitudes, suggesting that they hosted older stellar populations than is typical for their luminosities. Our findings have two other consequences. First, the Re-L relation of the early-type galaxy population as a whole (i.e., normal plus BCG) exhibits some curvature: the most luminous galaxies tend to have larger sizes than is expected from the Re ∝ L0.68 scaling—some of this curvature must be a consequence of the fact that an increasing fraction of the most luminous galaxies are BCGs. The second consequence is suggested by the fact that, despite following a steeper size-luminosity relation, BCGs tend to define a tight relation between dynamical mass Reσ2/G and luminosity. Although this relation is slightly different than that defined by the bulk of the population, the fact that their sizes are large for their luminosities suggests that their velocity dispersions are small. We find that, indeed, BCGs define a shallower σ-L relation than the bulk of the early-type galaxy population. This shallower relation suggests that there may be a curvature in the correlation between black hole mass and velocity dispersion; simple extrapolation of a single power law M•-σ relation to large σ will underestimate M•.

The Sloan Digital Sky Survey-II Supernova Survey: Search Algorithm and Follow-up Observations

Abstract: The Sloan Digital Sky Survey-II Supernova Survey has identified a large number of new transient sources in a 300 sq. deg. region along the celestial equator during its first two seasons of a three-season campaign. Multi-band (ugriz) light curves were measured for most of the sources, which include solar system objects, Galactic variable stars, active galactic nuclei, supernovae (SNe), and other astronomical transients. The imaging survey is augmented by an extensive spectroscopic follow-up program to identify SNe, measure their redshifts, and study the physical conditions of the explosions and their environment through spectroscopic diagnostics. During the survey, light curves are rapidly evaluated to provide an initial photometric type of the SNe, and a selected sample of sources are targeted for spectroscopic observations. In the first two seasons, 476 sources were selected for spectroscopic observations, of which 403 were identified as SNe. For the Type Ia SNe, the main driver for the Survey, our photometric typing and targeting efficiency is 90%. Only 6% of the photometric SN Ia candidates were spectroscopically classified as non-SN Ia instead, and the remaining 4% resulted in low signal-to-noise, unclassified spectra. This paper describes the search algorithm and the software, and the real-time processing of the SDSS imaging data. We also present the details of the supernova candidate selection procedures and strategies for follow-up spectroscopic and imaging observations of the discovered sources.

The 2dF-SDSS LRG and QSO Survey: the LRG 2-point correlation function and redshift-space distortions

Abstract: We present a clustering analysis of luminous red galaxies (LRGs) using nearly 9000 objects from the final, three-year catalogue of the 2dF-SDSS LRG and QSO (2SLAQ) Survey. We measure the redshift-space two-point correlation function, ξ(s) and find that, at the mean LRG redshift of shows the characteristic downturn at small scales (1 h−1 Mpc) expected from line-of-sight velocity dispersion. We fit a double power law to ξ(s) and measure an amplitude and slope of s0 = 17.3+2.5−2.0 h−1 Mpc, γ = 1.03 ± 0.07 at small scales (s 4.5 h−1 Mpc). In the semiprojected correlation function, wp(σ), we find a simple power law with γ = 1.83 ± 0.05 and r0 = 7.30 ± 0.34 h−1 Mpc fits the data in the range 0.4 < σ < 50 h−1 Mpc, although there is evidence of a steeper power law at smaller scales. A single power law also fits the deprojected correlation function ξ(r), with a correlation length of r0 = 7.45 ± 0.35 h−1 Mpc and a power-law slope of γ = 1.72 ± 0.06 in the 0.4 < r < 50 h−1 Mpc range. But it is in the LRG angular correlation function that the strongest evidence for non-power-law features is found where a slope of γ = −2.17 ± 0.07 is seen at 1 < r < 10 h−1 Mpc with a flatter γ = −1.67 ± 0.07 slope apparent at r 1 h−1 Mpc scales. We use the simple power-law fit to the galaxy ξ(r), under the assumption of linear bias, to model the redshift-space distortions in the 2D redshift-space correlation function, ξ(σ, π). We fit for the LRG velocity dispersion, wz, the density parameter, Ωm and β(z), where β(z) = Ω0.6m/b and b is the linear bias parameter. We find values of wz = 330 km s−1, Ωm = 0.10+0.35−0.10 and β = 0.40 ± 0.05. The low values for wz and β reflect the high bias of the LRG sample. These high-redshift results, which incorporate the Alcock–Paczynski effect and the effects of dynamical infall, start to break the degeneracy between Ωm and β found in low-redshift galaxy surveys such as 2dFGRS. This degeneracy is further broken by introducing an additional external constraint, which is the value β(z = 0.1) = 0.45 from 2dFGRS, and then considering the evolution of clustering from z 0 to zLRG 0.55. With these combined methods we find Ωm(z = 0) = 0.30 ± 0.15 and β(z = 0.55) = 0.45 ± 0.05. Assuming these values, we find a value for b(z = 0.55) = 1.66 ± 0.35. We show that this is consistent with a simple ����high-peak’ bias prescription which assumes that LRGs have a constant comoving density and their clustering evolves purely under gravity.

Clustering analyses of 300,000 photometrically classified quasars. I. Luminosity and redshift evolution in quasar bias

Abstract: 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.

Measuring the Matter Density Using Baryon Oscillations in the SDSS

Abstract: We measure the cosmological matter density by observing the positions of baryon acoustic oscillations in the clustering of galaxies in the Sloan Digital Sky Survey (SDSS). We jointly analyze the main galaxies and LRGs in the SDSS DR5 sample, using over half a million galaxies in total. The oscillations are detected with 99.74% confidence (3.0 σ assuming Gaussianity) compared to a smooth power spectrum. When combined with the observed scale of the peaks within the CMB, we find a best-fit value of ΩM = 0.256 (68% confidence interval) for a flat Λ cosmology when marginalizing over the Hubble parameter and the baryon density. This value of the matter density is derived from the locations of the baryon oscillations in the galaxy power spectrum and in the CMB, and does not include any information from the overall shape of the power spectra. This is an extremely clean cosmological measurement, as the physics of the baryon acoustic oscillation production is well understood, and the positions of the oscillations are expected to be independent of systematics such as galaxy bias.

The Mean and Scatter of the Velocity Dispersion-Optical Richness Relation for maxBCG Galaxy Clusters

Abstract: The distribution of galaxies in position and velocity around the centers of galaxy clusters encodes important information about cluster mass and structure. Using the maxBCG galaxy cluster catalog identified from imaging data obtained in the Sloan Digital Sky Survey, we study the BCG-galaxy velocity correlation function. By modeling its non-Gaussianity, we measure the mean and scatter in velocity dispersion at fixed richness. The mean velocity dispersion increases from 202 ± 10 km s-1 for small groups to more than 854 ± 102 km s-1 for large clusters. We show the scatter to be at most 40.5% ± 3.5%, declining to 14.9% ± 9.4% in the richest bins. We test our methods in the C4 cluster catalog, a spectroscopic cluster catalog produced from the Sloan Digital Sky Survey DR2 spectroscopic sample, and in mock galaxy catalogs constructed from N-body simulations. Our methods are robust, measuring the scatter to well within 1 σ of the true value, and the mean to within 10%, in the mock catalogs. By convolving the scatter in velocity dispersion at fixed richness with the observed richness space density function, we measure the velocity dispersion function of the maxBCG galaxy clusters. Although velocity dispersion and richness do not form a true mass-observable relation, the relationship between velocity dispersion and mass is theoretically well characterized and has low scatter. Thus, our results provide a key link between theory and observations up to the velocity bias between dark matter and galaxies.

Clustering analyses of 300,000 photometrically classified quasars. II. the excess on very small scales

Abstract: We study quasar clustering on small scales, modeling clustering amplitudes using halo-driven dark matter descriptions. From 91 pairs on scales 5 times higher at z > 2 than at z 2 could be consistent with reduced merger rates at z > 2 for the galaxies forming UV-excess quasars. Comparing our clustering at ~28 h-1 kpc to a ξ(r) = (r/4.8 h-1 Mpc)-1.53 power law, we find an upper limit on any excess of a factor of 4.3 ± 1.3, which, noting some caveats, differs from large excesses recently measured for binary quasars, at 2.2 σ. We speculate that binary quasar surveys that are biased to z > 2 may find inflated clustering excesses when compared to models fit at z < 2. We provide details of 111 photometrically classified quasar pairs with separations <0.1'. Spectroscopy of these pairs could significantly constrain quasar dynamics in merging galaxies.

MegaZ-LRG:a photometric redshift catalogue of one million SDSS luminous red galaxies

Abstract: We describe the construction of MegaZ-LRG, a photometric redshift catalogue of over one million luminous red galaxies (LRGs) in the redshift range 0.4 < z < 0.7 with limiting magnitude i < 20. The catalogue is selected from the imaging data of the Sloan Digital Sky Survey (SDSS) Data Release 4. The 2dF-SDSS LRG and Quasar (2SLAQ) spectroscopic redshift catalogue of 13 000 intermediate-redshift LRGs provides a photometric redshift training set, allowing use of annz, a neural network-based photometric-redshift estimator. The rms photometric redshift accuracy obtained for an evaluation set selected from the 2SLAQ sample is sigma(z) 0.049 averaged over all galaxies, and sigma(z) = 0.040 for a brighter subsample (i < 19.0). The catalogue is expected to contain similar to 5 per cent stellar contamination. The annz code is used to compute a refined star/galaxy probability based on a range of photometric parameters; this allows the contamination fraction to be reduced to 2 per cent with negligible loss of genuine galaxies. The MegaZ-LRG catalogue is publicly available on the World Wide Web from http://www.2slaq.info.

Radio galaxies in the 2SLAQ Luminous Red Galaxy Survey - I: The evolution of low-power radio galaxies to z ~ 0.7

Abstract: We have combined optical data from the 2dF-SDSS (Sloan Digital Sky Survey) LRG (Luminous Red Galaxy) and QSO (quasi-stellar object) (2SLAQ) redshift survey with radio measurements from the 1.4 GHz VLA (Very Large Array) FIRST (Faint Images of the Radio Sky at Twentycm) and NVSS (NRAO VLA Sky Survey) surveys to identify a volume-limited sample of 391 radio galaxies at redshift 0.4 < z < 0.7. By determining an accurate radio luminosity function for luminous early-type galaxies in this redshift range, we can investigate the cosmic evolution of the radio-galaxy population over a wide range in radio luminosity. The low-power radio galaxies in our LRG sample (those with 1.4 GHz radio luminosities in the range 10 24 to 10 25 WH z −1 , corresponding to Fanaroff‐Riley I (FR I) radio galaxies in the local Universe) undergo significant cosmic evolution over the redshift range 0 < z < 0.7, consistent with pure luminosity evolution of the form (1 + z) k , where k = 2.0 ± 0.3. Our results appear to rule out (at the 6‐7σ level) models in which low-power radio galaxies undergo no cosmic evolution. The most powerful radio galaxies in our sample (with radio luminosities above 10 26 WH z −1 ) may undergo more rapid evolution over the same redshift range. The evolution seen in the low-power radio-galaxy population implies that the total energy input into massive early-type galaxies from active galactic nucleus (AGN) heating increases with redshift, and was at least 50 per cent higher at z ∼ 0.55 (the median redshift of the 2SLAQ LRG sample) than in the local universe.

The three-point correlation function of luminous red galaxies in the Sloan Digital Sky Survey

Abstract: We present measurements of the redshift-space three-point correlation function of 50 967 luminous red galaxies (LRGs) from Data Release 3 (DR3) of the Sloan Digital Sky Survey (SDSS). We have studied the shape dependence of the reduced three-point correlation function (Qz(s, q, θ )) on three different scales, s = 4, 7 and 10 h −1 Mpc, and over the range of 1 < q < 3 and 0 ◦ <θ <180 ◦ . On small scales (s = 4 h −1 Mpc), Qz is nearly constant, with little change as a function of q and θ. However, there is evidence for a shallow U-shaped behaviour (with θ ) which is expected from theoretical modelling of Qz(s, q, θ ). On larger scales (s = 7 and 10 h −1 Mpc), the U-shaped anisotropy in Qz (with θ) is more clearly detected. We compare this shape dependence in Qz(s, q, θ) with that seen in mock galaxy catalogues which were generated by populating the dark matter haloes in large N-body simulations with mock galaxies using various halo occupation distributions (HOD). We find that the combination of the observed number density of LRGs, the (redshift-space) two-point correlation function and Qz(s, q, θ ) provides a strong constraint on the allowed HOD parameters (Mmin, M1, α) and breaks key degeneracies between these parameters. For example, our observed Qz(s, q, θ) disfavours mock catalogues that overpopulate massive dark matter haloes with many LRG satellites. We also estimate the linear bias of LRGs to be b = 1.87 ± 0.07 in excellent agreement with other measurements.

Modeling the three-point correlation function

Abstract: We present new theoretical predictions for the galaxy three-point correlation function (3PCF) using high-resolution dissipationless cosmological simulations of a flat {Lambda}CDM Universe which resolve galaxy-size halos and subhalos. We create realistic mock galaxy catalogs by assigning luminosities and colors to dark matter halos and subhalos, and we measure the reduced 3PCF as a function of luminosity and color in both real and redshift space. As galaxy luminosity and color are varied, we find small differences in the amplitude and shape dependence of the reduced 3PCF, at a level qualitatively consistent with recent measurements from the SDSS and 2dFGRS. We confirm that discrepancies between previous 3PCF measurements can be explained in part by differences in binning choices. We explore the degree to which a simple local bias model can fit the simulated 3PCF. The agreement between the model predictions and galaxy 3PCF measurements lends further credence to the straightforward association of galaxies with CDM halos and subhalos.

Optimizing baryon acoustic oscillation surveys – I. Testing the concordance ΛCDM cosmology

Abstract: We optimize the design of future spectroscopic redshift surveys for constraining the dark energy via precision measurements of the baryon acoustic oscillations (BAOs), with particular emphasis on the design of the Wide-Field Multi-Object Spectrograph. We develop a model that predicts the number density of possible target galaxies as a function of exposure time and redshift. We use this number counts model together with fitting formulae for the accuracy of the BAO measurements to determine the effectiveness of different surveys and instrument designs. We search through the available survey parameter space to find the optimal survey with respect to the dark energy equation-of-state parameters according to the Dark Energy Task Force Figure-of-Merit, including predictions of future measurements from the Planck satellite. We optimize the survey to test the Lambda cold dark matter model, assuming that galaxies are pre-selected using photometric redshifts to have a constant number density with redshift, and using a non-linear cut-off for the matter power spectrum that evolves with redshift. We find that line-emission galaxies are strongly preferred as targets over continuum emission galaxies. The optimal survey covers a redshift range 0.8 < z < 1.4, over the widest possible area (6000 deg2 from 1500 h observing time). The most efficient number of fibres for the spectrograph is 2000, and the survey performance continues to improve with the addition of extra fibres until a plateau is reached at 10 000 fibres. The optimal point in the survey parameter space is not highly peaked and is not significantly affected by including constraints from upcoming supernovae surveys and other BAO experiments.

Optimising Baryon Acoustic Oscillation Surveys - I: Testing the concordance LCDM cosmology

Abstract: We optimize the design of future spectroscopic redshift surveys for constraining the dark energy via precision measurements of the baryon acoustic oscillations (BAO), with particular emphasis on the design of the Wide-Field Multi-Object Spectrograph (WFMOS). We develop a model that predicts the number density of possible target galaxies as a function of exposure time and redshift. We use this number counts model together with fitting formulae for the accuracy of the BAO measurements to determine the effectiveness of different surveys and instrument designs. We search through the available survey parameter space to find the optimal survey with respect to the dark energy equation-of-state parameters according to the Dark Energy Task Force Figure-of-Merit, including predictions of future measurements from the Planck satellite. We optimize the survey to test the LambdaCDM model, assuming that galaxies are pre-selected using photometric redshifts to have a constant number density with redshift, and using a non-linear cut-off for the matter power spectrum that evolves with redshift. We find that line-emission galaxies are strongly preferred as targets over continuum emission galaxies. The optimal survey covers a redshift range 0.8 < z < 1.4, over the widest possible area (6000 sq. degs from 1500 hours observing time). The most efficient number of fibres for the spectrograph is 2,000, and the survey performance continues to improve with the addition of extra fibres until a plateau is reached at 10,000 fibres. The optimal point in the survey parameter space is not highly peaked and is not significantly affected by including constraints from upcoming supernovae surveys and other BAO experiments.

Optimizing future imaging survey of galaxies to confront dark energy and modified gravity models

Abstract: We consider the extent to which future imaging surveys of galaxies can distinguish between dark energy and modified gravity models for the origin of the cosmic acceleration. Dynamical dark energy models may have similar expansion rates as models of modified gravity, yet predict different growth of structure histories. We parametrize the cosmic expansion by the two parameters, w0 and wa, and the linear growth rate of density fluctuations by Linder's γ, independently. Dark energy models generically predict γ≈0.55, while the Dvali-Gabadadze-Porrati (DGP) model γ≈0.68. To determine if future imaging surveys can constrain γ within 20% (or Δγ<0.1), we perform the Fisher matrix analysis for a weak-lensing survey such as the ongoing Hyper Suprime-Cam (HSC) project. Under the condition that the total observation time is fixed, we compute the figure of merit (FoM) as a function of the exposure time texp. We find that the tomography technique effectively improves the FoM, which has a broad peak around texp several∼10min; a shallow and wide survey is preferred to constrain the γ parameter. While Δγ<0.1 cannot be achieved by the HSC weak-lensing survey alone, one can improve the constraints by combining with a follow-up spectroscopic survey like Wide-field Fiber-fed Multi-Object Spectrograph (WFMOS) and/or future cosmic microwave background (CMB) observations.

Mapping the cosmological confidence ball surface

Abstract: We present a new technique to compute simultaneously valid confidence intervals for a set of model parameters. We apply our method to the Wilkinson Microwave Anisotropy Probe's cosmic microwave background data, exploring a seven-dimensional space (τ,ΩDE,ΩM,ωDM,ωB,fν,ns). We find two distinct regions of interest: the standard concordance model and a region with large values of ωDM, ωB, and H0. This second peak in parameter space can be rejected by applying a constraint (or a prior) on the allowable values of the Hubble constant. Our new technique uses a nonparametric fit to the data, along with a frequentist approach and a smart search algorithm to map out a statistical confidence surface. The result is a confidence "ball," a set of parameter values that contains the true value with probability at least 1 - α. Our algorithm performs a role similar to the often-used Markov Chain Monte Carlo (MCMC), which samples from the posterior probability function in order to provide Bayesian credible intervals on the parameters. While the MCMC approach samples densely around a peak in the posterior, our new technique allows cosmologists to perform efficient analyses around any regions of interest, e.g., the peak itself or, possibly more importantly, the 1 - α confidence surface.

The Mean and Scatter of the Velocity Dispersion-Optical Richness Relation for maxBCG Galaxy Clusters

Abstract: The distribution of galaxies in position and velocity around the centers of galaxy clusters encodes important information about cluster mass and structure. Using the maxBCG galaxy cluster catalog identified from imaging data obtained in the Sloan Digital Sky Survey, we study the BCG-galaxy velocity correlation function. By modeling its non-Gaussianity, we measure the mean and scatter in velocity dispersion at fixed richness. The mean velocity dispersion increases from 202+/-10 km/s for small groups to more than 854+/-102 km/s for large clusters. We show the scatter to be at most 40.5+/-3.5%, declining to 14.9+/-9.4% in the richest bins. We test our methods in the C4 cluster catalog, a spectroscopic cluster catalog produced from the Sloan Digital Sky Survey DR2 spectroscopic sample, and in mock galaxy catalogs constructed from N-body simulations. Our methods are robust, measuring the scatter to well within one-sigma of the true value, and the mean to within 10%, in the mock catalogs. By convolving the scatter in velocity dispersion at fixed richness with the observed richness space density function, we measure the velocity dispersion function of the maxBCG galaxy clusters. Although velocity dispersion and richness do not form a true mass-observable relation, the relationship between velocity dispersion and mass is theoretically well characterized and has low scatter. Thus our results provide a key link between theory and observations up to the velocity bias between dark matter and galaxies.