Compact groups (CGs) of galaxies -relatively poor groups of galaxies in which the typical separat... more Compact groups (CGs) of galaxies -relatively poor groups of galaxies in which the typical separations between members is of the order of a galaxy diameter -offer an exceptional laboratory for the study of dense galaxian environments with short (< 1 Gyr) dynamical time-scales.
The mass function of clusters of galaxies is determined from 400 deg 2 of early commissioning ima... more The mass function of clusters of galaxies is determined from 400 deg 2 of early commissioning imaging data of the Sloan Digital Sky Survey; ∼300 clusters in the redshift range z = 0.1 -0.2 are used. Clusters are selected using two independent selection methods: a Matched Filter and a red-sequence color magnitude technique. The two methods yield consistent results. The cluster mass function is compared with large-scale cosmological simulations. We find a best-fit cluster normalization relation of σ 8 Ω m 0.6 = 0.33 ± 0.03 (for 0.1 Ω m 0.4), or equivalently σ 8 = ( 0.16 Ωm ) 0.6 . The amplitude of this relation is significantly lower than the previous canonical value, implying that either Ω m is lower than previously expected (Ω m = 0.16 if σ 8 = 1) or σ 8 is lower than expected (σ 8 = 0.7 if Ω m = 0.3) -2as suggested by recent results. The shape of the cluster mass function partially breaks this classic degeneracy; we find best-fit parameters of Ω m = 0.19 ± 0.08 0.07 and σ 8 = 0.9 ± 0.3 0.2 . High values of Ω m ( 0.4) and low σ 8 ( 0.6) are excluded at 2σ.
This is the first in a series of papers on the weak lensing effect caused by clusters of galaxies... more This is the first in a series of papers on the weak lensing effect caused by clusters of galaxies in Sloan Digital Sky Survey. The photometrically selected cluster sample, known as MaxBCG, includes ∼130,000 objects between redshift 0.1 and 0.3, ranging in size from small groups to massive clusters. We split the clusters into bins of richness and luminosity and stack the surface density contrast to produce mean radial profiles. The mean profiles are detected over a range of scales, from the inner halo (25 kpc/h) well into the surrounding large scale structure (30 Mpc/h), with a significance of 15 to 20 in each bin. The signal over this large range of scales is best interpreted in terms of the cluster-mass cross-correlation function. We pay careful attention to sources of systematic error, correcting for them where possible. The resulting signals are calibrated to the ∼10% level, with the dominant remaining uncertainty being the redshift distribution of the background sources. We find that the profiles scale strongly with richness and luminosity. We find the signal within a given richness bin depends upon luminosity, suggesting that luminosity is more closely correlated with mass than galaxy counts. We split the samples by redshift but detect no significant evolution. The profiles are not well described by power laws. In a subsequent series of papers we invert the profiles to three-dimensional mass profiles, show that they are well fit by a halo model description, measure mass-to-light ratios and provide a cosmological interpretation.
We investigate the evolution of the fractions of late type cluster galaxies as a function of reds... more We investigate the evolution of the fractions of late type cluster galaxies as a function of redshift, using one of the largest, most uniform cluster samples available. The sample consists of 514 clusters of galaxies in the range 0.02≤z≤0.3 from the Sloan Digital Sky Survey Cut & Enhance galaxy cluster catalog. This catalog was created using a single automated cluster finding algorithm applied to uniform data from a single telescope, with accurate CCD photometry, thus, minimizing selection biases. We use four independent methods to analyze the evolution of the late type galaxy fraction. Specifically, we select late type galaxies based on: restframe g −r color, u−r color, galaxy profile fitting and concentration index. The first criterion corresponds to 1 the one used in the classical Butcher-Oemler analyses. The last two criteria are more sensitive to the morphological type of the galaxies. In all the four cases, we find an increase in the fraction of late type galaxies with increasing redshift, significant at the 99.9% level. The results confirm that cluster galaxies do change colors with redshift (the Butcher-Oemler effect) and, in addition, they change their morphology to latertype toward higher redshift -indicating a morphological equivalent of the Butcher-Oemler effect. We also find a tendency of richer clusters to have lower fractions of late type galaxies. The trend is consistent with a ram pressure stripping model, where galaxies in richer clusters are affected by stronger ram pressure due to higher temperature of clusters.
We use the abundance and weak-lensing mass measurements of the Sloan Digital Sky Survey maxBCG cl... more We use the abundance and weak-lensing mass measurements of the Sloan Digital Sky Survey maxBCG cluster catalog to simultaneously constrain cosmology and the richness-mass relation of the clusters. Assuming a flat ΛCDM cosmology, we find σ 8 (Ω m /0.25) 0.41 = 0.832 ± 0.033 after marginalization over all systematics. In common with previous studies, our error budget is dominated by systematic uncertainties, the primary two being the absolute mass scale of the weak-lensing masses of the maxBCG clusters, and uncertainty in the scatter of the richness-mass relation. Our constraints are fully consistent with the WMAP five-year data, and in a joint analysis we find σ 8 = 0.807 ± 0.020 and Ω m = 0.265 ± 0.016, an improvement of nearly a factor of 2 relative to WMAP5 alone. Our results are also in excellent agreement with and comparable in precision to the latest cosmological constraints from X-ray cluster abundances. The remarkable consistency among these results demonstrates that cluster abundance constraints are not only tight but also robust, and highlight the power of optically selected cluster samples to produce precision constraints on cosmological parameters.
We present a lensing study of 42 galaxy clusters imaged in Sloan Digital Sky Survey (SDSS) commis... more We present a lensing study of 42 galaxy clusters imaged in Sloan Digital Sky Survey (SDSS) commissioning data. Cluster candidates are selected optically from SDSS imaging data and confirmed for this study by matching to X-ray sources found independently in the ROSAT all sky survey (RASS). Five color SDSS photometry is used to make accurate (∆z=0.018) photometric redshift estimates that are used to rescale and combine the lensing measurements. The mean shear from these clusters is detected to 2h −1 Mpc at the 7-σ level, corresponding to a mass within that radius of (4.2 ± 0.6)× 10 14 h −1 M ⊙ . The shear profile is well fit by a power law with index −0.9±0.3, consistent with that of an isothermal density profile. Clusters are divided by X-ray luminosity into two subsets, with mean L X of (0.14 ± 0.03) × 10 44 and (1.0 ± 0.09) × 10 44 h −2 ergs/s. The average lensing signal is converted to a projected mass density based on fits to isothermal density profiles. From this we calculate a mean r 500 (the radius at which the mean density falls to 500 times the critical density) and M(< r 500 ). The mass contained within r 500 differs substantially between the low-and high-L X bins, with (0.7 ± 0.2) × 10 14 and 2.7 +0.9 −1.1 × 10 14 h −1 M ⊙ respectively. This paper demonstrates our ability to measure ensemble cluster masses from SDSS imaging data. The full SDSS data set will include 1000 SDSS/RASS clusters. With this large data set we will measure the M-L X relation with high precision and put direct constraints on the mass density of the universe.
We interpret and model the statistical weak lensing measurements around 130,000 groups and cluste... more We interpret and model the statistical weak lensing measurements around 130,000 groups and clusters of galaxies in the Sloan Digital Sky Survey presented by . We present non-parametric inversions of the 2D shear profiles to the mean 3D cluster density and mass profiles in bins of both optical richness and cluster i-band luminosity. Since the mean cluster density profile is proportional to the cluster-mass correlation function, the mean profile is spherically symmetric by the assumptions of large-scale homogeneity and isotropy. We correct the inferred 3D profiles for systematic effects, including non-linear shear and the fact that cluster halos are not all precisely centered on their brightest galaxies. We also model the measured cluster shear profile as a sum of contributions from the brightest central galaxy, the cluster dark matter halo, and neighboring halos. We infer the relations between mean cluster virial mass and optical richness and luminosity over two orders of magnitude in cluster mass; the virial mass at fixed richness or luminosity is determined with a precision of ∼ 13% including both statistical and systematic errors. We also constrain the halo concentration parameter and halo bias as a function of cluster mass; both are in good agreement with predictions from N-body simulations of LCDM models. The methods employed here will be applicable to deeper, wide-area optical surveys that aim to constrain the nature of the dark energy, such as the Dark Energy Survey, the Large Synoptic Survey Telescope and space-based surveys.
This is the second paper in a series aimed at finding high-redshift quasars from five-color (u ′ ... more This is the second paper in a series aimed at finding high-redshift quasars from five-color (u ′ g ′ r ′ i ′ z ′ ) imaging data taken along the Celestial Equator by the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this paper, we present 22 high-redshift quasars (z > 3.6) discovered from ∼ 250 deg 2 of data in the spring Equatorial Stripe, plus photometry for two previously known high-redshift quasars in the same region of sky. Our success rate of identifying high-redshift quasars is 68%. Five of the newly discovered quasars have redshifts higher than 4.6 (z = 4. 62, 4.69, 4.70, 4.92 and 5.03). All the quasars have i * < 20.2 with absolute magnitude −28.8 < M B < −26.1 (h = 0.5, q 0 = 0.5). Several of the quasars show unusual emission and absorption features in their spectra, including an object at z = 4.62 without detectable emission lines, and a Broad Absorption Line (BAL) quasar at z = 4.92.
Galaxy masses can be estimated by a variety of methods; each applicable in different circumstance... more Galaxy masses can be estimated by a variety of methods; each applicable in different circumstances, and each suffering from different systematic uncertainties. Confirmation of results obtained by one technique with analysis by another is particularly important. Recent SDSS weak lensing measurements of the projected-mass correlation function reveal a linear relation between galaxy luminosities and the depth of their dark matter halos (measured on 260 h −1 kpc scales). In this work we use an entirely independent dynamical method to confirm these results. We begin by assembling a sample of 618 relatively isolated host galaxies, surrounded by a total of 1225 substantially fainter satellites. We observe the mean dynamical effect of these hosts on the motions of their satellites by assembling velocity difference histograms. Dividing the sample by host properties, we find significant variations in satellite velocity dispersion with host luminosity. We quantify these variations using a simple dynamical model, measuring M dyn 260 a dynamical mass within 260 h −1 kpc. The appropriateness of this mass reconstruction is checked by conducting a similar analysis within an N-body simulation. Comparison between the dynamical and lensing mass-to-light scalings shows reasonable agreement, providing some quantitative confirmation for the lensing results.
We present here results on the composite luminosity functions (LF) of galaxies in the clusters of... more We present here results on the composite luminosity functions (LF) of galaxies in the clusters of galaxies selected from the Cut and Enhance cluster catalog (CE) of the Sloan Digital Sky Survey. We constructed composite LFs in the five SDSS bands, u, g, r, i and z, using 204 CE clusters ranging from z = 0.02 to z = 0.25 . Background and foreground galaxies were subtracted from the LF using an annular region around clusters to take large-scale, galaxy-number-count variances into consideration. A LF of each cluster was weighted according to the richness and number of contributing galaxies to construct the composite LF. Taking advantage of accurate 1 photometry of SDSS, we used photometric redshifts to construct composite luminosity functions and thus study a large number of clusters. The robustness of the weighting scheme was tested using a Monte-Carlo simulation. The best-fit Schechter parameters are (M * ,α)=(−21.61±0.26, −1.40±0.11),(−22.01±0.11, −1.00±0.06),(−22.21±0.05, −0.85±0.03), (−22.31±0.08, −0.70±0.05) and (−21.36±0.06, −0.58±0.04) in u,g,r,i and z, respectively. We find that the slope of composite LFs becomes flatter toward a redder color band. Compared with the field LFs of the SDSS, the cluster LFs have brighter characteristic magnitude and flatter slopes in the g, r, i and z bands. These results are consistent with the hypothesis that the cluster LF has two distinct underlying populations i.e. the bright end of the LF is dominated by bright early types that follow a Gaussian-like luminosity distribution, while the faint-end of the cluster LF is a steep power-law like function dominated by star-forming (bluer) galaxies. We also studied the composite LFs for early-type and late-type galaxies using profile fits, a concentration parameter and u − r color to classify galaxy morphology. A strong dependence of LF on galaxy morphology was found. The faint end slope of the LF is always flatter for early-type galaxies than late-type, regardless of the passband and methodology. These results are consistent with the hypothesis that the cluster regions are dominated by bright elliptical galaxies. This work also provides a good low-redshift benchmark for on-going multi-color photometric studies of high redshift clusters of galaxies using 4-8 m class telescopes.
Measurements of galaxy cluster abundances, clustering properties, and massto-light ratios in curr... more Measurements of galaxy cluster abundances, clustering properties, and massto-light ratios in current and future surveys can provide important cosmological constraints. Digital wide-field imaging surveys, the recently-demonstrated fidelity of red-sequence cluster detection techniques, and a new generation of realistic mock galaxy surveys provide the means for construction of large, cosmologically-interesting cluster samples, whose selection and properties can be understood in unprecedented depth. We present the details of the "maxBCG" algorithm, a cluster-detection technique tailored to multi-band CCD-imaging data. MaxBCG primarily relies on an observational cornerstone of massive galaxy clusters: they are marked by an overdensity of bright, uniformly red galaxies. This detection scheme also exploits classical brightest cluster galaxies (BCGs), which are often found at the center of these same massive clusters. We study the algorithm herein through its performance on large, realistic, mock galaxy catalogs,
We present results on over 100 high-redshift quasars found in the Sloan Digital Sky Survey (SDSS)... more We present results on over 100 high-redshift quasars found in the Sloan Digital Sky Survey (SDSS), using automated selection algorithms applied to SDSS imaging data and with spectroscopic confirmation obtained during routine spectroscopic operations of the Sloan 2.5-m telescope. The SDSS spectra cover the wavelength range 3900 -9200Å at a spectral resolution of 1800, and have been obtained for 116 quasars with redshifts greater than 3.94; 92 of these objects were previously uncataloged, significantly increasing the current tally of published z > 4 quasars. The paper also reports observations of five additional new z > 4.6 quasars; all were found from the SDSS imaging survey and spectroscopically confirmed with data from the Apache Point Observatory's 3.5-m telescope. The i ′ magnitudes of the quasars range from 18.03 to 20.56. Of the 97 new objects in this paper, 13 are Broad Absorption Line quasars. Five quasars, including one object at a redshift of 5.11, have 20 cm peak flux densities greater than 1 mJy. Two of the quasars, both at z ≈ 4.5, have very weak emission lines; one of these objects is a radio source. Nineteen of the newly-discovered objects have redshifts above 4.6, and the maximum redshift is z=5.41; among objects reported to date, the latter is the third highest redshift AGN, and penultimate in redshift among luminous quasars.
Imaging data from the Sloan Digital Sky Survey is used to measure the empirical size-richness rel... more Imaging data from the Sloan Digital Sky Survey is used to measure the empirical size-richness relation for a large sample of galaxy clusters. Using population subtraction methods, we determine the radius at which the cluster galaxy number density is 200Ω −1 m times the mean galaxy density, without assuming a model for the radial distribution of galaxies in clusters. If these galaxies are unbiased on Mpc scales, this galaxy-density-based R 200 reflects the characteristic radii of clusters. We measure the scaling of this characteristic radius with richness over an order of magnitude in cluster richness, from rich clusters to poor groups. We use this information to examine the radial profiles of galaxies in clusters as a function of cluster richness, finding that the concentration of the galaxy distribution decreases with richness and is systematically lower than the concentrations measured for dark matter profiles in N-body simulations. Using these scaled radii, we investigate the behavior of the cluster luminosity function, and find that it is well matched by a Schechter function for galaxies brighter than M r = −18 only after the central galaxy has been removed. We find that the luminosity function varies with richness and with distance from the cluster center, underscoring the importance of using an aperture that scales with cluster mass to compare physically equivalent regions of these different systems. We note that the lowest richness systems in our catalog have properties consistent with those expected of the earliest-forming halos; our cluster-finding algorithm, in addition to reliably finding clusters, may be efficient at finding fossil groups.
We present a catalog of 799 clusters of galaxies in the redshift range z est ¼ 0:05 0:3 selected ... more We present a catalog of 799 clusters of galaxies in the redshift range z est ¼ 0:05 0:3 selected from $400 deg 2 of early Sloan Digital Sky Survey (SDSS) commissioning data along the celestial equator. The catalog is based on merging two independent selection methods-a color-magnitude red-sequence maxBCG technique (B), and a hybrid matched filter method (H). The BH catalog includes clusters with richness à ! 40 (matched filter) and N gal ! 13 (maxBCG), corresponding to typical velocity dispersion of v e400 km s À1 and mass (within 0.6 h À1 Mpc radius) e5  10 13 h À1 M . This threshold is below Abell richness class 0 clusters. The average space density of these clusters is 2  10 À5 h 3 Mpc À3 . All NORAS X-ray clusters and 53 of the 58 Abell clusters in the survey region are detected in the catalog; the five additional Abell clusters are detected below the BH catalog cuts. The cluster richness function is determined and found to exhibit a steeply decreasing cluster abundance with increasing richness. We derive observational scaling relations between cluster richness and observed cluster luminosity and cluster velocity dispersion; these scaling relations provide important physical calibrations for the clusters. The catalog can be used for studies of individual clusters, for comparisons with other sources such as X-ray clusters and active galactic nuclei, and, with proper correction for the relevant selection functions, also for statistical analyses of clusters.
We discuss optical and infrared photometric properties of stars matched in the Two Micron All Sky... more We discuss optical and infrared photometric properties of stars matched in the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) commissioning data for ∼50 deg 2 of sky along the Celestial Equator centered at l = 150 • , b = −60 • . About 98% (∼63,000) of objects listed in the 2MASS Point Source Catalog in the analyzed area are matched within 2 arcsec to an SDSS source. The matched sources represent 8% of the ∼800,000 SDSS sources in this area. They are predominantly red sources, as expected, and 15% of them are resolved in SDSS imaging data, although they are detected as point sources in 2MASS data. The distribution of positional discrepancies for the matched sources, and the astrometric statistics for the multiply observed SDSS sources, imply that the astrometric accuracy of both surveys is about 0.1 arcsec per coordinate (rms).
Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the ... more Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of 1 large-scale structures predicts 1−6 the existence of quasars gravitationally lensed by concentrations of dark matter 7 so massive that the quasar images would be split by over 7 arcsec. Numerous searches 8−11 for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known 12 , including the first lensed quasar discovered 13 , have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies 14 with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004+4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations 3−5 based on the cold-dark-matter model.
This paper describes the discovery of seven dwarf objects of spectral type 'L' (objects cooler th... more This paper describes the discovery of seven dwarf objects of spectral type 'L' (objects cooler than the latest M dwarfs) in commissioning imaging data taken by the Sloan Digital Sky Survey (SDSS). Low-resolution spectroscopy shows that these objects have spectral types from L0 to L8. Comparison of the SDSS and 2MASS photometry for several of these objects indicates the presence of significant opacity at optical wavelengths, perhaps due to atmospheric dust. This comparison also demonstrates the high astrometric accuracy (better than 1 ′′ for these faint sources) of both surveys.
Compact groups (CGs) of galaxies -relatively poor groups of galaxies in which the typical separat... more Compact groups (CGs) of galaxies -relatively poor groups of galaxies in which the typical separations between members is of the order of a galaxy diameter -offer an exceptional laboratory for the study of dense galaxian environments with short (< 1 Gyr) dynamical time-scales.
The mass function of clusters of galaxies is determined from 400 deg 2 of early commissioning ima... more The mass function of clusters of galaxies is determined from 400 deg 2 of early commissioning imaging data of the Sloan Digital Sky Survey; ∼300 clusters in the redshift range z = 0.1 -0.2 are used. Clusters are selected using two independent selection methods: a Matched Filter and a red-sequence color magnitude technique. The two methods yield consistent results. The cluster mass function is compared with large-scale cosmological simulations. We find a best-fit cluster normalization relation of σ 8 Ω m 0.6 = 0.33 ± 0.03 (for 0.1 Ω m 0.4), or equivalently σ 8 = ( 0.16 Ωm ) 0.6 . The amplitude of this relation is significantly lower than the previous canonical value, implying that either Ω m is lower than previously expected (Ω m = 0.16 if σ 8 = 1) or σ 8 is lower than expected (σ 8 = 0.7 if Ω m = 0.3) -2as suggested by recent results. The shape of the cluster mass function partially breaks this classic degeneracy; we find best-fit parameters of Ω m = 0.19 ± 0.08 0.07 and σ 8 = 0.9 ± 0.3 0.2 . High values of Ω m ( 0.4) and low σ 8 ( 0.6) are excluded at 2σ.
This is the first in a series of papers on the weak lensing effect caused by clusters of galaxies... more This is the first in a series of papers on the weak lensing effect caused by clusters of galaxies in Sloan Digital Sky Survey. The photometrically selected cluster sample, known as MaxBCG, includes ∼130,000 objects between redshift 0.1 and 0.3, ranging in size from small groups to massive clusters. We split the clusters into bins of richness and luminosity and stack the surface density contrast to produce mean radial profiles. The mean profiles are detected over a range of scales, from the inner halo (25 kpc/h) well into the surrounding large scale structure (30 Mpc/h), with a significance of 15 to 20 in each bin. The signal over this large range of scales is best interpreted in terms of the cluster-mass cross-correlation function. We pay careful attention to sources of systematic error, correcting for them where possible. The resulting signals are calibrated to the ∼10% level, with the dominant remaining uncertainty being the redshift distribution of the background sources. We find that the profiles scale strongly with richness and luminosity. We find the signal within a given richness bin depends upon luminosity, suggesting that luminosity is more closely correlated with mass than galaxy counts. We split the samples by redshift but detect no significant evolution. The profiles are not well described by power laws. In a subsequent series of papers we invert the profiles to three-dimensional mass profiles, show that they are well fit by a halo model description, measure mass-to-light ratios and provide a cosmological interpretation.
We investigate the evolution of the fractions of late type cluster galaxies as a function of reds... more We investigate the evolution of the fractions of late type cluster galaxies as a function of redshift, using one of the largest, most uniform cluster samples available. The sample consists of 514 clusters of galaxies in the range 0.02≤z≤0.3 from the Sloan Digital Sky Survey Cut & Enhance galaxy cluster catalog. This catalog was created using a single automated cluster finding algorithm applied to uniform data from a single telescope, with accurate CCD photometry, thus, minimizing selection biases. We use four independent methods to analyze the evolution of the late type galaxy fraction. Specifically, we select late type galaxies based on: restframe g −r color, u−r color, galaxy profile fitting and concentration index. The first criterion corresponds to 1 the one used in the classical Butcher-Oemler analyses. The last two criteria are more sensitive to the morphological type of the galaxies. In all the four cases, we find an increase in the fraction of late type galaxies with increasing redshift, significant at the 99.9% level. The results confirm that cluster galaxies do change colors with redshift (the Butcher-Oemler effect) and, in addition, they change their morphology to latertype toward higher redshift -indicating a morphological equivalent of the Butcher-Oemler effect. We also find a tendency of richer clusters to have lower fractions of late type galaxies. The trend is consistent with a ram pressure stripping model, where galaxies in richer clusters are affected by stronger ram pressure due to higher temperature of clusters.
We use the abundance and weak-lensing mass measurements of the Sloan Digital Sky Survey maxBCG cl... more We use the abundance and weak-lensing mass measurements of the Sloan Digital Sky Survey maxBCG cluster catalog to simultaneously constrain cosmology and the richness-mass relation of the clusters. Assuming a flat ΛCDM cosmology, we find σ 8 (Ω m /0.25) 0.41 = 0.832 ± 0.033 after marginalization over all systematics. In common with previous studies, our error budget is dominated by systematic uncertainties, the primary two being the absolute mass scale of the weak-lensing masses of the maxBCG clusters, and uncertainty in the scatter of the richness-mass relation. Our constraints are fully consistent with the WMAP five-year data, and in a joint analysis we find σ 8 = 0.807 ± 0.020 and Ω m = 0.265 ± 0.016, an improvement of nearly a factor of 2 relative to WMAP5 alone. Our results are also in excellent agreement with and comparable in precision to the latest cosmological constraints from X-ray cluster abundances. The remarkable consistency among these results demonstrates that cluster abundance constraints are not only tight but also robust, and highlight the power of optically selected cluster samples to produce precision constraints on cosmological parameters.
We present a lensing study of 42 galaxy clusters imaged in Sloan Digital Sky Survey (SDSS) commis... more We present a lensing study of 42 galaxy clusters imaged in Sloan Digital Sky Survey (SDSS) commissioning data. Cluster candidates are selected optically from SDSS imaging data and confirmed for this study by matching to X-ray sources found independently in the ROSAT all sky survey (RASS). Five color SDSS photometry is used to make accurate (∆z=0.018) photometric redshift estimates that are used to rescale and combine the lensing measurements. The mean shear from these clusters is detected to 2h −1 Mpc at the 7-σ level, corresponding to a mass within that radius of (4.2 ± 0.6)× 10 14 h −1 M ⊙ . The shear profile is well fit by a power law with index −0.9±0.3, consistent with that of an isothermal density profile. Clusters are divided by X-ray luminosity into two subsets, with mean L X of (0.14 ± 0.03) × 10 44 and (1.0 ± 0.09) × 10 44 h −2 ergs/s. The average lensing signal is converted to a projected mass density based on fits to isothermal density profiles. From this we calculate a mean r 500 (the radius at which the mean density falls to 500 times the critical density) and M(< r 500 ). The mass contained within r 500 differs substantially between the low-and high-L X bins, with (0.7 ± 0.2) × 10 14 and 2.7 +0.9 −1.1 × 10 14 h −1 M ⊙ respectively. This paper demonstrates our ability to measure ensemble cluster masses from SDSS imaging data. The full SDSS data set will include 1000 SDSS/RASS clusters. With this large data set we will measure the M-L X relation with high precision and put direct constraints on the mass density of the universe.
We interpret and model the statistical weak lensing measurements around 130,000 groups and cluste... more We interpret and model the statistical weak lensing measurements around 130,000 groups and clusters of galaxies in the Sloan Digital Sky Survey presented by . We present non-parametric inversions of the 2D shear profiles to the mean 3D cluster density and mass profiles in bins of both optical richness and cluster i-band luminosity. Since the mean cluster density profile is proportional to the cluster-mass correlation function, the mean profile is spherically symmetric by the assumptions of large-scale homogeneity and isotropy. We correct the inferred 3D profiles for systematic effects, including non-linear shear and the fact that cluster halos are not all precisely centered on their brightest galaxies. We also model the measured cluster shear profile as a sum of contributions from the brightest central galaxy, the cluster dark matter halo, and neighboring halos. We infer the relations between mean cluster virial mass and optical richness and luminosity over two orders of magnitude in cluster mass; the virial mass at fixed richness or luminosity is determined with a precision of ∼ 13% including both statistical and systematic errors. We also constrain the halo concentration parameter and halo bias as a function of cluster mass; both are in good agreement with predictions from N-body simulations of LCDM models. The methods employed here will be applicable to deeper, wide-area optical surveys that aim to constrain the nature of the dark energy, such as the Dark Energy Survey, the Large Synoptic Survey Telescope and space-based surveys.
This is the second paper in a series aimed at finding high-redshift quasars from five-color (u ′ ... more This is the second paper in a series aimed at finding high-redshift quasars from five-color (u ′ g ′ r ′ i ′ z ′ ) imaging data taken along the Celestial Equator by the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this paper, we present 22 high-redshift quasars (z > 3.6) discovered from ∼ 250 deg 2 of data in the spring Equatorial Stripe, plus photometry for two previously known high-redshift quasars in the same region of sky. Our success rate of identifying high-redshift quasars is 68%. Five of the newly discovered quasars have redshifts higher than 4.6 (z = 4. 62, 4.69, 4.70, 4.92 and 5.03). All the quasars have i * < 20.2 with absolute magnitude −28.8 < M B < −26.1 (h = 0.5, q 0 = 0.5). Several of the quasars show unusual emission and absorption features in their spectra, including an object at z = 4.62 without detectable emission lines, and a Broad Absorption Line (BAL) quasar at z = 4.92.
Galaxy masses can be estimated by a variety of methods; each applicable in different circumstance... more Galaxy masses can be estimated by a variety of methods; each applicable in different circumstances, and each suffering from different systematic uncertainties. Confirmation of results obtained by one technique with analysis by another is particularly important. Recent SDSS weak lensing measurements of the projected-mass correlation function reveal a linear relation between galaxy luminosities and the depth of their dark matter halos (measured on 260 h −1 kpc scales). In this work we use an entirely independent dynamical method to confirm these results. We begin by assembling a sample of 618 relatively isolated host galaxies, surrounded by a total of 1225 substantially fainter satellites. We observe the mean dynamical effect of these hosts on the motions of their satellites by assembling velocity difference histograms. Dividing the sample by host properties, we find significant variations in satellite velocity dispersion with host luminosity. We quantify these variations using a simple dynamical model, measuring M dyn 260 a dynamical mass within 260 h −1 kpc. The appropriateness of this mass reconstruction is checked by conducting a similar analysis within an N-body simulation. Comparison between the dynamical and lensing mass-to-light scalings shows reasonable agreement, providing some quantitative confirmation for the lensing results.
We present here results on the composite luminosity functions (LF) of galaxies in the clusters of... more We present here results on the composite luminosity functions (LF) of galaxies in the clusters of galaxies selected from the Cut and Enhance cluster catalog (CE) of the Sloan Digital Sky Survey. We constructed composite LFs in the five SDSS bands, u, g, r, i and z, using 204 CE clusters ranging from z = 0.02 to z = 0.25 . Background and foreground galaxies were subtracted from the LF using an annular region around clusters to take large-scale, galaxy-number-count variances into consideration. A LF of each cluster was weighted according to the richness and number of contributing galaxies to construct the composite LF. Taking advantage of accurate 1 photometry of SDSS, we used photometric redshifts to construct composite luminosity functions and thus study a large number of clusters. The robustness of the weighting scheme was tested using a Monte-Carlo simulation. The best-fit Schechter parameters are (M * ,α)=(−21.61±0.26, −1.40±0.11),(−22.01±0.11, −1.00±0.06),(−22.21±0.05, −0.85±0.03), (−22.31±0.08, −0.70±0.05) and (−21.36±0.06, −0.58±0.04) in u,g,r,i and z, respectively. We find that the slope of composite LFs becomes flatter toward a redder color band. Compared with the field LFs of the SDSS, the cluster LFs have brighter characteristic magnitude and flatter slopes in the g, r, i and z bands. These results are consistent with the hypothesis that the cluster LF has two distinct underlying populations i.e. the bright end of the LF is dominated by bright early types that follow a Gaussian-like luminosity distribution, while the faint-end of the cluster LF is a steep power-law like function dominated by star-forming (bluer) galaxies. We also studied the composite LFs for early-type and late-type galaxies using profile fits, a concentration parameter and u − r color to classify galaxy morphology. A strong dependence of LF on galaxy morphology was found. The faint end slope of the LF is always flatter for early-type galaxies than late-type, regardless of the passband and methodology. These results are consistent with the hypothesis that the cluster regions are dominated by bright elliptical galaxies. This work also provides a good low-redshift benchmark for on-going multi-color photometric studies of high redshift clusters of galaxies using 4-8 m class telescopes.
Measurements of galaxy cluster abundances, clustering properties, and massto-light ratios in curr... more Measurements of galaxy cluster abundances, clustering properties, and massto-light ratios in current and future surveys can provide important cosmological constraints. Digital wide-field imaging surveys, the recently-demonstrated fidelity of red-sequence cluster detection techniques, and a new generation of realistic mock galaxy surveys provide the means for construction of large, cosmologically-interesting cluster samples, whose selection and properties can be understood in unprecedented depth. We present the details of the "maxBCG" algorithm, a cluster-detection technique tailored to multi-band CCD-imaging data. MaxBCG primarily relies on an observational cornerstone of massive galaxy clusters: they are marked by an overdensity of bright, uniformly red galaxies. This detection scheme also exploits classical brightest cluster galaxies (BCGs), which are often found at the center of these same massive clusters. We study the algorithm herein through its performance on large, realistic, mock galaxy catalogs,
We present results on over 100 high-redshift quasars found in the Sloan Digital Sky Survey (SDSS)... more We present results on over 100 high-redshift quasars found in the Sloan Digital Sky Survey (SDSS), using automated selection algorithms applied to SDSS imaging data and with spectroscopic confirmation obtained during routine spectroscopic operations of the Sloan 2.5-m telescope. The SDSS spectra cover the wavelength range 3900 -9200Å at a spectral resolution of 1800, and have been obtained for 116 quasars with redshifts greater than 3.94; 92 of these objects were previously uncataloged, significantly increasing the current tally of published z > 4 quasars. The paper also reports observations of five additional new z > 4.6 quasars; all were found from the SDSS imaging survey and spectroscopically confirmed with data from the Apache Point Observatory's 3.5-m telescope. The i ′ magnitudes of the quasars range from 18.03 to 20.56. Of the 97 new objects in this paper, 13 are Broad Absorption Line quasars. Five quasars, including one object at a redshift of 5.11, have 20 cm peak flux densities greater than 1 mJy. Two of the quasars, both at z ≈ 4.5, have very weak emission lines; one of these objects is a radio source. Nineteen of the newly-discovered objects have redshifts above 4.6, and the maximum redshift is z=5.41; among objects reported to date, the latter is the third highest redshift AGN, and penultimate in redshift among luminous quasars.
Imaging data from the Sloan Digital Sky Survey is used to measure the empirical size-richness rel... more Imaging data from the Sloan Digital Sky Survey is used to measure the empirical size-richness relation for a large sample of galaxy clusters. Using population subtraction methods, we determine the radius at which the cluster galaxy number density is 200Ω −1 m times the mean galaxy density, without assuming a model for the radial distribution of galaxies in clusters. If these galaxies are unbiased on Mpc scales, this galaxy-density-based R 200 reflects the characteristic radii of clusters. We measure the scaling of this characteristic radius with richness over an order of magnitude in cluster richness, from rich clusters to poor groups. We use this information to examine the radial profiles of galaxies in clusters as a function of cluster richness, finding that the concentration of the galaxy distribution decreases with richness and is systematically lower than the concentrations measured for dark matter profiles in N-body simulations. Using these scaled radii, we investigate the behavior of the cluster luminosity function, and find that it is well matched by a Schechter function for galaxies brighter than M r = −18 only after the central galaxy has been removed. We find that the luminosity function varies with richness and with distance from the cluster center, underscoring the importance of using an aperture that scales with cluster mass to compare physically equivalent regions of these different systems. We note that the lowest richness systems in our catalog have properties consistent with those expected of the earliest-forming halos; our cluster-finding algorithm, in addition to reliably finding clusters, may be efficient at finding fossil groups.
We present a catalog of 799 clusters of galaxies in the redshift range z est ¼ 0:05 0:3 selected ... more We present a catalog of 799 clusters of galaxies in the redshift range z est ¼ 0:05 0:3 selected from $400 deg 2 of early Sloan Digital Sky Survey (SDSS) commissioning data along the celestial equator. The catalog is based on merging two independent selection methods-a color-magnitude red-sequence maxBCG technique (B), and a hybrid matched filter method (H). The BH catalog includes clusters with richness à ! 40 (matched filter) and N gal ! 13 (maxBCG), corresponding to typical velocity dispersion of v e400 km s À1 and mass (within 0.6 h À1 Mpc radius) e5  10 13 h À1 M . This threshold is below Abell richness class 0 clusters. The average space density of these clusters is 2  10 À5 h 3 Mpc À3 . All NORAS X-ray clusters and 53 of the 58 Abell clusters in the survey region are detected in the catalog; the five additional Abell clusters are detected below the BH catalog cuts. The cluster richness function is determined and found to exhibit a steeply decreasing cluster abundance with increasing richness. We derive observational scaling relations between cluster richness and observed cluster luminosity and cluster velocity dispersion; these scaling relations provide important physical calibrations for the clusters. The catalog can be used for studies of individual clusters, for comparisons with other sources such as X-ray clusters and active galactic nuclei, and, with proper correction for the relevant selection functions, also for statistical analyses of clusters.
We discuss optical and infrared photometric properties of stars matched in the Two Micron All Sky... more We discuss optical and infrared photometric properties of stars matched in the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) commissioning data for ∼50 deg 2 of sky along the Celestial Equator centered at l = 150 • , b = −60 • . About 98% (∼63,000) of objects listed in the 2MASS Point Source Catalog in the analyzed area are matched within 2 arcsec to an SDSS source. The matched sources represent 8% of the ∼800,000 SDSS sources in this area. They are predominantly red sources, as expected, and 15% of them are resolved in SDSS imaging data, although they are detected as point sources in 2MASS data. The distribution of positional discrepancies for the matched sources, and the astrometric statistics for the multiply observed SDSS sources, imply that the astrometric accuracy of both surveys is about 0.1 arcsec per coordinate (rms).
Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the ... more Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of 1 large-scale structures predicts 1−6 the existence of quasars gravitationally lensed by concentrations of dark matter 7 so massive that the quasar images would be split by over 7 arcsec. Numerous searches 8−11 for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known 12 , including the first lensed quasar discovered 13 , have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies 14 with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004+4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations 3−5 based on the cold-dark-matter model.
This paper describes the discovery of seven dwarf objects of spectral type 'L' (objects cooler th... more This paper describes the discovery of seven dwarf objects of spectral type 'L' (objects cooler than the latest M dwarfs) in commissioning imaging data taken by the Sloan Digital Sky Survey (SDSS). Low-resolution spectroscopy shows that these objects have spectral types from L0 to L8. Comparison of the SDSS and 2MASS photometry for several of these objects indicates the presence of significant opacity at optical wavelengths, perhaps due to atmospheric dust. This comparison also demonstrates the high astrometric accuracy (better than 1 ′′ for these faint sources) of both surveys.
Uploads
Papers by Timothy McKay