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.
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Papers by Timothy McKay