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 Delta *LCDM cosmology, we find Delta *s8( Delta *W 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 Delta *s8 = 0.807 +/- 0.020 and Delta *W 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 large catalog of optically selected galaxy clusters from the application of a new Gaussian Mixture Brightest Cluster Galaxy (GMBCG) algorithm to SDSS Data Release 7 data. The algorithm ...detects clusters by identifying the red-sequence plus brightest cluster galaxy (BCG) feature, which is unique for galaxy clusters and does not exist among field galaxies. Red-sequence clustering in color space is detected using an Error Corrected Gaussian Mixture Model. We run GMBCG on 8240 deg2 of photometric data from SDSS DR7 to assemble the largest ever optical galaxy cluster catalog, consisting of over 55,000 rich clusters across the redshift range from 0.1 < z < 0.55. We present Monte Carlo tests of completeness and purity and perform cross-matching with X-ray clusters and with the maxBCG sample at low redshift. These tests indicate high completeness and purity across the full redshift range for clusters with 15 or more members.
We study the mass distribution of a sample of 28 galaxy clusters using strong and weak lensing observations. The clusters are selected via their strong lensing properties as part of the Sloan Giant ...Arcs Survey (SGAS) from the Sloan Digital Sky Survey (SDSS). Mass modelling of the strong lensing information from the giant arcs is combined with weak lensing measurements from deep Subaru/Suprime-cam images to primarily obtain robust constraints on the concentration parameter and the shape of the mass distribution. We find that the concentration c
vir is a steep function of the mass, c
vir∝M
−0.59±0.12
vir, with the value roughly consistent with the lensing-bias-corrected theoretical expectation for high-mass (∼1015 h
−1 M⊙) clusters. However, the observationally inferred concentration parameters appear to be much higher at lower masses (∼1014 h
−1 M⊙), possibly a consequence of the modification to the inner density profiles provided by baryon cooling. The steep mass-concentration relation is also supported from direct stacking analysis of the tangential shear profiles. In addition, we explore the 2D shape of the projected mass distribution by stacking weak lensing shear maps of individual clusters with prior information on the position angle from strong lens modelling, and find significant evidence for a large mean ellipticity with the best-fitting value of 〈e〉= 0.47 ± 0.06 for the mass distribution of the stacked sample. We find that the luminous cluster member galaxy distribution traces the overall mass distribution very well, although the distribution of fainter cluster galaxies appears to be more extended than the total mass.
Abstract
We present strong gravitational lensing models for 37 galaxy clusters from the Sloan Digital Sky Survey Giant Arcs Survey. We combine data from multi-band
Hubble Space Telescope
Wide Field ...Camera 3 (WFC3) imaging, with ground-based imaging and spectroscopy from
Magellan
, Gemini, Apache Point Observatory, and the Multiple Mirror Telescope, in order to detect and spectroscopically confirm new multiply imaged lensed background sources behind the clusters. We report spectroscopic or photometric redshifts of sources in these fields, including cluster galaxies and background sources. Based on all available lensing evidence, we construct and present strong-lensing mass models for these galaxy clusters. The clusters span a redshift range of 0.176 <
z
< 0.66 with a median redshift of
z
= 0.45, and sample a wide range of dynamical masses, 1.5 <
M
200
< 35 × 10
14
, as estimated from their velocity dispersions. As these clusters were selected as lenses primarily owing to a fortuitous alignment with background galaxies that results in giant arcs, they exhibit a wide range in Einstein radii, 1.″3 <
θ
E
< 23.″1 for a source at
z
= 2, with a median
θ
E
= 10.″8. The reduced
HST
images and lens model outputs are made available to the scientific community as high-level data products with this publication.
We place constraints on the average density (Omegam) and clustering amplitude of matter using a combination of two measurements from the Sloan Digital Sky Survey: the galaxy two-point correlation ...function, wp(rp), and the mass-to-galaxy-number ratio within galaxy clusters, M/N, analogous to cluster M/L ratios. Our wp(rp) measurements are obtained from DR7 while the sample of clusters is the maxBCG sample, with cluster masses derived from weak gravitational lensing. We construct nonlinear galaxy bias models using the Halo Occupation Distribution (HOD) to fit both wp(rp) and M/N for different cosmological parameters. Our derived constraints are insensitive to the current level of uncertainties in the halo mass function and in the mass-richness relation of clusters and its scatter, making the M/N technique complementary to cluster abundances as a method for constraining cosmology with future galaxy surveys.
Significant gendered performance differences are signals of systemic inequity in higher education. Understanding of these inequities has been hampered by the local nature of prior studies; consistent ...measures of performance disparity across many disciplines and institutions have not been available. Here, we report the first wide-ranging, multi-institution measures of gendered performance difference, examining more than a million student enrollments in hundreds of courses at five universities. After controlling for factors that relate to academic performance using optimal matching, we identify patterns of gendered performance difference that are consistent across these universities. Biology, chemistry, physics, accounting, and economics lecture courses regularly exhibit gendered performance differences that are statistically and materially significant, whereas lab courses in the same subjects do not. These results reinforce the importance of broad investigation of performance disparities across higher education. They also help focus equity research on the structure and evaluative schemes of these lecture courses.
Residential higher education brings thousands of students together for multiple years and offers them an array of shared intellectual experiences and a network of social interactions. Many of these ...intellectual and social connections are formed during courses. Students are connected to students through courses they take together, and courses are connected to one another by students who take both. These courses and the students who take them form a bipartite network that encodes information about campus structures and student experiences. Because all institutions of higher education collect and maintain precise records of what courses students take, it is possible to assemble a student-course network that quantitatively describes the interactions among students and courses. We provide an example that demonstrates the identification of courses effective at creating unique connections among students and reveals how students and majors can be strongly connected or dispersed. We show how social network analysis can be used to improve our understanding of the learning environment at the University of Michigan, and we hope that this kind of analysis is of interest to persons at other institutions.
Minimizing the scatter between cluster mass and accessible observables is an important goal for cluster cosmology. In this work, we introduce a new matched filter richness estimator, and test its ...performance using the maxBCG cluster catalog. Our new estimator significantly reduces the variance in the LX -richness relation, from to. Relative to the maxBCG richness estimate, it also removes the strong redshift dependence of the LX -richness scaling relations, and is significantly more robust to photometric and redshift errors. These improvements are largely due to the better treatment of galaxy color data. We also demonstrate the scatter in the LX -richness relation depends on the aperture used to estimate cluster richness, and introduce a novel approach for optimizing said aperture which can easily be generalized to other mass tracers.
MaxBCG: A Red-Sequence Galaxy Cluster Finder Koester, Benjamin P; McKay, Timothy A; Annis, James ...
The Astrophysical journal,
05/2007, Letnik:
660, Številka:
1
Journal Article
PDF
