arXiv subjects

Risa H. Wechsler

Publications and source records attributed to Risa H. Wechsler.

At least 217 records · Page 12Linked to original sources

arXiv2008

Cross-correlation Weak Lensing of SDSS Galaxy Clusters I: Measurements

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.

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arXiv2008

Cross-correlation Weak Lensing of SDSS Galaxy Clusters III: Mass-to-light Ratios

We present measurements of the excess mass-to-light ratio measured aroundMaxBCG galaxy clusters observed in the SDSS. This red sequence cluster sample includes objects from small groups with masses ranging from ~5x10^{12} to ~10^{15} M_{sun}/h. Using cross-correlation weak lensing, we measure the excess mass density profile above the universal mean Δρ(r) = ρ(r) - \barρ for clusters in bins of richness and optical luminosity. We also measure the excess luminosity density Δl(r) = l(r) - \bar{l} measured in the z=0.25 i-band. For both mass and light, we de-project the profiles to produce 3D mass and light profiles over scales from 25 kpc/ to 22 Mpc/h. From these profiles we calculate the cumulative excess mass M(r) and excess light L(r) as a function of separation from the BCG. On small scales, where ρ(r) >> \barρ, the integrated mass-to-light profile may be interpreted as the cluster mass-to-light ratio. We find the M/L_{200}, the mass-to-light ratio within r_{200}, scales with cluster mass as a power law with index 0.33+/-0.02. On large scales, where ρ(r) ~ \barρ, the M/L approaches an asymptotic value independent of cluster richness. For small groups, the mean M/L_{200} is much smaller than the asymptotic value, while for large clusters it is consistent with the asymptotic value. This asymptotic value should be proportional to the mean mass-to-light ratio of the universe . We find /b^2_{ml} = 362+/-54 h (statistical). There is additional uncertainty in the overall calibration at the ~10% level. The parameter b_{ml} is primarily a function of the bias of the L <~ L_* galaxies used as light tracers, and should be of order unity. Multiplying by the luminosity density in the same bandpass we find Ω_m/b^2_{ml} = 0.02+/-0.03, independent of the Hubble parameter.

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arXiv2007

The Galaxy Content of SDSS Clusters and Groups

Imaging data from the Sloan Digital Sky Survey are used to characterize the population of galaxies in groups and clusters detected with the MaxBCG algorithm. We investigate the dependence of Brightest Cluster Galaxy (BCG) luminosity, and the distributions of satellite galaxy luminosity and satellite color, on cluster properties over the redshift range 0.1 < z < 0.3. The size of the dataset allows us to make measurements in many bins of cluster richness, radius and redshift. We find that, within r_200 of clusters with mass above 3e13 h-1 M_sun, the luminosity function of both red and blue satellites is only weakly dependent on richness. We further find that the shape of the satellite luminosity function does not depend on cluster-centric distance for magnitudes brighter than ^{0.25}M_i - 5log(h) < -19. However, the mix of faint red and blue galaxies changes dramatically. The satellite red fraction is dependent on cluster-centric distance, galaxy luminosity and cluster mass, and also increases by ~5% between redshifts 0.28 and 0.2, independent of richness. We find that BCG luminosity is tightly correlated with cluster richness, scaling as L_{BCG} ~ M_{200}^{0.3}, and has a Gaussian distribution at fixed richness, with sigma_{log L} ~ 0.17 for massive clusters. The ratios of BCG luminosity to total cluster luminosity and characteristic satellite luminosity scale strongly with cluster richness: in richer systems, BCGs contribute a smaller fraction of the total light, but are brighter compared to typical satellites. This study demonstrates the power of cross-correlation techniques for measuring galaxy populations in purely photometric data.

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arXiv2007

The DEEP2 Galaxy Redshift Survey: Color and Luminosity Dependence of Galaxy Clustering at z~1

We present measurements of the color and luminosity dependence of galaxy clustering at z~1 in the DEEP2 Galaxy Redshift Survey. Using volume-limited subsamples in bins of both color and luminosity, we find that: 1) The clustering dependence is much stronger with color than with luminosity and is as strong with color at z~1 as is found locally. We find no dependence of the clustering amplitude on color for galaxies on the red sequence, but a significant dependence on color for galaxies within the blue cloud. 2) For galaxies in the range L/L*~0.7-2, a stronger large-scale luminosity dependence is seen for all galaxies than for red and blue galaxies separately. The small-scale clustering amplitude depends significantly on luminosity for blue galaxies, with brighter samples having a stronger rise on scales r_p<0.5 Mpc/h. 3) Redder galaxies exhibit stronger small-scale redshift-space distortions ("fingers of god"), and both red and blue populations show large-scale distortions in xi(r_p,pi) due to coherent infall. 4) While the clustering length, r_0, increases smoothly with galaxy color (in narrow bins), its power-law exponent, gamma, exhibits a sharp jump from the blue cloud to the red sequence. The intermediate color `green' galaxy population likely includes transitional galaxies moving from the blue cloud to the red sequence; on large scales green galaxies are as clustered as red galaxies but show infall kinematics and a small-scale correlation slope akin to the blue galaxy population. 5) We compare our results to a semi-analytic galaxy formation model applied to the Millenium Run simulation. Differences between the data and the model suggest that in the model star formation is shut down too efficiently in satellite galaxies.

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arXiv2007

Cross-correlation Weak Lensing of SDSS galaxy Clusters II: Cluster Density Profiles and the Mass--Richness Relation

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 Sheldon et al. 2007 (Paper I). 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. 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 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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arXiv2007

Optical Cluster-Finding with An Adaptive Matched-Filter Technique: Algorithm and Comparison with Simulations

We present a modified adaptive matched filter algorithm designed to identify clusters of galaxies in wide-field imaging surveys such as the Sloan Digital Sky Survey. The cluster-finding technique is fully adaptive to imaging surveys with spectroscopic coverage, multicolor photometric redshifts, no redshift information at all, and any combination of these within one survey. It works with high efficiency in multi-band imaging surveys where photometric redshifts can be estimated with well-understood error distributions. Tests of the algorithm on realistic mock SDSS catalogs suggest that the detected sample is ~85% complete and over 90% pure for clusters with masses above 1.0*10^{14} h^{-1} M_solar and redshifts up to z=0.45. The errors of estimated cluster redshifts from maximum likelihood method are shown to be small (typically less that 0.01) over the whole redshift range with photometric redshift errors typical of those found in the Sloan survey. Inside the spherical radius corresponding to a galaxy overdensity of Delta=200, we find the derived cluster richness Lambda_{200} a roughly linear indicator of its virial mass M_{200}, which well recovers the relation between total luminosity and cluster mass of the input simulation.

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arXiv2007

Isolating Triggered Star Formation

Galaxy pairs provide a potentially powerful means of studying triggered star formation from galaxy interactions. We use a large cosmological N-body simulation coupled with a well-tested semi-analytic substructure model to demonstrate that the majority of galaxies in close pairs reside within cluster or group-size halos and therefore represent a biased population, poorly suited for direct comparison to ``field'' galaxies. Thus, the frequent observation that some types of galaxies in pairs have redder colors than ``field'' galaxies is primarily a selection effect. We select galaxy pairs that are isolated in their dark matter halos with respect to other massive subhalos (N=2 halos) and a control sample of isolated galaxies (N=1 halos) for comparison. We then apply these selection criteria to a volume-limited subset of the 2dF Galaxy Redshift Survey with M_Bj <= -19 and obtain the first clean measure of the typical fraction of galaxies affected by triggered star formation and the average elevation in the star formation rate. We find that 24% (30.5%) of these L^\star and sub-L^{\star} galaxies in isolated 50 (30) kpc/h pairs exhibit star formation that is boosted by a factor of >~ 5 above their average past value, while only 10% of isolated galaxies in the control sample show this level of enhancement. Thus, 14% (20 %) of the galaxies in these close pairs show clear triggered star formation. The isolation criteria we develop provide a means to constrain star formation and feedback prescriptions in hydrodynamic simulations and a very general method of understanding the importance of triggered star formation in a cosmological context. (Abridged.)

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arXiv2007

The Physics of Galaxy Clustering I: A Model for Subhalo Populations

(Abridged) We present a semi-analytic model for Cold Dark Matter halo substructure that can be used as a framework for studying galaxy formation and an ingredient in halo models of galaxy clustering. We perform a comprehensive comparison of the model to the results of a suite of high-resolution cosmological simulations. The comparisons reveal that subhalo statistics, such as velocity and mass functions, radial distributions, and the halo occupation distributions, agree well over three orders of magnitude in host halo mass at various redshifts. Both in the simulations and in our model, the radial distributions of subhalos are significantly shallower than that of the dark matter density. The abundance of subhalos in a host is set by competition between destruction and new accretion. High mass halos and halos at high redshift tend to host more subhalos because the subhalos have, on average, been accreted more recently. Similarly, at fixed mass, halos that formed more recently host more subhalos. Observed "fossil groups" may represent an extreme tail of this correlation. We find a related correlation between host halo concentration and subhalo abundance at fixed host mass, Nsat ~ c^-a, where a changes with redshift and host-to-subhalo mass ratio. Lastly, we use our model to populate host halos in one of our high-resolution cosmological simulations, replacing the subhalos resolved in the simulation with subhalos computed according to our model. We show that the resulting 2pt correlation function of such a "hybrid" halo ensemble is indistinguishable from that measured directly in the simulation. This supports one of the key tenets of the standard halo model -- the assumption that the halo occupation distribution is statistically independent of host halo environment.

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arXiv2007

Cosmological Constraints from SDSS maxBCG Cluster Abundances

We perform a maximum likelihood analysis of the cluster abundance measured in the SDSS using the maxBCG cluster finding algorithm. Our analysis is aimed at constraining the power spectrum normalization $σ_8$, and assumes flat cosmologies with a scale invariant spectrum, massless neutrinos, and CMB and supernova priors Omega_m*h^2=0.128+/-0.01 and h=0.72+/-0.05 respectively. Following the method described in the companion paper Rozo et al. 2007, we derive σ_8=0.92+/-0.10$ (1-sigma) after marginalizing over all major systematic uncertainties. We place strong lower limits on the normalization, sigma_8>0.76 (95% CL) (>0.68 at 99% CL). We also find that our analysis favors relatively low values for the slope of the Halo Occupation Distribution (HOD), alpha=0.83+/-0.06. The uncertainties of these determinations will substantially improve upon completion of an ongoing campaign to estimate dynamical, weak lensing, and X-ray cluster masses in the SDSS maxBCG cluster sample.

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arXiv2007

Optically-Selected Cluster Catalogs as a Precision Cosmology Tool

We introduce a framework for describing the halo selection function of optical cluster finders. We treat the problem as being separable into a term that describes the intrinsic galaxy content of a halo (the Halo Occupation Distribution, or HOD) and a term that captures the effects of projection and selection by the particular cluster finding algorithm. Using mock galaxy catalogs tuned to reproduce the luminosity dependent correlation function and the empirical color-density relation measured in the SDSS, we characterize the maxBCG algorithm applied by Koester et al. to the SDSS galaxy catalog. We define and calibrate measures of completeness and purity for this algorithm, and demonstrate successful recovery of the underlying cosmology and HOD when applied to the mock catalogs. We identify principal components -- combinations of cosmology and HOD parameters -- that are recovered by survey counts as a function of richness, and demonstrate that percent-level accuracies are possible in the first two components, if the selection function can be understood to ~15% accuracy.

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arXiv2007

The Hierarchical Build-Up of Massive Galaxies and the Intracluster Light since z=1

We use a set of simulation-based models for the dissipationless evolution of galaxies since z=1 to constrain the fate of accreted satellites embedded in dark matter subhalos. These models assign stellar mass to dark matter halos at z=1 by relating the observed galaxy stellar mass function (GSMF) to the halo+subhalo mass function monotonically. The evolution of the stellar mass content is then followed using halo merger trees extracted from N-body simulations. Our models are differentiated only in the fate assigned to satellite galaxies once subhalos, within which satellites are embedded, disrupt. These models are confronted with the observed evolution in the massive end of the GSMF, the z~0 brightest cluster galaxy (BCG)-cluster mass relation, and the combined BCG and intracluster light (ICL) luminosity distribution -- all observables expected to evolve approximately dissipationlessly since z=1. The combined observational constraints favor a model in which the vast majority (>80%) of satellite stars from disrupted subhalos go into the ICL. Conversely, models that leave behind a significant population of satellite galaxies once the subhalo has disrupted are strongly disfavored, as are models that put a significant fraction of satellite stars into the BCG. Our results show that observations of the ICL provide useful and unique constraints on models of galaxy merging and the dissipationless evolution of galaxies in groups and clusters.

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arXiv2007

MaxBCG: A Red Sequence Galaxy Cluster Finder

Measurements of galaxy cluster abundances, clustering properties, and mass to- 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, cosmologicallyinteresting 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. (ABRIDGED)

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arXiv2006

Close Galaxy Counts as a Probe of Hierarchical Structure Formation

Standard LCDM predicts that the major merger rate of galaxy-size dark matter halos rises rapidly with redshift. The average number of close companions per galaxy, Nc, is often used to infer the galaxy merger rate, however, recent observational studies suggest that Nc evolves very little with redshift. Here we use a "hybrid" N- body simulation plus analytic substructure model to predict Nc directly. We identify dark matter subhalos with galaxies and show that the observed lack of close pair count evolution arises because the high merger rate per halo at early times is counteracted by a decrease in the number of halos massive enough to host a galaxy pair. We compare our results to data compiled from the DEEP2, SSRS2, and the UZC redshift surveys. Observed pair counts match our predictions if we assume a monotonic mapping between galaxy luminosity and the maximum circular velocity that each subhalo had when it was first accreted onto its host halo. This suggests that satellite galaxies are significantly more resilient to destruction than are dissipationless dark matter subhalos. We argue that while Nc does not provide a direct measure of the halo merger rate, it offers a powerful means to constrain the Halo Occupation Distribution and the spatial distribution of galaxies within halos. Interpreted in this way, close pair counts provide a useful test of galaxy formation processes on < 100 kpc scales.

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arXiv2006

The Dependence of Halo Clustering on Halo Formation History, Concentration, and Occupation

We investigate the dependence of dark matter halo clustering on halo formation time, density profile concentration, and subhalo occupation number, using high-resolution numerical simulations of a LCDM cosmology. We confirm results that halo clustering is a function of halo formation time, and that this trend depends on halo mass. For the first time, we show unequivocally that halo clustering is a function of halo concentration and show that the dependence of halo bias on concentration, mass, and redshift can be accurately parameterized in a simple way: b(c,M|z) = b(M|z) b(c|M/M*). The scaling between bias and concentration changes sign with the value of M/M*: high concentration (early forming) objects cluster more strongly for M <~ M* while low concentration (late forming) objects cluster more strongly for rare high-mass halos, M >~ M*. We show the first explicit demonstration that host dark halo clustering depends on the halo occupation number (of dark matter subhalos) and discuss implications for halo model calculations of dark matter power spectra and galaxy clustering statistics. The effect of these halo properties on clustering is strongest for early-forming dwarf-mass halos, which are significantly more clustered than typical halos of their mass. Our results suggest that isolated low-mass galaxies (e.g. low surface-brightness dwarfs) should have more slowly-rising rotation curves than their clustered counterparts, and may have consequences for the dearth of dwarf galaxies in voids. They also imply that self calibrating richness-selected cluster samples with their clustering properties might overestimate cluster masses and bias cosmological parameter estimation.

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arXiv2006

Modeling Luminosity-Dependent Galaxy Clustering Through Cosmic Time

We employ high-resolution dissipationless simulations of the concordance LCDM cosmology to model the observed luminosity dependence and evolution of galaxy clustering through most of the age of the universe, from z~5 to z~0. We use a simple, non-parametric model which monotonically relates galaxy luminosities to the maximum circular velocity of dark matter halos (V_max) by preserving the observed galaxy luminosity function in order to match the halos in simulations with observed galaxies. The novel feature of the model is the use of the maximum circular velocity at the time of accretion, V_max,acc, for subhalos, the halos located within virial regions of larger halos. We argue that for subhalos in dissipationless simulations, V_max,acc reflects the luminosity and stellar mass of the associated galaxies better than the circular velocity at the epoch of observation, V_max,now. The simulations and our model L-V_max relation predict the shape, amplitude, and luminosity dependence of the two-point correlation function in excellent agreement with the observed galaxy clustering in the SDSS data at z~0 and in the DEEP2 samples at z~1 over the entire probed range of projected separations, 0.1<r_p/(Mpc/h)<10.0. In particular, the small-scale upturn of the correlation function from the power-law form in the SDSS and DEEP2 luminosity-selected samples is reproduced very well. At z~3-5, our predictions also match the observed shape and amplitude of the angular two-point correlation function of Lyman-break galaxies (LBGs) on both large and small scales, including the small-scale upturn.

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arXiv2005

Dark Energy Studies: Challenges to Computational Cosmology

The ability to test the nature of dark mass-energy components in the universe through large-scale structure studies hinges on accurate predictions of sky survey expectations within a given world model. Numerical simulations predict key survey signatures with varying degrees of confidence, limited mainly by the complex astrophysics of galaxy formation. As surveys grow in size and scale, systematic uncertainties in theoretical modeling can become dominant. Dark energy studies will challenge the computational cosmology community to critically assess current techniques, develop new approaches to maximize accuracy, and establish new tools and practices to efficiently employ globally networked computing resources.

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arXiv2005

Constraining Dark Energy with the Dark Energy Survey: Theoretical Challenges

The Dark Energy Survey (DES) will use a new imaging camera on the Blanco 4-m telescope at CTIO to image 5000 square degrees of sky in the South Galactic Cap in four optical bands, and to carry out repeat imaging over a smaller area to identify and measure lightcurves of Type Ia supernovae. The main imaging area overlaps the planned Sunyaev-Zel'dovich survey of the South Pole Telescope. The idea behind DES is to use four distinct and largely independent methods to probe the properties of dark energy: baryon oscillations of the power spectrum, abundance and spatial distribution of clusters, weak gravitational lensing, and Type Ia supernovae. This white paper outlines, in broad terms, some of the theoretical issues associated with the first three of these probes (the issues for supernovae are mostly different in character), and with the general task of characterizing dark energy and distinguishing it from alternative explanations for cosmic acceleration. A companion white paper discusses the kind of numerical simulations and other theoretical tools that will be needed to address the these issues and to create mock catalogs that allow end-to-end tests of analysis procedures. Although we have been thinking about these problems in the specific context of DES, many of them are also relevant to other planned dark energy studies.

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arXiv2005

Galaxy-galaxy Lensing: Dissipationless Simulations Versus the Halo Model

Galaxy-galaxy lensing is a powerful probe of the relation between galaxies and dark matter halos, but its theoretical interpretation requires a careful modeling of various contributions, such as the contribution from central and satellite galaxies. For this purpose, a phenomenological approach based on the halo model has been developed, allowing for fast exploration of the parameter space of models. In this paper, we investigate the ability of the halo model to extract information from the g-g weak lensing signal by comparing it to high-resolution dissipationless simulations that resolve subhalos. We find that the halo model reliably determines parameters such as the host halo mass of central galaxies, the fraction of galaxies that are satellites, and their radial distribution inside larger halos. If there is a significant scatter present in the central galaxy host halo mass distribution, then the mean and median mass of that distribution can differ significantly from one another, and the halo model mass determination lies between the two. This result suggests that when analyzing the data, galaxy subsamples with a narrow central galaxy halo mass distribution, such as those based on stellar mass, should be chosen for a simpler interpretation of the results.

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