arXiv subjects

Risa H. Wechsler

Publications and source records attributed to Risa H. Wechsler.

At least 235 records · Page 13Linked to original sources

arXiv2005

The Shape of Dark Matter Halos: Dependence on Mass, Redshift, Radius, and Formation

Using six high resolution dissipationless simulations with a varying box size in a flat LCDM universe, we study the mass and redshift dependence of dark matter halo shapes for M_vir = 9.0e11 - 2.0e14, over the redshift range z=0-3, and for two values of sigma_8=0.75 and 0.9. Remarkably, we find that the redshift, mass, and sigma_8 dependence of the mean smallest-to-largest axis ratio of halos is well described by the simple power-law relation = (0.54 +- 0.02)(M_vir/M_*)^(-0.050 +- 0.003), where s is measured at 0.3 R_vir and the z and sigma_8 dependences are governed by the characteristic nonlinear mass, M_*=M_*(z,sigma_8). We find that the scatter about the mean s is well described by a Gaussian with sigma ~ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the halos through ~100 snapshots in time. We determine the formation scalefactor a_c as defined by Wechsler et al. (2002) and find that it can be related to the halo shape at z = 0 and its evolution over time.

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arXiv2005

Measurement of Galaxy Cluster Sizes, Radial Profiles, and Luminosity Functions from SDSS Photometric Data

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 200/Omega_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.

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arXiv2004

The Ultimate Halo Mass in a LCDM Universe

In the far future of an accelerating LCDM cosmology, the cosmic web of large-scale structure consists of a set of increasingly isolated halos in dynamical equilibrium. We examine the approach of collisionless dark matter to hydrostatic equilibrium using a large N-body simulation evolved to scale factor a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h Gyr into the future for a concordance model universe (Omega_m ~ 0.3, Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically active accretion region -- simplifies at a > 10 a_eq when these surfaces merge to create a single zero-velocity surface, clearly defining the halo outer boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed physical size encompassing a mean interior density ~ 5 times the critical density, similar to the turnaround value in a classical Einstein-deSitter model. We relate mhalo to other scales currently used to define halo mass (m200, mvir, m180b) and find that m200 is approximately half of the total asymptotic cluster mass, while m180b follows the evolution of the inner zero velocity surface for a < 2 but becomes much larger than the total bound mass for a > 3. The radial density profile of all bound halo material is well fit by a truncated Hernquist profile. An NFW profile provides a somewhat better fit interior to r200 but is much too shallow in the range r200 < r < rhalo.

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arXiv2004

Modeling Galaxy-Mass Correlations in Dissipationless Simulations

We use high-resolution dissipationless simulations of the concordance flat LCDM model to make predictions for the galaxy--mass (GM) correlations and compare them to the recent SDSS weak lensing measurements.We use a simple observationally motivated scheme to assign luminosities and colors to the halos.This allows us to closely match the selection criteria used to define observational samples.The simulations reproduce the observed GM correlation function and its observed dependencies on luminosity and color.The luminosity dependence of the correlation function is primarily determined by the changing relative contribution of central and satellite galaxies at different luminosities. The color dependence of the GM correlations reflects the difference in the typical environments of blue and red galaxies. We also find agreement between the predicted and observed cross-bias, b_x=b/r,at all probed scales.The GM correlation coefficient, r, is close to unity on scales >1/h Mpc.The cross bias is thus expected to measure the actual bias of galaxy clustering on these scales.The aperture mass-to-light ratio is independent of galaxy color.The aperture mass scales approximately linearly with luminosity at L_r>10^{10}h^{-2} Lsun, while at lower luminosities the scaling is shallower: L_r^{0.5}. We show that most of the luminous galaxies (M_r<-21) are near the centers of their halos and their GM correlation function at r<100/h kpc can therefore be interpreted as the average dark matter density profile of these galaxies. We find that for galaxies in a given narrow luminosity range, there is a broad and possibly non-gaussian distribution of halo virial masses. Therefore, the average relation between mass and luminosity derived from the weak lensing analyses should be interpreted with caution.

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arXiv2004

The Dark Side of the Halo Occupation Distribution

We analyze the halo occupation distribution (HOD), the probability for a halo of mass M to host a number of subhalos N, and two-point correlation function of galaxy-size dark matter halos using high-resolution dissipationless simulations of the concordance flat LCDM model. The halo samples include both the host halos and the subhalos, distinct gravitationally-bound halos within the virialized regions of larger host systems. We find that the first moment of the HOD, (M), has a complicated shape consisting of a step, a shoulder, and a power law high-mass tail. The HOD can be described by a Poisson statistics at high halo masses but becomes sub-Poisson for <4. We show that the HOD can be understood as a combination of the probability for a halo of mass M to host a central galaxy and the probability to host a given number Ns of satellite galaxies. The former can be approximated by a step-like function, while the latter can be well approximated by a Poisson distribution, fully specified by its first moment (M). We find that ~M^b with b~1 for a wide range of number densities, redshifts, and different power spectrum normalizations. This formulation provides a simple but accurate model for the halo occupation distribution found in simulations. At z=0, the two-point correlation function (CF) of galactic halos can be well fit by a power law down to ~100/h kpc with an amplitude and slope similar to those of observed galaxies. At redshifts z>~1, we find significant departures from the power-law shape of the CF at small scales. If the deviations are as strong as indicated by our results, the assumption of the single power law often used in observational analyses of high-redshift clustering is likely to bias the estimates of the correlation length and slope of the correlation function.

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arXiv2003

The Asymptotic Structure of Space-Time

Astronomical observations strongly suggest that our universe is now accelerating and contains a substantial admixture of dark vacuum energy. Using numerical simulations to study this newly consolidated cosmological model (with a constant density of dark energy), we show that astronomical structures freeze out in the near future and that the density profiles of dark matter halos approach the same general form. Every dark matter halo grows asymptotically isolated and thereby becomes the center of its own island universe. Each of these isolated regions of space-time approaches a universal geometry and we calculate the corresponding form of the space-time metric.

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arXiv2003

Future Evolution of Structure in an Accelerating Universe

Current cosmological data indicate that our universe contains a substantial component of dark vacuum energy that is driving the cosmos to accelerate. We examine the immediate and longer term consequences of this dark energy (assumed here to have a constant density). Using analytic calculations and supporting numerical simulations, we present criteria for test bodies to remain bound to existing structures. We show that collapsed halos become spatially isolated and dynamically relax to a particular density profile with logarithmic slope steeper than -3 at radii beyond r_200. The asymptotic form of the space-time metric is then specified. We develop this scenario further by determining the effects of the accelerating expansion on the background radiation fields and individual particles. In an appendix, we generalize these results to include quintessence.

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arXiv2002

The Las Campanas Distant Cluster Survey -- The Correlation Function

We present the first non-local (z>0.2) measurement of the cluster-cluster spatial correlation length, using data from the Las Campanas Distant Cluster Survey (LCDCS). We measure the angular correlation function for velocity-dispersion limited subsamples of the catalog at estimated redshifts of 0.35<z_{est}<0.575, and derive spatial correlation lengths for these clusters via the cosmological Limber equation. The correlation lengths that we measure for clusters in the LCDCS are consistent both with local results for the APM cluster catalog and with theoretical expectations based upon the Virgo Consortium Hubble Volume simulations and the analytic predictions. Despite samples containing over 100 clusters, our ability to discriminate between cosmological models is limited because of statistical uncertainty.

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arXiv2001

The Masses of Lyman Break Galaxies

Data on galaxies at high redshift, identified by the Lyman-break photometric technique, can teach us about how galaxies form and evolve. The stellar masses and other properties of such Lyman break galaxies (LBGs) depend sensitively on the details of star formation. In this paper we consider three different star formation prescriptions, and use semi-analytic methods applied to the now-standard $Λ$CDM theory of hierarchical structure formation to show how these assumptions about star formation affect the predicted masses of the stars in these galaxies and the masses of the dark matter halos that host them. We find that, within the rather large uncertainties, recent estimates of the stellar masses of LBGs from multi-color photometry are consistent with the predictions of all three models. However, the estimated stellar masses are more consistent with the predictions of two of the models in which star formation is accelerated at high redshifts $z\gsim3$, and of these models the one in which many of the LBGs are merger-driven starbursts is also more consistent with indications that many high redshift galaxies are gas rich. The clustering properties of LBGs have put some constraints on the masses of their host halos, but due to similarities in the halo occupation of the three models we consider and degeneracies between model parameters, current constraints are not yet sufficient to distinguish between realistic models.

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arXiv2001

Concentrations and Assembly Histories of Dark Matter Halos

We study the relation between the density profiles of dark matter halos and their mass assembly histories, using a statistical sample of halos in a high-resolution N-body simulation of the LCDM cosmology. For each halo at z=0, we identify its merger-history tree, and determine concentration parameters c_vir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch a_c, defined when the log mass accretion rate dlog M/doga falls below a critical value. We find that late forming galaxies tend to be less concentrated, such that c_vir ``observed'' at any epoch a_obs is strongly correlated with a_c via c_vir=c_1 a_obs/a_c. Scatter about this relation is mostly due to measurement errors in c_vir and a_c, implying that the actual spread in c_vir for halos of a given mass can be mostly attributed to scatter in a_c. Because of the direct connection between halo concentration and velocity rotation curves, and because of probable connections between halo mass assembly history and star formation history, the tight correlation between these properties provides an essential new ingredient for galaxy formation modeling.

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arXiv2001

Concentrations of Dark Halos from their Assembly Histories

(abridged) We study the relation between the density profiles of dark matter halos and their mass assembly histories, using a statistical sample of halos in a high-resolution N-body simulation of the LCDM cosmology. For each halo at z=0, we identify its merger-history tree, and determine concentration parameters c_vir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch a_c, defined when the log mass accretion rate dlogM/dloga falls below a critical value S. We find that late forming galaxies tend to be less concentrated, such that c_vir ``observed'' at any epoch a_o is strongly correlated with a_c via c_vir=c_1*a_o/a_c. Scatter about this relation is mostly due to measurement errors in c_v and a_c, implying that the actual spread in c_vir for halos of a given mass can be mostly attributed to scatter in a_c. We demonstrate that this relation can also be used to predict the mass and redshift dependence of c_v, and the scatter about the median c_vir(M,z), using accretion histories derived from the Extended Press-Schechter (EPS) formalism, after adjusting for a constant offset between the formation times as predicted by EPS and as measured in the simulations;this new ingredient can thus be easily incorporated into semi-analytic models of galaxy formation. The correlation found between halo concentration and mass accretion rate suggests a physical interpretation: for high mass infall rates the central density is related to the background density; when the mass infall rate slows, the central density stays approximately constant and the halo concentration just grows as R_vir. The tight correlation demonstrated here provides an essential new ingredient for galaxy formation modeling.

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arXiv2001

Clustering in the Las Campanas Distant Cluster Survey

We utilize a sample of galaxy clusters at 0.35<z<0.6 drawn from the Las Campanas Distant Cluster Survey (LCDCS) to provide the first non-local constraint on the cluster-cluster spatial correlation function. The LCDCS catalog, which covers an effective area of 69 square degrees, contains over 1000 cluster candidates. Estimates of the redshift and velocity dispersion exist for all candidates, which enables construction of statistically completed, volume-limited subsamples. In this analysis we measure the angular correlation function for four such subsamples at z~0.5. After correcting for contamination, we then derive spatial correlation lengths via Limber inversion. We find that the resulting correlation lengths depend upon mass, as parameterized by the mean cluster separation, in a manner that is consistent with both local observations and CDM predictions for the clustering strength at z=0.5.

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arXiv2001

Tracing Cosmic Evolution with the Las Campanas Distant Cluster Survey

The Las Campanas Distant Cluster Survey (LCDCS), which contains over 1000 cluster candidates at z>0.3, is a unique sample with which to probe the evolution of both cluster galaxies and the properties of the cluster population. Programs are now underway to utilize the LCDCS for both purposes. We provide a brief overview of these programs, and also present new results for the LCDCS cluster correlation function. Utilizing well-defined, approximately dispersion-limited subsamples, we measure the angular correlation function for clusters at z~0.5. Spatial correlation lengths are then derived via Limber inversion. We find that the correlation length depends upon mass, as parameterized by the mean cluster separation, in a manner that is consistent with both local observations and CDM predictions for the clustering strength at z=0.5.

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arXiv2001

Galaxy Halo Occupation at High Redshift

We discuss how current and future data on the clustering and number density of z~3 Lyman-break galaxies (LBGs) can be used to constrain their relationship to dark matter haloes. We explore a three-parameter model in which the number of LBGs per dark halo scales like a power-law in the halo mass: N(M) = (M/M_1)^S for M>M_m. Here, M_m is the minimum mass halo that can host an LBG, M_1 is a normalization parameter, associated with the mass above which haloes host more than one observed LBG, and S determines the strength of the mass dependence. We show how these three parameters are constrained by three observable properties of LBGs: the number density, the large-scale bias, and the fraction of objects in close pairs. Given these three quantities, the three unknown model parameters may be estimated analytically, allowing a full exploration of parameter space. As an example, we assume an LCDM cosmology and consider the observed properties of a recent sample of spectroscopically confirmed LBGs. We find that the favored range for our model parameters is M_m = (0.4-8)x10^10 Msun/h, M_1 = (6-10)x10^12 Msun/h, and 0.9 < S < 1.1. The preferred region in M_m expands by an order of magnitude and slightly shallower slopes are acceptable if the allowed range of bias is permitted to span all recent observational estimates. We also discuss how the observed clustering of LBGs as a function of luminosity can be used to constrain halo occupation, although due to current observational uncertainties we are unable to reach any strong conclusions. Our methods and results can be used to constrain more realistic models that aim to derive the occupation function N(M) from first principles, and offer insight into how basic physical properties affect the observed properties of LBGs.

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arXiv2001

Galaxy Formation at z~3: Constraints from Spatial Clustering

We use N-body simulations combined with semi-analytic models to compute the clustering properties of modeled galaxies at z~3, and confront these predictions with the clustering properties of the observed population of Lyman-break galaxies (LBGs). Several scenarios for the nature of LBGs are explored, which may be broadly categorized into models in which high-redshift star formation is driven by collisional starbursts and those in which quiescent star formation dominates. For each model, we make predictions for the LBG overdensity distribution, the variance of counts-in-cells, the correlation length, and close pair statistics. Models which assume a one-to-one relationship between massive dark-matter halos and galaxies are disfavored by close pair statistics, as are models in which colliding halos are associated with galaxies in a simplified way. However, when modeling of gas consumption and star formation is included using a semi-analytic treatment, the quiescent and collisional starburst models predict similar clustering properties and none of these models can be ruled out based on the available clustering data. None of the ``realistic'' models predict a strong dependence of clustering amplitude on the luminosity threshold of the sample, in apparent conflict with some observational results.

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arXiv1999

Clustering of high-redshift galaxies: relating LBGs to dark matter halos

We investigate the clustering properties of high-redshift galaxies within three competing scenarios for assigning luminous galaxies to dark matter halos from N-body simulations: a one galaxy per massive halo model, a quiescent star formation model, and a collisional starburst model. We compare these models to observations of Lyman-Break galaxies at z~3$ With current data and the simple statistic used here, one cannot rule out any of these models, but we see potential for finding distinguishing features using statistics that are sensitive to the tails of the distribution, and statistics based on the number of multiple galaxies per halo, which we explore in an ongoing study.

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arXiv1998

Cosmological Implications of Lyman-Break Galaxy Clustering

We review our analysis of the clustering properties of ``Lyman-break'' galaxies (LBGs) at redshift z~3, previously discussed in Wechsler et al (1998). We examine the likelihood of spikes found by Steidel et al (1998) in the redshift distribution of LBGs, within a suite of models for the evolution of structure in the Universe. Using high-resolution dissipationless N-body simulations, we analyze deep pencil-beam surveys from these models in the same way that they are actually observed, identifying LBGs with the most massive dark matter halos. We find that all the models (with SCDM as a marginal exception) have a substantial probability of producing spikes similar to those observed, because the massive halos are much more clumped than the underlying matter -- i.e., they are biased. Therefore, the likelihood of such a spike is not a good discriminator among these models. The LBG correlation functions are less steep than galaxies today (gamma~1.4), but show similar or slightly longer correlation lengths. We have extened this analysis and include a preliminary comparison to the new data presented in Adelberger et al (1998). We also discuss work in progress, in which we use semi-analytic models to identify Lyman-break galaxies within dark-matter halos.

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arXiv1998

The nature of Lyman-break galaxies

Using semi-analytic models of galaxy formation, we investigate the properties of $z\sim3$ galaxies and compare them with the observed population of Lyman-break galaxies (LBGs). In addition to the usual quiescent mode of star formation, we introduce a physical model for starbursts triggered by galaxy-galaxy interactions. We find that with the merger rate that arises naturally in the CDM-based merging hierarchy, a significant fraction of bright galaxies identified at high redshift ($z \ga 2$) are likely to be low-mass, bursting satellite galaxies. The abundance of LBGs as a function of redshift and the luminosity function of LBGs both appear to be in better agreement with the data when the starburst mode is included, especially when effects of dust extinction are considered. The objects that we identify as LBGs have observable properties including low velocity dispersions that are in good agreement with the available data. In this ``Bursting Satellite'' scenario, quiescent star formation at $z\ga2$ is relatively inefficient and most of the observed LBGs are starbursts triggered by satellite mergers within massive halos. In high-resolution N-body simulations, we find that the most massive dark matter halos cluster at redshift $z\sim 3$ much as the LBGs are observed to do. This is true for both the $Ω=1$ CHDM model and low-$Ω$ \LCDM and OCDM models, all of which have fluctuation power spectra $P(k)$ consistent with the distribution of low-redshift galaxies. The Bursting Satellite scenario can resolve the apparent paradox of LBGs that cluster like massive dark matter halos but have narrow linewidths and small stellar masses.

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