Halo Mass Distribution Research Articles

THE STELLAR-TO-HALO MASS RELATION FOR LOCAL GROUP GALAXIES

We contend that a single power law halo mass distribution is appropriate for direct matching to the stellar masses of observed Local Group dwarf galaxies, allowing the determination of the slope of the stellar mass-halo mass relation for low mass galaxies. Errors in halo masses are well defined as the Poisson noise of simulated local group realisations, which we determine using constrained local universe simulations (CLUES). For the stellar mass range 10$^7$<M*<10$^8$M$_\odot$, for which we likely have a complete census of observed galaxies, we find that the stellar mass-halo mass relation follows a power law with slope of 3.1, significantly steeper than most values in the literature. The steep relation between stellar and halo masses indicates that Local Group dwarf galaxies are hosted by dark matter halos with a small range of mass. Our methodology is robust down to the stellar mass to which the census of observed Local Group galaxies is complete, but the significant uncertainty in the currently measured slope of the stellar-to halo mass relation will decrease dramatically if the Local Group completeness limit was $10^{6.5}$M$\odot$ or below, highlighting the importance of pushing such limit to lower masses and larger volumes.

CROSS-CORRELATION OF SDSS DR7 QUASARS AND DR10 BOSS GALAXIES: THE WEAK LUMINOSITY DEPENDENCE OF QUASAR CLUSTERING ATz∼ 0.5

We present the measurement of the two-point cross-correlation function (CCF) of 8,198 Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) quasars and 349,608 DR10 CMASS galaxies from the Baryonic Oscillation Spectroscopic Survey (BOSS) at redshift <z>~0.5 (0.3<z<0.9). The cross-correlation function can be reasonably well fit by a power-law model xi_QG(r)=(r/r_0)^(-gamma) on projected scales of r_p=2-25 Mpc/h with r_0=6.61+-0.25 Mpc/h and gamma=1.69+-0.07. We estimate a quasar linear bias of b_Q=1.38+-0.10 at <z>=0.53 from the CCF measurements. This linear bias corresponds to a characteristic host halo mass of ~4x10^12 M_sun/h, compared to ~10^13 M_sun/h characteristic host halo mass for CMASS galaxies. We divide the quasar sample in luminosity and constrain the luminosity dependence of quasar bias to be db_Q/dlogL=0.20+-0.34 or 0.11+-0.32 (depending on different luminosity divisions) for quasar luminosities -23.5>M_i(z=2)>-25.5, implying a weak luminosity dependence of quasar clustering for the bright end of the quasar population at <z>~0.5. We compare our measurements with theoretical predictions, Halo Occupation Distribution (HOD) models and mock catalogs. These comparisons suggest quasars reside in a broad range of host halos, and the host halo mass distributions significantly overlap with each other for quasars at different luminosities, implying a poor correlation between halo mass and instantaneous quasar luminosity. We also find that the quasar HOD parameterization is largely degenerate such that different HODs can reproduce the CCF equally well, but with different outcomes such as the satellite fraction and host halo mass distribution. These results highlight the limitations and ambiguities in modeling the distribution of quasars with the standard HOD approach and the need for additional information in populating quasars in dark matter halos with HOD. [Abridged]

Are group- and cluster-scale dark matter haloes overconcentrated?

We investigate the relationship between the halo mass, M_200, and concentration, c, for a sample of 26 group- and cluster-scale strong gravitational lenses. In contrast with previous results, we find that these systems are only ~ 0.1 dex more over-concentrated than similar-mass halos from dark matter simulations; the concentration of a halo with M_200 = 10^14 M_sun is log c = 0.78\pm0.05, while simulations of halos with this mass at similar redshifts (z ~ 0.4) predict log c ~ 0.56 - 0.71. We also find that we are unable to make informative inference on the slope of the M_200-c relation in spite of our large sample size; we note that the steep slopes found in previous studies tend to follow the slope in the covariance between M_200 and c, indicating that these results may be measuring the scatter in the data rather than the intrinsic signal. Furthermore, we conclude that our inability to constrain the M_200-c slope is due to a limited range of halo masses, as determined by explicitly modelling our halo mass distribution, and we suggest that other studies may be producing biased results by using an incorrect distribution for their halo masses.

Physical properties underlying observed kinematics of satellite galaxies

We study the kinematics of satellites around isolated galaxies selected from the Sloan Digital Sky Survey (SDSS) spectroscopic catalog. Using a model of the phase-space density previously measured for the halos of LCDM dark matter cosmological simulations, we determine the properties of the halo mass distribution and the orbital anisotropy of the satellites as a function of the colour-based morphological type and the stellar mass of the central host galaxy. We place constraints on the halo mass and the concentration parameter of dark matter and the satellite number density profiles. We obtain a concentration-mass relation for galactic dark matter haloes that is consistent with predictions of a standard LCDM cosmological model. At given halo or stellar mass, red galaxies have more concentrated halos than their blue counterparts. The fraction of dark matter within a few effective radii is minimal for 11.25<log M_star<11.5. The number density profile of the satellites appears to be shallower than of dark matter, with the scale radius typically 60 per cent larger than of dark matter. The orbital anisotropy around red hosts exhibits a mild excess of radial motions, in agreement with the typical anisotropy profiles found in cosmological simulations, whereas blue galaxies are found to be consistent with an isotropic velocity distribution. Our new constraints on the halo masses of galaxies are used to provide analytic approximations of the halo-to-stellar mass relation for red and blue galaxies.

A SECOND-ORDER BIAS MODEL FOR THE LOGARITHMIC HALO MASS DENSITY

We present an analytic model for the local bias of dark matter halos in a LCDM universe. The model uses the halo mass density instead of the halo number density and is searched for various halo mass cuts, smoothing lengths, and redshift epoches. We find that, when the logarithmic density is used, the second-order polynomial can fit the numerical relation between the halo mass distribution and the underlying matter distribution extremely well. In this model the logarithm of the dark matter density is expanded in terms of log halo mass density to the second order. The model remains excellent for all halo mass cuts (from M_{cut}=3\times10^{11}$ to $3\times10^{12}h^{-1}M_{\odot}$), smoothing scales (from $R=5h^{-1}$Mpc to $50h^{-1}$Mpc), and redshift ranges (from z=0 to 1.0) considered in this study. The stochastic term in the relation is found not entirely random, but a part of the term can be determined by the magnitude of the shear tensor.

Non-Gaussian halo mass function and non-spherical halo collapse: theory vs. simulations

The mass distribution of dark matter halos is a sensitive probe of primordial non-Gaussianity (NG). We derive an analytical formula of the halo mass function by perturbatively computing excursion set path-integrals for a non-Gaussian density field with non-vanishing skewness, fNL. We assume a stochastic barrier model which captures the main features of the ellipsoidal collapse of halos. Contrary to previous results based on extensions of the Press-Schechter formalism to NG initial conditions, we find that the non-spherical collapse of halos directly alter the signature of primordial NG. This points toward a potential degeneracy between the effect of primordial non-Gaussianity and that of non-linear halo collapse. The inferred mass function is found to be in remarkable agreement with N-body simulations of NG local type. Deviations are well within numerical uncertainties for all values of fNLloc in the range of validity of the perturbative calculation (|fnlloc|≲200). Moreover, the comparison with simulation results suggests that for |fNL|≳30 the non-linear collapse of halos, as described by our barrier model, strongly deviates from that of Gaussian initial conditions. This is not surprising since the effect of non-linear gravitational processes may be altered by initially large NG. Hence, in the lack of prior theoretical knowledge, halo collapse model parameters should be included in statistical halo mass function data analysis which aim to constrain the signature of primordial NG.

On the interaction of jet plasma with dark matter subhaloes in active galaxies

The interaction of fragmented plasma of active galactic nuclei jets with galactic haloes via gravitational scattering and lensing by dark matter subhaloes is studied using analytical calculations and numerical Monte-Carlo method. The lensing of jet radiation by halo masses is found to be negligible and unobservable. Moving through a galactic halo jet plasma fragments are sequentially deflected on hyperbolic orbits by gravitational field of subhaloes and deviates at some angles when leaving halo, causing widening of the jet. Based on this model jet opening angles are calculated numerically for various values of jet and halo characteristics. Though these angles are very small, gravitational scattering by halo masses results in specific radial profile of jet radiation intensity, that does not depend on halo mass distribution and jet properties. The intensity of jet radiation, obeying the derived profile, decreases by reasonable observable factors giving possibility to probe the presence of dark matter subhaloes.

Excursion set halo mass function and bias in a stochastic barrier model of ellipsoidal collapse

We use the excursion set formalism to compute the properties of the halo mass distribution for a stochastic barrier model which encapsulates the main features of the ellipsoidal collapse of dark matter halos. Non-Markovian corrections due to the sharp filtering of the linear density field in real space are computed with the path-integral technique introduced by Maggiore and Riotto [M. Maggiore and A. Riotto, Astrophys. J. 711, 907 (2010).]. Here, we provide a detailed derivation of the results presented in [P. S. Corasaniti and I. Achitouv, Phys. Rev. Lett. 106, 241302 (2011).] and extend the mass function analysis to higher redshift. We also derive an analytical expression for the linear halo bias. We find a remarkable agreement with $N$-body simulation data from Tinker et al. [J. L. Tinker et al., Astrophys. J. 688, 709 (2008).] with differences $\ensuremath{\lesssim}5%$ over the range of mass probed by simulations. The excursion set solution from Monte Carlo generated random walks shows the same level of agreement, thus confirming the validity of the path-integral approach for the barrier model considered here. Similarly, the analysis of the linear halo bias shows deviations no greater than 20%. Overall these results indicate that the excursion set formalism in combination with a realistic modeling of the conditions of halo collapse can provide an accurate description of the halo mass distribution.

MASS FUNCTION PREDICTIONS BEYOND ΛCDM

The mass distribution of halos, as specified by the halo mass function, is a key input for several cosmological probes. The sizes of $N$-body simulations are now such that, for the most part, results need no longer be statistics-limited, but are still subject to various systematic uncertainties. We investigate and discuss some of the reasons for these differences. Quantifying error sources and compensating for them as appropriate, we carry out a high-statistics study of dark matter halos from 67 $N$-body simulations to investigate the mass function and its evolution for a reference $\Lambda$CDM cosmology and for a set of $w$CDM cosmologies. For the reference $\Lambda$CDM cosmology (close to WMAP5), we quantify the breaking of universality in the form of the mass function as a function of redshift, finding an evolution of as much as 10% away from the universal form between redshifts $z=0$ and $z=2$. For cosmologies very close to this reference we provide a fitting formula to our results for the (evolving) $\Lambda$CDM mass function over a mass range of $6\cdot 10^{11}-3\cdot 10^{15}$ M$_{\odot}$ to an estimated accuracy of about 2%. The set of $w$CDM cosmologies is taken from the Coyote Universe simulation suite. The mass functions from this suite (which includes a $\Lambda$CDM cosmology and others with $w\simeq-1$) are described by the fitting formula for the reference $\Lambda$CDM case at an accuracy level of 10%, but with clear systematic deviations. We argue that, as a consequence, fitting formulae based on a universal form for the mass function may have limited utility in high precision cosmological applications.

STRONG FIELD-TO-FIELD VARIATION OF Lyα NEBULAE POPULATIONS ATz≃ 2.3

Understanding the nature of distant Ly-alpha nebulae ("blobs") and connecting them to their present-day descendants requires constraining their number density, clustering, and large-scale environment. To measure these basic quantities, we conduct a deep narrowband imaging survey in four different fields, Chandra Deep Field South (CDFS), Chandra Deep Field North, and two COSMOS subfields, for a total survey area of 1.2deg^2. We discover 25 blobs at z=2.3 with Ly-alpha luminosities of 0.7-8x10^43 erg/s and isophotal areas of Aiso = 10-60 arcsec^2. The transition from compact Ly-alpha emitters (Aiso ~ a few arcsec^2) to extended blobs (Aiso > 10 arcsec^2) is continuous, suggesting a single family perhaps governed by similar emission mechanisms. Surprisingly, most blobs (16/25) are in one survey field, the CDFS. The six brightest, largest blobs with L > 1.5x10^43 erg/s and Aiso > 16 arcsec^2 lie only in the CDFS. These large, bright blobs have a field-to-field variance of sigma_v >~ 1.5 (150%) about their number density n ~ 1.0x10^-5 Mpc^-3. This variance is large, significantly higher than that of unresolved LAEs (sigma_v ~ 0.3 or 30%), and can adversely affect comparisons of blob number densities and luminosity functions among different surveys. We compare the statistics of our blobs with dark matter halos in a 1 Gpc/h cosmological N-body simulation. At z=2.3, the number density (n) implies that each bright, large blob could occupy a halo of M_halo > 10^13 Msun if most halos have detectable blobs. The predicted variance in n is consistent with that observed and corresponds to a bias of ~7. Blob halos lie at the high end of the halo mass distribution at z=2.3 and are likely to evolve into the ~10^14 Msun halos typical of galaxy clusters today. On larger scales of ~10 co-moving Mpc, blobs cluster where compact LAEs do, indicating that blobs lie in coherent, highly overdense structures.

Merging and fragmentation in the Burgers dynamics

We explore the noiseless Burgers dynamics in the inviscid limit, the so-called "adhesion model" in cosmology, in a regime where (almost) all the fluid particles are embedded within pointlike massive halos. Following previous works, we focus our investigations on a "geometrical" model, where the matter evolution within the shock manifold is defined from a geometrical construction. This hypothesis is at variance with the assumption that the usual continuity equation holds but, in the inviscid limit, both models agree in the regular regions. Taking advantage of the formulation of the dynamics of this "geometrical model" in terms of Legendre transforms and convex hulls, we study the evolution with time of the distribution of matter and the associated partitions of the Lagrangian and Eulerian spaces. We describe how the halo mass distribution derives from a triangulation in Lagrangian space, while the dual Voronoi-like tessellation in Eulerian space gives the boundaries of empty regions with shock nodes at their vertices. We then emphasize that this dynamics actually leads to halo fragmentations for space dimensions greater or equal to 2 (for the inviscid limit studied in this paper). This is most easily seen from the properties of the Lagrangian-space triangulation and we illustrate this process in the two-dimensional (2D) case. In particular, we explain how pointlike halos only merge through three-body collisions while two-body collisions always give rise to two new massive shock nodes (in 2D). This generalizes to higher dimensions and we briefly illustrate the three-dimensional case. This leads to a specific picture for the continuous formation of massive halos through successive halo fragmentations and mergings.

THE HALO MASS FUNCTION CONDITIONED ON DENSITY FROM THE MILLENNIUM SIMULATION: INSIGHTS INTO MISSING BARYONS AND GALAXY MASS FUNCTIONS

The baryon content of high-density regions in the universe is relevant to two critical unanswered questions: the workings of nurture effects on galaxies and the whereabouts of the missing baryons. In this paper, we analyze the distribution of dark matter and semianalytical galaxies in the Millennium Simulation to investigate these problems. Applying the same density field reconstruction schemes as used for the overall matter distribution to the matter locked in halos we study the mass contribution of halos to the total mass budget at various background field densities, i.e., the conditional halo mass function. In this context, we present a simple fitting formula for the cumulative mass function accurate to ~ 5% for halo masses between 10^{10} and 10^{15}Msol/h. We find that in dense environments the halo mass function becomes top heavy and present corresponding fitting formulae for different redshifts. We demonstrate that the major fraction of matter in high-density fields is associated with galaxy groups. Since current X-ray surveys are able to nearly recover the universal baryon fraction within groups, our results indicate that the major part of the so-far undetected warm-hot intergalactic medium resides in low-density regions at low temperatures. Similarly, we show that the differences in galaxy mass functions with environment seen in observed and simulated data stem predominantly from differences in the mass distribution of halos. In particular, the hump in the galaxy mass function is associated with the central group galaxies, and the bimodality observed in the galaxy mass function is therefore interpreted as that of central galaxies versus satellites.

A simple model to link the properties of quasars to the properties of dark matter haloes out to high redshift

We present a simple model of how quasars occupy dark matter halos from z=0 to z=5 using the observed mBH-sigma relation and quasar luminosity functions. This provides a way for observers to statistically infer host halo masses for quasar observations using luminosity and redshift alone. Our model is deliberately simple and sidesteps any need to explicitly describe the physics. In spite of its simplicity, the model reproduces many key observations and has predictive power: 1) model quasars have the correct luminosity function (by construction) and spatial clustering (by consequence); 2) we predict high redshift quasars of a given luminosity live in less massive dark matter halos than the same luminosity quasars at low redshifts; 3) we predict a factor of ~5 more 10^8.5Msun black holes at z~2 than is currently observed; 4) we predict a factor of ~20 evolution in the amplitude of the mBH-Mhalo relation between z=5 and the present day; 5) we expect luminosity dependent quasar lifetimes of between tQ~10^(7-8)yr, but which may become as short as 10^(5-6)yr for quasars brighter than L*; 6) while little luminosity dependent clustering evolution is expected at z<1, increasingly strong evolution is predicted for L>L* quasars at higher redshifts. These last two results arise from the narrowing distribution of halo masses that quasars occupy as the Universe ages. We also deconstruct both "downsizing" and "upsizing" trends predicted by the model at different redshifts and space densities. Importantly, this work illustrates how current observations cannot distinguish between more complicated physically motivated quasar models and our simple phenomenological approach. It highlights the opportunities such methodologies provide.

The statistical properties of Λ cold dark matter halo formation

We present a comparison of the statistical properties of dark matter halo merger trees extracted from the Millennium Simulation with Extended Press–Schechter (EPS) formalism and the related galform Monte Carlo method for generating ensembles of merger trees. The volume, mass resolution and output frequency make the Millennium Simulation a unique resource for the study of the hierarchical growth of structure. We construct the merger trees of present-day friends-of-friends groups and calculate a variety of statistics that quantify the masses of their progenitors as a function of redshift, accretion rates, and the redshift distribution of their most recent major merger. We also look in the forward direction and quantify the present-day mass distribution of haloes into which high-redshift progenitors of a specific mass become incorporated. We find that the EPS formalism and its Monte Carlo extension capture the qualitative behaviour of all these statistics, but as redshift increases they systematically underestimate the masses of the most massive progenitors. This shortcoming is worst for the Monte Carlo algorithm. We present a fitting function to a scaled version of the progenitor mass distribution and show how it can be used to make more accurate predictions of both progenitor and final halo mass distributions.

The Influence of Baryons on the Mass Distribution of Dark Matter Halos

Using a set of high-resolution N-body/SPH cosmological simulations with identical initial conditions but run with different numerical setups, we investigate the influence of baryonic matter on the mass distribution of dark halos when radiative cooling is not included. We compare the concentration parameters of about 400 massive halos with virial mass from 1013 to 7.1 × 1014 h-1 M☉. We find that the concentration parameters for the total mass and dark matter distributions in nonradiative simulations are on average larger by ~3% and 10% than those in a pure dark matter simulation. Our results indicate that the total mass density profile is little affected by a hot gas component in the simulations. After carefully excluding the effects of resolutions and spurious two-body heating between dark matter (DM) and gas particles, we conclude that the increase of the DM concentration parameters is due to interactions between baryons and DM. We demonstrate this with the aid of idealized simulations of two-body mergers. The results of individual halos simulated with different mass resolutions show that in the gas profiles of densities, temperature and entropy are subjects of mass resolution of SPH particles. In particular, we find that in the inner parts of halos, as the SPH resolution increases the gas density becomes higher but both the entropy and temperature decrease.

VIMOS-IFU survey ofz~ 0.2 massive galaxy clusters

We present extensive multi-color imaging and low-resolution VIMOS integral field unit (IFU) spectroscopic observations of the X-ray luminous cluster Abell 2667 (). An extremely bright giant gravitational arc () is easily identified as part of a triple image system, and other fainter multiple images are also revealed by the Hubble Space Telescope Wide Field Planetary Camera-2 images. The VIMOS-IFU observations cover a field of view of and enable us to determine the redshift of all galaxies down to . Furthermore, redshifts could be identified for some sources down to . In particular we identify 21 members in the cluster core, from which we derive a velocity dispersion of km s-1, corresponding to a total mass of within a kpc (30 arcsec) radius. Using the multiple images constraints and priors on the mass distribution of cluster galaxy halos we construct a detailed lensing-mass model leading to a total mass of within the Einstein radius (16 arcsec). The lensing mass and dynamical mass are in good agreement, although the dynamical one is much less accurate. Within a 110 kpc radius, we find a rest-frame K-band M/L ratio of . Comparing these measurements with published X-ray analysis is, however, less conclusive. Although the X-ray temperature matches the dynamical and lensing estimates, the published NFW mass model derived from the X-ray measurement with its low concentration of cannot account for the large Einstein radius observed in this cluster. A higher concentration of ∼6 would, however, match the strong lensing measurements. These results very likely reflect the complex structure of the cluster mass distribution, underlying the importance of panchromatic studies from small to large scale in order to better understand cluster physics.

KINEMATIC PROPERTIES OF DAMPED LYMAN ALPHA SYSTEMS

In a previous paper, we determined the statistical distributions for various classes of QSO absorption systems in the framework of the CDM model of the universe with a mass distribution of dark matter halos as given by the Press Schechter mechanism. These were shown to be consistent with the observed distributions for reasonable choices of model parameters. In this paper, we generate Voigt profiles of C IV lines associated with Damped Lyman Alpha systems in the framework of this model, taking into account rotation of disks and random motion of clouds embedded in galactic halos. We compare these with the profiles for a sample of 32 Damped Lyman Alpha systems collected from the literature by performing several statistical tests. These tests compare the width and the degree of asymmetry in the line profiles produced by the rotation of randomly inclined disks and the random velocity of clouds in the galactic halos, with the corresponding quantities in the observed profiles. We find that the kinematic properties predicted by the model are in good agreement with observations provided the disk thickness is about ten thousand times smaller than its radius, which indicates that the material in the disks is concentrated in dense clouds with roughly unit covering factor.

Effects of the Complex Mass Distribution of Dark Matter Halos on Weak‐Lensing Cluster Surveys

Gravitational lensing effects arise from the light-ray deflection by all of the mass distribution along the line of sight. It is then expected that weak-lensing cluster surveys can provide us with true mass-selected cluster samples. With numerical simulations, we analyze the correspondence between peaks in the lensing convergence κ-map and dark matter halos. In particular, we emphasize the difference between the peak κ-value expected from a dark matter halo modeled as an isolated and spherical one, which exhibits a one-to-one correspondence with the halo mass at a given redshift, and that of the associated κ-peak from simulations. For halos with the same expected κ, their corresponding peak signals in the κ-map present a wide dispersion. At an angular smoothing scale of θG = 1', our study shows that for relatively large clusters, the complex mass distribution of individual clusters is the main reason for the dispersion. The projection effect of uncorrelated structures does not play significant roles. The triaxiality of dark matter halos accounts for a large part of the dispersion, especially for the tail on the high-κ side. Thus, lensing-selected clusters are not really mass-selected. To better predict κ-selected cluster abundance for a cosmological model, one has to take into account the triaxial mass distribution of dark matter halos. On the other hand, for a significant number of clusters, their mass distribution is even more complex than that described by the triaxial model. Our analyses find that large substructures affect the identification of lensing clusters considerably. They could show up as separate peaks in the κ-map and cause a misassociation of the whole cluster with a peak resulting only from a large substructure. The lower end dispersion of κ is attributed mostly to this substructure effect. For θG = 2', the projection effect can be significant and contributes to the dispersion at both high- and low-κ ends.

Halo model at its best: constraints on conditional luminosity functions from measured galaxy statistics

(Summary) Using the conditional luminosity function (CLF) -- the luminosity distribution of galaxies in a dark matter halo -- as the fundamental building block, we present an empirical model for the galaxy distribution. The model predictions are compared with the published luminosity function and clustering statistics from Sloan Digital Sky Survey (SDSS) at low redshifts, and galaxy correlation functions from COMBO-17 survey at a redshift of 0.6, DEEP2 survey at a redshift of unity, and Great Observatories Deep Origins Survey (GOODS) at a redshift around 3. The comparison with statistical measurements allows us to constrain certain parameters related to analytical descriptions that describe the relation between a dark matter halo and its central galaxy luminosity, its satellite galaxy luminosity, and the fraction of early- and late-type galaxies of that halo. The single parameter best constrained by clustering measurements is the total satellite galaxy luminosity averaged over the halo mass distribution probed by the galaxy sample. For SDSS, $ =(2.1^{+0.8}_{-0.4}) \times 10^{10}$ h${-2}$ L$_{\sun}$, while for GOODS at $z \sim 3$, $ < 2 \times 10^{11}$ h$^{-2}$ L$_{\sun}$. Quantities such as the fraction of satellite galaxies at a given galaxy luminosity are not strongly constrained with SDSS data. We use our CLFs to establish probability distribution of halo mass in which galaxies of a given luminosity could be found either at halo centers or as satellites. Finally, to help establish further properties of the galaxy distribution, we propose the measurement of cross-clustering between galaxies divided into two distinctly different luminosity bins.

Reconstructing the Triaxial Shapes of Dark Matter Halos from the Anisotropic Spatial Distributions of their Substructures in the Concordance Cosmology

We develop an algorithm to reconstruct the triaxial shapes of dark matter halos from the anisotropic spatial distributions of their substructures for the concordance background cosmology. First, we construct an analytic model for the anisotropic spatial distribution of dark halo substructures under the assumption that the tidal field with nonzero trace in the triaxial mass distribution of the host halos generates the substructure bulk motions toward the major principal axes of the hosts. Our analytic model implies that the degree of anisotropy depends sensitively on the triaxiality of the host halos as well as the correlation between the substructure locations and the tidal shear field. Second, we set the axis ratios of the triaxial host halos as free parameters in the analytic model, fit the model to the numerical results from high-resolution N-body simulation of the concordance cosmology, and reconstruct the two axis ratios of the host halos from the best-fit values of the free parameters. The comparison of the reconstructed axis ratios with the numerical results reveals good agreement. Finally, we conclude that our analytic model may provide a physical understanding of the anisotropic spatial distribution of dark halo substructures and a new way to reconstruct in principle the triaxial shapes of dark matter halos from the observables.