ABSTRACT
We measure the anisotropy of dark matter flows on small scales (∼500 kpc) in the near environment of haloes using a large set of simulations. We rely on two different approaches to quantify ...the anisotropy of the cosmic infall: we measure the flows at the virial radius of the haloes while describing the infalling matter via fluxes through a spherical shell; and we measure the spatial and kinematical distributions of satellites and substructures around haloes detected by the subclump finder adaptahop described for the first time in the appendix. The two methods are found to be in agreement both qualitatively and quantitatively via one- and two-point statistics.
The peripheral and advected momenta are correlated with the spin of the embedded halo at levels of 30 and 50 per cent. The infall takes place preferentially in the plane perpendicular to the direction defined by the spin of the halo. We computed the excess of equatorial accretion both through rings and via a harmonic expansion of the infall.
The level of anisotropy of infalling matter is found to be ∼15 per cent. The substructures have their spin orthogonal to their velocity vector in the rest frame of the halo at a level of about 5 per cent, suggestive of an image of a flow along filamentary structures, which provides an explanation for the measured anisotropy. Using a 'synthetic' stacked halo, it is shown that the positions and orientations of satellites relative to the direction of spin of the halo are not random even in projection. The average ellipticity of stacked haloes is 10 per cent, while the alignment excess in projection reaches 2 per cent. All measured correlations are fitted by a simple three-parameter model.
We conclude that a halo does not see its environment as an isotropic perturbation, we investigate how the anisotropy is propagated inwards using perturbation theory, and we discuss briefly the implications for weak lensing, warps and the thickness of galactic discs.
Towards the optimal window for the 2MASS dipole Chodorowski, Michał J.; Coiffard, Jean-Baptiste; Bilicki, Maciej ...
Monthly Notices of the Royal Astronomical Society,
09/2008, Letnik:
389, Številka:
2
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A comparison of the Two Micron All Sky Survey (2MASS) flux dipole to the cosmic microwave background (CMB) dipole can serve as a method to constrain a combination of the cosmological parameter Ωm and ...the luminosity bias of the 2MASS survey. For this constraint to be as tight as possible, it is necessary to maximize the correlation between the two dipoles. This can be achieved by optimizing the survey window through which the flux dipole is measured. Here we explicitly construct such a window for the 2MASS survey. The optimization in essence reduces to excluding from the calculation of the flux dipole galaxies brighter than some limiting magnitude Kmin of the near-infrared Ks band. This exclusion mitigates non-linear effects and shot noise from small scales, which decorrelate the 2MASS dipole from the CMB dipole. Under the assumption of negligible shot noise we find that the optimal value of Kmin is about 5. Inclusion of shot noise shifts the optimal Kmin to larger values. We present an analytical formula for shot noise for the 2MASS flux dipole, to be used in follow-up work with 2MASS data. The misalignment angle between the two dipoles is a sensitive measure of their correlation: the higher the correlation, the smaller the expectation value of the angle. A minimum of the misalignment is thus a sign of the optimal gravity window. We model analytically the distribution function for the misalignment angle and show that the misalignment estimated by Maller et al. is consistent with the assumed underlying model (though it is greater than the expectation value). We predict with about 90 per cent confidence that the misalignment will decrease if 2MASS galaxies brighter than Kmin= 5 mag are excluded from the calculation of the flux dipole. This prediction has been indirectly confirmed by the results of Erdoğdu et al. The measured misalignment constitutes thus an alternative way of finding the optimal value of Kmin: the latter corresponds to a minimum of the former.
Cosmic statistics of statistics Szapudi, István; Colombi, Stéphane; Bernardeau, Francis
Monthly Notices of the Royal Astronomical Society,
12/1999, Letnik:
310, Številka:
2
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The errors on statistics measured in finite galaxy catalogues are exhaustively investigated. The theory of errors on factorial moments by Szapudi & Colombi is applied to cumulants via a series ...expansion method. All results are subsequently extended to the weakly non-linear regime. Together with previous investigations this yields an analytic theory of the errors for moments and connected moments of counts in cells from highly non-linear to weakly non-linear scales. For non-linear functions of unbiased estimators, such as the cumulants, the phenomenon of cosmic bias is identified and computed. Since it is subdued by the cosmic errors in the range of applicability of the theory, correction for it is inconsequential. In addition, the method of Colombi, Szapudi & Szalay concerning sampling effects is generalized, adapting the theory for inhomogeneous galaxy catalogues. While previous work focused on the variance only, the present article calculates the cross-correlations between moments and connected moments as well for a statistically complete description. The final analytic formulae representing the full theory are explicit but somewhat complicated. Therefore we have made available a fortran program capable of calculating the described quantities numerically (for further details e-mail SC at colombi@iap.fr). An important special case is the evaluation of the errors on the two-point correlation function, for which this should be more accurate than any method put forward previously. This tool will be immensely useful in the future for assessing the precision of measurements from existing catalogues, as well as aiding the design of new galaxy surveys. To illustrate the applicability of the results and to explore the numerical aspects of the theory qualitatively and quantitatively, the errors and cross-correlations are predicted under a wide range of assumptions for the future Sloan Digital Sky Survey. The principal results concerning the cumulants ξ, Q3 and Q4 is that the relative error is expected to be smaller than 3, 5 and 15 per cent, respectively, in the scale range of 1–10 h−1 Mpc; the cosmic bias will be negligible.
It is possible to classify pixels of a smoothed cosmic microwave background (CMB) fluctuation map according to their local curvature in ‘hill’, ‘lake’ and ‘saddle’ regions. In the Gaussian case, ...fractional areas occupied by pixels of each kind can be computed analytically for families of excursion sets as functions of threshold and moments of the fluctuation power spectrum. We show how the shape of these functions can be used to constrain accurately the level of non-Gaussianity in the data by applying these new statistics to an hypothetical mixed model suggested by Bouchet et al. (2001). According to our simple test, with only one 12.5 × 12.5 deg2 map, Planck should be able to detect with a high significance a non-Gaussian level as weak as 10 per cent in temperature standard deviation (rms; 5 per cent in Cℓ), whereas a marginal detection would be possible for MAP with a non-Gaussian level around 30 per cent in temperature (15 per cent in Cℓ).
We use the galics hybrid model of galaxy formation to explore the nature of galaxy clustering in the local Universe. We bring the theoretical predictions of our model into the observational plane ...using the momaf software to build mock catalogues which mimic Sloan Digital Sky Survey (SDSS) observations. We measure low- and high-order angular clustering statistic from these mock catalogues, after selecting galaxies the same way as for observations, and compare them directly to estimates from the SDSS data. Note that we also present the first measurements of high-order statistics on the SDSS DR1. We find that our model is in general good agreement with observations in the scale/luminosity range where we can trust the predictions. This range is found to be limited (i) by the size of the dark matter simulation used – which introduces finite volume effects at large scales – and by the mass resolution of this simulation – which introduces incompleteness at apparent magnitudes fainter than r∼ 20. We then focus on the small-scale clustering properties of galaxies and investigate the behaviour of three different prescriptions for positioning galaxies within haloes of dark matter. We show that galaxies are poor tracers of either DM particles or DM substructures, within groups and clusters. Instead, SDSS data tells us that the distribution of galaxies lies somewhat in between these two populations. This confirms the general theoretical expectation from numerical simulations and semi-analytic modelling.
Haloes gone MAD: The Halo-Finder Comparison Project Knebe, Alexander; Knollmann, Steffen R.; Muldrew, Stuart I. ...
Monthly Notices of the Royal Astronomical Society,
08/2011, Letnik:
415, Številka:
3
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ABSTRACT
We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including ...friends‐of‐friends, spherical‐overdensity and phase‐space‐based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large‐scale structure of the universe.
All the halo‐finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30–40 particles. However, also here the phase‐space finders excelled by resolving substructure down to 10–20 particles. By comparing the halo finders using a high‐resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve).
We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge.
By comparing a collisionless cosmological N-body simulation (DM) to a smoothed particle hydrodynamics simulation (SPH) with the same initial conditions, we investigate the correspondence between dark ...matter subhalos produced by collisionless dynamics and galaxies produced by dissipative gas dynamics in a dark matter background. When galaxies in the SPH simulation fall into larger groups and become satellites, they retain local dark matter concentrations (SPH subhalos) whose mass is typically 5 times the galaxy baryonic mass (compared to the simulation's universal ratio image). The more massive subhalos of the SPH simulation generally have corresponding subhalos of similar mass and spatial position in the DM simulation; at lower masses, there is still fairly good correspondence, but some DM subhalos are in different spatial positions and some have suffered tidal stripping or disruption. The halo occupation statistics of DM subhalos-the mean number of subhalos, pairs, and triples as a function of host halo mass-are very similar to those of SPH subhalos and SPH galaxies. The gravity of the dissipative baryon component amplifies the density contrast of subhalos in the SPH simulation, making them more resistant to tidal disruption. Relative to SPH galaxies and SPH subhalos, the DM subhalo population is depleted in the densest regions of the most massive halos. The good agreement of halo occupation statistics between the DM subhalo and SPH galaxy populations leads to good agreement of their two-point correlation functions and higher order moments on large scales. The depletion of DM subhalos in dense regions depresses their clustering at image h super(-1) Mpc. In these simulations, the 'conversation' between dark matter and baryons is mostly one-way, with dark matter dynamics telling galaxies where to form and how to cluster, but the 'back talk' of the baryons influences small-scale clustering by enhancing the survival of substructure in the densest environments.
Experimental cosmic statistics — I. Variance Colombi, Stéphane; Szapudi, István; Jenkins, Adrian ...
Monthly notices of the Royal Astronomical Society,
04/2000, Letnik:
313, Številka:
4
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Counts-in-cells are measured in the τCDM Virgo Hubble Volume simulation. This large N-body experiment has 109 particles in a cubic box of size 2000 h−1 Mpc. The unprecedented combination of size and ...resolution allows, for the first time, a realistic numerical analysis of the cosmic errors and cosmic correlations of statistics related to counts-in-cells measurements, such as the probability distribution function PN itself, its factorial moments Fk and the related cumulants and SNs. These statistics are extracted from the whole simulation cube, as well as from 4096 subcubes of size 125 h−1 Mpc, each representing a virtual random realization of the local universe. The measurements and their scatter over the subvolumes are compared to the theoretical predictions of Colombi, Bouchet & Schaeffer for P0, and of Szapudi & Colombi and Szapudi, Colombi & Bernardeau for the factorial moments and the cumulants. The general behaviour of experimental variance and cross-correlations as functions of scale and order is well described by theoretical predictions, with a few per cent accuracy in the weakly non-linear regime for the cosmic error on factorial moments. On highly non-linear scales, however, all variants of the hierarchical model used by SC and SCB to describe clustering appear to become increasingly approximate, which leads to a slight overestimation of the error, by about a factor of two in the worst case. Because of the needed supplementary perturbative approach, the theory is less accurate for non-linear estimators, such as cumulants, than for factorial moments. The cosmic bias is evaluated as well, and, in agreement with SCB, is found to be insignificant compared with the cosmic variance in all regimes investigated. While higher order statistics were previously evaluated in several simulations, this work presents textbook quality measurements of SNs, 3≤N≤10, in an unprecedented dynamic range of. In the weakly non-linear regime the results confirm previous findings and agree remarkably well with perturbation theory predictions including the one-loop corrections based on spherical collapse by Fosalba & Gaztañaga. Extended perturbation theory is confirmed on all scales.
Experimental cosmic statistics — II. Distribution Szapudi, István; Colombi, Stéphane; Jenkins, Adrian ...
Monthly Notices of the Royal Astronomical Society,
04/2000, Letnik:
313, Številka:
4
Journal Article
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Colombi et al. (Paper I) investigated the counts-in-cells statistics and their respective errors in the τCDM Virgo Hubble Volume simulation. This extremely large N-body experiment also allows a ...numerical investigation of the cosmic distribution function, ϒ (Ã), itself for the first time. For a statistic A, ϒ (Ã) is the probability density of measuring the value à in a finite galaxy catalogue. ϒ was evaluated for the distribution of counts-in-cells, PN, the factorial moments, Fk, and the cumulants, ξ̄ and SNs, using the same subsamples as Paper I. While Paper I concentrated on the first two moments of ϒ, i.e. the mean, the cosmic error and the cross-correlations, here the function ϒ is studied in its full generality, including a preliminary analysis of joint distributions ϒ (Ã,B̃). The most significant, and reassuring result for the analyses of future galaxy data is that the cosmic distribution function is nearly Gaussian provided its variance is small. A good practical criterion for the relative cosmic error is that ΔAA≲0.2. This means that for accurate measurements, the theory of the cosmic errors, presented by Szapudi & Colombi and Szapudi, Colombi & Bernardeau, and confirmed empirically by Paper I, is sufficient for a full statistical description and thus for a maximum likelihood rating of models. As the cosmic error increases, the cosmic distribution function ϒ becomes increasingly skewed and is well described by a generalization of the lognormal distribution. The cosmic skewness is introduced as an additional free parameter. The deviation from Gaussianity of ϒ(F̃k) and ϒ(S̃ N) increases with order k, N and similarly for ϒ(P̃ N) when N is far from the maximum of PN, or when the scale approaches the size of the catalogue. For our particular experiment, ϒ(F̃k) and ϒ(ξ ̄ ̃) are well approximated with the standard lognormal distribution, as evidenced by both the distribution itself and the comparison of the measured skewness with that of the lognormal distribution.
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