We discuss the skeleton as a probe of the filamentary structures of a two-dimensional random field. It can be defined for a smooth field as the ensemble of pairs of field lines departing from saddle ...points, initially aligned with the major axis of local curvature and connecting them to local maxima. This definition is thus non-local and makes analytical predictions difficult, so we propose a local approximation: the local skeleton is given by the set of points where the gradient is aligned with the local curvature major axis and where the second component of the local curvature is negative. We perform a statistical analysis of the length of the total local skeleton, chosen for simplicity as the set of all points of space where the gradient is either parallel or orthogonal to the main curvature axis. In all our numerical experiments, which include Gaussian and various non-Gaussian realizations such as χ2 fields and Zel'dovich maps, the differential length f of the skeleton is found within a normalization factor to be very close to the probability distribution function (pdf) of the smoothed field, as expected and explicitly demonstrated in the Gaussian case where semi-analytical results are derived. As a result of the special nature of the skeleton, the differences between f and the pdf are small but noticeable. We find in the Gaussian case that they increase with the coherence parameter 0 ≤γ≤ 1 of the field: Here, is the total length of the skeleton and is the length of the skeleton in the excursion ρ > σx where σ is the variance of the density field. This result makes the skeleton an interesting alternative probe of non-Gaussianity. Our analyses furthermore assume that the total length of the skeleton is a free, adjustable parameter. This total length could in fact be used to constrain cosmological models, in cosmic microwave background maps but also in three-dimensional galaxy catalogues, where it estimates the total length of filaments in the Universe. Making the link with other works, we also show how the skeleton can be used to study the dynamics of large-scale structure.
We describe Ly alpha Mass Association Scheme (LyMAS), a method of predicting clustering statistics in the Ly alpha forest on large scales from moderate-resolution simulations of the dark matter (DM) ...distribution, with calibration from high-resolution hydrodynamic simulations of smaller volumes. In this study we adopt the spectral resolution of the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS) at z = 2.5, and we find optimal results for a DM smoothing length sigma = 0.3 h super(-1) Mpc (comoving). In its simplest form, LyMAS draws randomly from the hydro-calibrated P(F sub(s)| delta sub(s)) to convert DM skewers into Ly alpha forest pseudo-spectra, which are then used to compute cross-sightline flux statistics. LyMAS will be a powerful tool for interpreting 3D Ly alpha forest data, thereby transforming measurements from BOSS and other massive quasar absorption surveys into constraints on dark energy, DM, space geometry, and intergalactic medium physics.
We consider the growth of primordial dark matter halos seeded by three crossed initial sine waves of various amplitudes. Using a Lagrangian treatment of cosmological gravitational dynamics, we ...examine the convergence properties of a high-order perturbative expansion in the vicinity of shell crossing by comparing the analytical results with state-of-the-art high resolution Vlasov-Poisson simulations. Based on a quantitative exploration of parameter space, we study explicitly for the first time the convergence speed of the perturbative series and find, in agreement with intuition, that it slows down when going from quasi-one-dimensional initial conditions (one sine wave dominating) to quasitriaxial symmetry (three sine waves with same amplitude). In most cases, the system structure at collapse time is, as expected, very similar to what is obtained with simple one-dimensional dynamics, except in the quasitriaxial regime, where the phase-space sheet presents a velocity spike. In all cases, the perturbative series exhibits a generic convergence behavior as fast as an exponential of a power law of the order of the expansion, allowing one to numerically extrapolate it to infinite order. The results of such an extrapolation agree remarkably well with the simulations, even at shell crossing.
A large simulation reveals that most of the detectable signal from dark matter in our Milky Way probably comes from the main, smooth Galactic halo, rather than from small clumps.
An illuminating dark halo Colombi, Stéphane
Nature (London),
11/2008, Letnik:
456, Številka:
7218
Journal Article
Recenzirano
Most of the mass of the Universe is believed to be in the form of dark matter - an invisible component that has so far been only indirectly detected through the effects of its gravity on visible ...matter. In the theory of supersymmetry in particle physics, there is a corresponding dark-matter-particle candidate that interacts only very weakly with the rest of the Universe, and is thus very difficult to detect directly. There is, however, a general feeling in the astronomical community that the search for dark matter is now at a turning point. This feeling stems from the recent start of the largest particle accelerator in the world (the Large Hadron Collider), which could provide clues about the nature of dark matter, and from the advent of high-energy astrophysics observations, such as γ-ray observations carried out by NASA's Fermi Gamma-ray Space Telescope. Such observations are potentially able to detect dark-matter particles indirectly through their annihilation radiation. On page 73 of this issue, Springel et al.1 show that the primary and probably most easily observable annihilation signal is produced by the diffuse dark-matter component rather than the very small clumps in the main halo of our Galaxy (Fig. 1).
MoMaF: the Mock Map Facility Blaizot, Jérémy; Wadadekar, Yogesh; Guiderdoni, Bruno ...
Monthly notices of the Royal Astronomical Society,
06/2005, Letnik:
360, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present the Mock Map Facility, a powerful tool for converting theoretical outputs of hierarchical galaxy formation models into catalogues of virtual observations. The general principle is ...straightforward: mock observing cones can be generated using semi-analytically post-processed snapshots of cosmological N-body simulations. These cones can then be projected to synthesize mock sky images. To this end, the paper describes in detail an efficient technique for creating such mock cones and images from the galaxies in cosmological simulations (galics) semi-analytic model, providing the reader with an accurate quantification of the artefacts it introduces at every step. We show that replication effects introduce a negative bias on the clustering signal – typically peaking at less than 10 per cent around the correlation length. We also thoroughly discuss how the clustering signal is affected by finite-volume effects, and show that it vanishes at scales larger than approximately one-tenth of the simulation box size. For the purpose of analysing our method, we show that number counts and redshift distributions obtained with galics/momaf compare well with K-band observations and the two-degree field galaxy redshift survey. Given finite-volume effects, we also show that the model can reproduce the automatic plate measuring machine angular correlation function. The momaf results discussed here are made publicly available to the astronomical community through a public data base. Moreover, a user-friendly Web interface () allows any user to recover her/his own favourite galaxy samples through simple SQL queries. The flexibility of this tool should permit a variety of uses ranging from extensive comparisons between real observations and those predicted by hierarchical models of galaxy formation, to the preparation of observing strategies for deep surveys and tests of data processing pipelines.
Modelling Lyman a forest cross-correlations with LyMAS Lochhaas, Cassandra; Weinberg, David H; Peirani, Sebastien ...
Monthly notices of the Royal Astronomical Society,
10/2016, Letnik:
461, Številka:
4
Journal Article
Recenzirano
We use the Lya Mass Association Scheme (LyMAS) to predict cross-correlations at z = 2.5 between dark matter haloes and transmitted flux in the Lya forest, and compare to cross-correlations measured ...for quasars and damped Lya systems (DLAs) from the Baryon Oscillation Spectroscopic Survey (BOSS) by Font-Ribera et al. We calibrate LyMAS using Horizon-AGN hydrodynamical cosmological simulations of a (100 h super( -1) Mpc)3 comoving volume. We apply this calibration to a (1 h super( -1) Gpc)3 simulation realized with 20483 dark matter particles. In the 100 h super( -1) Mpc box, LyMAS reproduces the halo-flux correlations computed from the full hydrodynamic gas distribution very well. In the 1 h super( -1) Gpc box, the amplitude of the large-scale cross-correlation tracks the halo bias bh as expected. We provide empirical fitting functions that describe our numerical results. In the transverse separation bins used for the BOSS analyses, LyMAS cross-correlation predictions follow linear theory accurately down to small scales. Fitting the BOSS measurements requires inclusion of random velocity errors; we find best-fitting rms velocity errors of 399 and 252 km s super( -1) for quasars and DLAs, respectively. We infer bias-weighted mean halo masses of ... and 0.69+0.16-0.140.69-0.14+0.16 for the host haloes of quasars and DLAs, with ~0.2 dex systematic uncertainty associated with redshift evolution, intergalactic medium parameters, and selection of data fitting range. (ProQuest: ... denotes formulae/symbols omitted.)
We consider the Gumbel or extreme value statistics describing the distribution function p
G(νmax) of the maximum values of a random field ν within patches of fixed size. We present, for smooth ...Gaussian random fields in two and three dimensions, an analytical estimate of p
G which is expected to hold in a regime where local maxima of the field are moderately high and weakly clustered.
When the patch size becomes sufficiently large, the negative of the logarithm of the cumulative extreme value distribution is simply equal to the average of the Euler characteristic of the field in the excursion ν≥νmax inside the patches. The Gumbel statistics therefore represents an interesting alternative probe of the genus as a test of non-Gaussianity, e.g. in cosmic microwave background temperature maps or in 3D galaxy catalogues. It can be approximated, except in the remote positive tail, by a negative Weibull-type form, converging slowly to the expected Gumbel-type form for infinitely large patch size. Convergence is facilitated when large-scale correlations are weaker.
We compare the analytic predictions to numerical experiments for the case of a scale-free Gaussian field in two dimensions, achieving impressive agreement between approximate theory and measurements. We also discuss the generalization of our formalism to non-Gaussian fields.
Modelling Lyman α forest cross-correlations with LyMAS Lochhaas, Cassandra; Weinberg, David H; Peirani, Sébastien ...
Monthly notices of the Royal Astronomical Society,
10/2016, Letnik:
461, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We use the Lyα Mass Association Scheme (LyMAS) to predict cross-correlations at z = 2.5 between dark matter haloes and transmitted flux in the Lyα forest, and compare to cross-correlations measured ...for quasars and damped Lyα systems (DLAs) from the Baryon Oscillation Spectroscopic Survey (BOSS) by Font-Ribera et al. We calibrate LyMAS using Horizon-AGN hydrodynamical cosmological simulations of a (100 h
− 1 Mpc)3 comoving volume. We apply this calibration to a (1 h
− 1 Gpc)3 simulation realized with 20483 dark matter particles. In the 100 h
− 1 Mpc box, LyMAS reproduces the halo-flux correlations computed from the full hydrodynamic gas distribution very well. In the 1 h
− 1 Gpc box, the amplitude of the large-scale cross-correlation tracks the halo bias b
h as expected. We provide empirical fitting functions that describe our numerical results. In the transverse separation bins used for the BOSS analyses, LyMAS cross-correlation predictions follow linear theory accurately down to small scales. Fitting the BOSS measurements requires inclusion of random velocity errors; we find best-fitting rms velocity errors of 399 and
$252\ \rm {km}\ \rm {s}^{-1}$
for quasars and DLAs, respectively. We infer bias-weighted mean halo masses of
$M_{\rm h}/10^{12}\ h^{-1}\,\mathrm{M}_{\odot }=2.19^{+0.16}_{-0.15}$
and
$0.69^{+0.16}_{-0.14}$
for the host haloes of quasars and DLAs, with ∼0.2 dex systematic uncertainty associated with redshift evolution, intergalactic medium parameters, and selection of data fitting range.
ABSTRACT
The power spectrum response function of the large-scale structure of the Universe describes how the evolved power spectrum is modified by a small change in initial power through non-linear ...mode coupling of gravitational evolution. It was previously found that the response function for the coupling from small to large scales is strongly suppressed in amplitude, especially at late times, compared to predictions from perturbation theory (PT) based on the single-stream approximation. One obvious explanation for this is that PT fails to describe the dynamics beyond shell crossing. We test this idea by comparing measurements in N-body simulations to prescriptions based on PT but augmented with adaptive smoothing to account for the formation of non-linear structures of various sizes in the multistream regime. We first start with one-dimensional (1D) cosmology, where the Zel’dovich approximation provides the exact solution in the single-stream regime. Similarly to the three-dimensional (3D) case, the response function of the large-scale modes exhibits a strong suppression in amplitude at small scales that cannot be explained by the Zel’dovich solution alone. However, by performing adaptive smoothing of initial conditions to identify haloes of different sizes and solving approximately post-collapse dynamics in the three-stream regime, agreement between theory and simulations drastically improves. We extend our analyses to the 3D case using the pinocchio algorithm, in which similar adaptive smoothing is implemented on the Lagrangian PT fields to identify haloes and is combined with a spherical halo prescription to account for post-collapse dynamics. Again, a suppression is found in the coupling between small- and large-scale modes and the agreement with simulations is improved.
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