We compare the galaxy population of a smoothed particle hydrodynamics (SPH) simulation to those predicted by the GalICS (Galaxies In Cosmological Simulations) N-body + semi-analytic model and a ...stripped down version of GalICS that omits the effects of supernova and active galactic nucleus (AGN) feedback. The SPH simulation and the no-feedback GalICS model make similar predictions for the baryonic mass functions of galaxies and for the dependence of these mass functions on environment and redshift. The two methods also make similar predictions for the galaxy content of dark matter haloes as a function of halo mass and for the gas accretion history of galaxies. There is a fairly good correspondence between the ‘cold’ and ‘hot’ accretion modes of the SPH simulation and the rapid and slow cooling regimes of the GalICS calculation. Both the SPH and no-feedback GalICS models predict a bimodal galaxy population at z= 0. The ‘red’ sequence of gas poor, old galaxies is populated mainly by satellite systems, which are starved of fresh gas after they begin orbiting in larger haloes, while, contrary to observations, the central galaxies of massive haloes lie on the ‘blue’ star-forming sequence as a result of continuing hot gas accretion at late times. Furthermore, both models overpredict the observed baryonic mass function, especially at the high-mass end. In the full GalICS model, supernova-driven outflows reduce the masses of low and intermediate mass galaxies by about a factor of 2. AGN feedback suppresses gas cooling in large haloes, producing a sharp cut-off in the baryonic mass function and moving the central galaxies of these massive haloes to the red sequence. Our results imply that the observational failings of the SPH simulation and the no-feedback GalICS model are a consequence of missing input physics rather than computational inaccuracies. Truncating the accretion of gas in satellite galaxies automatically produces a bimodal distribution with a quenched population, but explaining the star formation shutdown in the most massive galaxies requires a mechanism like AGN feedback that suppresses the accretion of gas on to central galaxies in large haloes.
Reconstruction of primordial density fields Mohayaee, Roya; Mathis, Hugues; Colombi, Stéphane ...
Monthly notices of the Royal Astronomical Society,
January 2006, Letnik:
365, Številka:
3
Journal Article
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The Monge-Ampère-Kantorovich (MAK) reconstruction is tested against cosmological N-body simulations. Using only the present mass distribution sampled with particles, and the assumption of homogeneity ...of the primordial distribution, MAK recovers for each particle the non-linear displacement field between its present position and its Lagrangian position on a primordial uniform grid. To test the method, we examine a standard Lambda cold dark matter (λCDM) N-body simulation with Gaussian initial conditions and six models with non-Gaussian initial conditions: a χ2 model, a model with primordial voids and four weakly non-Gaussian models. Our extensive analyses of the Gaussian simulation show that the level of accuracy of the reconstruction of the non-linear displacement field achieved by MAK is unprecedented, at scales as small as ∼3 h−1 Mpc. In particular, it captures in a non-trivial way the non-linear contribution from gravitational instability, well beyond the Zel'dovich approximation. This is also confirmed by our analyses of the non-Gaussian samples. Applying the spherical collapse model to the probability distribution function of the divergence of the displacement field, we also show that from a well-reconstructed displacement field, such as that given by MAK, it is possible to accurately disentangle dynamical contributions induced by gravitational clustering from possible initial non-Gaussianities, allowing one to efficiently test the non-Gaussian nature of the primordial fluctuations. In addition, we test successfully a simple application of MAK using the Zel'dovich approximation to recover in real space the present-day peculiar velocity field on scales of 8 h−1 Mpc. Although non-trivial observational issues yet remain to be addressed, our numerical investigations suggest that MAK reconstruction represents a very promising tool to be applied to three-dimensional Galaxy catalogues.
We revisit, with a view to refinement and generalization, the elegant waterbag method for the numerical treatment of Vlasov–Poisson equations. In this method, the phase space is decomposed into ...patches of constant density, and by exploiting Liouville’s theorem, the dynamics is reduced to the evolution of the boundary of these patches (waterbags). We follow the boundary using an adaptive, oriented polygon, and recover the force by circulating along this polygon. We discuss sampling of initial conditions with a set of oriented isocontours, and propose a new refinement procedure for accurate rendering of the stretching and folding polygon. Time evolution is naturally undertaken with symplectic algorithms. Tools, initially developed for systems of self-gravitating sheets, generalize naturally to spherically symmetric systems. We conclude with examples of both cases.
The results from weak gravitational lensing analyses are subject to a cosmic variance error term that has previously been estimated assuming Gaussian statistics. In this Letter we address the issue ...of estimating cosmic variance errors for weak lensing surveys in the non-Gaussian regime. Using standard cold dark matter model ray-tracing simulations characterized by Ωm = 0.3, ΩΛ = 0.7, h = 0.7 and σ8 = 1 for different survey redshifts zs, we determine the variance of the two-point shear correlation function measured across 64 independent lines of sight. We compare the measured variance to the variance expected from a random Gaussian field and derive a redshift-dependent non-Gaussian calibration relation. We find that the ratio between the non-Gaussian and Gaussian variance at 1 arcmin can be as high as ∼30 for a survey with source redshift zs ∼ 0.5 and ∼10 for zs ∼ 1. The transition scale ϑc above which the ratio is consistent with unity is found to be ϑc ∼ 20 arcmin for zs ∼ 0.5 and ϑc∼ 10 arcmin for zs∼ 1. We provide fitting formulae to our results permitting the estimation of non-Gaussian cosmic variance errors, and discuss the impact on current and future surveys. A more extensive set of simulations will, however, be required to investigate the dependence of our results on cosmology, specifically on the amplitude of clustering.
We present a new method to calculate pixel space correlation functions via fast spherical harmonics transforms. Our present implementation of the algorithm extracts correlations from a Microwave ...Anisotropy Probe (MAP)-like cosmic microwave background map (~=3×106 pixels) in about 5 minutes on a 500 MHz computer, including Cl inversion; the analysis of one Planck-like map takes less than 1 hr. We use heuristic window and noise weighting in pixel space and include the possibility of additional signal weighting as well, either in l-space or pixel space. We apply the new code to an ensemble of MAP simulations to test the response of our method to the inhomogenous sky coverage/noise of MAP. We show that the resulting Cl and the corresponding variance are nearly optimal. The HEALPIX-based implementation of the method, SpICE (Spatially Inhomogenous Correlation Estimator), will be available to the public from the authors.
Cosmic velocity–gravity relation in redshift space Colombi, Stéphane; Chodorowski, Michał J.; Teyssier, Romain
Monthly notices of the Royal Astronomical Society,
2007, Letnik:
375, Številka:
1
Journal Article
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We propose a simple way to estimate the parameter β≃Ω0.6/b from 3D galaxy surveys, where Ω is the non-relativistic matter-density parameter of the Universe and b is the bias between the galaxy ...distribution and the total matter distribution. Our method consists in measuring the relation between the cosmological velocity and gravity fields, and thus requires peculiar velocity measurements. The relation is measured directly in redshift space, so there is no need to reconstruct the density field in real space. In linear theory, the radial components of the gravity and velocity fields in redshift space are expected to be tightly correlated, with a slope given, in the distant observer approximation, by We test extensively this relation using controlled numerical experiments based on a cosmological N-body simulation. To perform the measurements, we propose a new and rather simple adaptive interpolation scheme to estimate the velocity and the gravity field on a grid. One of the most striking results is that non-linear effects, including ‘fingers of God’, affect mainly the tails of the joint probability distribution function (PDF) of the velocity and gravity field: the 1–1.5 σ region around the maximum of the PDF is dominated by the linear theory regime, both in real and redshift space. This is understood explicitly by using the spherical collapse model as a proxy of non-linear dynamics. Applications of the method to real galaxy catalogues are discussed, including a preliminary investigation on homogeneous (volume-limited) ‘galaxy’ samples extracted from the simulation with simple prescriptions based on halo and substructure identification, to quantify the effects of the bias between the galaxy distribution and the total matter distribution, as well as the effects of shot noise.
We propose a new semi-Lagrangian Vlasov–Poisson solver. It employs metric elements to follow locally the flow and its deformation, allowing one to find quickly and accurately the initial phase-space ...position
$\boldsymbol{Q}(\boldsymbol{P})$
of any test particle
$\boldsymbol{P}$
, by expanding at second order the geometry of the motion in the vicinity of the closest element. It is thus possible to reconstruct accurately the phase-space distribution function at any time
$t$
and position
$\boldsymbol{P}$
by proper interpolation of initial conditions, following Liouville theorem. When distortion of the elements of metric becomes too large, it is necessary to create new initial conditions along with isotropic elements and repeat the procedure again until next resampling. To speed up the process, interpolation of the phase-space distribution is performed at second order during the transport phase, while third-order splines are used at the moments of remapping. We also show how to compute accurately the region of influence of each element of metric with the proper percolation scheme. The algorithm is tested here in the framework of one-dimensional gravitational dynamics but is implemented in such a way that it can be extended easily to four- or six-dimensional phase space. It can also be trivially generalised to plasmas.
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