5,668 research outputs found
A Cosmic Watershed: the WVF Void Detection Technique
On megaparsec scales the Universe is permeated by an intricate filigree of
clusters, filaments, sheets and voids, the Cosmic Web. For the understanding of
its dynamical and hierarchical history it is crucial to identify objectively
its complex morphological components. One of the most characteristic aspects is
that of the dominant underdense Voids, the product of a hierarchical process
driven by the collapse of minor voids in addition to the merging of large ones.
In this study we present an objective void finder technique which involves a
minimum of assumptions about the scale, structure and shape of voids. Our void
finding method, the Watershed Void Finder (WVF), is based upon the Watershed
Transform, a well-known technique for the segmentation of images. Importantly,
the technique has the potential to trace the existing manifestations of a void
hierarchy. The basic watershed transform is augmented by a variety of
correction procedures to remove spurious structure resulting from sampling
noise. This study contains a detailed description of the WVF. We demonstrate
how it is able to trace and identify, relatively parameter free, voids and
their surrounding (filamentary and planar) boundaries. We test the technique on
a set of Kinematic Voronoi models, heuristic spatial models for a cellular
distribution of matter. Comparison of the WVF segmentations of low noise and
high noise Voronoi models with the quantitatively known spatial characteristics
of the intrinsic Voronoi tessellation shows that the size and shape of the
voids are succesfully retrieved. WVF manages to even reproduce the full void
size distribution function.Comment: 24 pages, 15 figures, MNRAS accepted, for full resolution, see
http://www.astro.rug.nl/~weygaert/tim1publication/watershed.pd
The Universality of the Stellar IMF
We propose that the stellar initial mass function (IMF) is universal in the
sense that its functional form arises as a consequence of the statistics of
random supersonic flows.
A model is developed for the origin of the stellar IMF, that contains a
dependence on the average physical parameters (temperature, density, velocity
dispersion) of the large scale site of star formation. The model is based on
recent numerical experiments of highly supersonic random flows that have a
strong observational counterpart.
It is shown that a Miller-Scalo like IMF is naturally produced by the model
for the typical physical conditions in molecular clouds. A more ``massive'' IMF
in star bursts is also predicted.Comment: 22 pages; Latex; 6 figures included. MNRAS (in press
Averaging over Cosmic Structure: Cosmological Backreaction and the Gauge Problem
The observation that accelerated cosmic expansion appears to start around the
time that nonlinear cosmic structure is appearing seems like an extraordinary
coincidence, unless the acceleration is somehow driven by the emergence of the
structure. That has given rise to the controversial concept of a gravitational
backreaction through which inhomogeneity becomes a driver of accelerated
expansion. The standard route when studying strongly inhomogeneous cosmological
models is to take either a perturbative approach or a spatial averaging
approach. Here we argue that because backreaction is in fact a nonlinear
multiscale phenomenon, perturbative approaches may have a limited validity.
With respect to the currently proposed averaging approaches we here show that
they lead to gauge dependent backreaction and hence ambiguous estimates of its
magnitude. In the present study, we formalise inhomogeneous cosmic evolution
within the framework of foliations of spacetime. Fixing a foliation amounts to
making a gauge choice. Addressing the correspondence between the metric tensor
and the foliation allows us to clarify the implications of choosing a foliation
for the representation of equivalent cosmologies. It is important to note that,
within the context of backreaction, this formalism allows us to discuss the
vagaries of averaging in the framework of spacetime foliations. It reveals that
spatial averaging can induce artificial, i.e. gauge dependent, backreaction
terms that arise from any specific choice of gauge. Averaging methods presented
so far all encounter this problem. However, within our foliation framework, we
can produce a gauge invariant method of averaging by invoking the
gauge-invariant Bardeen formalism for cosmological perturbation theory. We
demonstrate that this implies the gauge invariance of the averaging procedure.
This makes it applicable to standard cosmological simulations.Comment: 21 pages, 3 figure
Caught in the cosmic web:Environmental effect on halo concentrations, shape, and spin
Using a set of high-resolution simulations we study the statistical
correlation of dark matter halo properties with the large-scale environment. We
consider halo populations split into four Cosmic Web (CW) elements: voids,
walls, filaments, and nodes. For the first time we present a study of CW
effects for halos covering six decades in mass: . We find that the fraction of halos living in various web
components is a strong function of mass, with the majority of
halos living in filaments and nodes. Low
mass halos are more equitably distributed in filaments, walls, and voids. For
halo density profiles and formation times we find a universal mass threshold of
below which these properties
vary with environment. Here, filament halos have the steepest
concentration-mass relation, walls are close to the overall mean, and void
halos have the flattest relation. This amounts to for filament and
void halos that are respectively higher and lower than the mean at
, with low-mass node halos being most
likely splashed-back. We find double power-law fits that very well describe
for the four environments in the whole probed mass range. A
complementary picture is found for the average formation times, with the
mass-formation time relations following trends shown for the concentrations:
the nodes halos being the oldest and void halo the youngest. The CW
environmental effect is much weaker when studying the halo spin and shapes. The
trends with halo mass is reversed: the small halos with seem to be unaffected by the CW environment. Some weak trends are
visible for more massive void and walls halos, which, on average, are
characterized by lower spin and higher triaxiality parameters.Comment: 18 pages, 9 figures, match the published version in Physical Review D
eid. 06351
Alignments of Voids in the Cosmic Web
We investigate the shapes and mutual alignment of voids in the large scale
matter distribution of a LCDM cosmology simulation. The voids are identified
using the novel WVF void finder technique. The identified voids are quite
nonspherical and slightly prolate, with axis ratios in the order of c:b:a
approx. 0.5:0.7:1. Their orientations are strongly correlated with significant
alignments spanning scales >30 Mpc/h.
We also find an intimate link between the cosmic tidal field and the void
orientations. Over a very wide range of scales we find a coherent and strong
alignment of the voids with the tidal field computed from the smoothed density
distribution. This orientation-tide alignment remains significant on scales
exceeding twice the typical void size, which shows that the long range external
field is responsible for the alignment of the voids. This confirms the view
that the large scale tidal force field is the main agent for the large scale
spatial organization of the Cosmic Web.Comment: 10 pages, 4 figures, submitted to MNRAS, for high resolution version,
see http://www.astro.rug.nl/~weygaert/tim1publication/voidshape.pd
Structure functions and form factors close to the chiral limit from lattice QCD
Results for nucleon matrix elements (arising from moments of structure
functions) and form factors from a mixture of runs using Wilson, clover and
overlap fermions (both quenched and unquenched) are presented and compared in
an effort to explore the size of the chiral `regime', lattice spacing errors
and quenching artefacts. While no run covers this whole range of effects the
partial results indicate a picture of small lattice spacing errors, small
quenching effects and only reaching the chiral regime at rather light quark
masses.Comment: 7 pages, 7 figures; contribution to the 2003 Workshop on Lattice
Hadron Physics, Cairns, Australia; error in Fig. 4 corrected; minor text
change
Sub-Megaparsec Individual Photometric Redshift Estimation from Cosmic Web Constraints
We present a method, PhotoWeb, for estimating photometric redshifts of
individual galaxies, and their equivalent distance, with megaparsec and even
sub-megaparsec accuracy using the Cosmic Web as a constraint over photo-z
estimates. PhotoWeb redshift errors for individual galaxies are of the order of
delta_z = 0.0007, compared to errors of delta_z = 0.02 for current photo-z
techniques. The mean redshift error is of the order of 0.00005-0.0004 compared
to mean errors in the range delta_z =z 0.001-0.01 for the best available
photo-z estimates in the literature. Current photo-z techniques based on the
spectral energy distribution of galaxies and projected clustering produce
redshift estimates with large errors due to the poor constraining power the
galaxy's spectral energy distribution and projected clustering can provide. The
Cosmic Web, on the other hand, provides the strongest constraints on the
position of galaxies. The network of walls, filaments and voids occupy ~%10 of
the volume of the Universe, yet they contain ~%95 of galaxies. The cosmic web,
being a cellular system with well-defined boundaries, sets a restricted set of
intermittent positions a galaxy can occupy along a given line-of-sight. Using
the information in the density field computed from spectroscopic redshifts we
can narrow the possible locations of a given galaxy along the line of sight
from a single broad probability distribution (from photo-z) to one or a few
narrow peaks. Our first results improve previous photo-z errors by more than
one order of magnitude allowing sub-megaparsec errors in some cases. Such
accurate estimates for tens of millions of galaxies will allow unprecedented
galaxy-LSS studies. In this work we apply our technique to the SDSS photo-z
galaxy sample and discuss its performance and future improvements.Comment: Final version submitted to MNRA
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