4,412 research outputs found
Automated Detection of Voids in Redshift Surveys
We present a new void search algorithm for automated detection of voids in
three-dimensional redshift surveys. Based on a model in which the main features
of the LSS of the Universe are voids and walls, we classify the galaxies into
wall galaxies and field galaxies and we define voids as continuous volumes that
are devoid of any wall galaxies. Field galaxies are allowed within the voids.
The algorithm makes no assumptions regarding the shapes of the voids and the
only constraint that is imposed is that the voids are always thicker than a
preset limit, thus eliminating connections between adjacent voids through smal
breaches in the walls. By appropriate scaling of the parameters with the
selection function this algorithm can be used to analyze flux-limited surveys.
We test the algorithm on Voronoi tessellations and apply it to the SSRS2
redshift survey to derive the spectrum of void sizes and other void properties.
We find that the average diameter of a void is 37\pm 8 \h Mpc. We suggest the
usage of this fully automated algorithm to derive a quantitative description of
the voids, as another tool in describing the large scale structure of the
Universe and for comparison with numerical simulations.Comment: Accepted for publication in ApJ Letters; 13 page AAS latex file
including 1 table, 3 PS figures. Complete uuencoded compressed PostScript
file is available at ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers.htm
A catalogue of the voids in the IRAS 1.2-Jy survey
Using the VOID FINDER algorithm we have compiled a catalogue of voids in the
IRAS 1.2-Jy sample. The positions of the voids correspond well to underdense
regions seen in the IRAS smoothed density map. However, since in our analysis
no smoothing is used, all structures appear much sharper: walls are not smeared
and the voids are not artificially reduced by them. Therefore the current
method based on the galaxy point distribution is better suited to determine the
diameter of voids in the galaxy distribution. We have identified 24 voids,
covering more than 30% of the volume considered. By comparing the results with
equivalent random catalogues we have determined that 12 voids are significant
at a 0.95 confidence level, having an average diameter of 40+-6 h^{-1} Mpc. Our
results serve not only for charting the cosmography of the nearby Universe, but
also to give support to the results recently obtained with the SSRS2 sample,
suggesting a void-filled Universe. Moreover, our results indicate that the
voids detected have a similar scale, demonstrating that both optically and
IRAS-selected galaxies delineate the same large-scale structures.Comment: Revised, matches the published MNRAS version, with some color
figures. 9 pages, MN LaTeX file, using EPSFIG, with 1 table, 5 PostScript
figures. Complete gzipped version is available at
http://shemesh.fiz.huji.ac.il/hagai/; uuencoded file is available at
http://shemesh.fiz.huji.ac.il/papers/epd2.uu or ftp://shemesh.fiz.huji.ac.i
On the utilization of meso-scale models for offshore wind atlases
Two different offshore wind atlases based on
the meso-scale model WRF are presented and
discussed in this paper. The Work is part of the EU-funded project NORSEWIND (Northern
Seas Wind Index Database). Validations show
that annual average wind speeds and windroses
at hub-height (100m) are well represented
by the model, while the model accuracy is
poorer for vertical wind profile, wind shear
parameters and static stability
Tactical Voting in Plurality Elections
How often will elections end in landslides? What is the probability for a
head-to-head race? Analyzing ballot results from several large countries rather
anomalous and yet unexplained distributions have been observed. We identify
tactical voting as the driving ingredient for the anomalies and introduce a
model to study its effect on plurality elections, characterized by the relative
strength of the feedback from polls and the pairwise interaction between
individuals in the society. With this model it becomes possible to explain the
polarization of votes between two candidates, understand the small margin of
victories frequently observed for different elections, and analyze the polls'
impact in American, Canadian, and Brazilian ballots. Moreover, the model
reproduces, quantitatively, the distribution of votes obtained in the Brazilian
mayor elections with two, three, and four candidates.Comment: 7 pages, 4 figure
The formation of voids in a universe with cold dark matter and a cosmological constant
A spherical Lagrangian hydrodynamical code has been written to study the
formation of cosmological structures in the early Universe. In this code we
take into account the presence of collisionless non-baryonic cold dark matter
(CDM), the cosmological constant and a series of physical processes present
during and after the recombination era, such as photon drag resulting from the
cosmic background radiation and hydrogen molecular production. We follow the
evolution of the structure since the recombination era until the present epoch.
As an application of this code we study the formation of voids starting from
negative density perturbations which evolved during and after the recombination
era. We analyse a set of COBE-normalized models, using different spectra to see
their influence on the formation of voids. Our results show that large voids
with diameters ranging from 10h^{-1} Mpc up to 50h^{-1} Mpc can be formed in a
universe model dominated by the cosmological constant (\Omega_\Lambda ~ 0.8).
This particular scenario is capable of forming large and deep empty regions
(with density contrasts \delta < -0.6). Our results also show that the physical
processes acting on the baryonic matter produce a transition region where the
radius of the dark matter component is greater than the baryonic void radius.
The thickness of this transition region ranges from about tens of kiloparsecs
up to a few megaparsecs, depending on the spectrum considered. Putative objects
formed near voids and within the transition region would have a different
amount of baryonic/dark matter when compared with \Omega_b/\Omega_d. If one
were to use these galaxies to determine, by dynamical effects or other
techniques, the quantity of dark matter present in the Universe, the result
obtained would be only local and not representative of the Universe as a whole.Comment: MNRAS (in press); 9 pages, no figure
A hierarchy of voids: Much ado about nothing
We present a model for the distribution of void sizes and its evolution in
the context of hierarchical scenarios of gravitational structure formation. We
find that at any cosmic epoch the voids have a size distribution which is
well-peaked about a characteristic void size which evolves self-similarly in
time. This is in distinct contrast to the distribution of virialized halo
masses which does not have a small-scale cut-off.
In our model, the fate of voids is ruled by two processes. The first process
affects those voids which are embedded in larger underdense regions: the
evolution is effectively one in which a larger void is made up by the mergers
of smaller voids, and is analogous to how massive clusters form from the
mergers of less massive progenitors. The second process is unique to voids, and
occurs to voids which happen to be embedded within a larger scale overdensity:
these voids get squeezed out of existence as the overdensity collapses around
them. It is this second process which produces the cut-off at small scales.
In the excursion set formulation of cluster abundance and evolution, solution
of the cloud-in-cloud problem, i.e., counting as clusters only those objects
which are not embedded in larger clusters, requires study of random walks
crossing one barrier. We show that a similar formulation of void evolution
requires study of a two-barrier problem: one barrier is required to account for
voids-in-voids, and the other for voids-in-clouds. Thus, in our model, the void
size distribution is a function of two parameters, one of which reflects the
dynamics of void formation, and the other the formation of collapsed objects.Comment: 23 pages, 9 figures, submitted to MNRA
Photometric Properties of Void Galaxies in the Sloan Digital Sky Survey DR7 Data Release
Using the sample presented in Pan:2011, we analyse the photometric properties
of 88,794 void galaxies and compare them to galaxies in higher density
environments with the same absolute magnitude distribution. In Pan et al.
(2011), we found a total of 1054 dynamically distinct voids in the SDSS with
radius larger than 10h^-1 Mpc. The voids are underdense, with delta rho/rho <
-0.9 in their centers. Here we study the photometric properties of these void
galaxies. We look at the u - r colours as an indication of star formation
activity and the inverse concentration index as an indication of galaxy type.
We find that void galaxies are statistically bluer than galaxies found in
higher density environments with the same magnitude distribution. We examine
the colours of the galaxies as a function of magnitude, and we fit each colour
distribution with a double-Gaussian model for the red and blue subpopulations.
As we move from bright to dwarf galaxies, the population of red galaxies
steadily decreases and the fraction of blue galaxies increases in both voids
and walls, however the fraction of blue galaxies in the voids is always higher
and bluer than in the walls. We also split the void and wall galaxies into
samples depending on galaxy type. We find that late type void galaxies are
bluer than late type wall galaxies and the same holds for early galaxies. We
also find that early type, dwarf void galaxies are blue in colour. We also
study the properties of void galaxies as a function of their distance from the
center of the void. We find very little variation in the properties, such as
magnitude, colour and type, of void galaxies as a function of their location in
the void. The only exception is that the dwarf void galaxies may live closer to
the center. The centers of voids have very similar density contrast and hence
all void galaxies live in very similar density environments (ABRIDGED)Comment: 10 pages, 25 figure
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