6,080 research outputs found
The Size and Shape of Local Voids
We study the size and shape of low density regions in the local universe
which we identify in the smoothed density field of the PSCz flux limited IRAS
galaxy catalogue. After quantifying the systematic biases that enter in the
detection of voids using our data set and method, we identify, using a
smoothing length of 5 Mpc, 14 voids within 80 Mpc and using a
smoothing length of 10 Mpc, 8 voids within 130 Mpc. We study
the void size distribution and morphologies and find that there is roughly an
equal number of prolate and oblate-like spheroidal voids. We compare the
measured PSCz void shape and size distributions with those expected in six
different CDM models and find that only the size distribution can discriminate
between models. The models preferred by the PSCz data are those with
intermediate values of , independent of cosmology.Comment: final version, Accepted in MNRA
AG Agriculture CK Coyote brush AK Alkaline flats CL Wedgeleaf ceanothus
AN Mendocino manzanita CM Upper montane mixed shru
The ecology of seamounts: structure, function, and human impacts.
In this review of seamount ecology, we address a number of key scientific issues concerning the structure and function of benthic communities, human impacts, and seamount management and conservation. We consider whether community composition and diversity differ between seamounts and continental slopes, how important dispersal capabilities are in seamount connectivity, what environmental factors drive species composition and diversity, whether seamounts are centers of enhanced biological productivity, and whether they have unique trophic architecture. We discuss how vulnerable seamount communities are to fishing and mining, and how we can balance exploitation of resources and conservation of habitat. Despite considerable advances in recent years, there remain many questions about seamount ecosystems that need closer integration of molecular, oceanographic, and ecological research
On an Analytical Framework for Voids: Their abundances, density profiles and local mass functions
We present a general analytical procedure for computing the number density of
voids with radius above a given value within the context of gravitational
formation of the large scale structure of the universe out of Gaussian initial
conditions. To this end we develop an accurate (under generally satisfied
conditions) extension of unconditional mass function to constrained
environments, which allowes us both to obtain the number density of collapsed
objects of certain mass at any distance from the center of the void, and to
derive the number density of voids defined by those collapsed objects. We have
made detailed calculations for the spherically averaged mass density and halo
number density profiles for individual voids. We also present a formal
expression for the number density of voids defined by galaxies of a given type
and luminosity. This expression contains the probability for a collapsed object
of certain mass to host a galaxy of that type and luminosity as a function of
the environmental density. We propose a procedure to infer this function, which
may provide useful clues as to the galaxy formation process, from the observed
void densities.Comment: 14 pages, 7 figures, MNRAS in pres
The DEEP2 Galaxy Redshift Survey: The Evolution of Void Statistics from z~1 to z~0
We present measurements of the void probability function (VPF) at z~1 using
data from the DEEP2 Redshift Survey and its evolution to z~0 using data from
the Sloan Digital Sky Survey (SDSS). We measure the VPF as a function of galaxy
color and luminosity in both surveys and find that it mimics trends displayed
in the two-point correlation function, ; namely that samples of brighter,
red galaxies have larger voids (i.e. are more strongly clustered) than fainter,
blue galaxies. We also clearly detect evolution in the VPF with cosmic time,
with voids being larger in comoving units at z~0. We find that the reduced VPF
matches the predictions of a `negative binomial' model for galaxies of all
colors, luminosities, and redshifts studied. This model lacks a physical
motivation, but produces a simple analytic prediction for sources of any number
density and integrated two-point correlation function, \bar{\xi}. This implies
that differences in the VPF across different galaxy populations are consistent
with being due entirely to differences in the population number density and
\bar{\xi}. The robust result that all galaxy populations follow the negative
binomial model appears to be due to primarily to the clustering of dark matter
halos. The reduced VPF is insensitive to changes in the parameters of the halo
occupation distribution, in the sense that halo models with the same \bar{\xi}
will produce the same VPF. For the wide range of galaxies studied, the VPF
therefore does not appear to provide useful constraints on galaxy evolution
models that cannot be gleaned from studies of \bar{\xi} alone. (abridged)Comment: 17 pages, 15 figures, ApJ accepte
Evaluating regional emission estimates using the TRACE-P observations
Measurements obtained during the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) experiment are used in conjunction with regional modeling analysis to evaluate emission estimates for Asia. A comparison between the modeled values and the observations is one method to evaluate emissions. Based on such analysis it is concluded that the inventory performs well for the light alkanes, CO, ethyne, SO2, and NOₓ. Furthermore, based on model skill in predicting important photochemical species such as O₃, HCHO, OH, HO₂, and HNO₃, it is found that the emissions inventories are of sufficient quality to support preliminary studies of ozone production. These are important finding in light of the fact that emission estimates for many species (such as speciated NMHCs and BC) for this region have only recently been estimated and are highly uncertain. Using a classification of the measurements built upon trajectory analysis, we compare observed species distributions and ratios of species to those modeled and to ratios estimated from the emissions inventory. It is shown that this technique can reconstruct a spatial distribution of propane/benzene that looks remarkably similar to that calculated from the emissions inventory. A major discrepancy between modeled and observed behavior is found in the Yellow Sea, where modeled values are systematically underpredicted. The integrated analysis suggests that this may be related to an underestimation of emissions from the domestic sector. The emission is further tested by comparing observed and measured species ratios in identified megacity plumes. Many of the model derived ratios (e.g., BC/CO, SOₓ/C₂H₂) fall within ∼25% of those observed and all fall outside of a factor of 2.5. (See Article file for details of the abstract.)Department of Civil and Environmental EngineeringAuthor name used in this publication: Wang, T
Voids in the Large-Scale Structure
Voids are the most prominent feature of the LSS of the universe. Still, they
have been generally ignored in quantitative analysis of it, essentially due to
the lack of an objective tool to identify and quantify the voids. To overcome
this, we present the Void-Finder algorithm, a novel tool for objectively
quantifying galaxy voids. The algorithm classifies galaxies as either wall- or
field-galaxies. Then it identifies voids in the wall-galaxy distribution. Voids
are defined as continuous volumes that do not contain any wall-galaxies. The
voids must be thicker than an adjustable limit, which is refined in successive
iterations. We test the algorithm using Voronoi tessellations. By appropriate
scaling of the parameters we apply it to the SSRS2 survey and to the IRAS 1.2
Jy. Both surveys show similar properties: ~50% of the volume is filled by the
voids, which have a scale of at least 40 Mpc, and a -0.9 under-density. Faint
galaxies populate the voids more than bright ones. These results suggest that
both optically and IRAS selected galaxies delineate the same LSS. Comparison
with the recovered mass distribution further suggests that the observed voids
in the galaxy distribution correspond well to under-dense regions in the mass
distribution. This confirms the gravitational origin of the voids.Comment: Submitted to ApJ; 33 pages, aaspp4 LaTeX file, using epsfig and
natbib, 1 table, 12 PS 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/ep3.uu or ftp://shemesh.fiz.huji.ac.i
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