We report the first measurement of the diminutive lensing signal arising from matter underdensities associated with cosmic voids. While undetectable individually, by stacking the weak gravitational ...shear estimates around 901 voids detected in Sloan Digital Sky Survey DR7 by Sutter et al., we find substantial evidence for a depression of the lensing signal compared to the cosmic mean. This depression is most pronounced at the void radius, in agreement with analytical models of void matter profiles. Even with the largest void sample and imaging survey available today, we cannot put useful constraints on the radial dark matter void profile. We invite independent investigations of our findings by releasing data and analysis code to the public at https://github.com/pmelchior/void-lensing.
We perform an Alcock–Paczyński test using stacked cosmic voids identified in the Sloan Digital Sky Survey (SDSS) Data Release 7 main sample and Data Release 10 LOWZ and CMASS samples. We find ∼1500 ...voids out to redshift 0.6 using a heavily modified and extended version of the watershed algorithm zobov, which we call vide (Void IDentification and Examination). To assess the impact of peculiar velocities, we use the mock void catalogues presented in Sutter et al. We find a constant uniform flattening of 14 per cent along the line of sight when peculiar velocities are included. This flattening appears universal for all void sizes at all redshifts and for all tracer densities. We also use these mocks to identify an optimal stacking strategy. After correcting for systematic effects, we find that our Alcock–Paczyński measurement leads to a preference of our best-fitting value of ΩM ∼ 0.15 over ΩM = 1.0 by a likelihood ratio of 10. Likewise, we find a factor of 4.5 preference of the likelihood ratio for a Λ cold dark matter ΩM = 0.3 model and a null measurement. Taken together, we find substantial evidence for the Alcock–Paczyński signal in our sample of cosmic voids. Our assessment using realistic mocks suggests that measurements with future SDSS releases and other surveys will provide tighter cosmological parameter constraints. The void-finding algorithm and catalogues used in this work will be made publicly available at http://www.cosmicvoids.net.
We produce the most comprehensive public void catalog to date using the Sloan Digital Sky Survey Data Release 7 main sample out to redshift z = 0.2 and the luminous red galaxy sample out to z = 0.44. ...Using a modified version of the parameter-free void finder ZOBOV, we fully take into account the presence of the survey boundary and masks. Our strategy for finding voids is thus appropriate for any survey configuration. We produce two distinct catalogs: a complete catalog including voids near any masks, which would be appropriate for void galaxy surveys, and a bias-free catalog of voids away from any masks, which is necessary for analyses that require a fair sampling of void shapes and alignments. Our discovered voids have effective radii from 5 to 135 h super(-1) Mpc. We discuss basic catalog statistics such as number counts and redshift distributions and describe some additional data products derived from our catalog, such as radial density profiles and projected density maps. We find that radial profiles of stacked voids show a qualitatively similar behavior across nearly two decades of void radii and throughout the full redshift range.
The ever increasing size and complexity of data coming from simulations of cosmic structure formation demand equally sophisticated tools for their analysis. During the past decade, the art of object ...finding in these simulations has hence developed into an important discipline itself. A multitude of codes based upon a huge variety of methods and techniques have been spawned yet the question remained as to whether or not they will provide the same (physical) information about the structures of interest. Here we summarize and extent previous work of the 'halo finder comparison project': we investigate in detail the (possible) origin of any deviations across finders. To this extent, we decipher and discuss differences in halo-finding methods, clearly separating them from the disparity in definitions of halo properties. We observe that different codes not only find different numbers of objects leading to a scatter of up to 20 per cent in the halo mass and V
max function, but also that the particulars of those objects that are identified by all finders differ. The strength of the variation, however, depends on the property studied, e.g. the scatter in position, bulk velocity, mass and the peak value of the rotation curve is practically below a few per cent, whereas derived quantities such as spin and shape show larger deviations. Our study indicates that the prime contribution to differences in halo properties across codes stems from the distinct particle collection methods and - to a minor extent - the particular aspects of how the procedure for removing unbound particles is implemented. We close with a discussion of the relevance and implications of the scatter across different codes for other fields such as semi-analytical galaxy formation models, gravitational lensing and observables in general.
A key physical quantity during reionization is the size of H ii regions. Previous studies found a characteristic bubble size which increases rapidly during reionization, with apparent agreement ...between simulations and analytic excursion set theory. Using four different methods, we critically examine this claim. In particular, we introduce the use of the watershed algorithm – widely used for void finding in galaxy surveys – which we show to be an unbiased method with the lowest dispersion and best performance on Monte Carlo realizations of a known bubble size probability density function (PDF). We find that a friends-of-friends algorithm declares most of the ionized volume to be occupied by a network of volume-filling regions connected by narrow tunnels. For methods tuned to detect the volume-filling regions, previous apparent agreement between simulations and theory is spurious, and due to a failure to correctly account for the window function of measurement schemes. The discrepancy is already obvious from visual inspection. Instead, H ii regions in simulations are significantly larger (by factors of 10–1000 in volume) than analytic predictions. The size PDF is narrower, and evolves more slowly with time, than predicted. It becomes more sharply peaked as reionization progresses. These effects are likely caused by bubble mergers, which are inadequately modelled by analytic theory. Our results have important consequences for high-redshift 21 cm observations, the mean free path of ionizing photons, and the visibility of Lyα emitters, and point to a fundamental failure in our understanding of the characteristic scales of the reionization process.
We report on the first application of the Alcock-Paczynski test to stacked voids in spectroscopic galaxy redshift surveys. We use voids from the Sutter et al. void catalog, which was derived from the ...Sloan Digital Sky Survey Data Release 7 main sample and luminous red galaxy catalogs. The construction of that void catalog removes potential shape measurement bias by using a modified version of the ZOBOV algorithm and by removing voids near survey boundaries and masks. We apply the shape-fitting procedure presented in Lavaux & Wandelt to 10 void stacks out to redshift z = 0.36. Combining these measurements, we determine the mean cosmologically induced "stretch" of voids in three redshift bins, with 1sigma errors of 5%-15%. The mean stretch is consistent with unity, providing no indication of a distortion induced by peculiar velocities. While the statistical errors are too large to detect the Alcock-Paczynski effect over our limited redshift range, this proof-of-concept analysis defines procedures that can be applied to larger spectroscopic galaxy surveys at higher redshifts to constrain dark energy using the expected statistical isotropy of structures that are minimally affected by uncertainties in galaxy velocity bias.
Haloes gone MAD: The Halo-Finder Comparison Project Knebe, Alexander; Knollmann, Steffen R.; Muldrew, Stuart I. ...
Monthly notices of the Royal Astronomical Society,
08/2011, Letnik:
415, Številka:
3
Journal Article
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ABSTRACT
We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including ...friends‐of‐friends, spherical‐overdensity and phase‐space‐based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large‐scale structure of the universe.
All the halo‐finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30–40 particles. However, also here the phase‐space finders excelled by resolving substructure down to 10–20 particles. By comparing the halo finders using a high‐resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve).
We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge.
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