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Mark C. Neyrinck

Publications and source records attributed to Mark C. Neyrinck.

54 records · Page 3Linked to original sources
arXiv2011

Removable Matter-Power-Spectrum Covariance from Bias Fluctuations

We find a simple, accurate model for the covariance matrix of the real-space cosmological matter power spectrum on slightly nonlinear scales (k~0.1-0.8 h/Mpc at z=0), where off-diagonal matrix elements become substantial. The model includes a multiplicative, scale-independent modulation of the power spectrum. It has only one parameter, the variance (among realizations) of the variance of the nonlinear density field in cells, with little dependence on the cell size between 2-8 Mpc/h. Furthermore, we find that this extra covariance can be modeled out by instead measuring the power spectrum of (delta/sigma_cell), i.e. the ratio of the overdensity to its dispersion in cells a few Mpc in size. Dividing delta by sigma_cell essentially removes the non-Gaussian part of the covariance matrix, nearly diagonalizing it.

astro-ph.CO↗
arXiv2011

Haloes gone MAD: The Halo-Finder Comparison Project

[abridged] 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 (FOF), spherical-overdensity (SO) and phase-space based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allows 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. Via 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).

astro-ph.CO↗
arXiv2011

Rejuvenating Power Spectra II: the Gaussianized galaxy density field

We find that, even in the presence of discreteness noise, a Gaussianizing transform (producing a more-Gaussian one-point distribution) reduces nonlinearities in the power spectra of cosmological matter and galaxy density fields, in many cases drastically. Although Gaussianization does increase the effective shot noise, it also increases the power spectrum's fidelity to the linear power spectrum on scales where the shot noise is negligible. Gaussianizing also increases the Fisher information in the power spectrum in all cases and resolutions, although the gains are smaller in redshift space than in real space. We also find that the gain in cumulative Fisher information from Gaussianizing peaks at a particular grid resolution that depends on the sampling level.

astro-ph.CO↗
arXiv2010

Redshift-Space Enhancement of Line-of-Sight Baryon Acoustic Oscillations in the SDSS Main-Galaxy Sample

We show that redshift-space distortions of galaxy correlations have a strong effect on correlation functions with distinct, localized features, like the signature of the baryon acoustic oscillations (BAO). Near the line of sight, the features become sharper as a result of redshift-space distortions. We demonstrate this effect by measuring the correlation function in Gaussian simulations and the Millennium Simulation. We also analyze the SDSS DR7 main-galaxy sample (MGS), splitting the sample into slices 2.5 degrees on the sky in various rotations. Measuring 2D correlation functions in each slice, we do see a sharp bump along the line of sight. Using Mexican-hat wavelets, we localize it to (110 +/- 10) Mpc/h. Averaging only along the line of sight, we estimate its significance at a particular wavelet scale and location at 2.2 sigma. In a flat angular weighting in the (pi,r_p) coordinate system, the noise level is suppressed, pushing the bump's significance to 4 sigma. We estimate that there is about a 0.2% chance of getting such a signal anywhere in the vicinity of the BAO scale from a power spectrum lacking a BAO feature. However, these estimates of the significances make some use of idealized Gaussian simulations, and thus are likely a bit optimistic.

astro-ph.CO↗
arXiv2010

Galaxy Counts on the CMB Cold Spot

The Cold Spot on the Cosmic Microwave Background could arise due to a supervoid at low redshift through the integrated Sachs-Wolfe effect. We imaged the region with MegaCam on the Canada-France-Hawai'i Telescope and present galaxy counts in photometric redshift bins. We rule out the existence of a 100Mpc radius spherical supervoid with underdensity delta=-0.3 at 0.5<z<0.9 at high significance. The data are consistent with an underdensity at low redshift, but the fluctuations are within the range of cosmic variance and the low density areas are not contiguous on the sky. Thus, we find no strong evidence for a supervoid. We cannot resolve voids smaller than 50Mpc radius; however, these can only make a minor contribution to the CMB temperature decrement.

astro-ph.CO↗
arXiv2009

A Map of the Integrated Sachs-Wolfe Signal from Luminous Red Galaxies

We construct a map of the time derivative of the gravitational potential traced by SDSS Luminous Red Galaxies. The potential decays on large scales due to cosmic acceleration, leaving an imprint on cosmic microwave background (CMB) radiation through the integrated Sachs-Wolfe (ISW) effect. With a template fit, we directly measure this signature on the CMB at a 2-sigma confidence level. The measurement is consistent with the cross-correlation statistic, strengthening the claim that dark energy is indeed the cause of the correlation. This new approach potentially simplifies the cosmological interpretation. Our constructed linear ISW map shows no evidence for degree-scale cold and hot spots associated with supervoid and supercluster structures. This suggests that the linear ISW effect in a concordance Lambda-CDM cosmology is insufficient to explain the strong CMB imprints from these structures that we previously reported.

astro-ph↗
arXiv2009

Rejuvenating the matter power spectrum: restoring information with a logarithmic density mapping

We find that nonlinearities in the dark-matter power spectrum are dramatically smaller if the density field first undergoes a logarithmic mapping. In the Millennium simulation, this procedure produces a power spectrum with a shape hardly departing from the linear power spectrum for k <~ 1 h/Mpc at all redshifts. Also, this procedure unveils pristine Fisher information on a range of scales reaching a factor of 2-3 smaller than in the standard power spectrum, yielding 10 times more cumulative signal-to-noise at z=0.

astro-ph.CO↗
arXiv2008

Dark Energy Detected with Supervoids and Superclusters

The observed apparent acceleration of the universe is usually attributed to negative pressure from a mysterious dark energy. This acceleration causes the gravitational potential to decay, heating or cooling photons travelling through crests or troughs of large-scale matter density fluctuations. This phenomenon, the late-time integrated Sachs-Wolfe (ISW) effect, has been detected, albeit at low significance, by cross-correlating various galaxy surveys with the Cosmic Microwave Background (CMB). Recently, the best evidence has come from the statistical combination of results from multiple correlated galaxy data sets. Here we show that vast structures identified in a galaxy survey project an image onto the CMB; stacking regions aligned with superclusters produces a hot spot, and supervoids, a cold spot. At over 4 sigma, this is the clearest evidence of the ISW effect to date. For the first time, our findings pin the effect to discrete structures. The ISW signal from supervoids and superclusters can be combined with other cosmological probes to constrain dark energy and cosmological parameters. In addition, our findings make it more plausible that the extreme Cold Spot and other anomalies in the CMB are caused by supervoids.

astro-ph↗
arXiv2008

An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect

We measure hot and cold spots on the microwave background associated with supercluster and supervoid structures identified in the Sloan Digital Sky Survey Luminous Red Galaxy catalog. The structures give a compelling visual imprint, with a mean temperature deviation of 9.6 +/- 2.2 microK, i.e. above 4 sigma. We interpret this as a detection of the late-time Integrated Sachs-Wolfe (ISW) effect, in which cosmic acceleration from dark energy causes gravitational potentials to decay, heating or cooling photons passing through density crests or troughs. In a flat universe, the linear ISW effect is a direct signal of dark energy.

astro-ph↗
arXiv2008

ZOBOV: a parameter-free void-finding algorithm

ZOBOV (ZOnes Bordering On Voidness) is an algorithm that finds density depressions in a set of points, without any free parameters, or assumptions about shape. It uses the Voronoi tessellation to estimate densities, which it uses to find both voids and subvoids. It also measures probabilities that each void or subvoid arises from Poisson fluctuations. This paper describes the ZOBOV algorithm, and the results from its application to the dark-matter particles in a region of the Millennium Simulation. Additionally, the paper points out an interesting high-density peak in the probability distribution of dark-matter particle densities.

astro-ph↗
arXiv2007

Baryon oscillations in galaxy and matter power-spectrum covariance matrices

We investigate large-amplitude baryon acoustic oscillations (BAO's) in off-diagonal entries of cosmological power-spectrum covariance matrices. These covariance-matrix BAO's describe the increased attenuation of power-spectrum BAO's caused by upward fluctuations in large-scale power. We derive an analytic approximation to covariance-matrix entries in the BAO regime, and check the analytical predictions using N-body simulations. These BAO's look much stronger than the BAO's in the power spectrum, but seem detectable only at about a one-sigma level in gigaparsec-scale galaxy surveys. In estimating cosmological parameters using matter or galaxy power spectra, including the covariance-matrix BAO's can have a several-percent effect on error-bar widths for some parameters directly related to the BAO's, such as the baryon fraction. Also, we find that including the numerous galaxies in small haloes in a survey can reduce error bars in these cosmological parameters more than the simple reduction in shot noise might suggest.

astro-ph↗
arXiv2007

Information content in the halo-model dark-matter power spectrum II: Multiple cosmological parameters

We investigate the cosmological Fisher information in the non-linear dark-matter power spectrum in the context of the halo model. We find that there is a plateau in information content on translinear scales which is generic to all cosmological parameters we tried. There is a rise in information on smaller scales, but we find that it is quite degenerate among different cosmological parameters (except, perhaps, the tilt). This suggests that it could be difficult to constrain cosmological parameters using the non-linear regime of the dark-matter power spectrum. We suggest ways to get around this problem, such as removing the largest haloes from consideration in survey analysis.

astro-ph↗
arXiv2006

Information content in the halo-model dark-matter power spectrum

Using the halo model, we investigate the cosmological Fisher information in the non-linear dark-matter power spectrum about the initial amplitude of linear power. We find that there is little information on `translinear' scales (where the one- and two-halo terms are both significant) beyond what is on linear scales, but that additional information is present on small scales, where the one-halo term dominates. This behavior agrees with the surprising results that Rimes & Hamilton (2005, 2006) found using N-body simulations. We argue that the translinear plateau in cumulative information arises largely from fluctuations in the numbers of large haloes in a finite volume. This implies that more information could be extracted on non-linear scales if the masses of the largest haloes in a survey are known.

astro-ph↗
arXiv2005

A galaxy-halo model of large-scale structure

We present a new, galaxy-halo model of large-scale structure, in which the galaxies entering a given sample are the fundamental objects. Haloes attach to galaxies, in contrast to the standard halo model, in which galaxies attach to haloes. The galaxy-halo model pertains mainly to the relationships between the power spectra of galaxies and mass, and their cross-power spectrum. With surprisingly little input, an intuition-aiding approximation to the galaxy-matter cross-correlation coefficient R(k) emerges, in terms of the halo mass dispersion. This approximation seems valid to mildly non-linear scales (k < ~3 h/Mpc), allowing measurement of the bias and the matter power spectrum from measurements of the galaxy and galaxy-matter power spectra (or correlation functions). This is especially relevant given the recent advances in precision in measurements of the galaxy-matter correlation function from weak gravitational lensing. The galaxy-halo model also addresses the issue of interpreting the galaxy-matter correlation function as an average halo density profile, and provides a simple description of galaxy bias as a function of scale.

astro-ph↗
arXiv2004

VOBOZ: An Almost-Parameter-Free Halo-Finding Algorithm

We have developed an algorithm to find haloes in an N-body dark matter simulation, called VOBOZ (VOronoi BOund Zones), which has as little dependence on free parameters as we can manage. By using the Voronoi diagram, we achieve nonparametric, `natural' measurements of each particle's density and set of neighbors. We then eliminate much of the ambiguity in merging sets of particles together by identifying every possible density peak, and measuring the probability that each does not arise from Poisson noise. This probability could be interpreted as a measure of a halo's sub-haloness, and might be of use to researchers of the halo model. The first parameter in VOBOZ controls the subtlety of particle unbinding, and may be eliminated if one is cavalier with processor time; even if one is not, the results saturate to the parameter-free answer when the parameter is sufficiently small. The only parameter which remains, an outer density cut-off, does not influence whether or not haloes are identified, nor does it have any effect on subhaloes; it only affects the masses returned for supercluster haloes.

astro-ph↗
arXiv2004

Understanding the PSCz Galaxy Power Spectrum with N-body Simulations

By comparing the PSCz galaxy power spectrum with the results of nested pure dark matter N-body simulations, we try to understand how infrared-selected galaxies populate dark-matter haloes, paying special attention to the method of halo identification in the simulations. We thus test the hypothesis that baryonic physics negligibly affects the distribution of galaxies down to the smallest scales yet observed. We are successful in reproducing the PSCz power spectrum on scales < ~40 h/Mpc, near our resolution limit, by imposing a central density cut-off on simulated haloes, which gives a rough minimum mass and circular velocity of haloes in which PSCz galaxies formed.

astro-ph↗
arXiv2003

The PSCz Galaxy Power Spectrum Compared to N-Body Simulations

By comparing the PSCz galaxy power spectrum with haloes from nested and phased N-body simulations, we try to understand how IRAS infrared-selected galaxies populate dark-matter haloes. We pay special attention to the way we identify haloes in the simulations.

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