Fiona Hoyle

The Three-Dimensional Power Spectrum of Galaxies from the Sloan Digital Sky Survey

We measure the large-scale real-space power spectrum P(k) using a sample of 205,443 galaxies from the Sloan Digital Sky Survey, covering 2417 square degrees with mean redshift z~0.1. We employ a matrix-based method using pseudo-Karhunen-Loeve eigenmodes, producing uncorrelated minimum-variance measurements in 22 k-bands of both the clustering power and its anisotropy due to redshift-space distortions, with narrow and well-behaved window functions in the range 0.02 h/Mpc < k < 0.3h/Mpc. We pay particular attention to modeling, quantifying and correcting for potential systematic errors, nonlinear redshift distortions and the artificial red-tilt caused by luminosity-dependent bias. Our final result is a measurement of the real-space matter power spectrum P(k) up to an unknown overall multiplicative bias factor. Our calculations suggest that this bias factor is independent of scale to better than a few percent for k<0.1h/Mpc, thereby making our results useful for precision measurements of cosmological parameters in conjunction with data from other experiments such as the WMAP satellite. As a simple characterization of the data, our measurements are well fit by a flat scale-invariant adiabatic cosmological model with h Omega_m =0.201+/- 0.017 and L* galaxy sigma_8=0.89 +/- 0.02 when fixing the baryon fraction Omega_b/Omega_m=0.17 and the Hubble parameter h=0.72; cosmological interpretation is given in a companion paper.We measure the large-scale real-space power spectrum P(k) by using a sample of 205,443 galaxies from the Sloan Digital Sky Survey, covering 2417 effective square degrees with mean redshift z ≈ 0.1. We employ a matrix-based method using pseudo-Karhunen-Loeve eigenmodes, producing uncorrelated minimum-variance measurements in 22 k-bands of both the clustering power and its anisotropy due to redshift-space distortions, with narrow and well-behaved window functions in the range 0.02 h Mpc-1 < k < 0.3 h Mpc-1. We pay particular attention to modeling, quantifying, and correcting for potential systematic errors, nonlinear redshift distortions, and the artificial red-tilt caused by luminosity-dependent bias. Our results are robust to omitting angular and radial density fluctuations and are consistent between different parts of the sky. Our final result is a measurement of the real-space matter power spectrum P(k) up to an unknown overall multiplicative bias factor. Our calculations suggest that this bias factor is independent of scale to better than a few percent for k < 0.1 h Mpc-1, thereby making our results useful for precision measurements of cosmological parameters in conjunction with data from other experiments such as the Wilkinson Microwave Anisotropy Probe satellite. The power spectrum is not well-characterized by a single power law but unambiguously shows curvature. As a simple characterization of the data, our measurements are well fitted by a flat scale-invariant adiabatic cosmological model with h Ωm = 0.213 ± 0.023 and σ8 = 0.89 ± 0.02 for L* galaxies, when fixing the baryon fraction Ωb/Ωm = 0.17 and the Hubble parameter h = 0.72; cosmological interpretation is given in a companion paper.

A Map of the Universe

We have produced a new conformal map of the universe illustrating recent discoveries, ranging from Kuiper belt objects in the Solar system, to the galaxies and quasars from the Sloan Digital Sky Survey. This map projection, based on the logarithm map of the complex plane, preserves shapes locally, and yet is able to display the entire range of astronomical scales from the Earth s neighborhood to the cosmic microwave background. The conformal nature of the projection, preserving shapes locally, may be of particular use for analyzing large scale structure. Prominent in the map is a Sloan Great Wall of galaxies 1.37 billion light years long, 80 percent longer than the Great Wall discovered by Geller and Huchra and therefore the largest observed structure in the universe.

Voids in the Two-Degree Field Galaxy Redshift Survey

We present an analysis of voids in the 2dF Galaxy Redshift Survey (2dFGRS). This analysis includes identification of void regions and measurement of void statistics. The 2dFGRS is the largest completed redshift survey to date, including a total of 245,591 galaxies covering 1500 deg2 to a median depth of zmed ~ 0.11. We use the voidfinder algorithm to identify a total of 289 voids in the 2dFGRS with radius larger than 10 h-1 Mpc. These voids have an average effective radius, the radius of a sphere with the same volume as the void, of h-1 Mpc in the North Galactic Pole region (NGP) and h-1 Mpc in the South Galactic Pole region (SGP). These voids are extremely underdense, with average density contrast of δρ/ρ = -0.94 ± 0.02. The centers of voids are even emptier, because the few galaxies within the voids typically lie close to the edges. The total volume of the universe filled by these void regions is approximately 40%. These results are very similar to results found from our analysis of the PSCz survey and the Updated Zwicky Catalog; here we detect almost a factor of 10 more voids. We measure the void probability function (VPF) of the 2dFGRS for volume-limited samples with limiting absolute magnitudes, Mlim - 5 log h, from -16 to -21 in bJ. We measure the underdensity probability function (with density contrast threshold δρ/ρ = -0.8) for samples with limiting absolute magnitudes, Mlim - 5 log h, from -18 to -21. We find that the SGP is more underdense than the NGP for all but the brightest sample under consideration. There is good agreement between the VPFs of the Center for Astrophysics survey and the 2dFGRS. Comparison of VPFs measured for the 2dFGRS with the distribution of simulated dark matter halos of similar number density indicates that voids in the matter distribution in ΛCDM simulations are not empty enough. However, semianalytic models of galaxy formation that include feedback effects yield VPFs that show excellent agreement with the data.

Voids in the Point Source Catalogue Survey and the Updated Zwicky Catalog

We describe an algorithm to detect voids in galaxy redshift surveys. The method is based on the void finder algorithm of El-Ad & Piran. We apply a series of tests to determine how accurately we are able to recover the volumes of voids using our detection method. We simulate voids of different ellipticity and find that if voids are approximately spherical, our algorithm will recover 100% of the volume of the void. The more elliptical the void, the smaller the fraction of the volume we can recover. We insist that voids lie completely within the survey. Voids close to the edge of the survey will therefore be underestimated in volume. By considering a deeper sample, we estimate that the maximal sphere diameters are correct to within 30%. We apply the algorithm to the Point Source Catalog redshift (PSCz) survey and the Updated Zwicky Catalog (UZC). The PSCz survey is an almost all-sky survey with objects selected from the IRAS catalog. The UZC covers a smaller area of sky but is optically selected and samples the structures more densely. We detect 35 voids in the PSCz and 19 voids in the UZC with diameter larger than 20 h-1 Mpc. Using this minimum size threshold, we find that voids have an average effective diameter of 29.8 ± 3.5 h-1 Mpc (PSCz) and 29.2 ± 2.7 h-1 Mpc (UZC) and that they are underdense regions with δρ/ρ values of -0.92 ± 0.03 (PSCz) and -0.96 ± 0.01 (UZC), respectively. Using this quite stringent threshold for void definition, voids fill up to 40% of the volume of the universe.

Cosmic Voids in Sloan Digital Sky Survey Data Release 7

We study the distribution of cosmic voids and void galaxies using Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Using the VoidFinder algorithm as described by Hoyle & Vogeley (2002), we identify 1054 statistically significant voids in the northern galactic hemisphere with radii > 10h 1 Mpc. The filling factor of voids in the sample volume is 62%. The largest void is just over 30h 1 Mpc in effective radius. The median effective radius is 17h 1 Mpc. The voids are found to be significantly underdense, with density contrast � < 0.85 at the edges of the voids. The radial density profiles of these voids are similar to predictions of dynamically distinct underdensities in gravitational theory. We find 8,046 galaxies brighter than Mr = 20.09 within the voids, accounting for 7% of the galaxies. We compare the results of VoidFinder on SDSS DR7 to mock catalogs generated from a SPH halo model simulation as well as other �-CDM simulations and find similar void fractions and void sizes in the data and simulations. This catalog is made publicly available at http://www.physics.drexel.edu/�pan/VoidCatalog for download.

The Aspen–Amsterdam void finder comparison project

Despite a history that dates back at least a quarter of a century studies of voids in the large–scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void–finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi– analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than mB = 20 is found to be smaller than about 0.8 by all our void finding algorithms.

Photometric Properties of Void Galaxies in the Sloan Digital Sky Survey

Using a nearest neighbor analysis, we construct a sample of void galaxies from the Sloan Digital Sky Survey (SDSS) and compare the photometric properties of these galaxies to the population of nonvoid (wall) galaxies. We trace the density field of galaxies using a volume-limited sample with zmax ¼ 0:089. Galaxies from the flux-limited SDSS with zzmax and fewer than three volume-limited neighbors within 7 h � 1 Mpc are classif ied as void gal- axies. This criterion implies a density contrast ��=� < � 0:6 around void galaxies. From 155,000 galaxies, we obtain a subsample of 13,742 galaxies with zzmax, from which we identify 1010 galaxies as void galaxies. To identify an additional 194 faint void galaxies from the SDSS in the nearby universe, r P 72 h � 1 Mpc, we employ volume-limited samples extracted from the Updated Zwicky Catalog and the Southern Sky Redshift Survey with zmax ¼ 0:025 to trace the galaxy distribution. Our void galaxies span a range of absolute magnitude from Mr ¼ � 13: 5t o� 22.5. Using SDSS photometry, we compare the colors, concentration indices, and Sersic indices of the void and wall samples. Void galaxies are significantly bluer than galaxies lying at higher density. The population of void galaxies with Mr P M � þ 1 and brighter is on average bluer and more concentrated (later type) than galaxies outside of voids. The latter behavior is only partly explained by the paucity of luminous red galaxies in voids. These results generally agree with the predictions of semianalytic models for galaxy formation in cold dark matter models, which indicate that void galaxies should be relatively bluer, more disklike, and have higher specific star formation rates. Subject heading gs: cosmology: observations — galaxies: photometry — galaxies: structure — large-scale structure of universe — methods: statistical

The luminosity function of void galaxies in the sloan digital sky survey

We measure the r-band luminosity function (LF) of a sample of 103 void galaxies over a large range of magnitude, -21.5 5 A], have brighter M but faint-end slopes similar to those of void galaxies. In contrast, the LFs of wall galaxies with red g - r color, elliptical-like profiles, or low star formation rates have significantly shallower faint-end slopes and brighter values of M than we find for void galaxies. We conclude that the void galaxy population is dominated by faint, late-type galaxies. The shift in M* between the void and wall galaxy LFs is consistent with the shift of the mass function in voids predicted by extended Press-Schechter theory.

SPECTROSCOPIC PROPERTIES OF VOID GALAXIES IN THE SLOAN DIGITAL SKY SURVEY

We study the spectroscopic properties of a sample of 103 void galaxies identified in the Sloan Digital Sky Survey (SDSS) and compare these with the properties of galaxies in higher density regions (wall galaxies). This sample of void galaxies covers the range of absolute magnitude from Mr = -13.5 to Mr = -22.5 in regions with density contrast δρ/ρ < -0.6. We compare the equivalent widths of Hα, [O II], [N II], Hβ, and [O III] of void and wall galaxies with similar luminosities. We find that void galaxies have larger emission line equivalent widths, indicating that they are forming stars at a higher rate. A comparison of the Balmer break, as measured by the parameter Dn(4000), reveals that void galaxies have younger stellar populations than wall galaxies. Using standard techniques, we estimate Hα and [O II] star formation rates (SFRs) of the void and wall galaxies. Combining these measurements with estimates of the stellar masses, we find specific star formation rates (SFR per unit stellar mass) for void galaxies that are generally higher than for wall galaxies, consistent with the results from the equivalent widths.

Galaxy voids in cold dark matter universes

We present predictions for numerous statistics related to the presence of voids in the distribution of galaxies in a cold dark matter model of structure formation using a semi-analytic model of galaxy formation. Our study is able to probe galaxies with masses as low as 10 9 h - 1 M O . corresponding to absolute magnitudes of M b - 5 log h = -18.1 and M r - 5 log h = -18.7. We quantify the void and underdense probability functions, distributions of nearest-neighbour distances and void sizes and compute the density profiles of voids. These results are contrasted with the expectations for dark matter (and the difference examined in terms of the galaxy/dark matter biasing relation) and are compared with analytic predictions and observational data where available. The predicted void probability functions are consistent with those measured from the Centre for Astrophysics redshift surveys given the rather large uncertainties in this relatively small (for studies of voids) observational sample. The size of the observational sample is too small to probe the bias between galaxies and dark matter that we predict. We also examine the predicted properties of galaxies existing within voids and contrast these with the general galaxy population. Our predictions are aimed at forthcoming large galaxy redshift surveys that should for the first time provide statistically accurate measures of the void population.