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Moorman C.M.,Drexel University | Vogeley M.S.,Drexel University | Hoyle F.,Pontifica University Catolica Of Ecuador | Pan D.C.,Shanghai Astronomical Observatory | And 2 more authors.
Astrophysical Journal | Year: 2015

We measure the r-band galaxy luminosity function (LF) across environments over the redshift range 0 < z < 0.107 using the Sloan Digital Sky Survey (SDSS). We divide our sample into galaxies residing in large-scale voids (void galaxies) and those residing in denser regions (wall galaxies). The best-fitting Schechter parameters for void galaxies are log(Mpc-3), = -19.88 ± 0.05, and α = -1.20 ± 0.02. For wall galaxies, the best-fitting parameters are log(Mpc-3), = -20.80 ± 0.03, and α = -1.16 ± 0.01. We find a shift in the characteristic magnitude, toward fainter magnitudes for void galaxies and find no significant difference between the faint-end slopes of the void and wall galaxy LFs. We investigate how low-surface-brightness selection effects can affect the galaxy LF. To attempt to examine a sample of galaxies that is relatively free of surface-brightness selection effects, we compute the optical galaxy LF of galaxies detected by the blind H i survey Arecibo Legacy Fast ALFA (ALFALFA). We find that the global LF of the ALFALFA sample is not well fit by a Schechter function because of the presence of a wide dip in the LF around Mr = -18 and an upturn at fainter magnitudes (α ∼ -1.47). We compare the H i selected r-band LF to various LFs of optically selected populations to determine where the H i selected optical LF obtains its shape. We find that sample selection plays a large role in determining the shape of the LF. © 2015. The American Astronomical Society. All rights reserved.

Lee J.,Seoul National University | Hoyle F.,Pontifica University Catolica Of Ecuador
Astrophysical Journal | Year: 2015

The shapes of cosmic voids are prone to distortions caused by external tidal forces since their low densities imply lower internal resistance. This susceptibility of the void shapes to tidal distortions makes them useful as indicators of large-scale tidal and density fields, despite the practical difficulty in defining them. Using the void catalog constructed by Pan et al. from the Seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7), we detect a clear 4s signal of spatial correlations of the void shapes on a scale of 20 h-1Mpc and show that the signal is robust against the projection of the void shapes onto the plane of sky. By constructing a simple analytic model for the void shape correlation, within the framework of tidal torque theory, we demonstrate that the void shape correlation function scales linearly with the two-point correlation function of the linear density field. We also find direct observational evidence for the cross-correlation of the void shapes with the large-scale velocity shear field that was linearly reconstructed by Lee et al. from SDSS DR7. We discuss the possibility of using the void shape correlation function to break the degeneracy between the density parameter and the power spectrum amplitude and to independently constrain the neutrino mass as well. © 2015. The American Astronomical Society. All rights reserved.

Pan D.C.,Drexel University | Vogeley M.S.,Drexel University | Hoyle F.,Widener University | Hoyle F.,Pontifica University Catolica Of Ecuador | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We study the distribution of cosmic voids and void galaxies using Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Using the VoidFinder algorithm based on the original VoidFinder method devised by El-Ad & Piran and implemented by Hoyle & Vogeley, we identify 1054 statistically significant voids in the Northern galactic hemisphere with radii > 10h -1Mpc. The filling factor of voids in the sample volume is 62per cent. The largest void is just over 30h -1Mpc in effective radius. The median effective radius is 17h -1Mpc. The voids are found to be significantly underdense, with density contrast δ < - 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 8046 galaxies brighter than M r=- 20.09 within the voids, accounting for 7per cent of the galaxies. We compare the results of VoidFinder on SDSS DR7 to mock catalogues generated from a smoothed particle hydrodynamics (SPH) halo model simulation as well as other Λ cold dark matter (ΛCDM) simulations and find similar void fractions and void sizes in the data and simulations. This catalogue is made publicly available at for download. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Eckhardt M.,Ostergotland County Council | Forsberg B.C.,Karolinska Institutet | Wolf D.,University of Trier | Crespo-Burgos A.,Pontifica University Catolica Of Ecuador
Revista Panamericana de Salud Publica/Pan American Journal of Public Health | Year: 2011

Objective: The main objective of this study was to assess people's willingness to join a community-based health insurance (CHI) model in El Páramo, a rural area in Ecuador, and to determine factors influencing this willingness. A second objective was to identify people's understanding and attitudes toward the presented CHI model. Methods: A cross-sectional survey was carried out using a structured questionnaire. Of an estimated 829 households, 210 were randomly selected by two-stage cluster sampling. Attitudes toward the scheme were assessed. Information on factors possibly influencing willingness to join was collected and related to the willingness to join. To gain an insight into a respondent's possible ability to pay, health care expenditure on the last illness episode was assessed. Feasibility was defined as at least 50% of household heads willing to join the scheme. Results: Willingness to join the CHI model for US$30 per year was 69.3%. With affiliation, 92.2% of interviewees stated that they would visit the local health facility more often. Willingness to join was found to be negatively associated with education. Other variables showed no significant association with willingness to join. The study showed a positive attitude toward the CHI scheme. Substantial health care expenditures on the last illness episode were documented. Conclusions: The investigation concludes that CHI in the study region is feasible. However, enrollments are likely to be lower than the stated willingness to join. Still, a CHI scheme should present an interesting financing alternative in rural areas where services are scarce and difficult to sustain.

Hoyle F.,Pontifica University Catolica Of Ecuador | Vogeley M.S.,Drexel University | Pan D.,Drexel University
Monthly Notices of the Royal Astronomical Society | Year: 2012

Using the sample presented by Pan et al., we analyse the photometric properties of 88794 void galaxies and compare them to galaxies that reside in higher density environments with the same absolute magnitude distribution as the void galaxies. We analysed the Sloan Digital Sky Survey Data Release 7 and found a total of 1054 dynamically distinct voids with radius larger than 10h -1Mpc. The voids are not empty, but are underdense, with δρ/ρ < -0.9 in their centres. In this paper, 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, dividing the galaxies into bright, medium, faint and dwarf groups, 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, as measured by the inverse concentration index. 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 and dwarf void galaxies are blue in colour. We also study the properties of void galaxies as a function of their distance from the centre 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 centres of voids. As shown by Pan et al., the centres of voids have very similar density contrast and hence all void galaxies live in very similar density environments, which may explain the lack of variation of galaxy properties with location within voids. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

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