Santiago G. Patiri

How far do they go? The outer structure of galactic dark matter halos

We study the density profiles of collapsed galaxy-size dark matter halos with masses 1011 to 5 ? 1012 M? focusing mostly on the halo outer regions from the formal virial radius Rvir up to 5Rvir-7Rvir. We find that isolated halos in this mass range extend well beyond Rvir exhibiting all properties of virialized objects up to 2Rvir-3Rvir: relatively smooth density profiles and no systematic infall velocities. The dark matter halos in this mass range do not grow as one naively may expect through a steady accretion of satellites; i.e., on average there is no mass infall. This is strikingly different from more massive halos, which have large infall velocities outside the virial radius. We provide an accurate fit for the density profile of these isolated galaxy-size halos. For a wide range 0.01Rvir-2Rvir of radii the halo density profiles are fitted with the approximation ? = ?s exp + ?m, where x ? r/rs, ?m is the mean matter density of the universe, and the index n is in the range n = 6-7.5. These profiles do not show a sudden change of behavior beyond the virial radius. For larger radii we combine the statistics of the initial fluctuations with the spherical collapse model to obtain predictions for the mean and most probable density profiles for halos of several masses. The model gives excellent results beyond 2-3 formal virial radii for the most probable profile and qualitatively correct predictions for the mean profile.

Void Statistics in Large Galaxy Redshift Surveys: Does Halo Occupation of Field Galaxies Depend on Environment?

We use measurements of the projected galaxy correlation function -->wp( -->rp) and galaxy void statistics to test whether the galaxy content of halos of fixed mass is systematically different in low-density environments. We present new measurements of the void probability function (VPF) and underdensity probability function (UPF) from Data Release 4 of the Sloan Digital Sky Survey (SDSS), as well as new measurements from the Two-Degree Field Galaxy Redshift Survey. We compare these measurements to predictions calculated from models of the halo occupation distribution (HOD) that are constrained to match both the projected correlation function -->wp( -->rp) and the space density of galaxies

Detection of the Effect of Cosmological Large-Scale Structure on the Orientation of Galaxies

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The properties of galaxies in voids

-->. The standard implementation of the HOD assumes that galaxy occupation depends on halo mass only, and is independent of local environment. For luminosity-defined samples, we find that the standard HOD prediction is a good match to the observations, and the data exclude models in which galaxy formation efficiency is reduced in low-density environments. More remarkably, we find that the void statistics of red and blue galaxies (at -->L ~ 0.4L) are perfectly predicted by standard HOD models matched to the correlation function of these samples, ruling out assembly bias models in which galaxy color is correlated with large-scale environment at fixed halo mass. We conclude that the luminosity and color of field galaxies are determined predominantly by the mass of the halo in which they reside and have little direct dependence on the environment in which the host halo formed. In broader terms, our results show that the sizes and emptiness of voids found in the distribution of -->L 0.2L galaxies are in excellent agreement with the predictions of a standard cosmological model with a simple connection between galaxies and dark matter halos.

Statistics of voids in the two-degree Field Galaxy Redshift Survey

Galaxies are not distributed randomly throughout space but are instead arranged in an intricate cosmic web of filaments and walls surrounding bubble-like voids. There is still no compelling observational evidence of a link between the structure of the cosmic web and how galaxies form within it. However, such a connection is expected on the basis of our understanding of the origin of galaxy angular momentum: disk galaxies should be highly inclined relative to the plane defined by the large-scale structure surrounding them. Using the two largest galaxy redshift surveys currently in existence (2dFGRS and SDSS), we show at the 99.7% confidence level that these alignments do indeed exist: spiral galaxies located on the shells of the largest cosmic voids have rotation axes that lie preferentially on the void surface.

Angular distribution of satellite galaxies from the Sloan Digital Sky Survey Data Release 4

We present a comparison of the properties of galaxies in the most underdense regions of the Universe, where the galaxy number density is less than 10 per cent of the mean density, with galaxies from more typical regions. We have compiled a sample of galaxies in 46 large nearby voids that were identified using the Sloan Digital Sky Survey DR4, which provides the largest coverage of the sky. We study the u - r colour distribution, morphology, specific star formation rate (SFR) and radial number density profiles for a total of 495 galaxies fainter than M r = -20.4 + 5 log h located inside the voids and compare these properties with a control sample of field galaxies. We show that there is an excess of blue galaxies inside the voids. However, inspecting the properties of blue and red galaxies separately, we find that galaxy properties such as colour distribution, bulge-to-total ratios and concentrations are remarkably similar between the void and overall sample. The void galaxies also show the same specific SFR at fixed colour as the control galaxies. We compare our results with the predictions of cosmological simulations of galaxy formation using the Millennium Run semi-analytic galaxy catalogue. We show that the properties of the simulated galaxies in large voids are in reasonably good agreement with those found in similar environments in the real Universe. To summarize, in spite of the fact that galaxies in voids live in the least dense large-scale environment, this environment makes very little impact on the properties of galaxies.

Spin alignment of dark matter haloes in the shells of the largest voids

We present a statistical analysis of voids in the two-degree Field Galaxy Redshift Survey (2dFGRS). In order to detect the voids, we have developed two robust algorithms. We define voids as non-overlapping maximal spheres empty of haloes or galaxies with mass or luminosity above a given value. We search for voids in cosmological N-Body simulations to test the performance of our void finders. We obtain and analyse the void statistics for several volume-limited samples for the North Galactic Pole (NGP) and the South Galactic Pole (SGP) constructed from the 2dFGRS full data release. We find that the results obtained from the NGP and the SGP are statistically compatible. From the results of several statistical tests we conclude that voids are essentially uncorrelated, with at most a mild anticorrelation and that at the 99.5 per cent confidence level there is a dependence of the void number density on redshift. We develop a technique to correct the distortion caused by the fact that we use the redshift as the radial coordinate. We calibrate this technique with mock catalogues and find that the correction might be of some relevance to carry out accurate inferences from void statistics. We study the statistics of the galaxies inside nine nearby voids. We find that galaxies in voids are not randomly distributed: they form structures like filaments. We also obtain the galaxy number density profile in voids. This profile follow a similar but steeper trend to that followed by haloes in voids.

The Alignment of Dark Matter Halos with the Cosmic Web

We explore the angular distribution of two samples of satellite galaxies orbiting isolated hosts extracted from the Sloan Digital Sky Survey Data Release 4. We find a clear alignment of the satellites along the major axis of their hosts when restricting the analysis to red-coloured hosts. The anisotropy is most pronounced for red satellites of red hosts. We find that the distribution of the satellites about blue, isolated hosts is consistent with isotropy. We show that under the assumption that the true, underlying distribution of satellites of blue hosts exhibits the same anisotropy as the satellites of red hosts, the sample of blue hosts is too small to measure this anisotropy at a statistically significant level. The anisotropy that we detect for satellites about red hosts is independent of the projected radius. In particular, it is evident at large projected distances from the hosts (300 < rp < 500 kpc).

On an analytical framework for voids: their abundances, density profiles and local mass functions

Using the high-resolution cosmological N-body simulation MareNostrum universe we study the orientation of shape and angular momentum of galaxy-size dark matter haloes around large voids. We find that haloes located on the shells of the largest cosmic voids have angular momenta that tend to be preferentially perpendicular to the direction that joins the centre of the halo and the centre of the void. This alignment has been found in spiral galaxies around voids using galaxy redshift surveys. We measure for the first time the strength of this alignment, showing how it falls off with increasing distance to the centre of the void. We also confirm the correlation between the intensity of this alignment and the halo mass. The analysis of the orientation of the halo main axes confirms the results of previous works. Moreover, we find a similar alignment for the baryonic matter inside dark matter haloes, which is much stronger in their inner parts.

The statistics of voids as a tool to constrain cosmological parameters: sigma_8 and Omega_m h

We investigate the orientation of the axes and angular momentum of dark matter halos with respect to their neighboring voids using high-resolution N-body cosmological simulations. We find that the minor axis of a halo tends to be aligned along the line joining the halo with the center of the void and that the major axis tends to lie in the plane perpendicular to this line. However, we find that the angular momentum of a halo does not have any particular orientation. These results may provide information about the mechanisms whereby the large-scale structure of the universe affects galaxy formation, and they may cast light on the issue of the orientation of galaxy disks with respect to their host halos.