195 research outputs found

    Large-Scale Mass Power Spectrum from Peculiar Velocities

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    This is a brief progress report on a long-term collaborative project to measure the power spectrum (PS) of mass density fluctuations from the Mark III and the SFI catalogs of peculiar velocities. The PS is estimated by applying maximum likelihood analysis, using generalized CDM models with and without COBE normalization. The application to both catalogs yields fairly similar results for the PS. The robust result is a relatively high PS, with P(k)\Omega^{1.2}=(4.5+/-2.0)X10^3 (Mpc/h)^3 at k=0.1 h/Mpc. An extrapolation to smaller scales using the different CDM models gives \sigma_8\Omega^{0.6}=0.85+/-0.2. The general constraint on the combination of cosmological parameters is of the sort \Omega \h_{50}^{\mu} n^{\nu}=0.75+/-0.25, where \mu=1.3 and \nu=3.7,2.0 for \Lambda CDM models with and without tensor fluctuations respectively. For open CDM, without tensor fluctuations, the powers are \mu=0.9 and \nu=1.4.Comment: 3 pages, 1 figure, uses mprocl.sty. To appear in Proceedings of the Eighth Marcel Grossmann Meetin

    The Growth of Galaxy Stellar Mass Within Dark Matter Halos

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    We study the evolution of stellar mass in galaxies as a function of host halo mass, using the "MPA" and "Durham" semi-analytic models, implemented on the Millennium Run simulation. The results from both models are similar. We find that about 45% of the stellar mass in central galaxies in present-day halos less massive than ~10^{12} Msun/h is already in place at z~1. This fraction increases to ~65% for more massive halos. The peak of star formation efficiency shifts toward lower mass halos from z~1 to z~0. The stellar mass in low-mass halos grows mostly by star formation since z~1, while in high-mass halos most of the stellar mass is assembled by mergers. These trends are clear indications of "halo downsizing". We compare our findings to the results of the phenomenological method developed by Zheng, Coil & Zehavi (2007). The theoretical predictions are in qualitative agreement with these results, however there are large discrepancies. The most significant one concerns the amount of stars already in place in the progenitor galaxies at z~1, which is about a factor of two larger in both semi-analytic models. We also use the semi-analytic catalogs to test different assumptions made in that work, and illustrate the importance of smooth accretion of dark matter when estimating the mergers contribution. We demonstrate that methods studying galaxy evolution from the galaxy-halo connection are powerful in constraining theoretical models and can guide future efforts of modeling galaxy evolution. Conversely, semi-analytic models serve an important role in improving such methods.Comment: 13 pages, 8 figures, submitted to Ap

    Properties and Origin of Galaxy Velocity Bias in the Illustris Simulation

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    We use the hydrodynamical galaxy formation simulations from the Illustris suite to study the origin and properties of galaxy velocity bias, i.e., the difference between the velocity distributions of galaxies and dark matter inside halos. We find that galaxy velocity bias is a decreasing function of the ratio of galaxy stellar mass to host halo mass. In general, central galaxies are not at rest with respect to dark matter halos or the core of halos, with a velocity dispersion above 0.04 times that of the dark matter. The central galaxy velocity bias is found to be mostly caused by the close interactions between the central and satellite galaxies. For satellite galaxies, the velocity bias is related to their dynamical and tidal evolution history after being accreted onto the host halos. It depends on the time after the accretion and their distances from the halo centers, with massive satellites generally moving more slowly than the dark matter. The results are in broad agreements with those inferred from modeling small-scale redshift-space galaxy clustering data, and the study can help improve models of redshift-space galaxy clustering.Comment: 15 pages, 11 figures. Accepted for publication in Ap

    On the Mass-to-Light Ratio of Large Scale Structure

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    We examine the dependence of the mass-to-light (M/L) ratio of large-scale structure on cosmological parameters, in models that are constrained to match observations of the projected galaxy correlation function w(rp). For a sequence of cosmological models with a fixed P(k) shape and increasing normalization \sig8, we find parameters of the galaxy halo occupation distribution (HOD) that reproduce SDSS measurements of w(rp) as a function of luminosity. Using these HOD models we calculate mean M/L ratios as a function of halo mass and populate halos of N-body simulations to compute M/L in larger scale environments, including cluster infall regions. For all cosmological models, the M/L ratio in high mass halos or high density regions is approximately independent of halo mass or smoothing scale. However, the "plateau" value of M/L depends on \sig8 as well as \Omega_m, and it represents the universal mass-to-light ratio only for models in which the galaxy correlation function is approximately unbiased, i.e., with \sig8 ~ \sig8_gal. Our results for cluster mass halos follow the trend M/L = 577(\Omega_m/0.3)(\sig8/0.9)^{1.7} h Msun/Lsun. Combined with Carlberg et al.'s (1996) mean M/L ratio of CNOC galaxy clusters, this relation implies (\sig8/0.9)(\Omega_m/0.3)^{0.6} = 0.75 +/- 0.06. M/L ratios of clusters from the SDSS and CAIRNS surveys yield similar results. This constraint is inconsistent with parameter values \Omega_m ~ 0.3, \sig8 ~ 0.9 favored by recent joint analyses of CMB measurements and other large-scale structure data. We discuss possible resolutions, none of which seems entirely satisfactory. Appendices present an improved formula for halo bias factors and an improved analytic technique for calculating the galaxy correlation function from a given cosmological model and HOD. (Abridged)Comment: Accepted to ApJ (v 630, no 2). Replaced with accepted versio

    The Linear Point: A cleaner cosmological standard ruler

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    We show how a characteristic length scale imprinted in the galaxy two-point correlation function, dubbed the "linear point", can serve as a comoving cosmological standard ruler. In contrast to the Baryon Acoustic Oscillation peak location, this scale is constant in redshift and is unaffected by non-linear effects to within 0.50.5 percent precision. We measure the location of the linear point in the galaxy correlation function of the LOWZ and CMASS samples from the Twelfth Data Release (DR12) of the Baryon Oscillation Spectroscopic Survey (BOSS) collaboration. We combine our linear-point measurement with cosmic-microwave-background constraints from the Planck satellite to estimate the isotropic-volume distance DV(z)D_{V}(z), without relying on a model-template or reconstruction method. We find DV(0.32)=1264±28D_V(0.32)=1264\pm 28 Mpc and DV(0.57)=2056±22D_V(0.57)=2056\pm 22 Mpc respectively, consistent with the quoted values from the BOSS collaboration. This remarkable result suggests that all the distance information contained in the baryon acoustic oscillations can be conveniently compressed into the single length associated with the linear point.Comment: The optimal two-point correlation function bin-size is employed. Results are updated and the distance constraints are improve

    The Conditional Colour-Magnitude Distribution: I. A Comprehensive Model of the Colour-Magnitude-Halo Mass Distribution of Present-Day Galaxies

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    We formulate a model of the conditional colour-magnitude distribution (CCMD) to describe the distribution of galaxy luminosity and colour as a function of halo mass. It consists of two populations of different colour distributions, dubbed pseudo-blue and pseudo-red, respectively, with each further separated into central and satellite galaxies. We define a global parameterization of these four colour-magnitude distributions and their dependence on halo mass, and we infer parameter values by simultaneously fitting the space densities and auto-correlation functions of 79 galaxy samples from the Sloan Digital Sky Survey defined by fine bins in the colour-magnitude diagram (CMD). The model deprojects the overall galaxy CMD, revealing its tomograph along the halo mass direction. The bimodality of the colour distribution is driven by central galaxies at most luminosities, though at low luminosities it is driven by the difference between blue centrals and red satellites. For central galaxies, the two pseudo-colour components are distinct and orthogonal to each other in the CCMD: at fixed halo mass, pseudo-blue galaxies have a narrow luminosity range and broad colour range, while pseudo-red galaxies have a narrow colour range and broad luminosity range. For pseudo-blue centrals, luminosity correlates tightly with halo mass, while for pseudo-red galaxies colour correlates more tightly (redder galaxies in more massive haloes). The satellite fraction is higher for redder and for fainter galaxies, with colour a stronger indicator than luminosity. We discuss the implications of the results and further applications of the CCMD model.Comment: 32 pages, 26 figures, accepted for publication in MNRA

    A New Method to Correct for Fiber Collisions in Galaxy Two-Point Statistics

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    In fiber-fed galaxy redshift surveys, the finite size of the fiber plugs prevents two fibers from being placed too close to one another, limiting the ability of studying galaxy clustering on all scales. We present a new method for correcting such fiber collision effects in galaxy clustering statistics based on spectroscopic observations. Our method makes use of observations in tile overlap regions to measure the contributions from the collided population, and to therefore recover the full clustering statistics. The method is rooted in solid theoretical ground and is tested extensively on mock galaxy catalogs. We demonstrate that our method can well recover the projected and the full three-dimensional redshift-space two-point correlation functions on scales both below and above the fiber collision scale, superior to the commonly used nearest neighbor and angular correction methods. We discuss potential systematic effects in our method. The statistical correction accuracy of our method is only limited by sample variance, which scales down with (the square root of) the volume probed. For a sample similar to the final SDSS-III BOSS galaxy sample, the statistical correction error is expected to be at the level of 1% on scales 0.1--30Mpc/h for the two-point correlation functions. The systematic error only occurs on small scales, caused by non-perfect correction of collision multiplets, and its magnitude is expected to be smaller than 5%. Our correction method, which can be generalized to other clustering statistics as well, enables more accurate measurements of full three-dimensional galaxy clustering on all scales with galaxy redshift surveys. (abridged)Comment: ApJ accepted. Matched to accepted version(improvements on systematics