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Nusser A.,Asher Space Science Institute Technion | Davis M.,University of California at Berkeley | Branchini E.,Third University of Rome | Branchini E.,National institute for astrophysics
Astrophysical Journal | Year: 2014

There is an ∼150 km s-1 discrepancy between the measured motion of the Local Group (LG) of galaxies with respect to the cosmic microwave background and the linear theory prediction based on the gravitational force field of the large-scale structure in full-sky redshift surveys. We perform a variety of tests which show that the LG motion cannot be recovered to better than 150-200 km s-1 in amplitude and within 10° in direction. The tests rely on catalogs of mock galaxies identified in the Millennium simulation using semi-analytic galaxy formation models. We compare these results to the Ks = 11.75 Two-Mass Galaxy Redshift Survey, which provides the deepest and most complete all-sky spatial distribution of galaxies with spectroscopic redshifts available thus far. In our analysis, we use a new concise relation for deriving the LG motion and bulk flow from the true distribution of galaxies in redshift space. Our results show that the main source of uncertainty is the small effective depth of surveys like the Two-Mass Redshift Survey (2MRS), which prevents a proper sampling of the large-scale structure beyond ∼100 h -1 Mpc. Deeper redshift surveys are needed to reach the "convergence scale" of 250 h -1 Mpc in a ΛCDM universe. Deeper surveys would also mitigate the impact of the "Kaiser rocket" which, in a survey like 2MRS, remains a significant source of uncertainty. Thanks to the quiet and moderate density environment of the LG, purely dynamical uncertainties of the linear predictions are subdominant at the level of ∼90 km s-1. Finally, we show that deviations from linear galaxy biasing and shot noise errors provide a minor contribution to the total error budget. © 2014. The American Astronomical Society. All rights reserved.. Source


Nusser A.,Asher Space Science Institute Technion | Branchini E.,Third University of Rome | Davis M.,University of California at Berkeley
Astrophysical Journal | Year: 2011

We present a simple method for measuring cosmological bulk flows from large redshift surveys, based on the apparent dimming or brightening of galaxies due to their peculiar motion. It is aimed at estimating bulk flows of cosmological volumes containing large numbers of galaxies. Constraints on the bulk flow are obtained by minimizing systematic variations in galaxy luminosities with respect to a reference luminosity function measured from the whole survey. This method offers two advantages over more popular bulk flow estimators: it is independent of error-prone distance indicators and of the poorly known galaxy bias. We apply the method to the Two Micron All Sky Survey redshift survey to measure the local bulk flows of spherical shells centered on the Milky Way (MW). The result is consistent with that obtained by Nusser & Davis using the SFI++ catalogue of Tully-Fisher distance indicators. We also make an assessment of the ability of the method to constrain bulk flows at larger redshifts (z = 0.1-0.5) from next-generation data sets. As a case study we consider the planned EUCLID survey. Using this method we will be able to measure a bulk motion of 200 km s-1 of 106 galaxies with photometric redshifts, at the 3σ level for both z 0.15 and z 0.5. Thus, the method will allow us to put strong constraints on dark energy models as well as alternative theories for structure formation. © 2011. The American Astronomical Society. All rights reserved.. Source


Branchini E.,Third University of Rome | Branchini E.,National institute for astrophysics | Davis M.,University of California at Berkeley | Nusser A.,Asher Space Science Institute Technion
Monthly Notices of the Royal Astronomical Society | Year: 2012

Using the nearly full-sky K s= 11.75 2MASS Redshift Survey (2MRS; Huchra et al.) of ∼45000 galaxies, we reconstruct the underlying peculiar velocity field and constrain the cosmological bulk flow within ∼100h -1Mpc. These results are obtained by maximizing the probability to estimate the absolute magnitude of a galaxy given its observed apparent magnitude and redshift. At a depth of ≈60h -1Mpc, we find a bulk flow in agreement with the theoretical predictions of the Λ cold dark matter model. The reconstructed peculiar velocity field that maximizes the likelihood is characterized by the parameter β= 0.32 ± 0.08. Both results are in agreement with those obtained previously using the ∼23000 galaxies of the shallower K s= 11.25 2MRS survey. In our analysis, we find that the luminosity function of 2MRS galaxies is poorly fitted by the Schechter form and that luminosity evolves such that objects become fainter with increasing redshift according to L(z) =L(z= 0)(1 +z) +2.7 ± 0.15. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source


Nusser A.,Asher Space Science Institute Technion | Branchini E.,Third University of Rome | Branchini E.,National Institute of Nuclear Physics, Italy | Branchini E.,National institute for astrophysics | Davis M.,University of California at Berkeley
Astrophysical Journal | Year: 2012

Given a redshift survey of galaxies with measurements of apparent magnitudes, we present a novel method for measuring the growth rate f(Ω) of cosmological linear perturbations. We use the galaxy distribution within the survey to solve for the peculiar velocity field which depends in linear perturbation theory on β = f(Ω)/b, where b is the bias factor of the galaxy distribution. The recovered line-of-sight peculiar velocities are subtracted from the redshifts to derive the distances, which thus allows an estimate of the absolute magnitude of each galaxy. A constraint on β is then found by minimizing the spread of the estimated magnitudes from their distribution function. We apply the method to the all sky K = 11.25 2MASS Redshift Survey and derive β = 0.35 0.1 at z 0, remarkably consistent with our previous estimate from the velocity-velocity comparison. The method could easily be applied to subvolumes extracted from the Sloan Digital Sky Survey to derive the growth rate at z 0.1. Further, it should also be applicable to ongoing and future spectroscopic redshift surveys to trace the evolution of f(Ω) to z 1. Constraints obtained from this method are entirely independent from those obtained from the two-dimensional distortion of ξ(s) and provide an important check on f(Ω), as alternative gravity models predict observable differences. © 2012 The American Astronomical Society. All rights reserved. Source


Nusser A.,Asher Space Science Institute Technion | Davis M.,University of California at Berkeley
Astrophysical Journal | Year: 2011

We derive estimates for the cosmological bulk flow from the SFI++ Tully-Fisher (TF) catalog. For a sphere of radius 40 h -1 Mpc centered on the Milky Way, we derive a bulk flow of 333 ± 38 km s -1 toward Galactic (l, b) = (276°, 14°) within a 3° 1σ error. Within a radius of 100h -1 Mpc we get 257 ± 44 km s-1 toward (l, b) = (279°, 10°) within a 6° error. These directions are at 40° to the Supergalactic plane, close to the apex of the motion of the Local Group of galaxies after the Virgocentric infall correction. Our findings are consistent with the ΛCDM model with the latest Wilkinson Microwave Anisotropy Probe (WMAP) best-fit cosmological parameters, but the bulk flow allows independent constraints. For the WMAP-inferred Hubble parameter h = 0.71 and baryonic mean density parameter Ωb = 0.0449, the constraint from the bulk flow on the matter density, Ωm, the normalization of the density fluctuations, σ8, and the growth index, γ, can be expressed as σ8Ω γ - 0.55m(Ωm/0.266)0.28 = 0.86 ± 0.11 (for Ωm ≈ 0.266). Fixing σ8 = 0.8 and Ωm = 0.266 as favored by WMAP, we get γ = 0.495 ± 0.096. The constraint derived here rules out popular Dvali-Gabadadze-Porrati models at more than the 99% confidence level. Our results are based on the All Space Constrained Estimate (ACSE) model which reconstructs the bulk flow from an all space three-dimensional peculiar velocity field constrained to match the TF measurements. At large distances, ASCE generates a robust bulk flow from the SFI++ survey that is insensitive to the assumed prior. For comparison, a standard straightforward maximum likelihood estimate leads to very similar results. © 2011. The American Astronomical Society. All rights reserved. Source

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