Willamette University Faculty Publications

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Consistently large cosmic flows on scales of 100 h−1 Mpc: a challenge for the standard ΛCDM cosmology
(Oxford Academic, 2009-12-23) Watkins, Richard; Feldman, Hume A.; Hudson, Michael J.
The bulk flow, i.e. the dipole moment of the peculiar velocity field, is a sensitive probe of matter density fluctuations on very large scales. However, the peculiar velocity surveys for which the bulk flow has been calculated have non-uniform spatial distributions of tracers, so that the bulk flow estimated does not correspond to that of a simple volume such as a sphere. Thus bulk flow estimates are generally not strictly comparable between surveys, even those whose effective depths are similar. In addition, the sparseness of typical surveys can lead to aliasing of small-scale power into what is meant to be a probe of the largest scales. Here we introduce a new method of calculating bulk flow moments where velocities are weighted to give an optimal estimate of the bulk flow of an idealized survey, with the variance of the difference between the estimate and the actual flow being minimized. These ‘minimum variance’ estimates can be designed to estimate the bulk flow on a particular scale with minimal sensitivity to small-scale power, and are comparable between surveys. We compile all major peculiar velocity surveys and apply this new method to them. We find that most surveys we studied are highly consistent with each other. Taken together the data suggest that the bulk flow within a Gaussian window of radius 50 h−1 Mpc is 407 ± 81 km s−1 toward l= 287°± 9°, b= 8°± 6°. The large-scale bulk motion is consistent with predictions from the local density field. This indicates that there are significant density fluctuations on very large scales. A flow of this amplitude on such a large scale is not expected in the WMAP5 (Wilkinson Microwave Anisotropy Probe) normalized Λ cold dark matter cosmology, for which the predicted one-dimensional rms velocity is ∼110 km s−1. The large amplitude of the observed bulk flow favours the upper values of the WMAP5 Ωmh2–σ8 error-ellipse, but even the point at the top of the WMAP595 per cent confidence ellipse predicts a bulk flow which is too low compared to that observed at >98 per cent confidence level.
Cosmic flows on 100 h−1 Mpc scales: standardized minimum variance bulk flow, shear and octupole moments
(Oxford Academic, 2010-07-22) Feldman, Hume A.; Watkins, Richard; Hudson, Michael J.
The low-order moments, such as the bulk flow and shear, of the large-scale peculiar velocity field are sensitive probes of the matter density fluctuations on very large scales. In practice, however, peculiar velocity surveys are usually sparse and noisy, which can lead to the aliasing of small-scale power into what is meant to be a probe of the largest scales. Previously, we developed an optimal ‘minimum variance’ (MV) weighting scheme, designed to overcome this problem by minimizing the difference between the measured bulk flow (BF) and that which would be measured by an ideal survey. Here we extend this MV analysis to include the shear and octupole moments, which are designed to have almost no correlations between them so that they are virtually orthogonal. We apply this MV analysis to a compilation of all major peculiar velocity surveys, consisting of 4536 measurements. Our estimate of the BF on scales of ∼100 h−1 Mpc has a magnitude of |v| = 416 ± 78 km s −1 towards Galactic l= 282°± 11° and b= 6°± 6°. This result is in disagreement with Λ cold dark matter with Wilkinson Microwave Anisotropy Probe 5 (WMAP5) cosmological parameters at a high confidence level, but is in good agreement with our previous MV result without an orthogonality constraint, showing that the shear and octupole moments did not contaminate the previous BF measurement. The shear and octupole moments are consistent with WMAP5 power spectrum, although the measurement noise is larger for these moments than for the BF. The relatively low shear moments suggest that the sources responsible for the BF are at large distances.
A slight excess of large-scale power from moments of the peculiar velocity field
(Oxford Academic, 2011-06-01) Macaulay, E.; Feldman, H.; Ferreira, P.G.; Hudson, M.J.; Watkins, R.
The peculiar motions of galaxies can be used to infer the distribution of matter in the Universe. It has recently been shown that measurements of the peculiar velocity field indicate an anomalously high bulk flow of galaxies in our local volume. In this paper, we find the implications of the high bulk flow for the power spectrum of density fluctuations. We find that analysing only the dipole moment of the velocity field yields an average power spectrum amplitude which is indeed much higher than the Λ cold dark matter (ΛCDM) value. However, by also including shear and octupole moments of the velocity field, and marginalizing over possible values for the growth rate, an average power spectrum amplitude which is consistent with the ΛCDM is recovered. We attempt to infer the shape of the matter power spectrum from moments of the velocity field, and find a slight excess of power on scales ∼1 h−1 Gpc.
Cosmic flows in the nearby universe from Type Ia supernovae
(Oxford Academic, 2012-01-23) Turnbull, Stephen J.; Hudson, Michael J.; Feldman, Hume A.; Hicken, Malcolm; Kirshner, Robert P.; Watkins, Richard
Peculiar velocities are one of the only probes of very large scale mass density fluctuations in the nearby Universe. We present new ‘minimal variance’ bulk flow measurements based upon the ‘First Amendment’ compilation of 245 Type Ia supernovae (SNe) peculiar velocities and find a bulk flow of 249 ± 76 km s−1 in the direction l= 319°± 18°, b= 7°± 14°. The SNe bulk flow is consistent with the expectations of Λ cold dark matter (ΛCDM). However, it is also marginally consistent with the bulk flow of a larger compilation of non-SNe peculiar velocities. By comparing the SNe peculiar velocities to predictions of the IRAS Point Source Catalogue Redshift Survey (PSCz) galaxy density field, we find Ω0.55mσ8,lin= 0.40 ± 0.07, which is in agreement with ΛCDM. However, we also show that the PSCz density field fails to account for 150 ± 43 km s−1 of the SNe bulk motion.
Testing the minimum variance method for estimating large-scale velocity moments
(Oxford Academic, 2012-09) Argarwal, Shankar; Feldman, Hume A.; Watkins, Richard
The estimation and analysis of large-scale bulk flow moments of peculiar velocity surveys is complicated by non-spherical survey geometry, the non-uniform sampling of the matter velocity field by the survey objects and the typically large measurement errors of the measured line-of-sight velocities. Previously, we have developed an optimal ‘;minimum variance’ (MV) weighting scheme for using peculiar velocity data to estimate bulk flow moments for idealized, dense and isotropic surveys with Gaussian radial distributions, that avoids many of these complications. These moments are designed to be easy to interpret and are comparable between surveys. In this paper, we test the robustness of our MV estimators using numerical simulations. Using MV weights, we estimate the bulk flow moments for various mock catalogues extracted from the LasDamas and the Horizon Run numerical simulations and compare these estimates to the moments calculated directly from the simulation boxes. We show that the MV estimators are unbiased and negligibly affected by non-linear flows.

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