6,870 research outputs found
An Unbiased Estimator of Peculiar Velocity with Gaussian Distributed Errors for Precision Cosmology
We introduce a new estimator of the peculiar velocity of a galaxy or group of
galaxies from redshift and distance estimates. This estimator results in
peculiar velocity estimates which are statistically unbiased and that have
errors that are Gaussian distributed, thus meeting the assumptions of analyses
that rely on individual peculiar velocities. We apply this estimator to the
SFI++ and the Cosmicflows-2 catalogs of galaxy distances and, using the fact
that peculiar velocity estimates of distant galaxies are error dominated,
examine their error distributions, The adoption of the new estimator
significantly improves the accuracy and validity of studies of the large-scale
peculiar velocity field and eliminates potential systematic biases, thus
helping to bring peculiar velocity analysis into the era of precision
cosmology. In addition, our method of examining the distribution of velocity
errors should provide a useful check of the statistics of large peculiar
velocity catalogs, particularly those that are compiled out of data from
multiple sources.Comment: 6 Pages, 5 Figure
Theoretical Expectations for Bulk Flows in Large Scale Surveys
We calculate the theoretical expectation for the bulk motion of a large scale
survey of the type recently carried out by Lauer and Postman. Included are the
effects of survey geometry, errors in the distance measurements, clustering
properties of the sample, and different assumed power spectra. We consider the
power spectrum calculated from the IRAS--QDOT survey, as well as spectra from
hot cold and standard cold dark matter models. We find that sparse sampling
and clustering can lead to an unexpectedly large bulk flow, even in a very deep
survey. Our results suggest that the expected bulk motion is inconsistent with
that reported by Lauer and Postman at the confidence level.Comment: 13 pages, uuencoded compressed postscript file with two figures and a
table enclosed, UM-AC-93-2
The Cosmic Mach Number: Comparison from Observations, Numerical Simulations and Nonlinear Predictions
We calculate the cosmic Mach number M - the ratio of the bulk flow of the
velocity field on scale R to the velocity dispersion within regions of scale R.
M is effectively a measure of the ratio of large-scale to small-scale power and
can be a useful tool to constrain the cosmological parameter space. Using a
compilation of existing peculiar velocity surveys, we calculate M and compare
it to that estimated from mock catalogues extracted from the LasDamas (a LCDM
cosmology) numerical simulations. We find agreement with expectations for the
LasDamas cosmology at ~ 1.5 sigma CL. We also show that our Mach estimates for
the mocks are not biased by selection function effects. To achieve this, we
extract dense and nearly-isotropic distributions using Gaussian selection
functions with the same width as the characteristic depth of the real surveys,
and show that the Mach numbers estimated from the mocks are very similar to the
values based on Gaussian profiles of the corresponding widths. We discuss the
importance of the survey window functions in estimating their effective depths.
We investigate the nonlinear matter power spectrum interpolator PkANN as an
alternative to numerical simulations, in the study of Mach number.Comment: 12 pages, 9 figures, 3 table
Structure in a Loitering Universe
We study the formation of structure for a universe that undergoes a recent
loitering phase. We compare the nonlinear mass distribution to that in a
standard, matter dominated cosmology. The statistical aspects of the clustered
matter are found to be robust to changes in the expansion law, an exception
being that the peculiar velocities are lower by a factor of in the
loitering model. Further, in the loitering scenario, nonlinear growth of
perturbation occurs more recently () than in the matter dominated
case. Differences in the high redshift appearances of the two models will
result but observable consequences depend critically on the chosen form, onset
and duration of the loitering phase.Comment: 8 pages, (uses revtex.sty), 5 figures not included, available on
request, UM AC 92-
Current Status Of Velocity Field Surveys: A Consistency Check
We present an analysis comparing the bulk--flow measurements for six recent
peculiar velocity surveys, namely, ENEAR, SFI, RFGC, SBF and the Mark III
singles and group catalogs. We study whether the direction of the bulk--flow
estimates are consistent with each other and construct the full three
dimensional bulk--flow vectors for each survey. We show that although the
surveys differ in their geometry, galaxy morphologies, distance measures and
measurement errors, their bulk flow vectors are expected to be highly
correlated and in fact show impressive agreement in all cases. We found a
combined weighted mean bulk motion of 330 km s km s
toward and
in a sphere with an effective depth of km s.Comment: 16 pages, 2 figures 2 tables, minor changes, reflects published
versio
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