138 research outputs found
Dynamics of pairwise motions
We derive a simple closed-form expression, relating \vs(r) -- the mean
relative velocity of pairs of galaxies at fixed separation -- to the
two-point correlation function of mass density fluctuations, . We
compare our analytic model for \vs(r) with N-body simulations, and find
excellent agreement in the entire dynamical range probed by the simulations
(0.1 \lsim \xi \lsim 1000). Our results can be used to estimate the
cosmological density parameter, \Om, directly from redshift-distance surveys,
like Mark III.Comment: 10 pages 2 Figs., submitted to ApJ Let
Omega from the skewness of the cosmic velocity divergence
We propose a method for measuring the cosmological density parameter
from the statistics of the divergence field, , the
divergence of peculiar velocity, expressed in units of the Hubble constant, . The velocity field is spatially smoothed over to remove strongly nonlinear effects. Assuming weakly-nonlinear
gravitational evolution from Gaussian initial fluctuations, and using
second-order perturbative analysis, we show that \propto
-\Omega^{-0.6} ^2. The constant of proportionality depends on the
smoothing window. For a top-hat of radius R and volume-weighted smoothing, this
constant is , where . If the
power spectrum is a power law, , then . A Gaussian
window yields similar results. The resulting method for measuring is
independent of any assumed biasing relation between galaxies and mass.
The method has been successfully tested with numerical simulations. A
preliminary application to real data, provided by the POTENT recovery procedure
from observed velocities favors . However, because of an
uncertain sampling error, this result should be treated as an assessment of the
feasibility of our method rather than a definitive measurement of .Comment: 16 pages + 2 figures, uuencoded postscript file, also available by
anonymous ftp from ftp.cita.utoronto.ca in directory
/cita/francis/div_skewness, CITA 94-1
Streaming velocities as a dynamical estimator of Omega
It is well known that estimating the pairwise velocity of galaxies, v_{12},
from the redshift space galaxy correlation function is difficult because this
method is highly sensitive to the assumed model of the pairwise velocity
dispersion. Here we propose an alternative method to estimate v_{12} directly
from peculiar velocity samples, which contain redshift-independent distances as
well as galaxy redshifts. In contrast to other dynamical measures which
determine beta = sigma_8 x Omega^{0.6}, our method can provide an estimate of
(sigma_8)^2 x Omega^{0.6} for a range of sigma_8 (here Omega is the
cosmological mass density parameter while sigma_8 is the standard normalization
parameter for the spectrum of matter density fluctuations). We demonstrate how
to measure this quantity from realistic catalogues.Comment: 8 pages of text, 4 figures Subject headings: Cosmology: theory -
observation - peculiar velocities: large scale flows Last name of one of the
authors was misspelled. It is now corrected. Otherwise the manuscript is
identical to its original versio
Measuring Omega with Galaxy Streaming Velocities
The mean pairwise velocity of galaxies has traditionally been estimated from
the redshift space galaxy correlation function. This method is notorious for
being highly sensitive to the assumed model of the pairwise velocity
dispersion. Here we propose an alternative method to estimate the streaming
velocity directly from peculiar velocity samples, which contain
redshift-independent distances as well as galaxy redshifts. This method can
provide an estimate of for a range of where
is the cosmological density parameter, while is the
standard normalization for the power spectrum of density fluctuations. We
demonstrate how to measure this quantity from realistic catalogues and identify
the main sources of bias and errorsComment: Proceedings of New Worlds in Astroparticle Physics, 6 pages, 2
figure
Evidence for a low-density Universe from the relative velocities of galaxies
The motions of galaxies can be used to constrain the cosmological density
parameter Omega and the clustering amplitude of matter on large scales. The
mean relative velocity of galaxy pairs, estimated from the Mark III survey,
indicates that Omega = 0.35 +0.35/-0.25. If the clustering of galaxies is
unbiased on large scales, Omega = 0.35 +/- 0.15, so that an unbiased
Einstein-de Sitter model (Omega = 1) is inconsistent with the data.Comment: 12 pages, 2 figures, to appear in the Jan.7 issue of ``Science''; In
the original version, the title appeared twice. This problem has now been
corrected. No other changes were mad
Previrialization
We propose a method to solve the "previrialization" problem of whether the non-linear interactions between perturbations at different scales increase or decrease the rate of growth of structure. As a measure of this effect we calculate the weakly non-linear corrections to the variance of the probability distribution function of the density field. We assume Gaussian initial conditions and use perturbative expansions to calculate these corrections for scale-free initial power spectra. As a realistic example, we also compute the corrections for the spectrum proposed by Peacock \& Dodds (1994). The calculations are performed for both a Gaussian and a top-hat smoothing of the evolved fields. We show that the effect of weakly non-linear interactions depends strongly on the spectral index; they increase the variance for the spectral index n=-2, but decrease it for n \ge -1. Finally, we compare our perturbative calculations to N-body simulations and a formula of a type proposed by Hamilton et al. (1991)
Dipole anisotropies of IRAS galaxies and the contribution of a large-scale local void
Recent observations of dipole anisotropies show that the velocity of the
Local Group (\Vec v_{\rm G}) induced by the clustering of IRAS galax ies has
an amplitude and direction similar to those of the velocity of Cosmic Microwave
Background dipole anisotropy (\Vec v_{\rm CMB}), but the difference | \Vec
v_{\rm G} - \Vec v_{\rm CMB} | is still km/s, which is about 28% of
|\Vec v_{\rm CMB} |. Here we consider the possibility that the origin of this
difference comes from a hypothetical large-scale local void, with which we can
account for the accelerating behavior of type Ia supernovae due to the spatial
inhomogeneity of the Hubble constant without dark energies and derive the
constraint to the model parameters of the local void. It is found as a result
that the distance between the Local Group and the center of the void must be
Mpc, whose accurate value depends on the background model
parameters.Comment: 13 pages, 1 figure, to be published in ApJ 584, No.2 (2003
Large-k Limit of Multi-Point Propagators in the RG Formalism
Renormalized versions of cosmological perturbation theory have been very
successful in recent years in describing the evolution of structure formation
in the weakly non-linear regime. The concept of multi-point propagators has
been introduced as a tool to quantify the relation between the initial matter
distribution and the final one and to push the validity of the approaches to
smaller scales. We generalize the n-point propagators that have been considered
until now to include a new class of multi-point propagators that are relevant
in the framework of the renormalization group formalism. The large-k results
obtained for this general class of multi-point propagators match the results
obtained earlier both in the case of Gaussian and non-Gaussian initial
conditions. We discuss how the large-k results can be used to improve on the
accuracy of the calculations of the power spectrum and bispectrum in the
presence of initial non-Gaussianities.Comment: 30 page
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