139 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
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
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
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
Cosmic ray acceleration at supergalactic accretion shocks: a new upper energy limit due to a finite shock extension
Accretion flows onto supergalactic-scale structures are accompanied with
large spatial scale shock waves. These shocks were postulated as possible
sources of ultra-high energy cosmic rays. The highest particle energies were
expected for perpendicular shock configuration in the so-called "Jokipii
diffusion limit", involving weakly turbulent conditions in the large-scale
magnetic field imbedded in the accreting plasma. For such configuration we
discuss the process limiting the highest energy that particles can obtain in
the first-order Fermi acceleration process due to finite shock extensions to
the sides, along and across the mean magnetic field. Cosmic ray outflow along
the shock structure can substantially lower (below ~10^18 eV for protons) the
upper particle energy limit for conditions considered for supergalactic shocks.Comment: A&A, accepte
Stochastic Biasing and Weakly Non-linear Evolution of Power Spectrum
Distribution of galaxies may be a biased tracer of the dark matter
distribution and the relation between the galaxies and the total mass may be
stochastic, non-linear and time-dependent. Since many observations of galaxy
clustering will be done at high redshift, the time evolution of non-linear
stochastic biasing would play a crucial role for the data analysis of the
future sky surveys. In this paper, we develop the weakly non-linear analysis
and attempt to clarify the non-linear feature of the stochastic biasing. We
compute the one-loop correction of the power spectrum for the total mass, the
galaxies and their cross correlation. Assuming the local functional form for
the initial galaxy distribution, we investigate the time evolution of the
biasing parameter and the correlation coefficient. On large scales, we first
find that the time evolution of the biasing parameter could deviate from the
linear prediction in presence of the initial skewness. However, the deviation
can be reduced when the initial stochasticity exists. Next, we focus on the
quasi-linear scales, where the non-linear growth of the total mass becomes
important. It is recognized that the scale-dependence of the biasing
dynamically appears and the initial stochasticity could affect the time
evolution of the scale-dependence. The result is compared with the recent
N-body simulation that the scale-dependence of the halo biasing can appear on
relatively large scales and the biasing parameter takes the lower value on
smaller scales. Qualitatively, our weakly non-linear results can explain this
trend if the halo-mass biasing relation has the large scatter at high redshift.Comment: 29pages, 7 postscript figures, submitted to Ap
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
Tailing Dam Zelazny Most Environmental Hazard
The factors governing the geotechnical stability of copper tailing dam Żelazny Most, the largest in Europe, have been discussed. The efficiency of pumping wells barrier against outflow of contaminated ground water has been predicted for a period of next 15 years
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
Skewness as a probe of non-Gaussian initial conditions
We compute the skewness of the matter distribution arising from non-linear
evolution and from non-Gaussian initial perturbations. We apply our result to a
very generic class of models with non-Gaussian initial conditions and we
estimate analytically the ratio between the skewness due to non-linear
clustering and the part due to the intrinsic non-Gaussianity of the models. We
finally extend our estimates to higher moments.Comment: 5 pages, 2 ps-figs., accepted for publication in PRD, rapid com
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