657 research outputs found
Large Scale Peculiar Velocities: Effects from Superclusters
We study the gravitational influence of very large scale structures, as
traced by clusters of galaxies, on the Local Group [LG] motion and the large
scale flows. We derive from the distribution of Abell clusters within
Mpc/h the overdensity field on a 3--D grid of spacing Mpc/h, then we
solve the Poisson equation for the peculiar potential and finally obtain the
peculiar velocity field. Quite interestingly, from this global solution we: i)
recover within the direction of the LG motion in the Cosmic
Microwave Background [CMB] frame, ii find that the Great Attractor itself moves
wrt to the CMB frame, iii) derive from a preliminary comparison with the Mark~II catalog of
peculiar velocities, iv) derive estimates for the bulk flow in spheres which
fairly agree with the level derived from POTENT and the Spiral samples, v) find
that the Lauer \& Postman [L\&P] bulk flow has too a large amplitude to be in
agreement with our results.Comment: To appear in XXXth Moriond "Clustering in the Universe". Four,
uuencoded, compressed (gzip -9), self unpacking postscript pages (figures
included). [if you want an hardcopy of Ref.5, please send an E-mail to
[email protected]
Measuring primordial non-gaussianity without cosmic variance
Non-gaussianity in the initial conditions of the universe is one of the most
powerful mechanisms to discriminate among the competing theories of the early
universe. Measurements using bispectrum of cosmic microwave background
anisotropies are limited by the cosmic variance, i.e. available number of
modes. Recent work has emphasized the possibility to probe non-gaussianity of
local type using the scale dependence of large scale bias from highly biased
tracers of large scale structure. However, this power spectrum method is also
limited by cosmic variance, finite number of structures on the largest scales,
and by the partial degeneracy with other cosmological parameters that can mimic
the same effect. Here we propose an alternative method that solves both of
these problems. It is based on the idea that on large scales halos are biased,
but not stochastic, tracers of dark matter: by correlating a highly biased
tracer of large scale structure against an unbiased tracer one eliminates the
cosmic variance error, which can lead to a high signal to noise even from the
structures comparable to the size of the survey. The square of error
improvement on non-gaussianity parameter f_nl relative to the power spectrum
method scales as Pn/2, where P and n is the power spectrum and the number
density of the biased tracer, respectively. For an ideal survey out to z=2 the
error reduction can be as large as a factor of seven, which should guarantee a
detection of non-gaussianity from an all sky survey of this type. The
improvements could be even larger if high density tracers that are sensitive to
non-gaussianity can be identified and measured over a large volume.Comment: 7 page
Mass - concentration relation and weak lensing peak counts
The statistics of peaks in weak lensing convergence maps is a promising tool
to investigate both the properties of dark matter haloes and constrain the
cosmological parameters. We study how the number of detectable peaks and its
scaling with redshift depend upon the cluster dark matter halo profiles and use
peak statistics to constrain the parameters of the mass - concentration (MC)
relation. We investigate which constraints the Euclid mission can set on the MC
coefficients also taking into account degeneracies with the cosmological
parameters. To this end, we first estimate the number of peaks and its redshift
distribution for different MC relations. We find that the steeper the mass
dependence and the larger the normalisation, the higher is the number of
detectable clusters, with the total number of peaks changing up to
depending on the MC relation. We then perform a Fisher matrix forecast of the
errors on the MC relation parameters as well as cosmological parameters. We
find that peak number counts detected by Euclid can determine the normalization
, the mass and redshift slopes and intrinsic scatter
of the MC relation to an unprecedented accuracy being
, , ,
if all cosmological parameters are assumed to
be known. Should we relax this severe assumption, constraints are degraded, but
remarkably good results can be restored setting only some of the parameters or
combining peak counts with Planck data. This precision can give insight on
competing scenarios of structure formation and evolution and on the role of
baryons in cluster assembling. Alternatively, for a fixed MC relation, future
peaks counts can perform as well as current BAO and SNeIa when combined with
Planck.Comment: 14 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
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