778 research outputs found
The evidence for unusual gravity from the large-scale structure of the Universe
Under the assumption that General Relativity (GR) correctly describes the
phenomenology of our Universe, astronomical observations provide compelling
evidence that (1) the dynamics of cosmic structure is dominated by dark matter
(DM), an exotic matter mostly made of hypothetical elementary particles, and
(2) the expansion of the Universe is currently accelerating because of the
presence of a positive cosmological constant Lambda. The DM particles have not
yet been detected and there is no theoretical justification for the tiny
positive Lambda implied by observations. Therefore, over the last decade, the
search for extended or alternative theories of gravity has flourished.Comment: Invited review to appear in the Proceedings of the 1st AFI symposium
"From the Vacuum to the Universe", Innsbruck, Austria, October 2007, to be
published by the Innsbruck University Press, ed. by S.D. Bass, F. Schallhart
and B. Tasse
Wide field imaging of distant clusters
Wide field imaging is key to understanding the build-up of distant clusters
and their galaxy population. By focusing on the so far unexplored outskirts of
clusters, where infalling galaxies first hit the cluster potential and the hot
intracluster medium, we can help separate cosmological field galaxy evolution
from that driven by environment. I present a selection of recent advancements
in this area, with particular emphasis on Hubble Space Telescope wide field
imaging, for its superior capability to deliver galaxy morphologies and precise
shear maps of distant clusters.Comment: Invited contribution. To appear in "Outskirts of galaxy clusters:
intense life in the suburbs", A. Diaferio et al. eds. 7 pages, 5 figures.
Refereed versio
Identification of members in the central and outer regions of galaxy clusters
The caustic technique measures the mass of galaxy clusters in both their
virial and infall regions and, as a byproduct, yields the list of cluster
galaxy members. Here we use 100 galaxy clusters with mass M200>=1E14 Msun/h
extracted from a cosmological N-body simulation of a LambdaCDM universe to test
the ability of the caustic technique to identify the cluster galaxy members. We
identify the true three-dimensional members as the gravitationally bound
galaxies. The caustic technique uses the caustic location in the redshift
diagram to separate the cluster members from the interlopers. We apply the
technique to mock catalogues containing 1000 galaxies in the field of view of
12 Mpc/h on a side at the cluster location. On average, this sample size
roughly corresponds to 180 real galaxy members within 3r200, similar to recent
redshift surveys of cluster regions. The caustic technique yields a
completeness, the fraction of identified true members, fc=0.95 (+- 0.03) within
3r200. The contamination increases from fi=0.020 (+0.046;-0.015) at r200 to
fi=0.08 (+0.11;-0.05) at 3r200. No other technique for the identification of
the members of a galaxy cluster provides such large completeness and small
contamination at these large radii. The caustic technique assumes spherical
symmetry and the asphericity of the cluster is responsible for most of the
spread of the completeness and the contamination. By applying the technique to
an approximately spherical system obtained by stacking the individual clusters,
the spreads decrease by at least a factor of two. We finally estimate the
cluster mass within 3r200 after removing the interlopers: for individual
clusters, the mass estimated with the virial theorem is unbiased and within 30
per cent of the actual mass; this spread decreases to less than 10 per cent for
the spherically symmetric stacked cluster.Comment: 13 pages, 10 figures, published on Ap
Topology and Geometry of the CfA2 Redshift Survey
We analyse the redshift space topology and geometry of the nearby Universe by
computing the Minkowski functionals of the Updated Zwicky Catalogue (UZC). The
UZC contains the redshifts of almost 20,000 galaxies, is 96% complete to the
limiting magnitude m_Zw=15.5 and includes the Center for Astrophysics (CfA)
Redshift Survey (CfA2). From the UZC we can extract volume limited samples
reaching a depth of 70 hMpc before sparse sampling dominates. We quantify the
shape of the large-scale galaxy distribution by deriving measures of planarity
and filamentarity from the Minkowski functionals. The nearby Universe shows a
large degree of planarity and a small degree of filamentarity. This quantifies
the sheet-like structure of the Great Wall which dominates the northern region
(CfA2N) of the UZC. We compare these results with redshift space mock
catalogues constructed from high resolution N-body simulations of two Cold Dark
Matter models with either a decaying massive neutrino (tauCDM) or a non-zero
cosmological constant (LambdaCDM). We use semi-analytic modelling to form and
evolve galaxies in these dark matter-only simulations. We are thus able, for
the first time, to compile redshift space mock catalogues which contain
galaxies, along with their observable properties, rather than dark matter
particles alone. In both models the large scale galaxy distribution is less
coherent than the observed distribution, especially with regard to the large
degree of planarity of the real survey. However, given the small volume of the
region studied, this disagreement can still be a result of cosmic variance.Comment: 14 pages including 10 figures. Accepted for publication in Monthly
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