841 research outputs found
The use of Minimal Spanning Tree to characterize the 2D cluster galaxy distribution
We use the Minimal Spanning Tree to characterize the aggregation level of
given sets of points. We test 3 distances based on the histogram of the MST
edges to discriminate between the distributions. We calibrate the method by
using artificial sets following Poisson, King or NFW distributions. The
distance using the mean, the dispersion and the skewness of the histogram of
MST edges provides the more efficient results. We apply this distance to a
subsample of the ENACS clusters and we show that the bright galaxies are
significantly more aggregated than the faint ones. The contamination provided
by uniformly distributed field galaxies is neglectible. On the other hand, we
show that the presence of clustered groups on the same cluster line of sight
masked the variation of the distance with the considered magnitude.Comment: 9 pages, 7 postscript figures, LateX A\{&}A, accepted in A\{&}
Group analysis in the SSRS2 catalog
We present an automated method to detect populations of groups in galaxy
redshift catalogs. This method uses both analysis of the redshift distribution
along lines of sight in fixed cells to detect elementary structures and a
friend-of-friend algorithm to merge these elementary structures into physical
structures. We apply this method to the SSRS2 galaxy redshift catalog. The
groups detected with our method are similar to group catalogs detected with
pure friend-of-friend algorithms. They have similar mass distribution, similar
abundance versus redshift, similar 2-point correlation function and the same
redshift completeness limit, close to 5000 km/s. If instead of SSRS2, we use
catalogs of new generation, it would lead to a completeness limit of
z0.7. We model the luminosity function for nearby galaxy groups by a
Schechter function with parameters M*=(-19.99+/-0.36)+5logh and alpha=-1.46 +/-
0.17 to compute the mass to light ratio. The median value of the mass to light
ratio is 360 h M/L and we deduce a relation between mass to light ratio and
velocity dispersion sigma (M/L=3.79 +/- 0.64)sigma -(294 +/- 570)). The more
massive the group, the higher the mass to light ratio, and therefore, the
larger the amount of dark matter inside the group. Another explanation is a
significant stripping of the gas of the galaxies in massive groups as opposed
to low mass groups. This extends to groups of galaxies the mild tendency
already detected for rich clusters of galaxies. Finally, we detect a barely
significant fundamental plane for these groups but much less narrow than for
clusters of galaxies.Comment: 8 pages, 5 figures, accepted in A&A, shortened abstrac
Cluster luminosity function and n^th ranked magnitude as a distance indicator
We define here a standard candle to determine the distance of clusters of
galaxies and to investigate their peculiar velocities by using the n^{th} rank
galaxy (magnitude m). We address the question of the universality of the
luminosity function for a sample of 28 rich clusters of galaxies () in order to model the influence on of cluster richness. This
luminosity function is found to be universal and the fit of a Schechter profile
gives and in the range
[-21,-17]. The uncorrected distance indicator is more efficient for the
first ranks n. With n=5, we have a dispersion of 0.61 magnitude for the
(m,5log(cz)) relation. When we correct for the richness effect and subtract
the background galaxies we reduce the uncertainty to 0.21 magnitude with n=15.
Simulations show that a large part of this dispersion originates from the
intrinsic scatter of the standard candle itself. These provide upper bounds on
the amplitude of cluster radial peculiar motions. At a confidence
level of 90%, the dispersion is 0.13 magnitude and is limited to
1200 km/s for our sample of clusters.Comment: 9 pages, 7 postscript figures, LateX A&A, accepted in A&
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