668 research outputs found
A substructure analysis of the A3558 cluster complex
The "algorithm driven by the density estimate for the identification of
clusters" (DEDICA, Pisani 1993, 1996) is applied to the A3558 cluster complex
in order to find substructures. This complex, located at the center of the
Shapley Concentration supercluster, is a chain formed by the ACO clusters
A3556, A3558 and A3562 and the two poor clusters SC 1327-312 and SC 1329-313.
We find a large number of clumps, indicating that strong dynamical processes
are active. In particular, it is necessary to use a fully three-dimensional
sample(i.e. using the galaxy velocity as third coordinate) in order to recover
also the clumps superimposed along the line of sight. Even if a great number of
detected substructures were already found in a previous analysis (Bardelli et
al. 1998), this method is more efficient and faster when compared with the use
of a wide battery of tests and permits the direct estimate of the detection
significance. Almost all subclusters previously detected by the wavelet
analyses found in the literature are recognized by DEDICA.
On the basis of the substructure analysis, we also briefly discuss the origin
of the A3558 complex by comparing two hypotheses: 1) the structure is a
cluster-cluster collision seen just after the first core-core encounter; 2)
this complex is the result of a series of incoherent group-group and
cluster-group mergings, focused in that region by the presence of the
surrounding supercluster. We studied the fraction of blue galaxies in the
detected substructures and found that the bluest groups reside between A3562
and A3558, i.e. in the expected position in the scenario of the cluster-cluster
collision.Comment: 10 pages with 12 encapsulated figures; MNRAS in pres
On the absence of gravitational lensing of the cosmic microwave background
The magnification of distant sources by mass clumps at lower ()
redshifts is calculated analytically. The clumps are initially assumed to be
galaxy group isothermal spheres with properties inferred from an extensive
survey. The average effect, which includes strong lensing, is exactly
counteracted by the beam divergence in between clumps (more precisely, the
average reciprocal magnification cancels the inverse Dyer-Roeder
demagnification). This conclusion is in fact independent of the matter density
function within each clump, and remains valid for arbitrary densities of matter
and dark energy. When tested against the CMB, a rather large lensing induced
{\it dispersion} in the angular size of the primary acoustic peaks of the TT
power spectrum is inconsistent with WMAP observations. The situation is
unchanged by the use of NFW profiles for the density distribution of groups.
Finally, our formulae are applied to an ensemble of NFW mass clumps or
isothermal spheres having the parameters of galaxy {\it clusters}. The acoustic
peak size dispersion remains unobservably large, and is also excluded by WMAP.
For galaxy groups, two possible ways of reconciling with the data are proposed,
both exploiting maximally the uncertainties in our knowledge of group
properties. The same escape routes are not available in the case of clusters,
however, because their properties are well understood. Here we have a more
robust conclusion: neither of the widely accepted models are good description
of clusters, or important elements of physics responsible for shaping zero
curvature space are missing from the standard cosmological model. When all the
effects are accrued, it is difficult to understand how WMAP could reveal no
evidence whatsoever of lensing by groups and clusters.Comment: ApJ v628, pp. 583-593 (August 1, 2005
Regularity bounds by minimal generators and Hilbert function
Let be the regularity of the Hilbert function of a projective curve in \mbox {P}^n_K over an algebraically closed field and
be degrees for which there exists a complete intersection of type () containing properly . Then the Castelnuovo-Mumford regularity of is bounded above by max .We investigate the sharpness of the above bound, which is achieved by curves algebraically linked to ones having degenerate general hyperplane section
K-band Properties of Well-Sampled Groups of Galaxies
We use a sample of 55 groups and 6 clusters of galaxies ranging in mass from
7 x 10^11 Msun to 1.5 x 10^15 Msun to examine the correlation of the Ks-band
luminosity with mass discovered by Lin et al. (2003). We use the 2MASS catalog
and published redshifts to construct complete magnitude limited redshift
surveys of the groups. From these surveys we explore the IR photometric
properties of groups members including their IR color distribution and
luminosity function. Although we find no significant difference between the
group Ks luminosity function and the general field, there is a difference
between the color distribution of luminous group members and their counterparts
(generally background) in the field. There is a significant population of
luminous galaxies with H-Ks > 0.35 which are rarely, if ever, members of the
groups in our sample. The most luminous galaxies which populate the groups have
a very narrow range of IR color. Over the entire mass range covered by our
sample, the Ks luminosity increases with mass as L ~ M^(0.64 +/- 0.06) implying
that the mass-to-light ratio in the Ks-band increases with mass. The agreement
between this result and earlier investigations of essentially non-overlapping
sets of systems shows that this window in galaxy formation and evolution is
insensitive to the selection of the systems and to the details of the mass and
luminosity computations.Comment: 38 pages, 9 figures, 2 tables. Accepted for publication on
Astronomical Journa
Confirming EIS Clusters. Optical and Infrared Imaging
Clusters of galaxies are important targets in observationally cosmology, as
they can be used both to study the evolution of the galaxies themselves and to
constrain cosmological parameters. Here we report on the first results of a
major effort to build up a sample of distant galaxy clusters to form the basis
for further studies within those fields. We search for simultaneous
overdensities in color and space to obtain supporting evidence for the reality
of the clusters. We find a confirmation rate for EIS clusters of 66%,
suggesting that a total of about 80 clusters with z>=0.6 are within reach using
the EIS cluster candidates.Comment: 4 pages, 2 figures, to appear in the proceedings of the IGRAP
International Conference 1999 on 'Clustering at high Redshift
Finding Galaxy Clusters using Voronoi Tessellations
We present an objective and automated procedure for detecting clusters of
galaxies in imaging galaxy surveys. Our Voronoi Galaxy Cluster Finder (VGCF)
uses galaxy positions and magnitudes to find clusters and determine their main
features: size, richness and contrast above the background. The VGCF uses the
Voronoi tessellation to evaluate the local density and to identify clusters as
significative density fluctuations above the background. The significance
threshold needs to be set by the user, but experimenting with different choices
is very easy since it does not require a whole new run of the algorithm. The
VGCF is non-parametric and does not smooth the data. As a consequence, clusters
are identified irrispective of their shape and their identification is only
slightly affected by border effects and by holes in the galaxy distribution on
the sky. The algorithm is fast, and automatically assigns members to
structures.Comment: 11 pages, 11 figures. It uses aa.cls (included). Accepted by A&
Spectroscopy of moderately high-redshift RCS-1 clusters
We present spectroscopic observations of 11 moderately high-redshift (z~0.7-
1.0) clusters from the first Red-Sequence Cluster Survey (RCS-1). We find
excellent agreement between the red-sequence estimated redshift and the
spectroscopic redshift, with a scatter of 10% at z>0.7. At the high-redshift
end (z>~0.9) of the sample, we find two of the systems selected are projections
of pairs of comparably rich systems, with red-sequences too close to
discriminate in (R-z') colour. In one of these systems, the two components are
close enough to be physically associated. For a subsample of clusters with
sufficient spectroscopic members, we examine the correlation between B_gcR
(optical richness) and the dynamical mass inferred from the velocity
dispersion. We find these measurements to be compatible, within the relatively
large uncertainties, with the correlation established at lower redshift for the
X-ray selected CNOC1 clusters and also for a lower redshift sample of RCS-1
clusters. Confirmation of this and calibration of the scatter in the relation
will require larger samples of clusters at these and higher redshifts.
[abridged]Comment: AJ accepted. 30 pages, 7 figures (figure 5 reduced quality
Spectroscopic confirmation of clusters from the ESO imaging survey
We measure redshifts for 67 galaxies in the field of six cluster candidates
from the ESO Imaging Survey (EIS). The cluster candidates are selected in the
EIS patches C and D among those with estimated mean redshifts between 0.5 and
0.7. The observations were made with EFOSC2 at the 3.6m ESO telescope.
In the six candidate cluster fields, we identify 19 possible sets of 2 to 7
galaxies in redshift space. In order to establish which of the 19 sets are
likely to correspond to real dense systems we compare our counts with those
expected from a uniform distribution of galaxies with given luminosity
function. In order to take into account the effect of the Large Scale
Structure, we modulate the probability computed from the luminosity function
with random samplings of the Canada-France Redshift Survey.
We find that four out of six candidate EIS clusters are likely to correspond
to real systems in redshift space (> 95 % confidence level). Two of these
systems have mean redshift in agreement with the redshift estimate given by the
matched filter algorithm. The other two systems have significantly lower
redshifts.
We discuss the implications of our results in the context of our ongoing
research projects aimed at defining high-redshift optically-selected cluster
samples.Comment: To appear in A&A, main journal -- 12 pages, 9 figure
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