859 research outputs found
The structure of Abell 1351: a bimodal galaxy cluster with peculiar diffuse radio emission
We aim to review the internal structure and dynamics of the Abell 1351
cluster, shown to host a radio halo with a quite irregular shape. Our analysis
is based on radial velocity data for 135 galaxies obtained at the Telescopio
Nazionale Galileo. We combine galaxy velocities and positions to select 95
cluster galaxy members and analyse the internal dynamics of the whole cluster.
We also examine X-ray data retrieved from Chandra and XMM archives. We measure
the cluster redshift, =0.325, the line-of-sight (LOS) velocity dispersion,
\sigma_v~1500 km/s, and the X-ray temperature, kT~9 keV. From both X-ray and
optical data independently, we estimate a large cluster mass, in the 1--4
M range. We attribute the extremely high value of \sigma_v to
the bimodality in the velocity distribution. We find evidence of a significant
velocity gradient and optical 3D substructure. The X-ray analysis also shows
many features in favour of a complex cluster structure, probably supporting an
ongoing merger of substructures in Abell 1351. The observational scenario
agrees with the presence of two main subclusters in the northern region, each
with its brightest galaxy (BCG1 and BCG2), detected as the two most important
X-ray substructures with a rest-frame LOS velocity difference of \Delta v~2500
km/s (in the rest frame) and probably being in large part aligned with the LOS.
We conclude that Abell 1351 is a massive merging cluster. The details of the
cluster structure allow us to interpret the quite asymmetric radio halo as a
`normal' halo plus a southern relic, strongly supporting a previous suggestion
based only on inspection of radio and preliminary X-ray data.Comment: 13 pages, 13 figures, 1 tabl
Consensus graph and spectral representation for one-step multi-view kernel based clustering
Recently, multi-view clustering has received much attention in the fields of machine learning and pattern recognition. Spectral clustering for single and multiple views has been the common solution. Despite its good clustering performance, it has a major limitation: it requires an extra step of clustering. This extra step, which could be the famous k-means clustering, depends heavily on initialization, which may affect the quality of the clustering result. To overcome this problem, a new method called Multiview Clustering via Consensus Graph Learning and Nonnegative Embedding (MVCGE) is presented in this paper. In the proposed approach, the consensus affinity matrix (graph matrix), consensus representation and cluster index matrix (nonnegative embedding) are learned simultaneously in a unified framework. Our proposed method takes as input the different kernel matrices corresponding to the different views. The proposed learning model integrates two interesting constraints: (i) the cluster indices should be as smooth as possible over the consensus graph and (ii) the cluster indices are set to be as close as possible to the graph convolution of the consensus representation. In this approach, no post-processing such as k-means or spectral rotation is required. Our approach is tested with real and synthetic datasets. The experiments performed show that the proposed method performs well compared to many state-of-the-art approaches
Fossil group origins - VI. Global X-ray scaling relations of fossil galaxy clusters
We present the first pointed X-ray observations of 10 candidate fossil galaxy
groups and clusters. With these Suzaku observations, we determine global
temperatures and bolometric X-ray luminosities of the intracluster medium (ICM)
out to for six systems in our sample. The remaining four systems show
signs of significant contamination from non-ICM sources. For the six objects
with successfully determined properties, we measure global
temperatures in the range ,
bolometric X-ray luminosities of , and estimate masses,
as derived from , of .
Fossil cluster scaling relations are constructed for a sample that combines our
Suzaku observed fossils with fossils in the literature. Using measurements of
global X-ray luminosity, temperature, optical luminosity, and velocity
dispersion, scaling relations for the fossil sample are then compared with a
control sample of non-fossil systems. We find the fits of our fossil cluster
scaling relations are consistent with the relations for normal groups and
clusters, indicating fossil clusters have global ICM X-ray properties similar
to those of comparable mass non-fossil systems.Comment: 17 pages, 7 figures, 8 tables. Accepted for publication in MNRA
Fossil Groups Origins III. The relation between optical and X-ray luminosities
This study is part of the FOssil Groups Origin (FOGO) project which aims at
carrying out a systematic and multiwavelength study of a large sample of fossil
systems. Here we focus on the relation between the optical luminosity (Lopt)
and X-ray luminosity (Lx). Out of a sample of 28 candidate fossil systems, we
consider a sample of 12 systems whose fossil classification has been confirmed
by a companion study. They are compared with the complementary sample of 16
systems whose fossil nature is not confirmed and with a subsample of 102 galaxy
systems from the RASS-SDSS galaxy cluster survey. Fossil and normal systems
span the same redshift range 0<z<0.5 and have the same Lx distribution. For
each fossil system, the Lx in the 0.1-2.4 keV band is computed using data from
the ROSAT All Sky Survey. For each fossil and normal system we homogeneously
compute Lopt in the r-band within the characteristic cluster radius, using data
from the SDSS DR7. We sample the Lx-Lopt relation over two orders of magnitude
in Lx. Our analysis shows that fossil systems are not statistically
distinguishable from the normal systems both through the 2D KS test and the fit
of the Lx-Lopt relation. The optical luminosity of the galaxy system does
strongly correlate with the X-ray luminosity of the hot gas component,
independently of whether the system is fossil or not. We conclude that our
results are consistent with the classical "merging scenario" of the brightest
galaxy formed via merger/cannibalism of other group galaxies, with conservation
of the optical light. We find no evidence for a peculiar state of the hot
intracluster medium.Comment: A&A, 12 pages, 4 figures, 3 tables, typos corr. and paper re-numbe
Fossil group origins V. The dependence of the luminosity function on the magnitude gap
In nature we observe galaxy aggregations that span a wide range of magnitude
gaps between the two first-ranked galaxies of a system (). There
are systems with gaps close to zero (e.g., the Coma cluster), and at the other
extreme of the distribution, the largest gaps are found among the so-called
fossil systems. Fossil and non-fossil systems could have different galaxy
populations that should be reflected in their luminosity functions. In this
work we study, for the first time, the dependence of the luminosity function
parameters on using data obtained by the fossil group origins
(FOGO) project. We constructed a hybrid luminosity function for 102 groups and
clusters at . We stacked all the individual luminosity functions,
dividing them into bins of , and studied their best-fit
Schechter parameters. We additionally computed a relative luminosity function,
expressed as a function of the central galaxy luminosity, which boosts our
capacity to detect differences, especially at the bright end. We find trends as
a function of at both the bright and faint ends of the
luminosity function. In particular, at the bright end, the larger the magnitude
gap, the fainter the characteristic magnitude . We also find
differences at the faint end. In this region, the larger the gap, the flatter
the faint-end slope . The differences found at the bright end support a
dissipationless, dynamical friction-driven merging model for the growth of the
central galaxy in group- and cluster-sized halos. The differences in the faint
end cannot be explained by this mechanism. Other processes, such as enhanced
tidal disruption due to early infall and/or prevalence of eccentric orbits, may
play a role. However, a larger sample of systems with is
needed to establish the differences at the faint end.Comment: 11 pages, 10 figures, accepted for publication in A&
- …