244,216 research outputs found
Covariance matrices for halo number counts and correlation functions
We study the mean number counts and two-point correlation functions, along
with their covariance matrices, of cosmological surveys such as for clusters.
In particular, we consider correlation functions averaged over finite redshift
intervals, which are well suited to cluster surveys or populations of rare
objects, where one needs to integrate over nonzero redshift bins to accumulate
enough statistics. We develop an analytical formalism to obtain explicit
expressions of all contributions to these means and covariance matrices, taking
into account both shot-noise and sample-variance effects. We compute low-order
as well as high-order (including non-Gaussian) terms. We derive expressions for
the number counts per redshift bins both for the general case and for the small
window approximation. We estimate the range of validity of Limber's
approximation and the amount of correlation between different redshift bins. We
also obtain explicit expressions for the integrated 3D correlation function and
the 2D angular correlation. We compare the relative importance of shot-noise
and sample-variance contributions, and of low-order and high-order terms. We
check the validity of our analytical results through a comparison with the
Horizon full-sky numerical simulations, and we obtain forecasts for several
future cluster surveys.Comment: 37 page
Star Formation and Relaxation in 379 Nearby Galaxy Clusters
We investigate the relationship between star formation (SF) and level of
relaxation in a sample of 379 galaxy clusters at z < 0.2. We use data from the
Sloan Digital Sky Survey to measure cluster membership and level of relaxation,
and to select star-forming galaxies based on mid-infrared emission detected
with the Wide-Field Infrared Survey Explorer. For galaxies with absolute
magnitudes M_r < -19.5, we find an inverse correlation between SF fraction and
cluster relaxation: as a cluster becomes less relaxed, its SF fraction
increases. Furthermore, in general, the subtracted SF fraction in all unrelaxed
clusters (0.117 +/- 0.003) is higher than that in all relaxed clusters (0.097
+/- 0.005). We verify the validity of our SF calculation methods and membership
criteria through analysis of previous work. Our results agree with previous
findings that a weak correlation exists between cluster SF and dynamical state,
possibly because unrelaxed clusters are less evolved relative to relaxed
clusters.Comment: 6 pages, 4 figures, accepted for publication in Ap
Relations between spatial correlations of rich clusters of galaxies
The authors express the N-point rich cluster correlation function in terms of the two-point rich cluster correlation function in a model where rich clusters formed wherever suitably averaged primordial energy density fluctuations are unusually large. The validity of the results is not restricted to regions where the connected (reduced) N-point correlation functions are small compared with unity
A correlation-based fuzzy cluster validity index with secondary options detector
The optimal number of clusters is one of the main concerns when applying
cluster analysis. Several cluster validity indexes have been introduced to
address this problem. However, in some situations, there is more than one
option that can be chosen as the final number of clusters. This aspect has been
overlooked by most of the existing works in this area. In this study, we
introduce a correlation-based fuzzy cluster validity index known as the
Wiroonsri-Preedasawakul (WP) index. This index is defined based on the
correlation between the actual distance between a pair of data points and the
distance between adjusted centroids with respect to that pair. We evaluate and
compare the performance of our index with several existing indexes, including
Xie-Beni, Pakhira-Bandyopadhyay-Maulik, Tang, Wu-Li, generalized C, and Kwon2.
We conduct this evaluation on four types of datasets: artificial datasets,
real-world datasets, simulated datasets with ranks, and image datasets, using
the fuzzy c-means algorithm. Overall, the WP index outperforms most, if not
all, of these indexes in terms of accurately detecting the optimal number of
clusters and providing accurate secondary options. Moreover, our index remains
effective even when the fuzziness parameter is set to a large value. Our R
package called WPfuzzyCVIs used in this work is also available in
https://github.com/nwiroonsri/WPfuzzyCVIs.Comment: 19 page
Clustering performance analysis using a new correlation-based cluster validity index
There are various cluster validity indices used for evaluating clustering
results. One of the main objectives of using these indices is to seek the
optimal unknown number of clusters. Some indices work well for clusters with
different densities, sizes, and shapes. Yet, one shared weakness of those
validity indices is that they often provide only one optimal number of
clusters. That number is unknown in real-world problems, and there might be
more than one possible option. We develop a new cluster validity index based on
a correlation between an actual distance between a pair of data points and a
centroid distance of clusters that the two points occupy. Our proposed index
constantly yields several local peaks and overcomes the previously stated
weakness. Several experiments in different scenarios, including UCI real-world
data sets, have been conducted to compare the proposed validity index with
several well-known ones. An R package related to this new index called NCvalid
is available at https://github.com/nwiroonsri/NCvalid.Comment: 19 page
Multiwavelength analysis of the young open cluster NGC 2362
We present a multiwavelength analysis of the young open cluster NGC 2362.
UBVRcIc CCD photometric observations, together with available data in the
Chandra data base, near infrared data from the Two Micron All Sky Survey
(2MASS), and recently published Halpha spectroscopy were used to get
information about the evolutionary stage of the cluster and the main physical
properties of its stellar content. Cluster membership is estimated for every
individual star by means of ZAMS and isochrone fitting. The cluster is
confirmed to host a richly populated pre-main sequence (PMS), and to contain a
large amount of X-ray emitting stars, which reach from the PMS members of GK
spectral type, up to the most luminous OB type main sequence (MS) members. The
PMS cluster members show no significant age spread, and the comparison to both
PMS and post-MS isochrones suggests a younger age for the more massive MS than
for lower mass PMS members. The analysis allows to asses the validity of
currently used pre-main sequence evolutionary models, and supports the
suggestion of a well defined positive correlation of the X-ray emission from
PMS stars with their bolometric luminosity. Clear differences are found on the
other hand, between the X-ray activity properties of MS and PMS cluster
members, both in the relation between X-ray luminosity and bolometric
luminosity, and in spectral properties as well.Comment: 1 gzipped file: 1 tex file with 9 pages text. 5 ps files with
figures. Submitted to Astrophysical Journa
The ACS survey of globular clusters. XIII. Photometric calibration in comparison with Stetson standards
In this study we compare the photometric data of 34 Milky Way globular
clusters, observed within the ACS Treasury Program (PI: Ata Sarajedini) with
the corresponding ground-based data, provided by the Photometric Standard Field
Catalogs of Stetson (2000, 2005). We focus on the transformation between the
HST/ACS F606W to V-band and F814W to I-band only. The goal is to assess the
validity of the filter transformation equations by Sirianni et al.(2005) with
respect to their dependence on metallicity, Horizontal Branch morphology, mass
and integrated (V-I) colour of the various globular clusters. Such a dependence
is expected due to the fact that the transformation equations are based on the
observations of only one globular cluster, i.e., NGC 2419. Surprisingly, the
correlation between offset and metallicity is found to be weak, with a low
level significance. The correlation between offset and Horizontal Branch
structure, as well as total cluster mass is still weaker. Based on the
available data we do not find the photometric offset to be linked to multiple
stellar populations, e.g., as found in NGC 0288, NGC 1851, and NGC 5139. The
results of this study show that there are small systematic offsets between the
transformed ACS- and observed ground based photometry, and that these are only
weakly correlated, if at all, with various cluster parameters and their
underlying stellar populations. As a result, investigators wishing to transform
globular cluster photometry from the Sirianni et al.(2005) ground-based V, I
system onto the Stetson (2000) system simply need to add 0.040 (+/-0.012) to
the V-band magnitudes and 0.047 (+/-0.011) to the I-band magnitudes. This in
turn means that the transformed ACS (V-I) colours match the ground-based values
from Stetson (2000) to within ~0.01 mag.Comment: 28 pages, 14 figures, accepted for publication in ApJ
A numerical and analytical study of two holes doped into the 2D t--J model
Exact diagonalization numerical results are presented for a 32-site square
cluster, with two holes propagating in an antiferromagnetic background
described by the t-J model. We characterize the wave function of the lowest
energy bound state found in this calculation, which has d_{x^2-y^2} symmetry.
Analytical work is presented, based on a Lang-Firsov-type canonical
transformation derived quasiparticle Hamiltonian, that accurately agrees with
numerically determined values for the electron momentum distribution function
and the pair correlation function. We interpret this agreement as strong
support for the validity of this description of the hole quasiparticles.Comment: 3 pages, REVTeX, to appear in the proceedings of the Fifth
International Conference on Spectroscopies in Novel Superconductors,
September 14-18, 1997, Cape Cod, Massachusett
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