1,532 research outputs found
Sizes of Voids as a test for Dark Matter Models
We use the void probability statistics to study the redshift-space galaxy
distribution as described by a volume-limited subsample of the Perseus-Pisces
survey. We compare the results with the same analysis realized on artificial
samples, extracted from high-resolution N-body simulations by reproducing the
observational biases of the real data set. Simulations are run for the
Cold+HotDM model (CHDM) and for unbiased and biased (b=1.5) CDM models in a 50
Mpc/h box. We identify galaxies as residing in peaks of the evolved density
field. We fragment overmerged structures into individual galaxies so as to
reproduce both the correct luminosity function (after assuming M/ L values for
the resulting galaxy groups) and the two-point correlation function. Our main
result is that a void-probability function (VPF) from the standard CHDM model
with fractions 60% cold, 30% hot, 10% barions, exceeds the observational VPF
with a high confidence level. CDM models produce smaller VPF independent of the
biasing parameter. We verify the robustness of this result against changing the
observer position in the simulations and the galaxy identification in the
evolved density field.Comment: 15 pages, postscrip
The BMW (Brera-Multiscale-Wavelet) Catalogue of Serendipitous X-ray Sources
In collaboration with the Observatories of Palermo and Rome and the SAX-SDC
we are constructing a multi-site interactive archive system featuring specific
analysis tools. In this context we developed a detection algorithm based on the
Wavelet Transform (WT) and performed a systematic analysis of all ROSAT-HRI
public data (~3100 observations +1000 to come). The WT is specifically suitable
to detect and characterize extended sources while properly detecting point
sources in very crowded fields. Moreover, the good angular resolution of HRI
images allows the source extension and position to be accurately determined.
This effort has produced the BMW (Brera Multiscale Wavelet) catalogue, with
more than 19,000 sources detected at the 4.2 sigma level. For each source
detection we have information on the X-ray flux and extension, allowing for
instance to select complete samples of extended X-ray sources such as candidate
clusters of galaxies or SNR's. Here we present an overview of first results
from several undergoing projects which make use of the BMW catalogue.Comment: 7 pages, 6 postscript files, 2 gif images, to appear in the
proceedings of the conference "Mining the Sky", August 2000, Garching,
German
The BMW Deep X-ray Cluster Survey
We briefly describe the main features of the Brera Multi-Wavelet (BMW) survey
of serendipitous X-ray clusters, based on the still unexploited ROSAT-HRI
archival observations. Cluster candidates are selected from the general BMW
catalogue of 20,000 sources based exclusively on their X-ray extension.
Contrary to common wisdom, a clever selection of the HRI energy channels allows
us to significantly reduce the background noise, thus greatly improving the
ability to detect low surface-brightness sources as clusters. The resulting
sample of ~250 candidates shows a very good sky coverage down to a flux
\~3x10^-14 erg/s/cm^2 ([0.5-2.0] keV band), i.e comparable to existing
PSPC-based deep survey, with a particularly interesting area of ~100 sq.deg.
around fluxes ~10^-13 erg/s/cm^2, i.e. where highly-luminous, rare systems at
z~0.6-1 can be detected. At the same time, the superior angular resolution of
the instrument should avoid biases against intrinsically small systems, while
easing the identification process (e.g. by spotting blends and AGN
contaminants). While about 20% of the candidates are already identified with
groups/clusters at z<0.3 on the DSS2 images, we have started a deep CCD imaging
campaign to observe all sources associated to "blank fields". First results
from these observations reveal a distant (z>0.5) bonafide cluster counterpart
for ~80% of the targets.Comment: 3 pages, 2 figures; to appear in Proc. of the ESO/ECF/STSCI workshop
on "Deep Fields", Garching Oct 2000, (Publ: Springer
Augmenting photometric redshift estimates using spectroscopic nearest neighbours
As a consequence of galaxy clustering, close galaxies observed on the plane
of the sky should be spatially correlated with a probability that is inversely
proportional to their angular separation. In principle, this information can be
used to improve photometric redshift estimates when spectroscopic redshifts are
available for some of the neighbouring objects. Depending on the depth of the
survey, however, this angular correlation is reduced by chance projections. In
this work, we implement a deep-learning model to distinguish between apparent
and real angular neighbours by solving a classification task. We adopted a
graph neural network architecture to tie together photometry, spectroscopy, and
the spatial information between neighbouring galaxies. We trained and validated
the algorithm on the data of the VIPERS galaxy survey, for which photometric
redshifts based on spectral energy distribution are also available. The model
yields a confidence level for a pair of galaxies to be real angular neighbours,
enabling us to disentangle chance superpositions in a probabilistic way. When
objects for which no physical companion can be identified are excluded, all
photometric redshift quality metrics improve significantly, confirming that
their estimates were of lower quality. For our typical test configuration, the
algorithm identifies a subset containing ~75% high-quality photometric
redshifts, for which the dispersion is reduced by as much as 50% (from 0.08 to
0.04), while the fraction of outliers reduces from 3% to 0.8%. Moreover, we
show that the spectroscopic redshift of the angular neighbour with the highest
detection probability provides an excellent estimate of the redshift of the
target galaxy, comparable to or even better than the corresponding
template-fitting estimate.Comment: 9 pages, 12 figures, matching the accepted version. NezNet is
available at https://github.com/tos-1/NezNe
Cluster Alignments in the Edinburgh/Milano Cluster Redshift Survey
We present here the results of a statistical search for cluster alignments
using the Edinburgh/Milano cluster redshift survey. This survey is a unique
cluster database which has been objectively constructed to help minimise the
systematic biases associated with previous optical cluster catalogues. We find
some evidence for cluster alignments out to spatial separations of <10\mpc,
however, it is not statistically significant. On larger scales, we find no
evidence, statistically significant or not, for cluster alignments. These
results are in most disagreement with the recent observations of West and
Plionis; both of whom see significant cluster alignments out to \simeq30\mpc
and beyond in the Abell \& Lick catalogues of clusters. Our findings are
consistent with other searches for cluster alignments that do not involve these
catalogues.Comment: postscript file of text (8pages), but not figures. Four figures
available via anon ftp on oddjob.uchicago.edu (pub/align_fig*.ps), rest
available from [email protected]. Paper accepted for publication in
Monthly Notice
Redshift-Space Distortions and the Real-Space Clustering of Different Galaxy Types
We study the distortions induced by peculiar velocities on the redshift-space
correlation function of galaxies of different morphological types in the
Pisces-Perseus redshift survey. Redshift-space distortions affect early- and
late-type galaxies in different ways. In particular, at small separations, the
dominant effect comes from virialized cluster cores, where ellipticals are the
dominant population. The net result is that a meaningful comparison of the
clustering strength of different morphological types can be performed only in
real space, i.e., after projecting out the redshift distortions on the
two-point correlation function xi(r_p,pi). A power-law fit to the projected
function w_p(r_p) on scales smaller than 10/h Mpc gives r_o =
8.35_{-0.76}^{+0.75} /h Mpc, \gamma = 2.05_{-0.08}^{+0.10} for the early-type
population, and r_o = 5.55_{-0.45}^{+0.40} /h Mpc, \gamma =
1.73_{-0.08}^{+0.07} for spirals and irregulars. These values are derived for a
sample luminosity brighter than M_{Zw} = -19.5. We detect a 25% increase of r_o
with luminosity for all types combined, from M_{Zw} = -19 to -20. In the
framework of a simple stable-clustering model for the mean streaming of pairs,
we estimate sigma_12(1), the one-dimensional pairwise velocity dispersion
between 0 and 1 /h Mpc, to be 865^{+250}_{-165} km/s for early-type galaxies
and 345^{+95}_{-65} km/s for late types. This latter value should be a fair
estimate of the pairwise dispersion for ``field'' galaxies; it is stable with
respect to the presence or absence of clusters in the sample, and is consistent
with the values found for non-cluster galaxies and IRAS galaxies at similar
separations.Comment: 17 LaTeX pages including 3 tables, plus 11 PS figures. Uses AASTeX
macro package (aaspp4.sty) and epsf.sty. To appear on ApJ, 489, Nov 199
The X-ray Luminosity Function of Bright Clusters in the Local Universe
We present the X-ray luminosity function (XLF) for clusters of galaxies
derived from the RASS1 Bright Sample. The sample, selected from the ROSAT
All-Sky Survey in a region of 2.5 sr within the southern Galactic cap, contains
130 clusters with flux limits in the range ~ 3-4 x 10^-12 ergs/cm^2/s in the
0.5-2.0 keV band. A maximum-likelihood fit with a Schechter function of the XLF
over the entire range of luminosities (0.045 - 28. x 10^44 ergs/s), gives alpha
= 1.52 +/- 0.11, L_* = 3.80 +0.70 -0.55 x 10^44 ergs/s, and A = 5.07 +/- 0.45 x
10^-7 Mpc^-3 (10^44 ergs/s)^(\alpha-1). We investigate possible evolutionary
effects within the sample, out to our redshift limit (z ~ 0.3), finding no
evidence for evolution. Our results are in good agreement with other local
estimates of the XLF, implying that this statistic for the local universe is
now well determined. Comparison with XLFs for distant clusters (0.3 < z < 0.6),
shows that no evolution is present for L_X < 10^{44} ergs/s. However, we detect
differences at the 3 sigma level, between our local XLF and the distant one
estimated by Henry et al. for the EMSS sample. This difference is still present
when considering the EMSS sample revised by Nichol et al.Comment: 13 pages with 3 figures included, LaTex, aaspp4.sty and epsf.sty,
accepted for publication in ApJ Letters, only minor changes, added reference
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