32 research outputs found
The Edinburgh/Durham Southern Galaxy Catalogue: an investigation into the large-scale structure of the Universe
This thesis describes the construction and application of the Edinburgh/Durham Southern Galaxy Catalogue, which is a database of information on ~ 1-5 million galaxies, covering ~ 2000 deg2 of the South Galactic Cap. This catalogue is based on objective image detection and classification techniques, rather than the visual searches of photographic plates used in previous galaxy catalogues. The raw data for this project are digitised scans of 60 ESO/SERC Atlas plates using the COSMOS high-speed plate measuring machine. The quality controls employed during the production of the ESO/SERCAtlas, ensures that it is deeper and more uniform than set of plates used previously to construct a galaxy catalogue. The COSMOS machine objectively detects and parameterises ~ 2 X 105 images on each photographic plate. Image deblending software has been introduced to ensure the accurate detection and parameterisation of images in the crowded regions of compact clusters.Star-galaxy classification and photometric calibration techniques have been investigated and optimised to reduce and quantify any systematic variations that could introduce spurious structure. A classification algorithm has been used to automatically classify images over the whole range of magnitudes in the survey. Accurate intra-plate phoÂtometry is possible for galaxies, because a COSMOS magnitude can be defined which is linearly related to the object magnitude. Inter-plate calibration is carried out using CCD galaxy sequences for every second field in the catalogue. Unlike global calibration techniques used previously, this arrangement of CCDâs prevents propagation of calibration errors. Statistics are given to show that the final catalogue of galaxies will be > 95% complete for bj 18-75. The variation in the amplitude of the counts across the survey cannot be accounted for by systematic variations in limiting magnitude and so is probably due large-scale clustering of galaxies. The two-point correlation functions calculated for this 35-plate mosaic confirm a break from power-law, though at larger scales (~ 20h-1 Mpc ) than previously estimated. In the context of current theories of galaxy formation, models involving standard cold dark matter with extra large-scale power would still seem to be excluded
Quasar-galaxy and AGN-galaxy cross-correlations
We compute quasar-galaxy and AGN-galaxy cross-correlation functions for
samples taken from the \cite{VCV98} catalog of quasars and active galaxies,
using tracer galaxies taken from the Edinburgh/Durham Southern Catalog. The
sample of active galaxy targets shows positive correlation at projected
separations consistent with the usual power-law. On the
other hand, we do not find a statistically significant positive quasar-galaxy
correlation signal except in the range
where we find similar AGN-galaxy and quasar-galaxy correlation amplitudes. At
separations a strong decline of quasar-galaxy correlations
is observed, suggesting a significant local influence of quasars in galaxy
formation. In an attempt to reproduce the observed cross-correlation between
quasars and galaxies, we have performed CDM cosmological hydrodynamical
simulations and tested the viability of a scenario based on the model developed
by \cite{silkrees98}. In this scheme a fraction of the energy released by
quasars is considered to be transferred into the baryonic component of the
intergalactic medium in the form of winds. The results of the simulations
suggest that the shape of the observed quasar-galaxy cross-correlation function
could be understood in a scenario where a substantial amount of energy is
transferred to the medium at the redshift of maximum quasar activity.Comment: 11 pages, 9 figures. Accepted for publication in Ap
The Hawaii K-Band Galaxy Survey. II. Bright K-band Imaging
We present the results of a wide-field K-selected galaxy survey with
complementary optical I- and B-band imaging in six fields with a total coverage
of 9.8 square degrees. This survey establishes the bright-end K-band galaxy
number counts in the magnitude range 13<K<16 with high precision. We find that
our bright-end counts have a significantly steeper slope than the prediction of
a no-evolution model, which cannot be accounted for by known observational or
theoretical error. We also argue against the likelihood of sufficient evolution
at such low redshifts to account for this effect, we describe an alternative
picture in which there is a local deficiency of galaxies by a factor of 2 on
scale sizes of around 300 Mpc. Taken at face value, this would imply that local
measurements of \Omega_0 underestimate the true value of the cosmological mass
density by this factor and that local measurements of could be high by as
much as 33%.Comment: 24 pages Latex + 8 PostScript figures, to appear in Astrophysical
Journal(02/97
A Quantitative Evaluation of the Galaxy Component of COSMOS and APM Catalogs
We have carried out an independent quantitative evaluation of the galaxy
component of the "COSMOS/UKST Southern Sky Object Catalogue" (SSC) and the
"APM/UKST J Catalogue" (APM). Using CCD observations our results corroborate
the accuracy of the photometry of both catalogs, which have an overall
dispersion of about 0.2 mag in the range 17 <= b_J <= 21.5. The SSC presents
externally calibrated galaxy magnitudes that follow a linear relation, while
the APM instrumental magnitudes of galaxies, only internally calibrated by the
use of stellar profiles, require second-order corrections. The completeness of
both catalogs in a general field falls rapidly fainter than b_J = 20.0, being
slightly better for APM. The 90% completeness level of the SSC is reached
between b_J = 19.5 and 20.0, while for APM this happens between b_J = 20.5 and
21.0. Both SSC and APM are found to be less complete in a galaxy cluster field.
Galaxies misclassified as stars in the SSC receive an incorrect magnitude
because the stellar ones take saturation into account besides using a different
calibration curve. In both cases, the misclassified galaxies show a large
diversity of colors that range from typical colors of early-types to those of
blue star-forming galaxies. A possible explanation for this effect is that it
results from the combination of low sampling resolutions with properties of the
image classifier for objects with characteristic sizes close to the
instrumental resolution. We find that the overall contamination by stars
misclassified as galaxies is < 5% to b_J = 20.5, as originally estimated for
both catalogs. Although our results come from small areas of the sky, they are
extracted from two different plates and are based on the comparison with two
independent datasets.Comment: 14 pages of text and tables, 8 figures; to be published in the
Astronomical Journal; for a single postscript version file see
ftp://danw.on.br/outgoing/caretta/caretta.p
A Bayesian approach to star-galaxy classification
Star-galaxy classification is one of the most fundamental data-processing
tasks in survey astronomy, and a critical starting point for the scientific
exploitation of survey data. For bright sources this classification can be done
with almost complete reliability, but for the numerous sources close to a
survey's detection limit each image encodes only limited morphological
information. In this regime, from which many of the new scientific discoveries
are likely to come, it is vital to utilise all the available information about
a source, both from multiple measurements and also prior knowledge about the
star and galaxy populations. It is also more useful and realistic to provide
classification probabilities than decisive classifications. All these
desiderata can be met by adopting a Bayesian approach to star-galaxy
classification, and we develop a very general formalism for doing so. An
immediate implication of applying Bayes's theorem to this problem is that it is
formally impossible to combine morphological measurements in different bands
without using colour information as well; however we develop several
approximations that disregard colour information as much as possible. The
resultant scheme is applied to data from the UKIRT Infrared Deep Sky Survey
(UKIDSS), and tested by comparing the results to deep Sloan Digital Sky Survey
(SDSS) Stripe 82 measurements of the same sources. The Bayesian classification
probabilities obtained from the UKIDSS data agree well with the deep SDSS
classifications both overall (a mismatch rate of 0.022, compared to 0.044 for
the UKIDSS pipeline classifier) and close to the UKIDSS detection limit (a
mismatch rate of 0.068 compared to 0.075 for the UKIDSS pipeline classifier).
The Bayesian formalism developed here can be applied to improve the reliability
of any star-galaxy classification schemes based on the measured values of
morphology statistics alone.Comment: Accepted 22 November 2010, 19 pages, 17 figure
Decision Tree Classifiers for Star/Galaxy Separation
We study the star/galaxy classification efficiency of 13 different decision
tree algorithms applied to photometric objects in the Sloan Digital Sky Survey
Data Release Seven (SDSS DR7). Each algorithm is defined by a set of parameters
which, when varied, produce different final classification trees. We
extensively explore the parameter space of each algorithm, using the set of
SDSS objects with spectroscopic data as the training set. The
efficiency of star-galaxy separation is measured using the completeness
function. We find that the Functional Tree algorithm (FT) yields the best
results as measured by the mean completeness in two magnitude intervals: () and (). We compare the performance of the
tree generated with the optimal FT configuration to the classifications
provided by the SDSS parametric classifier, 2DPHOT and Ball et al. (2006). We
find that our FT classifier is comparable or better in completeness over the
full magnitude range , with much lower contamination than all but
the Ball et al. classifier. At the faintest magnitudes (), our classifier
is the only one able to maintain high completeness (80%) while still
achieving low contamination (). Finally, we apply our FT classifier
to separate stars from galaxies in the full set of SDSS
photometric objects in the magnitude range .Comment: Submitted to A
The Northern Sky Optical Cluster Survey II: An Objective Cluster Catalog for 5800 Square Degrees
We present a new, objectively defined catalog of candidate galaxy clusters
based on the galaxy catalogs from the Digitized Second Palomar Observatory Sky
Survey (DPOSS). This cluster catalog, derived from the best calibrated plates
in the high latitude (|b|>30) Northern Galactic Cap region, covers 5,800 square
degrees, and contains 8,155 candidate clusters. A simple adaptive kernel
density mapping technique, combined with the SExtractor object detection
algorithm, is used to detect galaxy overdensities, which we identify as
clusters. Simulations of the background galaxy distribution and clusters of
varying richnesses and redshifts allow us to optimize detection parameters, and
measure the completeness and contamination rates for our catalog. Cluster
richnesses and photometric redshifts are measured, using integrated colors and
magnitudes for each cluster. An extensive spectroscopic survey is used to
confirm the photometric results.
This catalog, with well-characterized sample properties, provides a sound
basis for future studies of cluster physics and large scale structure.Comment: 49 pages, 16 figures. Accepted to AJ; appearing in April. Version
with full resolution figures, and full length tables available at
http://dposs.caltech.edu:8080/NoSOCS.htm
The Aquarius Superclusters - I. Identification of Clusters and Superclusters
We study the distribution of galaxies and galaxy clusters in a 10^deg x 6^deg
field in the Aquarius region. In addition to 63 clusters in the literature, we
have found 39 new candidate clusters using a matched-filter technique and a
counts-in-cells analysis. From redshift measurements of galaxies in the
direction of these cluster candidates, we present new mean redshifts for 31
previously unobserved clusters, while improved mean redshifts are presented for
35 other systems. About 45% of the projected density enhancements are due to
the superposition of clusters and/or groups of galaxies along the line of
sight, but we could confirm for 72% of the cases that the candidates are real
physical associations similar to the ones classified as rich galaxy clusters.
On the other hand, the contamination due to galaxies not belonging to any
concentration or located only in small groups along the line of sight is ~ 10%.
Using a percolation radius of 10 h^{-1} Mpc (spatial density contrast of about
10), we detect two superclusters of galaxies in Aquarius, at z = 0.086 and at z
= 0.112, respectively with 5 and 14 clusters. The latter supercluster may
represent a space overdensity of about 160 times the average cluster density as
measured from the Abell et al. (1989) cluster catalog, and is possibly
connected to a 40 h^{-1} Mpc filament from z ~ 0.11 to 0.14.Comment: LateX text (21 pages) and 12 (ps/eps/gif) figures; figures 5a, 5b and
6 are not included in the main LateX text; to be published in the
Astronomical Journal, March issu
The SuperCOSMOS Sky Survey. Paper II: Image detection, parameterisation, classification and photometry
In this, the second in a series of three papers concerning the SuperCOSMOS
Sky Survey, we describe the methods for image detection, parameterisation,
classification and photometry. We demonstrate the internal and external
accuracy of our object parameters. Using examples from the first release of
data, the South Galactic Cap survey, we show that our image detection
completeness is close to 100% to within 1.5 mag of the nominal plate limits. We
show that for the Bj survey data, the image classification is externally > 99%
reliable to Bj = 19.5. Internally, the image classification is reliable at a
level of > 90% to Bj=21, R=19. The photometric accuracy of our data is
typically 0.3 mag with respect to external data for m > 15. Internally, the
relative photometric accuracy in restricted position and magnitude ranges can
be as accurate as 5% for well exposed stellar images. Colours (B-R or R-I) are
externally accurate to 0.07 mag at Bj = 16.5 rising to 0.16 mag at Bj = 20.Comment: 22 pages, 16 figures; accepted for publication in MNRA
Power Spectrum Analysis of the ESP Galaxy Redshift Survey
We measure the power spectrum of the galaxy distribution in the ESO Slice
Project (ESP) galaxy redshift survey. We develope a technique to describe the
survey window function analytically, and then deconvolve it from the measured
power spectrum using a variant of the Lucy method. We test the whole
deconvolution procedure on ESP mock catalogues drawn from large N-body
simulations, and find that it is reliable for recovering the correct amplitude
and shape of at Mpc. In general, the technique is
applicable to any survey composed by a collection of circular fields with
arbitrary pattern on the sky, as typical of surveys based on fibre
spectrographs. The estimated power spectrum has a well-defined power-law shape
with for Mpc, and a smooth bend to a
flatter shape () for smaller 's. The smallest wavenumber,
where a meaningful reconstruction can be performed ( Mpc),
does not allow us to explore the range of scales where other power spectra seem
to show a flattening and hints for a turnover. We also find, by direct
comparison of the Fourier transforms, that the estimate of the two-point
correlation function is much less sensitive to the effect of a
problematic window function as that of the ESP, than the power spectrum.
Comparison to other surveys shows an excellent agreement with estimates from
blue-selected surveys. In particular, the ESP power spectrum is virtually
indistinguishable from that of the Durham-UKST survey over the common range of
's, an indirect confirmation of the quality of the deconvolution technique
applied.Comment: 15 pages, 11 postscript figures. Version accepted for publication in
MNRAS. Complete description of window function computation, 1 additional
figur