199 research outputs found
The Cut & Enhance method : selecting clusters of galaxies from the SDSS commissioning data
We describe an automated method, the Cut & Enhance method (CE) for detecting
clusters of galaxies in multi-color optical imaging surveys. This method uses
simple color cuts, combined with a density enhancement algorithm, to up-weight
pairs of galaxies that are close in both angular separation and color. The
method is semi-parametric since it uses minimal assumptions about cluster
properties in order to minimize possible biases. No assumptions are made about
the shape of clusters, their radial profile or their luminosity function. The
method is successful in finding systems ranging from poor to rich clusters of
galaxies, of both regular and irregular shape. We determine the selection
function of the CE method via extensive Monte Carlo simulations which use both
the real, observed background of galaxies and a randomized background of
galaxies. We use position shuffled and color shuffled data to perform the false
positive test. We have also visually checked all the clusters detected by the
CE method. We apply the CE method to the 350 deg^2 of the SDSS (Sloan Digital
Sky Survey) commissioning data and construct a SDSS CE galaxy cluster catalog
with an estimated redshift and richness for each cluster. The CE method is
compared with other cluster selection methods used on SDSS data such as the
Matched Filter (Postman et al. 1996, Kim et al. 2001), maxBCG technique (Annis
et al. 2001) and Voronoi Tessellation (Kim et al. 2001). The CE method can be
adopted for cluster selection in any multi-color imaging surveys.Comment: 62 pages, 32 figures, Accepted for publication in the Astronomical
Journal, "the CE galaxy cluster catalog can be downloaded from,
http://astrophysics.phys.cmu.edu/~tomo/ce/
Photometric observations of selected, optically bright quasars for Space Interferometry Mission and other future celestial reference frames
Photometric observations of 235 extragalactic objects that are potential
targets for the Space Interferometry Mission (SIM) are presented. Mean B, V, R,
I magnitudes at the 5% level are obtained at 1 - 4 epochs between 2005 and 2007
using the 1-m telescopes at Cerro Tololo Inter-American Observatory and Naval
Observatory Flagstaff Station. Of the 134 sources which have V magnitudes in
the Veron & Veron-Cetty catalog a difference of over 1.0 mag is found for the
observed-catalog magnitudes for about 36% of the common sources, and 10 sources
show over 3 mag difference. Our first set of observations presented here form
the basis of a long-term photometric variability study of the selected
reference frame sources to assist in mission target selection and to support in
general QSO multi-color photometric variability studies.Comment: 40 pages, 13 figures, 4 table
Spectroscopy of Quasar Candidates from SDSS Commissioning Data
The Sloan Digital Sky Survey has obtained images in five broad-band colors
for several hundred square degrees. We present color-color diagrams for stellar
objects, and demonstrate that quasars are easily distinguished from stars by
their distinctive colors. Follow-up spectroscopy in less than ten nights of
telescope time has yielded 22 new quasars, 9 of them at , and one with
, the second highest-redshift quasar yet known. Roughly 80% of the
high-redshift quasar candidates selected by color indeed turn out to be
high-redshift quasars.Comment: 4 pages, 3 figures, to appear in the proceedings of "After the Dark
Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October
Astrophysics Conference in Marylan
The Angular Clustering of Galaxy Pairs
We identify close pairs of galaxies from 278 deg^2 of Sloan Digital Sky
Survey commissioning imaging data. The pairs are drawn from a sample of 330,041
galaxies with 18 < r^* < 20. We determine the angular correlation function of
galaxy pairs, and find it to be stronger than the correlation function of
single galaxies by a factor of 2.9 +/- 0.4. The two correlation functions have
the same logarithmic slope of 0.77. We invert Limber's equation to estimate the
three-dimensional correlation functions; we find clustering lengths of r_0= 4.2
+/- 0.4 h^{-1} Mpc for galaxies and 7.8 +/- 0.7 h^{-1} Mpc for galaxy pairs.
These results agree well with the global richness dependence of the correlation
functions of galaxy systems.Comment: 12 pages. ApJ, in pres
The Worst Distortions of Astrometric Instruments and Orthonormal Models for Rectangular Fields of View
The non-orthogonality of algebraic polynomials of field coordinates
traditionally used to model field-dependent corrections to astrometric
measurements, gives rise to subtle adverse effects. In particular, certain
field dependent perturbations in the observational data propagate into the
adjusted coefficients with considerable magnification. We explain how the worst
perturbation, resulting in the largest solution error, can be computed for a
given non-orthogonal distortion model. An algebraic distortion model of full
rank can be converted into a fully orthonormal model based on the Zernike
polynomials for a circular field of view, or a basis of functions constructed
from the original model by a variant of the Gram-Schmidt orthogonalization
process for a rectangular field of view. The relative significance of
orthonormal distortion terms is assessed simply by the numerical values of the
corresponding coefficients. Orthonormal distortion models are easily extendable
when the distribution of residuals indicate the presence of higher order terms.Comment: 1 figure; submitted in PAS
An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey
An initial assessment is made of white dwarf and hot subdwarf stars observed
in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees),
observed much like the full survey will be, 269 white dwarfs and 56 hot
subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot
subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and
DB (helium) types. In addition, in the full survey to date, a number of WDs
have been found with uncommon spectral types. Among these are blue DQ stars
displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2
with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na,
and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic
field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry
alone allows identification of stars hotter than 12000 K, and the density of
these stars for 15<g<20 is found to be ~2.2 deg^{-2} at Galactic latitudes
29-62 deg. Spectra are obtained for roughly half of these hot stars. The
spectra show that, for 15<g<17, 40% of hot stars are WDs and the fraction of
WDs rises to ~90% at g=20. The remainder are hot sdB and sdO stars.Comment: Accepted for AJ; 43 pages, including 12 figures and 5 table
Optimal neighborhood indexing for protein similarity search
Background: Similarity inference, one of the main bioinformatics tasks, has to face an exponential growth of the biological data. A classical approach used to cope with this data flow involves heuristics with large seed indexes. In order to speed up this technique, the index can be enhanced by storing additional information to limit the number of random memory accesses. However, this improvement leads to a larger index that may become a bottleneck. In the case of protein similarity search, we propose to decrease the index size by reducing the amino acid alphabet.\ud
\ud
Results: The paper presents two main contributions. First, we show that an optimal neighborhood indexing combining an alphabet reduction and a longer neighborhood leads to a reduction of 35% of memory involved into the process, without sacrificing the quality of results nor the computational time. Second, our approach led us to develop a new kind of substitution score matrices and their associated e-value parameters. In contrast to usual matrices, these matrices are rectangular since they compare amino acid groups from different alphabets. We describe the method used for computing those matrices and we provide some typical examples that can be used in such comparisons. Supplementary data can be found on the website http://bioinfo.lifl.fr/reblosum.\ud
\ud
Conclusions: We propose a practical index size reduction of the neighborhood data, that does not negatively affect the performance of large-scale search in protein sequences. Such an index can be used in any study involving large protein data. Moreover, rectangular substitution score matrices and their associated statistical parameters can have applications in any study involving an alphabet reduction
Early-type galaxies in the SDSS. I. The sample
A sample of nearly 9000 early-type galaxies, in the redshift range 0.01 < z <
0.3, was selected from the Sloan Digital Sky Survey using morphological and
spectral criteria. This paper describes how the sample was selected, presents
examples of images and seeing corrected fits to the observed surface brightness
profiles, describes our method for estimating K-corrections, and shows that the
SDSS spectra are of sufficiently high quality to measure velocity dispersions
accurately. It also provides catalogs of the measured photometric and
spectroscopic parameters. In related papers, these data are used to study how
early-type galaxy observables, including luminosity, effective radius, surface
brightness, color, and velocity dispersion, are correlated with one another.Comment: 63 pages, 21 figures. Accepted by AJ (scheduled for April 2003). This
paper is part I of a revised version of astro-ph/0110344. The full version of
Tables 2 and 3, i.e. the tables listing the photometric and spectroscopic
parameters of ~ 9000 galaxies, are available at
http://astrophysics.phys.cmu.edu/~bernardi/SDSS/Etypes/TABLE
Optical and Radio Properties of Extragalactic Sources Observed by the FIRST and SDSS Surveys
We discuss the optical and radio properties of 30,000 FIRST sources
positionally associated with an SDSS source in 1230 deg of sky. The
majority (83%) of the FIRST sources identified with an SDSS source brighter
than r=21 are optically resolved. We estimate an upper limit of 5% for the
fraction of quasars with broad-band optical colors indistinguishable from those
of stars. The distribution of quasars in the radio flux -- optical flux plane
supports the existence of the "quasar radio-dichotomy"; 8% of all quasars with
i<18.5 are radio-loud and this fraction seems independent of redshift and
optical luminosity. The radio-loud quasars have a redder median color by 0.08
mag, and a 3 times larger fraction of objects with red colors. FIRST galaxies
represent 5% of all SDSS galaxies with r<17.5, and 1% for r<20, and are
dominated by red galaxies. Magnitude and redshift limited samples show that
radio galaxies have a different optical luminosity distribution than non-radio
galaxies selected by the same criteria; when galaxies are further separated by
their colors, this result remains valid for both blue and red galaxies. The
distributions of radio-to-optical flux ratio are similar for blue and red
galaxies in redshift-limited samples; this similarity implies that the
difference in their luminosity functions, and resulting selection effects, are
the dominant cause for the preponderance of red radio galaxies in flux-limited
samples. We confirm that the AGN-to-starburst galaxy number ratio increases
with radio flux, and find that radio emission from AGNs is more concentrated
than radio emission from starburst galaxies (abridged).Comment: submitted to AJ, color gif figures, PS figures available from
[email protected]
- …