3,095 research outputs found
Detection of a Dipole in the Handedness of Spiral Galaxies with Redshifts z ~ 0.04
A preference for spiral galaxies in one sector of the sky to be left-handed
or right-handed spirals would indicate a parity violating asymmetry in the
overall universe and a preferred axis. This study uses 15158 spiral galaxies
with redshifts <0.085 from the Sloan Digital Sky Survey. An unbinned analysis
for a dipole component that made no prior assumptions for the dipole axis gives
a dipole asymmetry of -0.0408\pm0.011 with a probability of occurring by chance
of 7.9 x 10-4. A similar asymmetry is seen in the Southern Galaxy spin catalog
of Iye and Sugai. The axis of the dipole asymmetry lies at approx. (l, b)
=(52{\deg}, 68.5{\deg}), roughly along that of our Galaxy and close to
alignments observed in the WMAP cosmic microwave background distributions. The
observed spin correlation extends out to separations ~210 Mpc/h, while spirals
with separations < 20 Mpc/h have smaller spin correlations.Comment: To be published in Physics Letters
Photometric Response Functions of the SDSS Imager
The monochromatic illumination system is constructed to carry out in situ
measurements of the response function of the mosaicked CCD imager used in the
Sloan Digital Sky Survey (SDSS). The system is outlined and the results of the
measurements, mostly during the first 6 years of the SDSS, are described. We
present the reference response functions for the five colour passbands derived
from these measurements, and discuss column to column variations and variations
in time, and also their effects on photometry. We also discuss the effect
arising from various, slightly different response functions of the associated
detector systems that were used to give SDSS photometry. We show that the
calibration procedures of SDSS remove these variations reasonably well with the
resulting final errors from variant response functions being unlikely to be
larger than 0.01 mag for g, r, i, and z bands over the entire duration of the
survey. The considerable aging effect is uncovered in the u band, the response
function showing a 30% decrease in the throughput in the short wavelength side
during the survey years, which potentially causes a systematic error in
photometry. The aging effect is consistent with variation of the instrumental
sensitivity in u-band, which is calibrated out. The expected colour variation
is consistent with measured colour variation in the catalog of repeated
photometry. The colour variation is delta (u-g) ~ 0.01 for most stars, and at
most delta (u-g) ~ 0.02 mag for those with extreme colours. We verified in the
final catalogue that no systematic variations in excess of 0.01 mag are
detected in the photometry which can be ascribed to aging and/or seasonal
effects except for the secular u-g colour variation for stars with extreme
colours.Comment: 54 pages, 18 figures, 7 tables, accepted for publication in A
Inclination-Independent Galaxy Classification
We present a new method to classify galaxies from large surveys like the
Sloan Digital Sky Survey using inclination-corrected concentration,
inclination-corrected location on the color-magnitude diagram, and apparent
axis ratio. Explicitly accounting for inclination tightens the distribution of
each of these parameters and enables simple boundaries to be drawn that
delineate three different galaxy populations: Early-type galaxies, which are
red, highly concentrated, and round; Late-type galaxies, which are blue, have
low concentrations, and are disk dominated; and Intermediate-type galaxies,
which are red, have intermediate concentrations, and have disks. We have
validated our method by comparing to visual classifications of high-quality
imaging data from the Millennium Galaxy Catalogue. The inclination correction
is crucial to unveiling the previously unrecognized Intermediate class.
Intermediate-type galaxies, roughly corresponding to lenticulars and early
spirals, lie on the red sequence. The red sequence is therefore composed of two
distinct morphological types, suggesting that there are two distinct mechanisms
for transiting to the red sequence. We propose that Intermediate-type galaxies
are those that have lost their cold gas via strangulation, while Early-type
galaxies are those that have experienced a major merger that either consumed
their cold gas, or whose merger progenitors were already devoid of cold gas
(the ``dry merger'' scenario).Comment: Accepted for publication in ApJ. 7 pages in emulateap
Investigating Heating and Cooling in the BCS & B55 Cluster Samples
We study clusters in the BCS cluster sample which are observed by Chandra and
are more distant than redshift, z>0.1. We select from this subsample the
clusters which have both a short central cooling time and a central temperature
drop, and also those with a central radio source. Six of the clusters have
clear bubbles near the centre. We calculate the heating by these bubbles and
express it as the ratio r_heat/r_cool=1.34+/-0.20. This result is used to
calculate the average size of bubbles expected in all clusters with central
radio sources. In three cases the predicted bubble sizes approximately match
the observed radio lobe dimensions.
We combine this cluster sample with the B55 sample studied in earlier work to
increase the total sample size and redshift range. This extended sample
contains 71 clusters in the redshift range 0<z<0.4. The average distance out to
which the bubbles offset the X-ray cooling in the combined sample is at least
r_heat/r_cool=0.92+/-0.11. The distribution of central cooling times for the
combined sample shows no clusters with clear bubbles and t_cool>1.2Gyr. An
investigation of the evolution of cluster parameters within the redshift range
of the combined samples does not show any clear variation with redshift.Comment: 12 pages, 9 figures, accepted for publication in MNRA
Extremely metal-poor stars from the SDSS
We give a progress report about the activities within the CIFIST Team related
to the search for extremely metal-poor stars in the Sloan Digital Sky Survey's
spectroscopic catalog. So far the search has provided 25 candidates with
metallicities around or smaller -3. For 15 candidates high resolution
spectroscopy with UVES at the VLT has confirmed their extremely metal-poor
status. Work is under way to extend the search to the SDSS's photometric
catalog by augmenting the SDSS photometry, and by gauging the capabilities of
X-shooter when going to significantly fainter targets.Comment: 6 pages, 6 figures, Proceedings paper of the conference "A stellar
journey: A symposium in celebration of Bengt Gustafsson's 65th birthday
Flux calculations in an inhomogeneous Universe: weighting a flux-limited galaxy sample
Many astrophysical problems arising within the context of ultra-high energy
cosmic rays, very-high energy gamma rays or neutrinos, require calculation of
the flux produced by sources tracing the distribution of galaxies in the
Universe. We discuss a simple weighting scheme, an application of the method
introduced by Lynden-Bell in 1971, that allows the calculation of the flux sky
map directly from a flux-limited galaxy catalog without cutting a
volume-limited subsample. Using this scheme, the galaxy distribution can be
modeled up to large scales while representing the distribution in the nearby
Universe with maximum accuracy. We consider fluctuations in the flux map
arising from the finiteness of the galaxy sample. We show how these
fluctuations are reduced by the weighting scheme and discuss how the remaining
fluctuations limit the applicability of the method.Comment: 8 pages, 10 figures, accepted for publication in MNRA
Cross-Identification of Stars with Unknown Proper Motions
The cross-identification of sources in separate catalogs is one of the most
basic tasks in observational astronomy. It is, however, surprisingly difficult
and generally ill-defined. Recently Budav\'ari & Szalay (2008) formulated the
problem in the realm of probability theory, and laid down the statistical
foundations of an extensible methodology. In this paper, we apply their
Bayesian approach to stars that, we know, can move measurably on the sky, with
detectable proper motion, and show how to associate their observations. We
study models on a sample of stars in the Sloan Digital Sky Survey, which allow
for an unknown proper motion per object, and demonstrate the improvements over
the analytic static model. Our models and conclusions are directly applicable
to upcoming surveys such as PanSTARRS, the Dark Energy Survey, Sky Mapper, and
the LSST, whose data sets will contain hundreds of millions of stars observed
multiple times over several years.Comment: 10 pages, 5 figure
The Sloan Bright Arcs Survey : Six Strongly Lensed Galaxies at z=0.4-1.4
We present new results of our program to systematically search for strongly
lensed galaxies in the Sloan Digital Sky Survey (SDSS) imaging data. In this
study six strong lens systems are presented which we have confirmed with
follow-up spectroscopy and imaging using the 3.5m telescope at the Apache Point
Observatory. Preliminary mass models indicate that the lenses are group-scale
systems with velocity dispersions ranging from 466-878 km s^{-1} at z=0.17-0.45
which are strongly lensing source galaxies at z=0.4-1.4. Galaxy groups are a
relatively new mass scale just beginning to be probed with strong lensing. Our
sample of lenses roughly doubles the confirmed number of group-scale lenses in
the SDSS and complements ongoing strong lens searches in other imaging surveys
such as the CFHTLS (Cabanac et al 2007). As our arcs were discovered in the
SDSS imaging data they are all bright (), making them ideally
suited for detailed follow-up studies.Comment: 13 pages, 3 figures, submitted to ApJL, the Sloan Bright Arcs page is
located here: http://home.fnal.gov/~kubo/brightarcs.htm
An Optical Catalog of Galaxy Clusters Obtained from an Adaptive Matched Filter Finder Applied to SDSS DR6
We present a new cluster catalog extracted from the Sloan Digital Sky Survey
Data Release 6 (SDSS DR6) using an adaptive matched filter (AMF) cluster
finder. We identify 69,173 galaxy clusters in the redshift range 0.045 0.78 in 8420 sq. deg. of the sky. We provide angular position, redshift,
richness, core and virial radii estimates for these clusters, as well as an
error analysis for each of these quantities. We also provide a catalog of more
than 205,000 galaxies representing the three brightest galaxies in the band
which are possible BCG candidates. We show basic properties of the BCG
candidates and study how their luminosity scales in redshift and cluster
richness. We compare our catalog with the maxBCG and GMBCG catalogs, as well as
with that of Wen, Han, and Liu. We match between 30% and 50% of clusters
between catalogs over all overlapping redshift ranges. We find that the
percentage of matches increases with the richness for all catalogs. We cross
match the AMF catalog with available X-ray data in the same area of the sky and
find 539 matches, 119 of which with temperature measurements. We present
scaling relations between optical and X-ray properties and cluster center
comparison. We find that both and correlate well with
both and , with no significant difference in trend if we restrict
the matches to flux-limited X-ray samples.Comment: 59 pages, 28 figures; accepted for publication in The Astrophysical
Journa
Post common envelope binaries from SDSS. II : identification of 9 close binaries with VLT/FORS2
Context. Post common envelope binaries (PCEBs) consisting of a white dwarf and a main sequence star are ideal systems to use to calibrate current theories of angular momentum loss in close compact binary stars. The potential held by PCEBs for further development of close binary evolution could so far not be exploited due to the small number of known systems and the inhomogeneity of the sample. The Sloan Digital Sky Survey is changing this scene dramatically, as it is very efficient in identifying white dwarf/main sequence (WDMS) binaries, including both wide systems whose stellar components evolve like single stars and â more interesting in the context of close binary evolution â PCEBs.
Aims. We pursue a large-scale follow-up survey to identify and characterise the PCEBs among the WDMS binaries that have been found with SDSS. We use a two-step strategy with the identification of PCEBs among WDMS binaries in the first phase and orbital period determinations in the second phase. Here we present first results of our ESO-VLT/FORS2 pilot study that targets the identification of the PCEBs among the fainter (g >âŒ18.5) SDSSWDMS binaries.
Methods. From published SDSS catalogues we selected 26 WDMS binaries to be observed with ESO-VLT/FORS2 in service mode.
The design of the observations was to get two spectra per object separated by at least one night.We used the Na I λλ 8183.27, 8194.81 doublet to measure radial velocity variations of our targets and a spectral decomposition/fitting technique to determine the white dwarf
effective temperatures and surface gravities, masses, and secondary star spectral types for all WDMS binaries in our sample.
Results. Among the 26 observed WDMS binaries, we find 9 strong PCEB candidates showing clear (â„3Ï) radial velocity variations, and we estimate the fraction of PCEBs among SDSS WDMS binaries to be âŒ35 ± 12%. We find indications of a dependence of the relative number of PCEBs among SDSSWDMS binaries on the spectral type of the secondary star. These results are subject to small number statistics and need to be confirmed by additional observations. Using Magellan-Clay/LDSS3, we measured the orbital periods of two PCEB candidates, SDSS J1047+0523 and SDSS J1414â0132, to be 9.17 h and 17.48 h, respectively.
Conclusions. This pilot study demonstrates that our survey is highly efficient in identifying PCEBs among the SDSSWDMS binaries, and it will indeed provide the observational parameters that are needed to constrain the theoretical models of close binary evolution
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