265 research outputs found
Spectroscopy of brown dwarf candidates in IC 348 and the determination of its substellar IMF down to planetary masses
Context. Brown dwarfs represent a sizable fraction of the stellar content of
our Galaxy and populate the transition between the stellar and planetary mass
regime. There is however no agreement on the processes responsible for their
formation. Aims. We have conducted a large survey of the young, nearby cluster
IC 348, to uncover its low-mass brown dwarf population and study the cluster
properties in the substellar regime. Methods. Deep optical and near-IR images
taken with MegaCam and WIRCam at the Canada-France-Hawaii Telescope (CFHT) were
used to select photometric candidate members. A spectroscopic follow-up of a
large fraction of the candidates was conducted to assess their youth and
membership. Results. We confirmed spectroscopically 16 new members of the IC
348 cluster, including 13 brown dwarfs, contributing significantly to the
substellar census of the cluster, where only 30 brown dwarfs were previously
known. Five of the new members have a L0 spectral type, the latest-type objects
found to date in this cluster. At 3 Myr, evolutionary models estimate these
brown dwarfs to have a mass of ~13 Jupiter masses. Combining the new members
with previous census of the cluster, we constructed the IMF complete down to 13
Jupiter masses. Conclusions. The IMF of IC 348 is well fitted by a log-normal
function, and we do not see evidence for variations of the mass function down
to planetary masses when compared to other young clusters.Comment: Accepted to A&A (8 November 2012
The XXL Survey X: K-band luminosity - weak-lensing mass relation for groups and clusters of galaxies
We present the K-band luminosity-halo mass relation, ,
for a subsample of 20 of the 100 brightest clusters in the XXL Survey observed
with WIRCam at the Canada-France-Hawaii Telescope (CFHT). For the first time,
we have measured this relation via weak-lensing analysis down to . This allows us to investigate whether the slope
of the relation is different for groups and clusters, as seen in other
works. The clusters in our sample span a wide range in mass, , at . The K-band luminosity
scales as with and an
intrinsic scatter of . Combining our
sample with some clusters in the Local Cluster Substructure Survey (LoCuSS)
present in the literature, we obtain a slope of and an
intrinsic scatter of . The flattening in the seen
in previous works is not seen here and might be a result of a bias in the mass
measurement due to assumptions on the dynamical state of the systems. We also
study the richness-mass relation and find that group-sized halos have more
galaxies per unit halo mass than massive clusters. However, the brightest
cluster galaxy (BCG) in low-mass systems contributes a greater fraction to the
total cluster light than BCGs do in massive clusters; the luminosity gap
between the two brightest galaxies is more prominent for group-sized halos.
This result is a natural outcome of the hierarchical growth of structures,
where massive galaxies form and gain mass within low-mass groups and are
ultimately accreted into more massive clusters to become either part of the BCG
or one of the brighter galaxies. [Abridged]Comment: A&A, in pres
The VIPERS Multi-Lambda Survey. I. UV and NIR Observations, multi-color catalogues and photometric redshifts
We present observations collected in the CFHTLS-VIPERS region in the
ultraviolet (UV) with the GALEX satellite (far and near UV channels) and the
near infrared with the CFHT/WIRCam camera (-band) over an area of 22 and
27 deg, respectively. The depth of the photometry was optimized to measure
the physical properties (e.g., SFR, stellar masses) of all the galaxies in the
VIPERS spectroscopic survey. The large volume explored by VIPERS will enable a
unique investigation of the relationship between the galaxy properties and
their environment (density field and cosmic web) at high redshift (0.5 < z <
1.2). In this paper, we present the observations, the data reductions and the
build-up of the multi-color catalogues. The CFHTLS-T0007 (gri-{\chi}^2) images
are used as reference to detect and measure the -band photometry, while
the T0007 u-selected sources are used as priors to perform the GALEX photometry
based on a dedicated software (EMphot). Our final sample reaches ~25
(at 5{\sigma}) and ~22 (at 3{\sigma}). The large spectroscopic sample
(~51,000 spectroscopic redshifts) allows us to highlight the robustness of our
star/galaxy separation, and the reliability of our photometric redshifts with a
typical accuracy 0.04 and a catastrophic failure rate {\eta} <
2% down to i~23. We present various tests on the band completeness and
photometric redshift accuracy by comparing with existing, overlapping deep
photometric catalogues. Finally, we discuss the BzK sample of passive and
active galaxies at high redshift and the evolution of galaxy morphology in the
(NUV-r) vs (r-K_s) diagram at low redshift (z < 0.25) thanks to the high image
quality of the CFHTLS. The images, catalogues and photometric redshifts for 1.5
million sources (down to 25 or 22) are released and
available at this URL: http://cesam.lam.fr/vipers-mls/Comment: 14 pages, 16 figures. Accepted for publication in A&A. Version to be
publishe
Extragalactic Fields Optimized for Adaptive Optics
In this paper we present the coordinates of 67 55' x 55' patches of sky which
have the rare combination of both high stellar surface density (>0.5
arcmin^{-2} with 13<R<16.5 mag) and low extinction (E(B-V)<0.1). These fields
are ideal for adaptive-optics based follow-up of extragalactic targets. One
region of sky, situated near Baade's Window, contains most of the patches we
have identified. Our optimal field, centered at RA: 7h24m3s, Dec: -1deg27'15",
has an additional advantage of being accessible from both hemispheres. We
propose a figure of merit for quantifying real-world adaptive optics
performance, and use this to analyze the performance of multi-conjugate
adaptive optics in these fields. We also compare our results to those that
would be obtained in existing deep fields. In some cases adaptive optics
observations undertaken in the fields given in this paper would be orders of
magnitude more efficient than equivalent observations undertaken in existing
deep fields.Comment: 28 pages, 15 figures, 1 table; accepted for publication in PAS
LBQS 1429-0053: a binary quasar rather than a lensed quasar
Very deep ESO/VLT FORS1 and ISAAC images, as well as HST NICMOS2 data are
used to infer the nature of the quasar pair LBQS 1429-0053 A and B, either a
binary quasar or a doubly-imaged lensed quasar. Direct search of a putative
lensing galaxy is unsuccessful . Moreover, no galaxy overdensity close to the
quasar pair is found. A weak shear analysis of the field also fails at
detecting any concentration of dark matter and weakens the hypothesis of a dark
lens. The only sign of a possible lens consists in a group of 5 objects at z~1,
within a radius of 5'', from the quasar pair. Considering this group as the
lensing potential does not allow to reproduce the image position and flux ratio
of the quasars. Our deep R-band image shows a blue, extended object at the
position of quasar A, which is consistent with either being the lensed quasar A
host, or being an intervening galaxy at lower redshift. Unless future very deep
optical images demonstrate that this object is actually the lensed host of LBQS
1429-0053, we conclude that there is very little evidence for this quasar being
lensed. Therefore, we are led to declare LBQS 1429-0053 A and B a genuine
binary quasar.Comment: 9 pages, 6 jpg images, accepted for publication in A&
R-Coffee: a web server for accurately aligning noncoding RNA sequences
The R-Coffee web server produces highly accurate multiple alignments of noncoding RNA (ncRNA) sequences, taking into account predicted secondary structures. R-Coffee uses a novel algorithm recently incorporated in the T-Coffee package. R-Coffee works along the same lines as T-Coffee: it uses pairwise or multiple sequence alignment (MSA) methods to compute a primary library of input alignments. The program then computes an MSA highly consistent with both the alignments contained in the library and the secondary structures associated with the sequences. The secondary structures are predicted using RNAplfold. The server provides two modes. The slow/accurate mode is restricted to small datasets (less than 5 sequences less than 150 nucleotides) and combines R-Coffee with Consan, a very accurate pairwise RNA alignment method. For larger datasets a fast method can be used (RM-Coffee mode), that uses R-Coffee to combine the output of the three packages which combines the outputs from programs found to perform best on RNA (MUSCLE, MAFFT and ProbConsRNA). Our BRAliBase benchmarks indicate that the R-Coffee/Consan combination is one of the best ncRNA alignment methods for short sequences, while the RM-Coffee gives comparable results on longer sequences. The R-Coffee web server is available at http://www.tcoffee.org
R-Coffee: a web server for accurately aligning noncoding RNA sequences
The R-Coffee web server produces highly accurate multiple alignments of noncoding RNA (ncRNA) sequences, taking into account predicted secondary structures. R-Coffee uses a novel algorithm recently incorporated in the T-Coffee package. R-Coffee works along the same lines as T-Coffee: it uses pairwise or multiple sequence alignment (MSA) methods to compute a primary library of input alignments. The program then computes an MSA highly consistent with both the alignments contained in the library and the secondary structures associated with the sequences. The secondary structures are predicted using RNAplfold. The server provides two modes. The slow/accurate mode is restricted to small datasets (less than 5 sequences less than 150 nucleotides) and combines R-Coffee with Consan, a very accurate pairwise RNA alignment method. For larger datasets a fast method can be used (RM-Coffee mode), that uses R-Coffee to combine the output of the three packages which combines the outputs from programs found to perform best on RNA (MUSCLE, MAFFT and ProbConsRNA). Our BRAliBase benchmarks indicate that the R-Coffee/Consan combination is one of the best ncRNA alignment methods for short sequences, while the RM-Coffee gives comparable results on longer sequences. The R-Coffee web server is available at http://www.tcoffee.org
The galaxy-halo connection from a joint lensing, clustering and abundance analysis in the CFHTLenS/VIPERS field
We present new constraints on the relationship between galaxies and their
host dark matter halos, measured from the location of the peak of the
stellar-to-halo mass ratio (SHMR), up to the most massive galaxy clusters at
redshift and over a volume of nearly 0.1~Gpc. We use a unique
combination of deep observations in the CFHTLenS/VIPERS field from the near-UV
to the near-IR, supplemented by secure spectroscopic redshifts,
analysing galaxy clustering, galaxy-galaxy lensing and the stellar mass
function. We interpret our measurements within the halo occupation distribution
(HOD) framework, separating the contributions from central and satellite
galaxies. We find that the SHMR for the central galaxies peaks at with an amplitude of ,
which decreases to for massive halos (). Compared to central galaxies only, the total SHMR (including
satellites) is boosted by a factor 10 in the high-mass regime (cluster-size
halos), a result consistent with cluster analyses from the literature based on
fully independent methods. After properly accounting for differences in
modelling, we have compared our results with a large number of results from the
literature up to : we find good general agreement, independently of the
method used, within the typical stellar-mass systematic errors at low to
intermediate mass () and the statistical
errors above. We have also compared our SHMR results to semi-analytic
simulations and found that the SHMR is tilted compared to our measurements in
such a way that they over- (under-) predict star formation efficiency in
central (satellite) galaxies.Comment: 31 pages, 18 figures, 4 table. Accepted for publication in MNRAS.
Online material available at http://www.cfhtlens.or
The Shear TEsting Programme 1: Weak lensing analysis of simulated ground-based observations
The Shear TEsting Programme, STEP, is a collaborative project to improve the accuracy and reliability of all weak lensing measurements in preparation for the next generation of wide-field surveys. In this first STEP paper we present the results of a blind analysis of simulated ground-based observations of relatively simple galaxy morphologies. The most successful methods are shown to achieve percent level accuracy. From the cosmic shear pipelines that have been used to constrain cosmology, we find weak lensing shear measured to an accuracy that is within the statistical errors of current weak lensing analyses, with shear measurements accurate to better than 7%. The dominant source of measurement error is shown to arise from calibration uncertainties where the measured shear is over or under-estimated by a constant multiplicative factor. This is of concern as calibration errors cannot be detected through standard diagnostic tests. The measured calibration errors appear to result from stellar contamination, false object detection, the shear measurement method itself, selection bias and/or the use of biased weights. Additive systematics (false detections of shear) resulting from residual point-spread function anisotropy are, in most cases, reduced to below an equivalent shear of 0.001, an order of magnitude below cosmic shear distortions on the scales probed by current surveys. Our results provide a snapshot view of the accuracy of current ground-based weak lensing methods and a benchmark upon which we can improve. To this end we provide descriptions of each method tested and include details of the eight different implementations of the commonly used Kaiser, Squires and Broadhurst (1995) method (KSB+) to aid the improvement of future KSB+ analyses
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