228 research outputs found
The Top Ten List of Gravitational Lens Candidates from the HST Medium Deep Survey
A total of 10 good candidates for gravitational lensing have been discovered
in the WFPC2 images from the HST Medium Deep Survey (MDS) and archival primary
observations. These candidate lenses are unique HST discoveries, i.e. they are
faint systems with sub-arcsecond separations between the lensing objects and
the lensed source images. Most of them are difficult objects for ground-based
spectroscopic confirmation or for measurement of the lens and source redshifts.
Seven are ``strong lens'' candidates which appear to have multiple images of
the source. Three are cases where the single image of the source galaxy has
been significantly distorted into an arc. The first two quadruply lensed
candidates were reported in Ratnatunga et al 1995 (ApJL, 453, L5) We report on
the subsequent eight candidates and describe them with simple models based on
the assumption of singular isothermal potentials. Residuals from the simple
models for some of the candidates indicate that a more complex model for the
potential will probably be required to explain the full structural detail of
the observations once they are confirmed to be lenses. We also discuss the
effective survey area which was searched for these candidate lens objects.Comment: 26 pages including 12 figures and 10 tables. AJ Vol. 117, No.
On the kinematic deconvolution of the local neighbourhood luminosity function
A method for inverting the statistical star counts equation, including proper
motions, is presented; in order to break the degeneracy in that equation it
uses the supplementary constraints required by dynamical consistency. The
inversion gives access to both the kinematics and the luminosity function of
each population in three r\'egimes: the singular ellipsoid, the constant ratio
Schwarzschild ellipsoid plane parallel models and the epicyclic model. This
more realistic model is taylored to account for local neighbourhood density and
velocity distribution.
The first model is fully investigated both analytically and via means of a
non-parametric inversion technique, while the second model is shown to be
formally its equivalent. The effect of noise and incompleteness in apparent
magnitude is investigated. The third model is investigated via a 5D+2D
non-parametric inversion technique where positivity of the underlying
luminosity function is explicitely accounted for.
It is argued that its future application to data such as the Tycho catalogue
(and in the upcoming satellite GAIA) could lead -- provided the vertical
potential, and/or the asymmetric drift or w_0 are known -- to a non-parametric
determination of the local neighbourhood luminosity function without any
reference to stellar evolution tracks. It should also yield the proportion of
stars for each kinematic component and a kinematic diagnostic to split the thin
disk from the thick disk or the halo.Comment: 18 pages, LateX (or Latex, etc), mnras, accepted for publicatio
Quantitative Morphology of Moderate Redshift Galaxies : How Many Peculiars are There ?
The advent of the Hubble Space Telescope (HST) has provided images of
galaxies at moderate and high redshifts and changed the scope of galaxy
morphologies considerably. It is evident that the Hubble Sequence requires
modifications in order to incorporate all the various morphologies one
encounters at such redshifts. We investigate and compare different approaches
to quantifying peculiar galaxy morphologies on images obtained from the Medium
Deep Survey (MDS) and other surveys using the Wide Field Planetary Camera 2
(WFPC2) on board the HST, in the I band (filter F814W). We define criteria for
peculiarity and put them to use on a sample of 978 galaxies, classifying them
by eye as either normal or peculiar. Based on our criteria and on concepts
borrowed from digital image processing we design a set of four purely
morphological parameters, which comprise the overall texture (or
``blobbiness'') of the image; the distortion of isophotes; the filling-factor
of isophotes; and the skeleta of detected structures. We also examine the
parameters suggested by Abraham et al. (1995). An artificial neural network
(ANN) is trained to distinguish between normal and peculiar galaxies. While the
majority of peculiar galaxies are disk-dominated, we also find evidence for a
significant population of bulge-dominated peculiars. Consequently, peculiar
galaxies do not all form a ``natural'' continuation of the Hubble sequence
beyond the late spirals and the irregulars. The trained neural network is
applied to a second, larger sample of 1999 WFPC2 images and its probabilistic
capabilities are used to estimate the frequency of peculiar galaxies at
moderate redshifts as .Comment: 32 pages, latex and 9 figures, Ap. J., accepte
Vertical distribution of Galactic disk stars : II. The surface mass density in the Galactic plane
High resolution spectra data of red clump stars towards the NGP have been
obtained with the high resolution spectrograph Elodie at OHP for Tycho-2
selected stars. Combined with Hipparcos local analogues, we determine both the
gravitational force law perpendicaular to the Galactic plane, and the total
surface mass density and thickness of the Galactic disk. The surface mass
density of the Galactic disk within 800 pc derived from this analysis is
Sigma(|z|<800pc)=76 Msol.pc-2 and, removing the dark halo contribution, the
total disk mass density is Sigma0=67 Msol.pc-2 at solar radius. The thickness
of the total disk mass distribution is dynamicaly measured for the first time
and is found to be 390pc in relative agreement with the old stellar disk scale
height. All dynamical evidences concerning the structure of the disk (its local
volume density -i.e. the Oort limit-, its surface density and its thickness)
are compatible with our knowledge of the corresponding stellar disk properties.Comment: 11 pages, 9 figures, Accepted for publication in A&
PHotometry Assisted Spectral Extraction (PHASE) and identification of SNLS supernovae
Aim: We present new extraction and identification techniques for supernova
(SN) spectra developed within the Supernova Legacy Survey (SNLS) collaboration.
Method: The new spectral extraction method takes full advantage of
photometric information from the Canada-France-Hawai telescope (CFHT) discovery
and reference images by tracing the exact position of the supernova and the
host signals on the spectrogram. When present, the host spatial profile is
measured on deep multi-band reference images and is used to model the host
contribution to the full (supernova + host) signal. The supernova is modelled
as a Gaussian function of width equal to the seeing. A chi-square minimisation
provides the flux of each component in each pixel of the 2D spectrogram. For a
host-supernova separation greater than <~ 1 pixel, the two components are
recovered separately and we do not use a spectral template in contrast to more
standard analyses. This new procedure permits a clean extraction of the
supernova separately from the host in about 70% of the 3rd year ESO/VLT spectra
of the SNLS. A new supernova identification method is also proposed. It uses
the SALT2 spectrophotometric template to combine the photometric and spectral
data. A galaxy template is allowed for spectra for which a separate extraction
of the supernova and the host was not possible.
Result: These new techniques have been tested against more standard
extraction and identification procedures. They permit a secure type and
redshift determination in about 80% of cases. The present paper illustrates
their performances on a few sample spectra.Comment: 27 pages, 18 Figures, 1 Table. Accepted for publication in A&
A seven square degrees survey for galaxy-scale gravitational lenses with the HST imaging archive
We present the results of a visual search for galaxy-scale gravitational
lenses in nearly 7 square degrees of Hubble Space Telescope (HST) images. The
dataset comprises the whole imaging data ever taken with the Advanced Camera
for Surveys (ACS) in the filter F814W (I-band) up to August 31st, 2011, i.e.
6.03 square degrees excluding the field of the Cosmic Evolution Survey (COSMOS)
which has been the subject of a separate visual search. In addition, we have
searched for lenses in the whole Wide Field Camera 3 (WFC3) near-IR imaging
dataset in all filters (1.01 square degrees) up to the same date. Our primary
goal is to provide a sample of lenses with a broad range of different
morphologies and lens-source brightness contrast in order estimate a lower
limit to the number of galaxy-scale strong lenses in the future Euclid survey
in its VIS band. Our criteria to select lenses are purely morphological as we
do not use any colour or redshift information.The final candidate selection is
very conservative hence leading to a nearly pure but incomplete sample. We find
49 new lens candidates: 40 in the ACS images and 9 in the WFC3 images. Out of
these, 16 candidates are secure lenses owing to their striking morphology, 21
more are very good candidates, and 12 more have morphologies compatible with
gravitational lensing but also compatible with other astrophysical objects. It
is therefore insensitive to cosmic variance and allows to estimate the number
of galaxy-scale strong lenses on the sky for a putative survey depth, which is
the main result of the present work. Because of the incompleteness of the
sample, the estimated lensing rates should be taken as lower limits. Using
these, we anticipate that a 15 000 square degrees space survey such as Euclid
will find at least 60 000 galaxy-scale strong lenses down to a limiting AB
magnitude of I = 24.5 (10-sigma) or I = 25.8 (3-sigma).Comment: 13 pages, 12 figures, Accepted for publication in MNRA
Image reduction pipeline for the detection of variable sources in highly crowded fields
We present a reduction pipeline for CCD (charge-coupled device) images which
was built to search for variable sources in highly crowded fields like the M31
bulge and to handle extensive databases due to large time series. We describe
all steps of the standard reduction in detail with emphasis on the realisation
of per pixel error propagation: Bias correction, treatment of bad pixels,
flatfielding, and filtering of cosmic rays. The problems of conservation of PSF
(point spread function) and error propagation in our image alignment procedure
as well as the detection algorithm for variable sources are discussed: We build
difference images via image convolution with a technique called OIS (Alard &
Lupton, 1998), proceed with an automatic detection of variable sources in noise
dominated images and finally apply a PSF-fitting, relative photometry to the
sources found. For the WeCAPP project (Riffeser et al., 2001) we achieve 3
sigma detections for variable sources with an apparent brightness of e.g. m =
24.9 mag at their minimum and a variation of dm = 2.4 mag (or m = 21.9 mag
brightness minimum and a variation of dm = 0.6 mag) on a background signal of
18.1 mag/arcsec^2 based on a 500 s exposure with 1.5 arcsec seeing at a 1.2 m
telescope. The complete per pixel error propagation allows us to give accurate
errors for each measurement.Comment: 15 pages, 4 figures, accepted by A&
VLT spectroscopy of XTE J2123-058 during quiescence
We present VLT low resolution spectroscopy of the neutron star X-ray
transient XTE J2123-058 during its quiescent state. Our data reveal the
presence of a K7V companion which contributes 77 % to the total flux at 6300 A
and orbits the neutron star at K_2 = 287 +/- 12 km/s. Contrary to other soft
X-ray transients (SXTs), the Halpha emission is almost exactly in antiphase
with the velocity curve of the optical companion. Using the light-center
technique we obtain K_1 = 140 +/- 27 km/s and hence q=K_1/K_2=M_2/M_1= 0.49 +/-
0.10. This, combined with a previous determination of the inclination angle
(i=73 +/- 4) yields M_1 = 1.55 +/- 0.31 Msun and M_2 = 0.76 +/- 0.22 Msun. M_2
agrees well with the observed spectral type. Doppler tomography of the Halpha
emission shows a non-symmetric accretion disc distribution mimicking that seen
in SW Sex stars. Although we find a large systemic velocity of -110 +/- 8 km/s
this value is consistent with the galactic rotation velocity at the position of
J2123-058, and hence a halo origin. The formation scenario of J2123-058 is
still unresolved.Comment: 10 pages, 3 figures, accepted for publication in MNRAS with very
minor change
Anomalies in T cell function are associated with individuals at risk of mycobacterium abscessus complex infection
The increasing global incidence and prevalence of non-tuberculous mycobacteria (NTM) infection is of growing concern. New evidence of person-to-person transmission of multidrug-resistant NTM adds to the global concern. The reason why certain individuals are at risk of NTM infections is unknown. Using high definition flow cytometry, we studied the immune profiles of two groups that are at risk of Mycobacterium abscessus complex infection and matched controls. The first group was cystic fibrosis (CF) patients and the second group was elderly individuals. CF individuals with active M. abscessus complex infection or a history of M. abscessus complex infection exhibited a unique surface T cell phenotype with a marked global deficiency in TNFa production during mitogen stimulation. Importantly, immune-based signatures were identified that appeared to predict at baseline the subset of CF individuals who were at risk of M. abscessus complex infection. In contrast, elderly individuals with M. abscessus complex infection exhibited a separate T cell phenotype underlined by the presence of exhaustion markers and dysregulation in type 1 cytokine release during mitogen stimulation. Collectively, these data suggest an association between T cell signatures and individuals at risk of M. abscessus complex infection, however, validation of these immune anomalies as robust biomarkers will require analysis on larger patient cohorts
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