We present the Strong Lensing Legacy Survey-ARCS (SARCS) sample compiled from the final T0006 data release of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) covering a total ...non-overlapping area of 159 deg super(2). We adopt a semi-automatic method to find gravitational arcs in the survey that makes use of an arc-finding algorithm. The candidate list is pruned by visual inspection and ranking to form the final SARCS sample. This list also includes some serendipitously discovered lens candidates which the automated algorithm did not detect. The SARCS sample consists of 127 lens candidates which span arc radii ~2"-18" within the unmasked area of ~150 deg super(2). Within the sample, 54 systems are promising lenses among which, we find 12 giant arcs (length-to-width ratio > or =, slanted8). We also find two radial arc candidates in SL2SJ141447+544704. From our sample, we detect a systematic alignment of the giant arcs with the major axis of the baryonic component of the putative lens in concordance with previous studies. This alignment is also observed for all arcs in the sample and does not vary significantly with increasing arc radius. The mean values of the photometric redshift distributions of lenses corresponding to the giant arcs and all arcs sample are at z ~ 0.6. Owing to the large area and depth of the CFHTLS, we find the largest sample of lenses probing mass scales that are intermediate to cluster and galaxy lenses for the first time. We compare the observed image separation distribution (ISD) of our arcs with theoretical models. A two-component density profile for the lenses which accounts for both the central galaxy and the dark matter component is required by the data to explain the observed ISD. Unfortunately, current levels of uncertainties and degeneracies accommodate models both with and without adiabatic contraction. We also show the effects of changing parameters of the model that predict the ISD and that a larger lens sample might constrain relations such as the concentration-mass relation, mass-luminosity relation, and the faint-end slope of the luminosity function.
Aims. We present data from the CFHTLS Strong Lensing Legacy Survey (SL2S). Due to the unsurpassed combined depth, area and image quality of the Canada-France-Hawaii Legacy Survey it is becoming ...possible to uncover a large, statistically well-defined sample of strong gravitational lenses which spans the dark halo mass spectrum predicted by the concordance model from galaxy to cluster haloes. Methods. We describe the development of several automated procedures to find strong lenses of various mass regimes in CFHTLS images. Results. The preliminary sample of about 40 strong lensing candidates discovered in the CFHTLS T0002 release, covering an effective field of view of 28 deg super(2) is presented. These strong lensing systems were discovered using an automated search and consist mainly of gravitational arc systems with splitting angles between 2 and 15 arcsec. This sample shows for the first time that it is possible to uncover a large population of strong lenses from galaxy groups with typical halo masses of about 10 6h 6M_\odot. We discuss the future evolution of the SL2S project and its main scientific aims for the next 3 years, in particular our observational strategy to extract the hundreds of gravitational rings also present in these fields.
Large-scale magnetic topologies of mid M dwarfs Morin, J.; Donati, J.-F.; Petit, P. ...
Monthly notices of the Royal Astronomical Society,
10/2008, Letnik:
390, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present in this paper, the first results of a spectropolarimetric analysis of a small sample (∼20) of active stars ranging from spectral type M0 to M8, which are either fully convective or possess ...a very small radiative core. This study aims at providing new constraints on dynamo processes in fully convective stars. This paper focuses on five stars of spectral type ∼M4, i.e. with masses close to the full convection threshold (≃0.35 M⊙), and with short rotational periods. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarized profiles. We find that all stars host mainly axisymmetric large-scale poloidal fields. Three stars were observed at two different epochs separated by ∼1 yr; we find the magnetic topologies to be globally stable on this time-scale. We also provide an accurate estimation of the rotational period of all stars, thus allowing us to start studying how rotation impacts the large-scale magnetic field.
Based on a new sample of 355 quasars with significant optical polarization and using complementary statistical methods, we confirm that quasar polarization vectors are not randomly oriented over the ...sky with a probability often in excess of 99.9%. The polarization vectors appear coherently oriented or aligned over huge (~1 Gpc) regions of the sky located at both low ($z \sim 0.5$) and high ($z \sim 1.5$) redshifts and characterized by different preferred directions of the quasar polarization. In fact, there seems to exist a regular alternance along the line of sight of regions of randomly and aligned polarization vectors with a typical comoving length scale of 1.5 Gpc. Furthermore, the mean polarization angle $\bar{\theta}$ appears to rotate with redshift at the rate of ~30° per Gpc. The symmetry of the the $\bar{\theta} -z$ relation is mirror-like, the mean polarization angle rotating clockwise with increasing redshift in North Galactic hemisphere and counter-clockwise in the South Galactic one. These characteristics make the alignment effect difficult to explain in terms of local mechanisms, namely a contamination by interstellar polarization in our Galaxy. While interpretations like a global rotation of the Universe can potentially explain the effect, the properties we observe qualitatively correspond to the dichroism and birefringence predicted by photon-pseudoscalar oscillation within a magnetic field. Interestingly, the alignment effect seems to be prominent along an axis not far from preferred directions tentatively identified in the Cosmic Microwave Background maps. Although many questions and more particularly the interpretation of the effect remain open, alignments of quasar polarization vectors appear as a promising new way to probe the Universe and its dark components at extremely large scales.
Context.
Cosmological probes based on galaxy clusters rely on cluster number counts and large-scale structure information. X-ray cluster surveys are well suited for this purpose because they are far ...less affected by projection effects than optical surveys, and cluster properties can be predicted with good accuracy.
Aims.
The XMM Cluster Archive Super Survey, X-CLASS, is a serendipitous search of X-ray-detected galaxy clusters in 4176
XMM-Newton
archival observations until August 2015. All observations are clipped to exposure times of 10 and 20 ks to obtain uniformity, and they span ∼269 deg
2
across the high-Galactic latitude sky (|
b
| > 20°). The main goal of the survey is the compilation of a well-selected cluster sample suitable for cosmological analyses.
Methods.
We describe the detection algorithm, the visual inspection, the verification process, and the redshift validation of the cluster sample, as well as the cluster selection function computed by simulations. We also present the various metadata that are released with the catalogue, along with two different count-rate measurements, an automatic one provided by the pipeline, and a more detailed and accurate interactive measurement. Furthermore, we provide the redshifts of 124 clusters obtained with a dedicated multi-object spectroscopic follow-up programme.
Results.
With this publication, we release the new X-CLASS catalogue of 1646 well-selected X-ray-detected clusters over a wide sky area, along with their selection function. The sample spans a wide redshift range, from the local Universe up to
z
∼ 1.5, with 982 spectroscopically confirmed clusters, and over 70 clusters above
z
= 0.8. The redshift distribution peaks at
z
∼ 0.1, while if we remove the pointed observations it peaks at
z
∼ 0.3. Because of its homogeneous selection and thorough verification, the cluster sample can be used for cosmological analyses, but also as a test-bed for the upcoming eROSITA observations and other current and future large-area cluster surveys. It is the first time that such a catalogue is made available to the community via an interactive database which gives access to a wealth of supplementary information, images, and data.
We present here additional results of a spectropolarimetric survey of a small sample of stars ranging from spectral type M0 to M8 aimed at investigating observationally how dynamo processes operate ...in stars on both sides of the full convection threshold (spectral type M4). The present paper focuses on early M stars (M0–M3), that is above the full convection threshold. Applying tomographic imaging techniques to time series of rotationally modulated circularly polarized profiles collected with the NARVAL spectropolarimeter, we determine the rotation period and reconstruct the large-scale magnetic topologies of six early M dwarfs. We find that early-M stars preferentially host large-scale fields with dominantly toroidal and non-axisymmetric poloidal configurations, along with significant differential rotation (and long-term variability); only the lowest-mass star of our subsample is found to host an almost fully poloidal, mainly axisymmetric large-scale field resembling those found in mid-M dwarfs. This abrupt change in the large-scale magnetic topologies of M dwarfs (occurring at spectral type M3) has no related signature on X-ray luminosities (measuring the total amount of magnetic flux); it thus suggests that underlying dynamo processes become more efficient at producing large-scale fields (despite producing the same flux) at spectral types later than M3. We suspect that this change relates to the rapid decrease in the radiative cores of low-mass stars and to the simultaneous sharp increase of the convective turnover times (with decreasing stellar mass) that models predict to occur at M3; it may also be (at least partly) responsible for the reduced magnetic braking reported for fully convective stars.
Context. Studying the density profiles of galaxy groups offers an important insight into the formation and evolution of the structures in the universe, since galaxy groups bridge the gap between ...single galaxies and massive clusters. Aims. We aim to probe the mass of SL2S J02140-0535, a galaxy group at z = 0.44 from the Strong Lensing Legacy Survey (SL2S), which has uncovered this new population of group-scale strong lenses. Methods. We combine strong lensing modeling and dynamical constraints. The strong lensing analysis is based on multi-band HST/ACS observations exhibiting strong lensing features that we have followed-up spectroscopically with VLT/FORS2. To constrain the scale radius of an NFW mass profile that cannot be constrained by strong lensing, we propose a new method by taking advantage of the large-scale dynamical information provided by VLT/FORS2 and KECK/LRIS spectroscopy of group members. Results. In constrast to other authors, we show that the observed lensing features in SL2S J02140-0535 belong to different background sources: one at z = 1.7 ± 0.1 (photometric redshift) produces three images, while the other at z = 1.023 ± 0.001 (spectroscopic redshift) has only a single image. Our unimodal NFW mass model reproduces these images very well. It is characterized by a concentration parameter c200 = 6.0 ± 0.6, which is slightly greater than the value expected from ΛCDM simulations for a mass of M200 ≈ 1 × 1014 M⊙. The spectroscopic analysis of group members also reveals a unimodal structure that exhibits no evidence of merging. The position angle of the halo is θ = 111.6 ± 0.2, which agrees with the direction defined by the luminosity contours. We compare our dynamic mass estimate with an independent weak-lensing based mass estimate finding that both are consistent. Conclusions. Our combined lensing and dynamical analysis of SL2S J02140-0535 demonstrates the importance of spectroscopic information in reliably identifying the lensing features. Our findings argue that the system is a relaxed, massive galaxy group where mass is traced by light. This work shows a potentially useful method for constraining large-scale properties inaccessible to strong lensing, such as the scale radius of the NFW profile.
We report on the X-ray observation of a strong lensing selected group, SL2S J08544-0121, with a total mass of 2.4 ± 0.6 × 1014 M⊙ which revealed a separation of 124 ± 20 kpc between the X-ray ...emitting collisional gas and the collisionless galaxies and dark matter (DM), traced by strong lensing. This source allows to put an order of magnitude estimate to the upper limit to the interaction cross-section of DM of 10 cm2 g−1. It is the lowest mass object found to date showing a DM–baryons separation, and it reveals that the detection of bullet-like objects is not rare and confined to mergers of massive objects opening the possibility of a statistical detection of DM–baryons separation with future surveys.
Aims. We observe the nearby, weakly-active single giant, Pollux, in order to directly study and infer the nature of its magnetic field. Methods. We used the new generation spectropolarimeters ...ESPaDOnS and NARVAL to observe and detect circular polarization within the photospheric absorption lines of Pollux. Our observations span 18 months from 2007–2009. We treated the spectropolarimetric data using the Least-Squares Deconvolution method to create high signal-to-noise ratio mean Stokes V profiles. We also measured the classical activity indicator S-index for the Ca ii H and K lines, and the stellar radial velocity (RV). Results. We have unambiguously detected a weak Stokes V signal in the spectral lines of Pollux, and measured the related surface-averaged longitudinal magnetic field Bl. The longitudinal field averaged over the span of the observations is below one gauss. Our data suggest variations of the longitudinal magnetic field, but no significant variation of the S-index. We observe variations of RV which are qualitatively consistent with the published ephemeris for a proposed exoplanet orbiting Pollux. The observed variations of Bl appear to mimic those of RV, but additional data for this relationship to be established. Using evolutionary models including the effects of rotation, we derive the mass of Pollux and we discuss its evolutionary status and the origin of its magnetic field. Conclusions. This work presents the first direct detection of the magnetic field of Pollux, and demonstrates that ESPaDOnS and NARVAL are capable of obtaining sub-G measurements of the surface-averaged longitudinal magnetic field of giant stars, and of directly studying the relationships between magnetic activity, stellar evolution and planet hosting of these stars.
SARCS strong-lensing galaxy groups Foex, G; Motta, V; Limousin, M ...
Astronomy and astrophysics (Berlin),
11/2013, Letnik:
559
Journal Article
Recenzirano
We present the weak-lensing and optical analysis of the sL25-ARCS (SARCS) sample of strong-lensing candidates. The sample is based on the Strong Lensing Legacy Survey (SL2S), a systematic search of ...strong-lensing systems in the photometric Canada- France-Hawaii Telescope Legacy Survey (CFHTLS). The SARCS sample focuses on arc-like features and is designed to contain mostly galaxy groups. We briefly present the weak-lensing methodology that we used to estimate the mass of the SARCS objects. We finally combined the lensing and optical analyses to define a sample of the 80 most secure group candidates, i.e. weak-lensing detection and over-density at the lens position in the luminosity map, to remove false detections and galaxy-scale systems from the initial sample. This paper talks about the SARCS strong-lensing galaxy groups.
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