255 research outputs found
SHARP - I. A high-resolution multi-band view of the infra-red Einstein ring of JVAS B1938+666
We present new mass models for the gravitational lens system B1938+666, using
multi-wavelength data acquired from Keck adaptive optics (AO) and Hubble Space
Telescope (HST) observations. These models are the first results from the
Strong-lensing at High Angular Resolution Program (SHARP), a project designed
to study known quadruple-image and Einstein ring lenses using high-resolution
imaging, in order to probe their mass distributions in unprecedented detail.
Here, we specifically highlight differences between AO- and HST-derived lens
models, finding that -- at least when the lens and source galaxies are both
bright and red, and the system has a high degree of circular symmetry --
AO-derived models place significantly tighter constraints on model parameters.
Using this improved precision, we infer important physical properties about the
B1938+666 system, including the mass density slope of the lensing galaxy (gamma
= 2.045), the projected dark matter mass fraction within the Einstein radius
(M_dark/M_lens = 0.55), and the total magnification factor of the source galaxy
(~ 13). Additionally, we measure an upper-limit constraint on luminous
substructure (M_V > 16.2), based on the non-detection of bright satellite
galaxies in all data sets. Finally, we utilize the improved image resolution of
the AO data to reveal the presence of faint arcs outside of the primary
Einstein ring. The positions and orientations of these arcs raise the
intriguing possibility that B1938+666 has a second source galaxy, located at a
more distant redshift. However, future work is needed to verify this
hypothesis.Comment: 12 pages, 7 figures; Accepted for publication in MNRA
The X-shooter Lens Survey - II. Sample presentation and spatially resolved kinematics
We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is
to disentangle the stellar and dark matter content of massive early-type
galaxies (ETGs), through combined strong gravitational lensing, dynamics and
spectroscopic stellar population studies. The sample consists of 11 lens
galaxies covering the redshift range from to and having stellar
velocity dispersions between and . All
galaxies have multi-band, high-quality HST imaging. We have obtained long-slit
spectra of the lens galaxies with X-shooter on the VLT. We are able to
disentangle the dark and luminous mass components by combining lensing and
extended kinematics data-sets, and we are also able to precisely constrain
stellar mass-to-light ratios and infer the value of the low-mass cut-off of the
IMF, by adding spectroscopic stellar population information. Our goal is to
correlate these IMF parameters with ETG masses and investigate the relation
between baryonic and non-baryonic matter during the mass assembly and structure
formation processes. In this paper we provide an overview of the survey,
highlighting its scientific motivations, main goals and techniques. We present
the current sample, briefly describing the data reduction and analysis process,
and we present the first results on spatially resolved kinematics.Comment: Accepted for publication in MNRA
Selection functions of strong lens finding neural networks
Convolution Neural Networks trained for the task of lens finding with similar
architecture and training data as is commonly found in the literature are
biased classifiers. An understanding of the selection function of lens finding
neural networks will be key to fully realising the potential of the large
samples of strong gravitational lens systems that will be found in upcoming
wide-field surveys. We use three training datasets, representative of those
used to train galaxy-galaxy and galaxy-quasar lens finding neural networks. The
networks preferentially select systems with larger Einstein radii and larger
sources with more concentrated source-light distributions. Increasing the
detection significance threshold to 12 from 8 results in 50 per
cent of the selected strong lens systems having Einstein radii
1.04 arcsec from 0.879
arcsec, source radii 0.194 arcsec from 0.178 arcsec and
source S\'ersic indices 2.62 from
2.55. The model trained to find lensed
quasars shows a stronger preference for higher lens ellipticities than those
trained to find lensed galaxies. The selection function is independent of the
slope of the power-law of the mass profiles, hence measurements of this
quantity will be unaffected. The lens finder selection function reinforces that
of the lensing cross-section, and thus we expect our findings to be a general
result for all galaxy-galaxy and galaxy-quasar lens finding neural networks.Comment: Submitted to MNRA
Separating inner and outer contributions in gravitational lenses using the perturbative method
This paper presents a reconstruction of the gravitational lens SL2S02176-0513
using the singular perturbative method presented in Alard 2007, MNRAS Letters,
382, 58 and Alard, C., 2008, MNRAS, 388, 375. The ability of the perturbative
method to separate the inner and outer contributions of the potential in
gravitational lenses is tested using SL2S02176-0513. In this lens, the
gravitational field of the central galaxy is dominated by a nearby group of
galaxies located at a distance of a few critical radius. The perturbative
functionals are re-constructed using local polynomials. The polynomial
interpolation is smoothed using Fourier series, and numerically fitted to HST
data using a non-linear minimization procedure. The potential inside and
outside the critical circle is derived from the reconstruction of the
perturbative fields. The inner and outer potential contours are very
different.The inner contours are consistent with the central galaxy, while the
outer contours are fully consistent with the perturbation introduced by the
group of galaxies. The ability of the perturbative method to separate the inner
and outer contribution is confirmed, and indicates that in the perturbative
approach the field of the central deflector can be separated from outer
perturbations. The separation of the inner and outer contribution is especially
important for the study of the shape of dark matter halo's as well as for the
statistical analysis of the effect of dark matter substructures
Gravitational detection of a low-mass dark satellite at cosmological distance
The mass-function of dwarf satellite galaxies that are observed around Local
Group galaxies substantially differs from simulations based on cold dark
matter: the simulations predict many more dwarf galaxies than are seen. The
Local Group, however, may be anomalous in this regard. A massive dark satellite
in an early-type lens galaxy at z = 0.222 was recently found using a new method
based on gravitational lensing, suggesting that the mass fraction contained in
substructure could be higher than is predicted from simulations. The lack of
very low mass detections, however, prohibited any constraint on their mass
function. Here we report the presence of a 1.9 +/- 0.1 x 10^8 M_sun dark
satellite in the Einstein-ring system JVAS B1938+666 at z = 0.881, where M_sun
denotes solar mass. This satellite galaxy has a mass similar to the Sagittarius
galaxy, which is a satellite of the Milky Way. We determine the logarithmic
slope of the mass function for substructure beyond the local Universe to be
alpha = 1.1^+0.6_-0.4, with an average mass-fraction of f = 3.3^+3.6_-1.8 %, by
combining data on both of these recently discovered galaxies. Our results are
consistent with the predictions from cold dark matter simulations at the 95 per
cent confidence level, and therefore agree with the view that galaxies formed
hierarchically in a Universe composed of cold dark matter.Comment: 25 pages, 7 figures, accepted for publication in Nature (19 January
2012
Constraining the mass density of free-floating black holes using razor-thin lensing arcs
Strong lensing of active galactic nuclei in the radio can result in
razor-thin arcs, with a thickness of less than a milli-arcsecond, if observed
at the resolution achievable with very long baseline interferometry (VLBI).
Such razor-thin arcs provide a unique window on the coarseness of the matter
distribution between source and observer. In this paper, we investigate to what
extent such razor-thin arcs can constrain the number density and mass function
of `free-floating' black holes, defined as black holes that do not, or no
longer, reside at the centre of a galaxy. These can be either primordial in
origin or arise as by-products of the evolution of super-massive black holes in
galactic nuclei. When sufficiently close to the line of sight, free-floating
black holes cause kink-like distortions in the arcs, which are detectable by
eye in the VLBI images as long as the black hole mass exceeds Solar
masses. Using a crude estimate for the detectability of such distortions, we
analytically compute constraints on the matter density of free-floating black
holes resulting from null-detections of distortions along a realistic, fiducial
arc, and find them to be comparable to those from quasar milli-lensing. We also
use predictions from a large hydrodynamical simulation for the demographics of
free-floating black holes that are not primordial in origin, and show that
their predicted mass density is roughly four orders of magnitude below the
constraints achievable with a single razor-thin arc.Comment: 17 pages, 13 figures, 1 table, comments welcom
Estructura del disemÃnulo en especies de Andropogoneae (Poaceae)
The disseminule in Andropogoneae is studied, analizing its structure and characteristics in the different subtribes. The shape and function of the components of the dispersal units (rachis segment and spikelets, especially the glumes) are discussed in relation to the protection of the floral organs and the dispersal of the fruit. Some evolutionary trends are suggested.Se estudia exomorfológica y anatómicamente el disemÃnulo en especies de Andropogoneae pertenecientes a distintas subtribus. Se relacionan las caracterÃsticas del artejo del raquis y del par de espiguillas desarrolladas en su nudo proximal, con la protección dada a las flores y con la dispersión del fruto. Se proponen posibles tendencias filogenético
Perturbative reconstruction of a gravitational lens: when mass does not follow light
The structure and potential of a complex gravitational lens is reconstructed
using the perturbative method presented in Alard 2007, MNRAS, 382L, 58; Alard
2008, MNRAS, 388, 375. This lens is composed of 6 galaxies belonging to a small
group. The lens inversion is reduced to the problem of reconstructing
non-degenerate quantities: the 2 fields of the perturbative theory of strong
gravitational lenses. Since in the perturbative theory the circular source
solution is analytical, the general properties of the perturbative solution can
be inferred directly from the data. As a consequence, the reconstruction of the
perturbative fields is not affected by degeneracy, and finding the best
solution is only a matter of numerical refinement. The local shape of the
potential and density of the lens are inferred from the perturbative solution,
revealing the existence of an independent dark component that does not follow
light. The most likely explanation is that the particular shape of the dark
halo is due to the merging of cold dark matter halos. This is a new result
illustrating the structure of dark halos at the scale of galaxies.Comment: Final version (Astronomy and Astrophysics in press
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