231 research outputs found
Analytic Solutions for Navarro--Frenk--White Lens Models for Low Characteristic Convergences
The Navarro-Frenk-White (NFW) density profile is often used to model
gravitational lenses. For low values of the characteristic convergence
() of this model - corresponding to galaxy and galaxy group
mass scales - a high numerical precision is required in order to accurately
compute several quantities in the strong lensing regime. An alternative for
fast and accurate computations is to derive analytic approximations in this
limit. In this work we obtain analytic solutions for several lensing quantities
for elliptical (ENFW) and pseudo-elliptical (PNFW) NFW lens models on the
typical scales where gravitational arcs are expected to be formed, in the
limit, establishing their domain of validity. We derive
analytic solutions for the convergence and shear for these models, obtaining
explicit expressions for the iso-convergence contours and constant distortion
curves (including the tangential critical curve). We also compute the
deformation cross section, which is given in closed form for the circular NFW
model and in terms of a one-dimensional integral for the elliptical ones. In
addition, we provide a simple expression for the ellipticity of the
iso-convergence contours of the pseudo-elliptical models and the connection of
characteristic convergences among the PNFW and ENFW models. We conclude that
the set of solutions derived here is generally accurate for . For low ellipticities, values up to are allowed.
On the other hand, the mapping between PNFW and the ENFW models is valid up to
. The solutions derived in this work can be used to speed
up numerical codes and ensure their accuracy in the low regime,
including applications to arc statistics and other strong lensing observables.
(Abridged)Comment: Accepted for publication in A&
CLASH-VLT: A highly precise strong lensing model of the galaxy cluster RXC J2248.7−4431 (Abell S1063) and prospects for cosmography
Aims. We perform a comprehensive study of the total mass distribution of the galaxy cluster RXC J2248.7−4431 (z = 0.348) with a set of high-precision strong lensing models, which take advantage of extensive spectroscopic information on many multiply lensed systems. In the effort to understand and quantify inherent systematics in parametric strong lensing modelling, we explore a collection of 22 models in which we use different samples of multiple image families, different parametrizations of the mass distribution and cosmological parameters.
Methods. As input information for the strong lensing models, we use the Cluster Lensing And Supernova survey with Hubble (CLASH) imaging data and spectroscopic follow-up observations, with the VIsible Multi-Object Spectrograph (VIMOS) and Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT), to identify and characterize bona fide multiple image families and measure their redshifts down to m_(F814W) ≃ 26. A total of 16 background sources, over the redshift range 1.0−6.1, are multiply lensed into 47 images, 24 of which are spectroscopically confirmed and belong to ten individual sources. These also include a multiply lensed Lyman-α blob at z = 3.118. The cluster total mass distribution and underlying cosmology in the models are optimized by matching the observed positions of the multiple images on the lens plane. Bayesian Markov chain Monte Carlo techniques are used to quantify errors and covariances of the best-fit parameters.
Results. We show that with a careful selection of a large sample of spectroscopically confirmed multiple images, the best-fit model can reproduce their observed positions with a rms scatter of 0.̋3 in a fixed flat ΛCDM cosmology, whereas the lack of spectroscopic information or the use of inaccurate photometric redshifts can lead to biases in the values of the model parameters. We find that the best-fit parametrization for the cluster total mass distribution is composed of an elliptical pseudo-isothermal mass distribution with a significant core for the overall cluster halo and truncated pseudo-isothermal mass profiles for the cluster galaxies. We show that by adding bona fide photometric-selected multiple images to the sample of spectroscopic families, one can slightly improve constraints on the model parameters. In particular, we find that the degeneracy between the lens total mass distribution and the underlying geometry of the Universe, which is probed via angular diameter distance ratios between the lens and sources and the observer and sources, can be partially removed. Allowing cosmological parameters to vary together with the cluster parameters, we find (at 68% confidence level) Ω_m = 0.25^(+0.13)_(-0.16) and w = −1.07^(+0.16)_(-0.42) for a flat ΛCDM model, and Ω_m = 0.31^(+0.12)_(-0.13) and Ω_Λ = 0.38^(+0.38)_(-0.27) for a Universe with w = −1 and free curvature. Finally, using toy models mimicking the overall configuration of multiple images and cluster total mass distribution, we estimate the impact of the line-of-sight mass structure on the positional rms to be 0.̋3 ± 0. We argue that the apparent sensitivity of our lensing model to cosmography is due to the combination of the regular potential shape of RXC J2248, a large number of bona fide multiple images out to z = 6.1, and a relatively modest presence of intervening large-scale structure, as revealed by our spectroscopic survey
Discovery of a faint, star-forming, multiply lensed, Lyman-alpha blob
We report the discovery of a multiply lensed Lyman- blob (LAB) behind
the galaxy cluster AS1063 using the Multi Unit Spectroscopic Explorer (MUSE) on
the Very Large Telescope (VLT). The background source is at 3.117 and is
intrinsically faint compared to almost all previously reported LABs. We used
our highly precise strong lensing model to reconstruct the source properties,
and we find an intrinsic luminosity of =
erg s, extending to 33 kpc. We find that the LAB is associated with a
group of galaxies, and possibly a protocluster, in agreement with previous
studies that find LABs in overdensities. In addition to Lyman-
(Ly) emission, we find \ion{C}{IV}, \ion{He}{II}, and \ion{O}{III}]
ultraviolet (UV) emission lines arising from the centre of the nebula. We used
the compactness of these lines in combination with the line ratios to conclude
that the \Lya nebula is likely powered by embedded star formation. Resonant
scattering of the \Lya photons then produces the extended shape of the
emission. Thanks to the combined power of MUSE and strong gravitational
lensing, we are now able to probe the circumgalatic medium of sub-
galaxies at .Comment: 7 pages, 7 figures; moderate changes to match the accepted A&A
versoi
Massive Star cluster formation under the microscope at z=6
We report on a superdense star-forming region with an effective radius (R_e)
smaller than 13 pc identified at z=6.143 and showing a star-formation rate
density \Sigma_SFR~1000 Msun/yr/kpc2 (or conservatively >300 Msun/yr/kpc2).
Such a dense region is detected with S/N>40 hosted by a dwarf extending over
440 pc, dubbed D1 (Vanzella et al. 2017b). D1 is magnified by a factor
17.4+/-5.0 behind the Hubble Frontier Field galaxy cluster MACS~J0416 and
elongated tangentially by a factor 13.2+/-4.0 (including the systematic
errors). The lens model accurately reproduces the positions of the confirmed
multiple images with a r.m.s. of 0.35", and the tangential stretch is well
depicted by a giant multiply-imaged Lya arc. D1 is part of an interacting
star-forming complex extending over 800 pc. The SED-fitting, the very blue
ultraviolet slope (\beta ~ -2.5, F(\lambda) ~ \lambda^\beta) and the prominent
Lya emission of the stellar complex imply that very young (< 10-100 Myr),
moderately dust-attenuated (E(B-V)<0.15) stellar populations are present and
organised in dense subcomponents. We argue that D1 (with a stellar mass of 2 x
10^7 Msun) might contain a young massive star cluster of M < 10^6 Msun and
Muv~-15.6 (or m_uv=31.1), confined within a region of 13 pc, and not dissimilar
from some local super star clusters (SSCs). The ultraviolet appearance of D1 is
also consistent with a simulated local dwarf hosting a SSC placed at z=6 and
lensed back to the observer. This compact system fits into some popular
globular cluster formation scenarios. We show that future high spatial
resolution imaging (e.g., E-ELT/MAORY-MICADO and VLT/MAVIS) will allow us to
spatially resolve light profiles of 2-8 pc.Comment: 21 pages, 14 figures, 1 table, MNRAS accepte
CLASH-VLT: Testing the Nature of Gravity with Galaxy Cluster Mass Profiles
We use high-precision kinematic and lensing measurements of the total mass
profile of the dynamically relaxed galaxy cluster MACS J1206.2-0847 at
to estimate the value of the ratio between the two scalar
potentials in the linear perturbed Friedmann-Lemaitre-Robertson-Walker
metric.[...] Complementary kinematic and lensing mass profiles were derived
from exhaustive analyses using the data from the Cluster Lensing And Supernova
survey with Hubble (CLASH) and the spectroscopic follow-up with the Very Large
Telescope (CLASH-VLT). Whereas the kinematic mass profile tracks only the
time-time part of the perturbed metric (i.e. only ), the lensing mass
profile reflects the contribution of both time-time and space-space components
(i.e. the sum ). We thus express as a function of the mass
profiles and perform our analysis over the radial range . Using a spherical Navarro-Frenk-White mass profile, which
well fits the data, we obtain \eta(r_{200})=1.01\,_{-0.28}^{+0.31} at the
68\% C.L. We discuss the effect of assuming different functional forms for mass
profiles and of the orbit anisotropy in the kinematic reconstruction.
Interpreting this result within the well-studied modified gravity model,
the constraint on translates into an upper bound to the interaction
length (inverse of the scalaron mass) smaller than 2 Mpc. This tight constraint
on the interaction range is however substantially relaxed when
systematic uncertainties in the analysis are considered. Our analysis
highlights the potential of this method to detect deviations from general
relativity, while calling for the need of further high-quality data on the
total mass distribution of clusters and improved control on systematic effects.Comment: 18 pages, 3 figures, submitted to JCA
On the accuracy of time-delay cosmography in the Frontier Fields Cluster MACS J1149.5+2223 with supernova Refsdal
We study possible systematic effects on the values of the cosmological
parameters measured through strong lensing analyses of the HFF galaxy cluster
MACS J1149.5+2223. We use the observed positions of a large set of
spectroscopically selected multiple images, including those of supernova
"Refsdal" with their estimated time delays. Starting from our reference model
in a flat CDM cosmology, published in Grillo et al. (2018), we confirm
the relevance of the longest measurable time delay, between SX and S1, and an
approximately linear relation between its value and that of . We perform
true blind tests by considering a range of time delays around its original
estimate of days, as an accurate measurement of this time delay
was not known at the time of analysis and writing. We investigate separately
the impact of a constant sheet of mass at the cluster redshift, of a power-law
profile for the mass density of the cluster main halo and of some scatter in
the cluster member scaling relations. Remarkably, we find that these systematic
effects do not introduce a significant bias on the inferred values of
and and that the statistical uncertainties dominate the total
error budget: a 3% uncertainty on the time delay of image SX translates into
approximately 6% and 40% (including both statistical and systematic )
uncertainties for and , respectively. Furthermore, our
model accurately reproduces the extended surface brightness distribution of the
supernova host, covering more than pixels. We also
present the interesting possibility of measuring the value of the
equation-of-state parameter of the dark energy density, currently with a
30% uncertainty. We conclude that time-delay cluster lenses have the potential
to become soon an alternative and competitive cosmological probe.Comment: 10 pages, 8 figures, 1 table; ApJ in press (all main results and
figures unchanged, only minor changes during revision
Ionising the Intergalactic Medium by Star Clusters: The first empirical evidence
We present a VLT/X-Shooter spectroscopy of the Lyman continuum (LyC) emitting
galaxy 'Ion2' at z=3.2121 and compare it to that of the recently discovered
strongly lensed LyC-emitter at z=2.37, known as the 'Sunburst' arc. Three main
results emerge from the X-Shooter spectrum: (a) the Lya has three distinct
peaks with the central one at the systemic redshift, indicating a ionised
tunnel through which both Lya and LyC radiation escape; (b) the large O32
oxygen index ([OIII]4959-5007 / [OII]3727-3729) of 9.18(-1.32/+1.82) is
compatible to those measured in local (z~0.4) LyC leakers; (c) there are narrow
nebular high-ionisation metal lines with \sigma_v < 20 km/s, which confirms the
presence of young hot, massive stars. The HeII1640 appears broad, consistent
with a young stellar component including Wolf-Rayet stars. Similarly, the
Sunburst LyC-emitter shows a triple-peaked Lya profile and from VLT/MUSE
spectroscopy the presence of spectral features arising from young hot and
massive stars. The strong lensing magnification, (\mu > 20), suggests that this
exceptional object is a gravitationally-bound star cluster observed at a
cosmological distance, with a stellar mass M <~ 10^7 Msun and an effective
radius smaller than 20 pc. Intriguingly, sources like Sunburst but without
lensing magnification might appear as Ion2-like galaxies, in which unresolved
massive star clusters dominate the ultraviolet emission. This work supports the
idea that dense young star clusters can contribute to the ionisation of the IGM
through holes created by stellar feedback.Comment: 13 pages, 9 figures and 1 table, MNRAS accepted. Some typos fixe
CLASH-VLT: Environment-driven evolution of galaxies in the z=0.209 cluster Abell 209
The analysis of galaxy properties and the relations among them and the
environment, can be used to investigate the physical processes driving galaxy
evolution. We study the cluster A209 by using the CLASH-VLT spectroscopic data
combined with Subaru photometry, yielding to 1916 cluster members down to a
stellar mass of 10^{8.6} Msun. We determine: i) the stellar mass function of
star-forming and passive galaxies; ii) the intra-cluster light and its
properties; iii) the orbits of low- and high-mass passive galaxies; and iv) the
mass-size relation of ETGs. The stellar mass function of the star-forming
galaxies does not depend on the environment, while the slope found for passive
galaxies becomes flatter in the densest region. The color distribution of the
intra-cluster light is consistent with the color of passive members. The
analysis of the dynamical orbits shows that low-mass passive galaxies have
tangential orbits, avoiding small pericenters around the BCG. The mass-size
relation of low-mass passive ETGs is flatter than that of high mass galaxies,
and its slope is consistent with that of field star-forming galaxies. Low-mass
galaxies are also more compact within the scale radius of 0.65 Mpc. The ratio
between stellar and number density profiles shows a mass segregation in the
center. The comparative analysis of the stellar and total density profiles
indicates that this effect is due to dynamical friction. Our results are
consistent with a scenario in which the "environmental quenching" of low-mass
galaxies is due to mechanisms such as harassment out to R200, starvation and
ram-pressure stripping at smaller radii, as supported by the analysis of the
mass function, of the dynamical orbits and of the mass-size relation of passive
early-types in different regions. Our analyses support the idea that the
intra-cluster light is formed through the tidal disruption of subgiant
galaxies.Comment: 17 pages, 20 figures, A&A in pres
Constraining the multi-scale dark-matter distribution in CASSOWARY 31 with strong gravitational lensing and stellar dynamics
We study the inner structure of the group-scale lens CASSOWARY 31 (CSWA 31)
by adopting both strong lensing and dynamical modeling. CSWA 31 is a peculiar
lens system. The brightest group galaxy (BGG) is an ultra-massive elliptical
galaxy at z = 0.683 with a weighted mean velocity dispersion of km s. It is surrounded by group members and several lensed arcs
probing up to ~150 kpc in projection. Our results significantly improve
previous analyses of CSWA 31 thanks to the new HST imaging and MUSE
integral-field spectroscopy. From the secure identification of five sets of
multiple images and measurements of the spatially-resolved stellar kinematics
of the BGG, we conduct a detailed analysis of the multi-scale mass distribution
using various modeling approaches, both in the single and multiple lens-plane
scenarios. Our best-fit mass models reproduce the positions of multiple images
and provide robust reconstructions for two background galaxies at z = 1.4869
and z = 2.763. The relative contributions from the BGG and group-scale halo are
remarkably consistent in our three reference models, demonstrating the
self-consistency between strong lensing analyses based on image position and
extended image modeling. We find that the ultra-massive BGG dominates the
projected total mass profiles within 20 kpc, while the group-scale halo
dominates at larger radii. The total projected mass enclosed within =
27.2 kpc is M. We find that CSWA
31 is a peculiar fossil group, strongly dark-matter dominated towards the
central region, and with a projected total mass profile similar to higher-mass
cluster-scale halos. The total mass-density slope within the effective radius
is shallower than isothermal, consistent with previous analyses of early-type
galaxies in overdense environments.Comment: 22 pages, 12 figures, 5 tables, submitted to Astronomy &
Astrophysics. We welcome the comments from reader
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