Search

SuperBIT was a 0.5-meter near-ultraviolet to near-infrared wide-field telescope that launched on a NASA superpressure balloon into the stratosphere from New Zealand for a 45-night flight. SuperBIT acquired multi-band images of galaxy clusters to study the properties of dark matter using weak gravitational lensing. We provide an overview of the instrument and its various subsystems. We then present the instrument performance from the flight, including the telescope and image stabilization system, the optical system, the power system, and the thermal system. SuperBIT successfully met the instrument's technical requirements, achieving a telescope pointing stability of 0.34 +/- 0.10 arcseconds, a focal plane image stability of 0.055 +/- 0.027 arcseconds, and a PSF FWHM of ~ 0.35 arcseconds over 5-minute exposures throughout the 45-night flight. The telescope achieved a near-diffraction limited point-spread function in all three science bands (u, b, and g). SuperBIT served as a pathfinder to the GigaBIT observatory, which will be a 1.34-meter near-ultraviolet to near-infrared balloon-borne telescope., Comment: 17 pages, 25 pages, published in the Astronomical Journal

We present new free-form and hybrid mass reconstructions of the galaxy cluster lens MACS J0416.1$-$2403 at $z=0.396$ using the lens inversion method GRALE. The reconstructions use 237 spectroscopically confirmed multiple images from Bergamini et. al. 2023 as the main input. Our primary model reconstructs images to a positional accuracy of 0.191", thus representing one of the most precise reconstructions of this lens to date. Our models find broad agreement with previous reconstructions, and identify two $\sim 10^{12} M_{\odot}$ light-unaffiliated substructures. We focus on two highly magnified arcs: Spock and Mothra. Our model features a unique critical curve structure around the Spock arc with 2 crossings. This structure enables sufficient magnification across this arc to potentially explain the large number of transients as microlensing events of supergiant stars. Additionally, we develop a model of the millilens substructure expected to be magnifying Mothra, which may be a binary pair of supergiants with $\mu \sim 6000$. This model accounts for flexibility in the millilens position while preserving the observed flux and minimizing image position displacements along the Mothra arc. We constrain the millilens mass and core radius to $\lesssim 10^6 M_{\odot}$ and $\lesssim 17$ pc, respectively, which would render it one of the smallest and most compact substructures constrained by lensing. If the millilens is dominated by wave dark matter, the axion mass is constrained to be $\lesssim 3.0 \times 10^{-21}$ eV. Further monitoring of this lens with JWST will uncover more transients, permitting tighter constraints on the structure surrounding these two arcs., Comment: Accepted to MNRAS

The Super-pressure Balloon-borne Imaging Telescope (SuperBIT) is a near-diffraction-limited 0.5m telescope that launched via NASA's super-pressure balloon technology on April 16, 2023. SuperBIT achieved precise pointing control through the use of three nested frames in conjunction with an optical Fine Guidance System (FGS), resulting in an average image stability of 0.055" over 300-second exposures. The SuperBIT FGS includes a tip-tilt fast-steering mirror that corrects for jitter on a pair of focal plane star cameras. In this paper, we leverage the empirical data from SuperBIT's successful 45-night stratospheric mission to inform the FGS design for the next-generation balloon-borne telescope. The Gigapixel Balloon-borne Imaging Telescope (GigaBIT) is designed to be a 1.35m wide-field, high resolution imaging telescope, with specifications to extend the scale and capabilities beyond those of its predecessor SuperBIT. A description and analysis of the SuperBIT FGS will be presented along with methodologies for extrapolating this data to enhance GigaBIT's FGS design and fine pointing control algorithm. We employ a systems engineering approach to outline and formalize the design constraints and specifications for GigaBIT's FGS. GigaBIT, building on the SuperBIT legacy, is set to enhance high-resolution astronomical imaging, marking a significant advancement in the field of balloon-borne telescopes., Comment: 13 pages, 7 figures, SPIE Astronomical Telescopes + Instrumentation 2024

BlueMUSE is a blue-optimised, medium spectral resolution, panoramic integral field spectrograph under development for the Very Large Telescope (VLT). With an optimised transmission down to 350 nm, spectral resolution of R$\sim$3500 on average across the wavelength range, and a large FoV (1 arcmin$^2$), BlueMUSE will open up a new range of galactic and extragalactic science cases facilitated by its specific capabilities. The BlueMUSE consortium includes 9 institutes located in 7 countries and is led by the Centre de Recherche Astrophysique de Lyon (CRAL). The BlueMUSE project development is currently in Phase A, with an expected first light at the VLT in 2031. We introduce here the Top Level Requirements (TLRs) derived from the main science cases, and then present an overview of the BlueMUSE system and its subsystems fulfilling these TLRs. We specifically emphasize the tradeoffs that are made and the key distinctions compared to the MUSE instrument, upon which the system architecture is built., Comment: 20 pages, 10 figures, proceedings of the SPIE astronomical telescopes and instrumentation conference, Yokohama, 16-21 June

We present a combined strong and weak gravitational-lensing analysis of the massive galaxy cluster MACS J1423.8+2404 ($z=0.545$, MACS J1423 hereafter), one of the most dynamically relaxed and massive cool-core clusters discovered in the MAssive Cluster Survey at $z>0.5$. We combine high-resolution imaging from the Hubble Space Telescope (HST) in the F606W, F814W, and F160W pass-bands with spectroscopic observations taken as part of the KALEIDOSCOPE survey with the Multi-Unit Spectroscopic Explorer mounted on the Very Large Telescope. Our strong lensing analysis of the mass distribution in the cluster core is constrained by four multiple-image systems (17 individual images) within redshift range $1.779

Strong gravitational lensing offers a powerful probe of the detailed distribution of matter in lenses, while magnifying and bringing faint background sources into view. Observed strong lensing by massive galaxy clusters, which are often in complex dynamical states, has also been used to map their dark matter substructures on smaller scales. Deep high resolution imaging has revealed the presence of strong lensing events associated with these substructures, namely galaxy-scale sub-halos. However, an inventory of these observed galaxy-galaxy strong lensing (GGSL) events is noted to be discrepant with state-of-the-art {\Lambda}CDM simulations. Cluster sub-halos appear to be over-concentrated compared to their simulated counterparts yielding an order of magnitude higher value of GGSL. In this paper, we explore the possibility of resolving this observed discrepancy by redistributing the mass within observed cluster sub-halos in ways that are consistent within the {\Lambda}CDM paradigm of structure formation. Lensing mass reconstructions from data provide constraints on the mass enclosed within apertures and are agnostic to the detailed mass profile within them. Therefore, as the detailed density profile within cluster sub-halos currently remains unconstrained by data, we are afforded the freedom to redistribute the enclosed mass. We investigate if rearranging the mass to a more centrally concentrated density profile helps alleviate the GGSL discrepancy. We report that refitting cluster sub-halos to the ubiquitous {\Lambda}CDM-motivated Navarro-Frenk-White profile, and further modifying them to include significant baryonic components, does not resolve this tension. A resolution to this persisting GGSL discrepancy may require more careful exploration of alternative dark matter models., Comment: 20 pages, 5 figures

MACS J0600.1-2008 (MACS0600) is an X-ray luminous, massive galaxy cluster at $z_{\mathrm{d}}=0.43$, studied previously by the REionization LensIng Cluster Survey (RELICS) and ALMA Lensing Cluster Survey (ALCS) projects which revealed a complex, bimodal mass distribution and an intriguing high-redshift object behind it. Here, we report on the results of a combined analysis of the extended strong lensing (SL), X-ray, Sunyaev-Zeldovich (SZ), and galaxy luminosity-density properties of this system. Using new JWST and ground-based Gemini-N and Keck data, we obtain 13 new spectroscopic redshifts of multiply imaged galaxies and identify 12 new photometric multiple-image systems and candidates, including two multiply imaged $z\sim7$ objects. Taking advantage of the larger areal coverage, our analysis reveals an additional bimodal, massive SL structure which we measure spectroscopically to lie adjacent to the cluster and whose existence was implied by previous SL-modeling analyses. While based in part on photometric systems identified in ground-based imaging requiring further verification, our extended SL model suggests that the cluster may have the second-largest critical area and effective Einstein radius observed to date, $A_{\mathrm{crit}}\simeq2.16 \mathrm{arcmin}^2$ and $\theta_{\mathrm{E}}=49.7''\pm5.0''$ for a source at $z_{\mathrm{s}}=2$, enclosing a total mass of $M(<\theta_{\mathrm{E}})=(4.7\pm0.7)\times10^{14} \mathrm{M}_{\odot}$. These results are also supported by the galaxy luminosity distribution, the SZ and X-ray data. Yet another, probably related massive cluster structure, discovered in X-rays $5'$ (1.7 Mpc) further north, suggests that MACS0600 is part of an even larger filamentary structure. This discovery adds to several recent detections of massive structures around SL galaxy clusters and establishes MACS0600 as a prime target for future high-redshift surveys with JWST., Comment: Accepted for publication in MNRAS. Version updated to the accepted paper

In April to May 2023, the superBIT telescope was lifted to the Earth's stratosphere by a helium-filled super-pressure balloon, to acquire astronomical imaging from above (99.5% of) the Earth's atmosphere. It was launched from New Zealand then, for 40 days, circumnavigated the globe five times at a latitude 40 to 50 degrees South. Attached to the telescope were four 'DRS' (Data Recovery System) capsules containing 5 TB solid state data storage, plus a GNSS receiver, Iridium transmitter, and parachute. Data from the telescope were copied to these, and two were dropped over Argentina. They drifted 61 km horizontally while they descended 32 km, but we predicted their descent vectors within 2.4 km: in this location, the discrepancy appears irreducible below 2 km because of high speed, gusty winds and local topography. The capsules then reported their own locations to within a few metres. We recovered the capsules and successfully retrieved all of superBIT's data - despite the telescope itself being later destroyed on landing., Comment: 12 pages

Whilst X-rays and Sunyaev-Zel'dovich observations allow to study the properties of the intra-cluster medium (ICM) of galaxy clusters, their gravitational potential may be constrained using strong gravitational lensing. Although being physically related, these two components are often described with different physical models. Here, we present a unified technique to derive the ICM properties from strong lensing for clusters in hydrostatic equilibrium. In order to derive this model, we present a new universal and self-similar polytropic temperature profile, which we fit using the X-COP sample of clusters. We subsequently derive an analytical model for the electron density, which we apply to strong lensing clusters MACS J0242.5-2132 and MACS J0949.8+1708. We confront the inferred ICM reconstructions to XMM-Newton and ACT observations. We contrast our analytical electron density reconstructions with the best canonical $\beta$-model. The ICM reconstructions obtained prove to be compatible with observations. However they appear to be very sensitive to various dark matter halo parameters constrained through strong lensing (such as the core radius), and to the halo scale radius (fixed in the lensing optimisations). With respect to the important baryonic effects, we make the sensitivity on the scale radius of the reconstruction an asset, and use the inferred potential to constrain the dark matter density profile using ICM observations. The technique here developed should allow to take a new, and more holistic path to constrain the content of galaxy clusters., Comment: 27 pages, 21 figures, published in MNRAS

This manuscript presents new astronomical source catalogs using data from the BUFFALO Survey. These catalogs contain detailed information for over 100,000 astronomical sources in the 6 BUFFALO clusters: Abell 370, Abell 2744, Abell S1063, MACS 0416, MACS 0717, and MACS 1149 spanning a total 240 arcmin^2. The catalogs include positions and forced photometry measurements of these objects in the F275W, F336W, F435W, F606W, F814W, F105W, F125W, F140W, and F160W HST -bands, Keck-NIRC2/VLT-HAWKI Ks band, and IRAC Channel 1 and 2 bands. Additionally, we include photometry measurements in the F475W, F625W, and F110W bands for Abell 370. This catalog also includes photometric redshift estimates computed via template fitting using LePhare. When comparing to spectroscopic reference, we obtain an outlier fraction of 9.2% and scatter, normalized median absolute deviation (NMAD), of 0.062. The catalogs are publicly available for their use by the community., Comment: 16 pages, to be submitted to ApJS

The HST treasury program BUFFALO provides extended wide-field imaging of the six Hubble Frontier Fields galaxy clusters. Here we present the combined strong and weak-lensing analysis of Abell 370, a massive cluster at z=0.375. From the reconstructed total projected mass distribution in the 6arcmin x 6arcmin BUFFALO field-of-view, we obtain the distribution of massive substructures outside the cluster core and report the presence of a total of seven candidates, each with mass $\sim 5 \times 10^{13}M_{\odot}$. Combining the total mass distribution derived from lensing with multi-wavelength data, we evaluate the physical significance of each candidate substructure, and conclude that 5 out of the 7 substructure candidates seem reliable, and that the mass distribution in Abell 370 is extended along the North-West and South-East directions. While this finding is in general agreement with previous studies, our detailed spatial reconstruction provides new insights into the complex mass distribution at large cluster-centric radius. We explore the impact of the extended mass reconstruction on the model of the cluster core and in particular, we attempt to physically explain the presence of an important external shear component, necessary to obtain a low root-mean-square separation between the model-predicted and observed positions of the multiple images in the cluster core. The substructures can only account for up to half the amplitude of the external shear, suggesting that more effort is needed to fully replace it by more physically motivated mass components. We provide public access to all the lensing data used as well as the different lens models., Comment: 29 pages, 17 figures, 3 tables

The Superpressure Balloon-borne Imaging Telescope (SuperBIT) is a diffraction-limited, wide-field, 0.5 m, near-infrared to near-ultraviolet observatory designed to exploit the stratosphere's space-like conditions. SuperBIT's 2023 science flight will deliver deep, blue imaging of galaxy clusters for gravitational lensing analysis. In preparation, we have developed a weak lensing measurement pipeline with modern algorithms for PSF characterization, shape measurement, and shear calibration. We validate our pipeline and forecast SuperBIT survey properties with simulated galaxy cluster observations in SuperBIT's near-UV and blue bandpasses. We predict imaging depth, galaxy number (source) density, and redshift distribution for observations in SuperBIT's three bluest filters; the effect of lensing sample selections is also considered. We find that in three hours of on-sky integration, SuperBIT can attain a depth of b = 26 mag and a total source density exceeding 40 galaxies per square arcminute. Even with the application of lensing-analysis catalog selections, we find b-band source densities between 25 and 30 galaxies per square arcminute with a median redshift of z = 1.1. Our analysis confirms SuperBIT's capability for weak gravitational lensing measurements in the blue., Comment: Submitted to Astronomical Journal

In late 2014, four images of Supernova (SN) "Refsdal," the first known example of a strongly lensed SN with multiple resolved images, were detected in the MACS J1149 galaxy-cluster field. Following the images' discovery, the SN was predicted to reappear within hundreds of days at a new position ~8 arcseconds away in the field. The observed reappearance in late 2015 makes it possible to carry out Refsdal's (1964) original proposal to use a multiply imaged SN to measure the Hubble constant H0, since the time delay between appearances should vary inversely with H0. Moreover, the position, brightness, and timing of the reappearance enable a novel test of the blind predictions of galaxy-cluster models, which are typically constrained only by the positions of multiply imaged galaxies. We have developed a new photometry pipeline that uses DOLPHOT to measure the fluxes of the five images of SN Refsdal from difference images. We apply four separate techniques to perform a blind measurement of the relative time delays and magnification ratios (mu_i/mu_1) between the last image SX and the earlier images S1-S4. We measure the relative time delay of SX-S1 to be 376.0+5.6-5.5 days and the relative magnification to be 0.30+0.05-0.03. This corresponds to a 1.5% precision on the time delay and 17% precision for the magnification ratios, and includes uncertainties due to millilensing and microlensing. In an accompanying paper, we place initial and blind constraints on the value of the Hubble constant., Comment: Published in ApJ. Companion paper presenting H0 constraints published in Science (DOI: 10.1126/science.abh1322)

The gravitationally lensed Supernova Refsdal appeared in multiple images, produced through gravitational lensing by a massive foreground galaxy cluster. After the supernova appeared in 2014, lens models of the galaxy cluster predicted an additional image of the supernova would appear in 2015, which was subsequently observed. We use the time delays between the images to perform a blinded measurement of the expansion rate of the Universe, quantified by the Hubble constant (H0). Using eight cluster lens models, we infer H0 = 64.8 +4.4-4.3 km / s / Mpc, where Mpc is the megaparsec. Using the two models most consistent with the observations, we find H0 = 66.6 +4.1-3.3 km / s / Mpc. The observations are best reproduced by models that assign dark-matter halos to individual galaxies and the overall cluster., Comment: Published in Science on May 11, 2023; this version updated to reflect minor edits to galley proofs. Companion paper presenting time-delay and relative magnification measurements published in ApJ (DOI: 10.3847/1538-4357/ac4ccb)

We present a constraint on the abundance of supergiant (SG) stars at redshift z approx. 1, based on recent observations of a strongly lensed arc at this redshift. First we derive a free-form model of MACS J0416.1-2403 using data from the BUFFALO program. The new lens model is based on 72 multiply lensed galaxies that produce 214 multiple images, making it the largest sample of spectroscopically confirmed lensed galaxies on this cluster. The larger coverage in BUFFALO allows us to measure the shear up to the outskirts of the cluster, and extend the range of lensing constraints up to ~ 1 Mpc from the central region, providing a mass estimate up to this radius. As an application, we make predictions for the number of high-redshift multiply-lensed galaxies detected in future observations with JWST. Then we focus on a previously known lensed galaxy at z=1.0054, nicknamed Spock, which contains four previously reported transients. We interpret these transients as microcaustic crossings of SG stars and compute the probability of such events. Based on simplifications regarding the stellar evolution, we find that microlensing (by stars in the intracluster medium) of SG stars at z=1.0054 can fully explain these events. The inferred abundance of SG stars is consistent with either (1) a number density of stars with bolometric luminosities beyond the Humphreys-Davidson (HD) limit (L ~ $6\times10^5 L_{\odot}$) that is below 400 stars per sq. kpc, or (2) the absence of stars beyond the HD limit but with a SG number density of ~ 9000 per sq. kpc for stars with luminosities between $10^5$ and $6\times10^5$. This is equivalent to one SG star per 10x10 pc$^2$. We finally make predictions for future observations with JWST's NIRcam. We find that in observations made with the F200W filter that reach 29 mag AB, if cool red SG stars exist at z~1 beyond the HD limit, they should be easily detected in this arc, Comment: 24 pages & 18 figures

We present the first strong-gravitational-lensing analysis of the galaxy cluster RXJ0437.1+0043 (RXJ0437; z = 0.285). Newly obtained, deep MUSE observations, Keck/MOSFIRE near-infrared spectroscopy, and Hubble Space Telescope SNAPshot imaging reveal 13 multiply imaged background galaxies, three of them (at z=1.98, 2.97, and 6.02, respectively) in hyperbolic umbilic (H-U) lensing configurations. The H-U images are located only 20 -- 50 kpc from the cluster centre, i.e., at distances well inside the Einstein radius where images from other lens configurations are demagnified and often unobservable. Extremely rare (only one H-U lens was known previously) these systems are able to constrain the inner slope of the mass distribution -- and unlike radial arcs, the presence of H-U configurations is not biased towards shallow cores. The galaxies lensed by RXJ0437 are magnified by factors ranging from 30 to 300 and (in the case of H-U systems) stretched nearly isotropically. Taking advantage of this extreme magnification, we demonstrate how the source galaxies in H-U systems can be used to probe for small-scale ($\sim 10^{9} M_{\odot}$) substructures, providing additional insight into the nature of dark matter., Comment: 17 pages, 13 figures, 3 tables (including 1 in an appendix). Accepted in MNRAS

We present a statistical study of 180 dust continuum sources identified in 33 massive cluster fields by the ALMA Lensing Cluster Survey (ALCS) over a total of 133 arcmin$^{2}$ area, homogeneously observed at 1.2 mm. ALCS enables us to detect extremely faint mm sources by lensing magnification, including near-infrared (NIR) dark objects showing no counterparts in existing {\it Hubble Space Telescope} and {\it Spitzer} images. The dust continuum sources belong to a blind sample ($N=141$) with S/N $\gtrsim$ 5.0 (a purity of $>$ 0.99) or a secondary sample ($N=39$) with S/N= $4.0-5.0$ screened by priors. With the blind sample, we securely derive 1.2-mm number counts down to $\sim7$ $\mu$Jy, and find that the total integrated 1.2mm flux is 20.7$^{+8.5}_{-6.5}$ Jy deg$^{-2}$, resolving $\simeq$ 80 % of the cosmic infrared background light. The resolved fraction varies by a factor of $0.6-1.1$ due to the completeness correction depending on the spatial size of the mm emission. We also derive infrared (IR) luminosity functions (LFs) at $z=0.6-7.5$ with the $1/V_{\rm max}$ method, finding the redshift evolution of IR LFs characterized by positive luminosity and negative density evolution. The total (=UV+IR) cosmic star-formation rate density (SFRD) at $z>4$ is estimated to be $161^{+25}_{-21}$ % of the established measurements, which were almost exclusively based on optical$-$NIR surveys. Although our general understanding of the cosmic SFRD is unlikely to change beyond a factor of 2, these results add to the weight of evidence for an additional ($\approx 60$ %) SFRD component contributed by the faint-mm population, including NIR dark objects., Comment: ApJS in press, 34 pages, 8 Tables, and 18 Figures (+38 pages, 5 Tables, and 6 Figures in Appendix)

We present a new method to simultaneously/self-consistently model the mass distribution of galaxy clusters that combines constraints from strong lensing features, X-ray emission and galaxy kinematics measurements. We are able to successfully decompose clusters into their collisionless and collisional mass components thanks to the X-ray surface brightness, as well as using the dynamics of cluster members to obtain more accurate masses with the fundamental plane of elliptical galaxies. Knowledge from all observables is included through a consistent Bayesian approach in the likelihood or in physically motivated priors. We apply this method to the galaxy cluster Abell S1063 and produce a mass model that we publicly release with this paper. The resulting mass distribution presents a different ellipticities for the intra-cluster gas and the other large-scale mass components; and deviation from elliptical symmetry in the main halo. We assess the ability of our method to recover the masses of the different elements of the cluster using a mock cluster based on a simplified version of our Abell S1063 model. Thanks to the wealth of information provided by the mass model and the X-ray emission, we also found evidence for an on-going merger event with gas sloshing from a smaller infalling structure into the main cluster. In agreement with previous findings, the total mass, gas profile and gas mass fraction are consistent with small deviations from the hydrostatic equilibrium. This new mass model for Abell S1063 is publicly available as is the software used to construct it through the \textsc{Lenstool} package., Comment: 29 pages, 23 figures, Accepted for publication in MNRAS

The distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus 'external shear', which notionally accounts for neighbouring galaxies and cosmic shear. We show that it does not. Except in a handful of rare systems, the best-fit values of external shear do not correlate with independent measurements of shear: from weak lensing in 45 Hubble Space Telescope images, or in 50 mock images of lenses with complex distributions of mass. Instead, the best-fit shear is aligned with the major or minor axis of 88% of lens galaxies; and the amplitude of the external shear increases if that galaxy is disky. We conclude that 'external shear' attached to a power law model is not physically meaningful, but a fudge to compensate for lack of model complexity. Since it biases other model parameters that are interpreted as physically meaningful in several science analyses (e.g. measuring galaxy evolution, dark matter physics or cosmological parameters), we recommend that future studies of galaxy-scale strong lensing should employ more flexible mass models., Comment: 13 pages, 11 figures, submitted to MNRAS and Review Response Submitted

The statistical power of weak lensing measurements is principally driven by the number of high redshift galaxies whose shapes are resolved. Conventional wisdom and physical intuition suggest this is optimised by deep imaging at long (red or near IR) wavelengths, to avoid losing redshifted Balmer break and Lyman break galaxies. We use the synthetic Emission Line EL-COSMOS catalogue to simulate lensing observations using different filters, from various altitudes. Here were predict the number of exposures to achieve a target z > 0.3 source density, using off-the-shelf and custom filters. Ground-based observations are easily better at red wavelengths, as (more narrowly) are space-based observations. However, we find that SuperBIT, a diffraction-limited observatory operating in the stratosphere, should instead perform its lensing-quality observations at blue wavelengths.

We present the strong lensing analysis of two galaxy clusters: MACS J0242.5-2132 (MACS J0242, $z=0.313$) and MACS J0949.8+1708 (MACS J0949, $z=0.383$). Their total matter distributions are constrained thanks to the powerful combination of observations with the Hubble Space Telescope and the MUSE instrument. Using these observations, we precisely measure the redshift of six multiple image systems in MACS J0242, and two in MACS J0949. We also include four multiple image systems in the latter cluster identified in HST imaging without MUSE redshift measurements. For each cluster, our best-fit mass model consists of a single cluster-scale halo, and 57 (170) galaxy-scale halos for MACS J0242 (MACS J0949). Multiple images positions are predicted with a $rms$ 0.39 arcsec and 0.15 arcsec for MACS J0242 and MACS J0949 models respectively. From these mass models, we derive aperture masses of $M(R<$200 kpc$) = 1.67_{-0.05}^{+0.03}\times 10^{14} M_{\odot}$, and $M(R<$200 kpc$) = 2.00_{-0.20}^{+0.05}\times 10^{14} M_{\odot}$. Combining our analysis with X-ray observations from the XMM-Newton Observatory, we show that MACS J0242 appears to be a relatively relaxed cluster, while conversely, MACS J0949 shows a relaxing post-merger state. At 200 kpc, X-ray observations suggest the hot gas fraction to be respectively $f_g = 0.115^{+0.003}_{-0.004}$ and $0.053^{+0.007}_{-0.006}$ for MACS J0242 and MACS J0949. MACS J0242 being relaxed, its density profile is very well fitted by a NFW distribution, in agreement with X-ray observations. Finally, the strong lensing analysis of MACS J0949 suggests a flat dark matter density distribution in the core, between 10 and 100 kpc. This appears consistent with X-ray observations., Comment: 20 pages, 11 figures, published in MNRAS

Exploiting the fundamentally achromatic nature of gravitational lensing, we present a lens model for the massive galaxy cluster SMACSJ0723.3-7323 (SMACSJ0723, z=0.388) that significantly improves upon earlier work. Building on strong-lensing constraints identified in prior Hubble Space Telescope (HST) observations, the mass model utilizes 21 multiple-image systems, 17 of which were newly discovered in Early Release Observation (ERO) data from the James Webb Space Telescope (JWST). The resulting lens model maps the cluster mass distribution to an RMS spatial precision of 0.32'' and is publicly available. Consistent with previous analyses, our study shows SMACSJ0723.3-7323 to be well described by a single large-scale component centered on the location of the brightest cluster galaxy. However, satisfying all lensing constraints provided by the JWST data, the model point to the need for the inclusion of an additional, diffuse component west of the cluster. A comparison of the galaxy, mass, and gas distributions in the core of SMACSJ0723 based on HST, JWST, and Chandra data reveals a concentrated regular elliptical profile along with tell-tale signs of a recent merger, possibly proceeding almost along our line of sight. The exquisite sensitivity of JWST's NIRCAM reveals in spectacular fashion both the extended intra-cluster-light distribution and numerous star-forming clumps in magnified background galaxies. The high-precision lens model derived here for SMACSJ0723-7323 demonstrates the unprecedented power of combining HST and JWST data for studies of structure formation and evolution in the distant Universe., Comment: Updated to match the revised version submitted to ApJ - analysis, results and conclusions are unchanged. Link to the lensing outputs: https://github.com/guillaumemahler/SMACS0723-mahler2022

Cross-referencing a watchlist of galaxy groups and clusters with transient detections from real-time streams of wide-field survey data is a promising method for discovering gravitationally lensed explosive transients including supernovae, kilonovae, gravitational waves and gamma-ray bursts in the next ten years. However, currently there exists no catalogue of objects with both sufficient angular extent and depth to adequately perform such a search. In this study, we develop a cluster-finding method capable of creating an all-sky list of galaxy group- and cluster-scale objects out to $z\simeq1$ based on their lens-plane properties and using only existing data from wide-field infrared surveys such as VHS and UHS, and all-sky \textit{WISE} data. In testing this method, we recover 91 per cent of a sample containing known and candidate lensing objects with Einstein radii of $\theta_E \geq 5\arcsec$. We also search the surrounding regions of this test sample for other groups and clusters using our method and verify the existence of any significant findings by visual inspection, deriving estimates of the false positive rate that are as low as 6 per cent. The method is also tested on simulated Rubin data from their DP0 programme, which yields complementary results of a good recovery rate of $\gtrsim 80$ per cent for $M_{200}\geq7\times10^{13}$M$_\odot$ clusters and with no false positives produced in our test region. Importantly, our method is positioned to create a watchlist in advance of Rubin's LSST, as it utilises only existing data, therefore enabling the discovery of lensed transients early within the survey's lifetime., Comment: 11 pages, 9 figures. Published, MNRAS

We search for gravitational wave (GW) events from LIGO-Virgo's third run that may have been affected by gravitational lensing. Gravitational lensing delays the arrival of GWs, and alters their amplitude -- thus biasing the inferred progenitor masses. This would provide a physically well-understood interpretation of GW detections in the ''mass gap'' between neutron stars and black holes, as gravitationally lensed binary neutron star (BNS) mergers. We selected three GW detections in LIGO-Virgo's third run for which the probability of at least one of the constituent compact objects being in the mass gap was reported as high with low latency -- i.e. candidate lensed BNS mergers. Our observations of powerful strong lensing clusters located adjacent to the peak of their sky localisation error maps reached a sensitivity $\rm AB\simeq25.5$ in the $z'$-band with the GMOS instruments on the Gemini telescopes, and detected no candidate lensed optical counterparts. We combine recent kilonova lightcurve models with recent predictions of the lensed BNS population and the properties of the objects that we followed up to show that realistic optical counterparts were detectable in our observations. Further detailed analysis of two of the candidates suggests that they are a plausible pair of images of the same low-mass binary black hole merger, lensed by a local galaxy or small group of galaxies. This further underlines that access to accurate mass information with low latency would improve the efficiency of candidate lensed BNS selection., Comment: Accepted for publication on MNRAS

Secure confirmation that a gravitational wave (GW) has been gravitationally lensed would bring together these two pillars of General Relativity for the first time. This breakthrough is challenging for many reasons, including: GW sky localization uncertainties dwarf the angular scale of gravitational lensing, the mass and structure of gravitational lenses is diverse, the mass function of stellar remnant compact objects is not yet well constrained, and GW detectors do not operate continuously. We introduce a new approach that is agnostic to the mass and structure of the lenses, compare the efficiency of different methods for lensed GW discovery, and explore detection of lensed kilonova counterparts as a direct method for localising candidates. Our main conclusions are: (1) lensed neutron star mergers (NS-NS) are magnified into the "mass gap" between NS and black holes, therefore selecting candidates from public GW alerts with high mass gap probability is efficient, (2) the rate of detectable lensed NS-NS will approach one per year in the mid-2020s, (3) the arrival time difference between lensed NS-NS images is $1\,\rm sec\lesssim\Delta t\lesssim1\,year$, and thus well-matched to the operations of GW detectors and optical telescopes, (4) lensed kilonova counterparts are faint at peak (e.g.\ $r_{\rm AB}\simeq24-26$ in the mid-2020s), fade quickly ($d<2\,\rm days$), and are detectable with target of opportunity observations with large wide-field telescopes. For example, just $\lesssim0.25$ per cent of Vera C.\ Rubin Observatory's observing time will be sufficient to follow up one well-localized candidate per year. Our predictions also provide a physically well-defined basis for exploring electromagnetically the exciting new "mass gap" discovery space., Comment: Accepted by MNRAS. 20 pages, 10 figures

The pressure exerted by the ambient hot X-ray gas on cluster galaxies can lead to the presence of ram pressure stripped (RPS) galaxies, characterized by asymmetric shapes, and, in some cases, tails of blue stars and/or X-ray gas, with increased star formation. We searched for such galaxies in the cluster MS 0451.6-0305 at z~0.5, based on Hubble Space Telescope (HST) imaging covering a region of about 6x6 Mpc^2, an eight magnitude ground-based catalogue with photometric redshifts, and a spectroscopic redshift catalogue. We defined as cluster members a spectroscopic redshift sample of 359 galaxies within 4sigma_v of the mean cluster velocity, and a photometric redshift sample covering the [0.48,0.61] range. We searched for RPS galaxies and tested the error on their classification with a Zooniverse collaboration, and computed the phase space diagram for the spectroscopic sample. We ran the LePhare stellar population synthesis code to analyze and compare the properties of RPS and non-RPS galaxies. We find 56 and 273 RPS candidates in the spectroscopic and photometric redshift samples, respectively, distributed throughout the cluster and tending to avoid high density regions. The phase space diagram gives the percentages of virialized, backsplash, and infall galaxies. RPS galaxy candidates typically show rather high star formation rates, young ages, and relatively low masses. This study confirms that RPS galaxies host, on average, younger stellar populations and strongly form stars when compared with non-RPS counterparts. The fact that RPS candidates with spectroscopic and with photometric redshifts have comparable properties shows that large samples of such objects could be gathered based on multi-band photometry only, a promising result in view of future very large imaging surveys., Comment: Accepted for publication in Astronomy & Astrophysics

We investigate the strong-lensing cluster Abell 370 (A370) using a wide Integral Field Unit (IFU) spectroscopic mosaic from the Multi-Unit Spectroscopic Explorer (MUSE). IFU spectroscopy provides significant insight into the structure and mass content of galaxy clusters, yet IFU-based cluster studies focus almost exclusively on the central Einstein-radius region. Covering over 14 arcmin$^2$, the new MUSE mosaic extends significantly beyond the A370 Einstein radius, providing, for the first time, a detailed look at the cluster outskirts. Combining these data with wide-field, multi-band Hubble Space Telescope (HST) imaging from the BUFFALO project, we analyse the distribution of objects within the cluster and along the line of sight. Identifying 416 cluster galaxies, we use kinematics to trace the radial mass profile of the halo, providing a mass estimate independent from the lens model. We also measure radially-averaged properties of the cluster members, tracking their evolution as a function of infall. Thanks to the high spatial resolution of our data, we identify six cluster members acting as galaxy-galaxy lenses, which constrain localized mass distributions beyond the Einstein radius. Finally, taking advantage of MUSE's 3D capabilities, we detect and analyse multiple spatially extended overdensities outside of the cluster that influence lensing-derived halo mass estimates. We stress that much of this work is only possible thanks to the robust, extended IFU coverage, highlighting its importance even in less optically dense cluster regions. Overall, this work showcases the power of combining HST+MUSE, and serves as the initial step towards a larger and wider program targeting several clusters., Comment: 20 pages, 12 figures, 3 tables (including 1 in an appendix). Accepted in MNRAS. Data Release products available at https://astro.dur.ac.uk/~hbpn39/pilot-wings.html

This study explores the gravitational lensing effects of supermassive black holes (SMBHs) in galaxy clusters. While the presence of central SMBHs in galaxies is firmly established, recent work from high-resolution simulations predict the existence of an additional population of wandering SMBHs. Though the masses of these SMBHs are a minor perturbation on the larger scale and individual galaxy scale dark matter components in the cluster, they can impact statistical lensing properties and individual lensed image configurations. Probing for these potentially observable signatures, we find that SMBHs imprint detectable signatures in rare, higher-order strong lensing image configurations although they do not manifest any statistically significant detectable evidence in either the magnification distribution or the integrated shear profile. Investigating specific lensed image geometries, we report that a massive, near point-like, potential of an SMBH causes the following detectable effects: (i) image splitting leading to the generation of extra images; (ii) positional and magnification asymmetries in multiply imaged systems; and (iii) the apparent disappearance of a lensed counter-image. Of these, image splitting inside the cluster tangential critical curve, is the most prevalent notable observational signature. We demonstrate these possibilities in two cases of observed giant arcs in $SGAS\,J003341.5+024217$ and $RX\,J1347.5-1145$, wherein specific image configurations seen can be reproduced with SMBHs. Future observations with high-resolution instrumentation (e.g. MAVIS-Very Large Telescope, MICADO-Extremely Large Telescope, and the upgraded ngVLA, along with data from the \textit{Euclid} \& \textit{Nancy Grace Roman} Space Telescopes and the Rubin LSST Observatory are likely to allow us to probe these unique yet rare SMBHs lensing signatures., Comment: 12 pages, 12 figures, submitted to MNRAS. For associated online animation, see https://sites.google.com/view/guillaume-mahler-astronomer/paper-animation

The connection between galaxies and their dark matter haloes is often described with the Stellar-to-Halo Mass relation (SHMR). Satellite galaxies in clusters have been shown to follow a SHMR distinct from central galaxies because of the environmental processes that they are subject to. In addition, the variety of accretion histories leads to an important scatter in this relation, even more for satellites than for central galaxies. In this work, we use the hydrodynamical simulation IllustrisTNG to study the scatter in the satellite galaxy SHMR, and extract the parameters that can best allow to understand it. Active galaxies, that represent a very small fraction of cluster galaxies, follow a very different relation than their passive counterparts, mainly because they were accreted much more recently. For this latter population, we find that the distance to the cluster centre is a good predictor of variations in the SHMR, but some information on the galaxy orbital history, such as the distance of closest approach to the host centre, is an even better one, although it is in practice more difficult to measure. In addition, we found that galaxy compactness is also correlated with the SHMR, while the host cluster properties (mass and concentration, formation redshift, mass and size of BCG) do not play a significant role. We provide accurate fitting functions and scaling relations to the scientific community, useful to predict the subhalo mass given a set of observable parameters. Finally, we connect the scatter in the SHMR to the physical processes affecting galaxies in clusters, and how they impact the different satellite sub-populations., Comment: 17 pages, 8 figures. Accepted in MNRAS

We present a Spitzer/IRAC survey of H-faint ($H_{160} \gtrsim 26.4$, $<5\sigma$) sources in 101 lensing cluster fields. Across a CANDELS/Wide-like survey area of $\sim$648 arcmin$^2$ (effectively $\sim$221 arcmin$^2$ in the source plane), we have securely discovered 53 sources in the IRAC Channel-2 band (CH2, 4.5 $\mathrm{\mu m}$; median CH2$=22.46\pm0.11$ AB mag) that lack robust HST/WFC3-IR F160W counterparts. The most remarkable source in our sample, namely ES-009 in the field of Abell 2813, is the brightest H-faint galaxy at 4.5 $\mathrm{\mu m}$ known so far ($\mathrm{CH2}=20.48\pm0.03$ AB mag). We show that the H-faint sources in our sample are massive (median $M_\mathrm{star} = 10^{10.3\pm 0.3}$ $M_\odot$), star-forming (median star formation rate $=100_{-40}^{+60}$ $M_\odot$yr$^{-1}$) and dust-obscured ($A_V=2.6\pm0.3$) galaxies around a median photometric redshift of $z=3.9\pm0.4$. The stellar continua of 14 H-faint galaxies can be resolved in the CH2 band, suggesting a median circularized effective radius ($R_\mathrm{e,circ}$; lensing corrected) of $1.9\pm0.2$ kpc and $<1.5$ kpc for the resolved and whole samples, respectively. This is consistent with the sizes of massive unobscured galaxies at $z\sim4$, indicating that H-faint galaxies represent the dusty tail of the distribution of a wider galaxy population. Comparing with the ALMA dust continuum sizes of similar galaxies reported previously, we conclude that the heavy dust obscuration in H-faint galaxies is related to the compactness of both stellar and dust continua ($R_\mathrm{e,circ}\sim 1$ kpc). These H-faint galaxies make up $16_{-7}^{+13}$% of the galaxies in the stellar mass range of $10^{10}-10^{11.2}$ $M_\odot$ at $z=3\sim5$, contributing to $8_{-4}^{+8}$% of the cosmic star formation rate density in this epoch and likely tracing the early phase of massive galaxy formation., Comment: 26 pages, 10 figures, 3 tables. Accepted for publication in ApJ

We use the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) strong lensing image catalog of the merging galaxy cluster Abell 370 to obtain a mass model using the free-form lens inversion algorithm GRALE. The improvement of the strong lensing data quality results in a lens plane rms of only 0.45 arcsec, about a factor of two lower than that of our existing HFF v4 reconstruction. We attribute the improvement to spectroscopic data and use of the full reprocessed HST mosaics. In our reconstructed mass model, we found indications of three distinct mass features in Abell 370: (i) a $\sim 35$ kpc offset between the northern BCG and the nearest mass peak, (ii) a $\sim 100$ kpc mass concentration of roughly critical density $\sim 250$ kpc east of the main cluster, and (iii) a probable filament-like structure passing N-S through the cluster. While (i) is present in some form in most publicly available reconstructions spanning the range of modeling techniques: parametric, hybrid, and free-form, (ii) and (iii) are recovered by only about half of the reconstructions. We tested our hypothesis on the presence of the filament-like structure by creating a synthetic cluster - Irtysh IIIc - mocking the situation of a cluster with external mass. We also computed the source plane magnification distributions. Using them we estimated the probabilities of magnifications in the source plane, and scrutinized their redshift dependence. Finally, we explored the lensing effects of Abell 370 on the luminosity functions of sources at $z_s=9.0$, finding it consistent with published results., Comment: 16 pages, 13 figures

Galaxies in clusters undergo several phenomena such as ram pressure stripping and tidal interactions, that can trigger or quench their star formation and, in some cases, lead to galaxies acquiring unusual shapes and long tails. We searched for jellyfish galaxy candidates in a sample of 40 clusters from the DAFT/FADA and CLASH surveys covering the redshift range 0.2

If properly calibrated, the shapes of galaxy clusters can be used to investigate many physical processes: from feedback and quenching of star formation, to the nature of dark matter. Theorists frequently measure shapes using moments of inertia of simulated particles'. We instead create mock (optical, X-ray, strong- and weak-lensing) observations of the twenty-two most massive ($\sim10^{14.7}\,M_\odot$) relaxed clusters in the BAHAMAS simulations. We find that observable measures of shape are rounder. Even when moments of inertia are projected into 2D and evaluated at matched radius, they overestimate ellipticity by 56\% (compared to observable strong lensing) and 430\% (compared to observable weak lensing). Therefore, we propose matchable quantities and test them using observations of eight relaxed clusters from the {\emph Hubble Space Telescope} and {\emph Chandra X-Ray Observatory}. We also release our HST data reduction and lensing analysis software to the community. In real clusters, the ellipticity and orientation angle at all radii are strongly correlated. In simulated clusters, the ellipticity of inner ($

This paper presents optical night sky brightness measurements from the stratosphere using CCD images taken with the Super-pressure Balloon-borne Imaging Telescope (SuperBIT). The data used for estimating the backgrounds were obtained during three commissioning flights in 2016, 2018, and 2019 at altitudes ranging from 28 km to 34 km above sea level. For a valid comparison of the brightness measurements from the stratosphere with measurements from mountain-top ground-based observatories (taken at zenith on the darkest moonless night at high Galactic and high ecliptic latitudes), the stratospheric brightness levels were zodiacal light and diffuse Galactic light subtracted, and the airglow brightness was projected to zenith. The stratospheric brightness was measured around 5.5 hours, 3 hours, and 2 hours before the local sunrise time in 2016, 2018, and 2019 respectively. The $B$, $V$, $R$, and $I$ brightness levels in 2016 were 2.7, 1.0, 1.1, and 0.6 mag arcsec$^{-2}$ darker than the darkest ground-based measurements. The $B$, $V$, and $R$ brightness levels in 2018 were 1.3, 1.0, and 1.3 mag arcsec$^{-2}$ darker than the darkest ground-based measurements. The $U$ and $I$ brightness levels in 2019 were 0.1 mag arcsec$^{-2}$ brighter than the darkest ground-based measurements, whereas the $B$ and $V$ brightness levels were 0.8 and 0.6 mag arcsec$^{-2}$ darker than the darkest ground-based measurements. The lower sky brightness levels, stable photometry, and lower atmospheric absorption make stratospheric observations from a balloon-borne platform a unique tool for astronomy. We plan to continue this work in a future mid-latitude long duration balloon flight with SuperBIT., Comment: 17 pages, 7 figures. Accepted for publication in the Astronomical Journal

We examine the outskirts of galaxy clusters in the C-EAGLE simulations to quantify the `edges' of the stellar and dark matter distribution. The radius of the steepest slope in the dark matter, commonly used as a proxy for the splashback radius, is located at ~r_200m; the strength and location of this feature depends on the recent mass accretion rate, in good agreement with previous work. Interestingly, the stellar distribution (or intracluster light, ICL) also has a well-defined edge, which is directly related to the splashback radius of the halo. Thus, detecting the edge of the ICL can provide an independent measure of the physical boundary of the halo, and the recent mass accretion rate. We show that these caustics can also be seen in the projected density profiles, but care must be taken to account for the influence of substructures and other non-diffuse material, which can bias and/or weaken the signal of the steepest slope. This is particularly important for the stellar material, which has a higher fraction bound in subhaloes than the dark matter. Finally, we show that the `stellar splashback' feature is located beyond current observational constraints on the ICL, but these large projected distances (>> 1 Mpc) and low surface brightnesses (mu >> 32 mag/arcsec^2) can be reached with upcoming observational facilities such as the Vera C. Rubin Observatory, the Nancy Grace Roman Space Telescope, and Euclid., Comment: 12 pages, 9 figures. Accepted for publication in MNRAS

We present strong-lensing analyses of three galaxy clusters, RXJ2129.4+0009 (z=0.235), MS0451.6-0305 (z=0.55), and MACSJ2129.4-0741 (z=0.589), using the powerful combination of Hubble Space Telescope (HST) multi-band observations, and Multi-Unit Spectroscopic Explorer (MUSE) spectroscopy. In RXJ2129, we newly spectroscopically confirm 15 cluster members. Our resulting mass model uses 8 multiple image systems as we include a galaxy-galaxy lensing system North-East of the cluster, and is composed of 71 halos including one dark matter cluster-scale halo and 2 galaxy-scale halos optimized individually. For MS0451, we report the spectroscopic identification of 2 new systems of multiple images in the Northern region, and 112 cluster members. Our mass model uses 16 multiple image systems, and 146 halos, including 2 large-scale halos, and 7 galaxy-scale halos independently optimized. For MACSJ2129, we report the spectroscopic identification of one new multiple image system at z=4.41, and newly measure spectroscopic redshifts for 4 cluster members. Our mass model uses 14 multiple image systems, and is composed of 151 halos, including 2 large-scale halos and 4 galaxy-scale halos independently optimized. Our best models have rms of 0.29'', 0.6'', 0.74'' in the image plane for RXJ2129, MS0451, and MACSJ2129 respectively. This analysis presents a detailed comparison with the existing literature showing excellent agreements, and discuss specific studies of lensed galaxies, e.g. a group of submilimeter galaxies at z=2.9 in MS0451, and a bright z=2.1472 red singly imaged galaxy in MACSJ2129., Comment: 25 pages, 10 figures, 13 tables, accepted in MNRAS

We quantify the performance of mass mapping techniques on mock imaging and gravitational lensing data of galaxy clusters. The optimum method depends upon the scientific goal. We assess measurements of clusters' radial density profiles, departures from sphericity, and their filamentary attachment to the cosmic web. We find that mass maps produced by direct (KS93) inversion of shear measurements are unbiased, and that their noise can be suppressed via filtering with MRLens. Forward-fitting techniques, such as Lenstool, suppress noise further, but at a cost of biased ellipticity in the cluster core and over-estimation of mass at large radii. Interestingly, current searches for filaments are noise-limited by the intrinsic shapes of weakly lensed galaxies, rather than by the projection of line-of-sight structures. Therefore, space-based or balloon-based imaging surveys that resolve a high density of lensed galaxies, could soon detect one or two filaments around most clusters., Comment: 19 pages, 10 figures, updated to match MNRAS version

Using the largest mosaic of Hubble Space Telescope images around a galaxy cluster, we map the distribution of dark matter throughout a $\sim$$6\times6$ Mpc$^2$ area centred on the cluster MS 0451-03 ($z=0.54$, $M_{200}=1.65\times10^{15} \rm{M}_\odot$). Our joint strong- and weak-lensing analysis shows three possible filaments extending from the cluster, encompassing six group-scale substructures. The dark-matter distribution in the cluster core is elongated, consists of two distinct components, and is characterized by a concentration parameter of $c_{200}=3.79\pm0.36$. By contrast, XMM-Newton observations show the gas distribution to be more spherical, with excess entropy near the core, and a lower concentration of $c_{200}=2.35^{+0.89}_{-0.70}$ (assuming hydrostatic equilibrium). Such a configuration is predicted in simulations of major mergers 2-7Gyr after the first core passage, when the two dark-matter halos approach second turnaround, and before their gas has relaxed. This post-merger scenario finds further support in optical spectroscopy of the cluster's member galaxies, which shows that star formation was abruptly quenched 5 Gyr ago. MS 0451-03 will be an ideal target for future studies of the growth of structure along filaments, star-formation processes after a major merger, and the late-stage evolution of cluster collisions., Comment: 21 pages, 13 figures, updated to match MNRAS version

Motivated by discovering strongly-lensed supernovae, gravitational waves, and kilonovae in the 2020s, we investigate whether to build a watch-list of clusters based on observed cluster properties (i.e. lens-plane selection) or on the detectability of strongly-lensed background galaxies (i.e. source-plane selection). First, we estimate the fraction of high-redshift transient progenitors that reside in galaxies that are themselves too faint to be detected as being strongly-lensed. We find $\sim15-50$ per cent of transient progenitors reside in $z = 1-2$ galaxies too faint to be detected in surveys that reach ${\rm AB}\simeq23$, such as the Dark Energy Survey. This falls to $\ls10$ per cent at depths that will be probed by early data releases of LSST (${\rm AB}\simeq25$). Second, we estimate a conservative lower limit on the fraction of strong lensing clusters that will be missed by magnitude limited searches for multiply-imaged galaxies and giant arcs due to the faintness of such images. We find that DES-like surveys will miss $\sim75$ per cent of $10^{15}$M$_\odot$ strong lensing clusters, rising to $\sim100$ per cent of $10^{14}$M$_\odot$ clusters. Deeper surveys, such as LSST, will miss $\sim40$ per cent at $10^{15}$M$_\odot$, and $\sim95$ per cent at $10^{14}$M$_\odot$. Our results motivate building a cluster watch-list for strongly-lensed transients that includes those found by lens-plane selection., Comment: 7 pages, 2 figures. MNRAS, accepted

We present a new galaxy cluster lens modeling approach, hybrid-Lenstool, that is implemented in the publicly available modeling software Lenstool. hybrid-Lenstool combines a parametric approach to model the core of the cluster, and a non-parametric (free-form) approach to model the outskirts. hybrid-Lenstool optimizes both strong- and weak-lensing constraints simultaneously (Joint-Fit), providing a self-consistent reconstruction of the cluster mass distribution on all scales. In order to demonstrate the capabilities of the new algorithm, we tested it on a simulated cluster. hybrid-Lenstool yields more accurate reconstructed mass distributions than the former Sequential-Fit approach where the parametric and the non-parametric models are optimized successively. Indeed, we show with the simulated cluster that the mass density profile reconstructed with a Sequential-Fit deviates form the input by $2-3\sigma$ at all scales while the Joint-Fit gives a profile that is within $1-1.5\sigma$ of the true value. This gain in accuracy is consequential for recovering mass distributions exploiting cluster lensing and therefore for all applications of clusters as cosmological probes. Finally we found that the Joint-Fit approach yields shallower slope of the inner density profile than the Sequential-Fit approach, thus revealing possible biases in previous lensing studies., Comment: 11 pages, 4 figures, 1 table

Many distant objects can only be detected, or become more scientifically valuable, if they have been highly magnified by strong gravitational lensing. We use EAGLE and BAHAMAS, two recent cosmological hydrodynamical simulations, to predict the probability distribution for both the lens mass and lens redshift when point sources are highly magnified by gravitational lensing. For sources at a redshift of two, we find the distribution of lens redshifts to be broad, peaking at z=0.6. The contribution of different lens masses is also fairly broad, with most high-magnification lensing due to lenses with halo masses between 10^12 and 10^14 solar masses. Lower mass haloes are inefficient lenses, while more massive haloes are rare. We find that a simple model in which all haloes have singular isothermal sphere density profiles can approximately reproduce the simulation predictions, although such a model over-predicts the importance of haloes with mass <10^12 solar masses for lensing. We also calculate the probability that point sources at different redshifts are strongly lensed. At low redshift, high magnifications are extremely unlikely. Each z=0.5 source produces, on average, 5x10^-7 images with magnification greater than ten; for z =2 this increases to about 2x10^-5. Our results imply that searches for strongly lensed optical transients, including the optical counterparts to strongly lensed gravitational waves, can be optimized by monitoring massive galaxies, groups and clusters rather than concentrating on an individual population of lenses., Comment: 15 pages, 5 figures, updated to match MNRAS version

The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 parallel Cycle 25 Hubble Space Telescope Treasury program taking data from 2018-2020. BUFFALO will expand existing coverage of the Hubble Frontier Fields (HFF) in WFC3/IR F105W, F125W, and F160W and ACS/WFC F606W and F814W around each of the six HFF clusters and flanking fields. This additional area has not been observed by HST but is already covered by deep multi-wavelength datasets, including Spitzer and Chandra. As with the original HFF program, BUFFALO is designed to take advantage of gravitational lensing from massive clusters to simultaneously find high-redshift galaxies which would otherwise lie below HST detection limits and model foreground clusters to study properties of dark matter and galaxy assembly. The expanded area will provide a first opportunity to study both cosmic variance at high redshift and galaxy assembly in the outskirts of the large HFF clusters. Five additional orbits are reserved for transient followup. BUFFALO data including mosaics, value-added catalogs and cluster mass distribution models will be released via MAST on a regular basis, as the observations and analysis are completed for the six individual clusters., Comment: Accepted ApJS; MAST archive will be live concurrent with publication

In the context of the BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) survey, we present a new analysis of the merging galaxy cluster MACS\,J0416.1-2403 ($z = 0.397$) and its parallel field using the data collected by the Hubble Frontier Fields (HFF) campaign. In this work, we measure the surface mass density from a weak-lensing analysis, and characterise the overall matter distribution in both the cluster and parallel fields. The surface mass distribution derived for the parallel field shows clumpy overdensities connected by filament-like structures elongated in the direction of the cluster core. We also characterise the X-ray emission of the cluster, and compare it with the lensing mass distribution. We identify five substructures at the $>5\sigma$ level over the two fields, four of them being in the cluster one. Furthermore, three of them are located close to the edges of the field of view, and border issues can significantly hamper the determination of their physical parameters. Finally, we compare our results with the predicted subhalo distribution of one of the Hydrangea/C-EAGLE simulated cluster. Significant differences are obtained suggesting the simulated cluster is at a more advanced evolutionary state than MACS\,J0416.1-2403. Our results anticipate the upcoming BUFFALO observations that will link the two HFF fields, extending further the \emph{HST} coverage, and thus allowing a better characterisation of the reported substructures., Comment: 15 pages, 8 figures, 3 tables, submitted to MNRAS

At a fraction the total cost of an equivalent orbital mission, scientific balloon-borne platforms, operating above 99.7% of the Earth's atmosphere, offer attractive, competitive, and effective observational capabilities -- namely space-like resolution, transmission, and backgrounds -- that are well suited for modern astronomy and cosmology. SuperBIT is a diffraction-limited, wide-field, 0.5 m telescope capable of exploiting these observing conditions in order to provide exquisite imaging throughout the near-IR to near-UV. It utilizes a robust active stabilization system that has consistently demonstrated a 1 sigma sky-fixed pointing stability at 48 milliarcseconds over multiple 1 hour observations at float. This is achieved by actively tracking compound pendulations via a three-axis gimballed platform, which provides sky-fixed telescope stability at < 500 milliarcseconds and corrects for field rotation, while employing high-bandwidth tip/tilt optics to remove residual disturbances across the science imaging focal plane. SuperBIT's performance during the 2019 commissioning flight benefited from a customized high-fidelity science-capable telescope designed with exceptional thermo- and opto-mechanical stability as well as tightly constrained static and dynamic coupling between high-rate sensors and telescope optics. At the currently demonstrated level of flight performance, SuperBIT capabilities now surpass the science requirements for a wide variety of experiments in cosmology, astrophysics and stellar dynamics., Comment: The following article has been submitted to Review of Scientific Instruments (RSI)

We study the shape of eight dynamically relaxed galaxy clusters observed with the Hubble Space Telescope and Chandra X-Ray Observatory. Using strong and weak gravitational lensing, the shape of the Brightest Cluster Galaxy and the X-ray isophote we study the ellipticity of the cluster halo at four different radii. We find that the proxies probing the inner regions of the cluster are strongly coupled with the BCG shape correlated with both the shape predicted by strong gravitational lensing and the X-ray isophote. Conversely we find no such correlation between the shape as predicted by the weak lensing and the other three probes suggesting any coupling between the inner and outer regions is weak. We also present in this paper the public release of the HST weak lensing shape measurement code pyRRG, directly available from PyPi (https://pypi.org/project/pyRRG/). This python3.7 code, based on Rhodes et al. (2000) adopts an automated star-galaxy classifier based on a Random Forest and outputs scientifically useful products such as weak lensing catalogues suitable for the mass mapping algorithm Lenstool.

LSST's wide-field of view and sensitivity will revolutionize studies of the transient sky by finding extraordinary numbers of new transients every night. The recent discovery of a kilonova counterpart to LIGO/Virgo's first detection of gravitational waves (GWs) from a double neutron star (NS-NS) merger also creates an exciting opportunity for LSST to offer a Target of Opportunity (ToO) mode of observing. We have been exploring the possibility of detecting strongly lensed GWs, that would enable new tests of GR, extend multi-messenger astronomy out to $z\gtrsim1$, and deliver a new class of sub-millisecond precision time-delay constraints on lens mass distributions. We forecast that the rate of detection of lensed NS-NS mergers in the 2020s will be $\sim0.1$ per Earth year, that the typical source will be at $z\simeq2$, and that the multiply-imaged kilonova counterpart will have a magnitude of ${\rm AB}\simeq25.4$ in $g/r/i$-band filters - i.e. fainter than the sensitivity of a single LSST WFD visit. We therefore advocate (1) creating a flexible and efficient Target of Opportunity programme within the LSST observing strategy that is capable of discovering sources fainter than single-visit depth, and (2) surveying the entire observable extragalactic sky as rapidly as possible in the WFD survey. The latter will enable a very broad range of early science that relies on wide survey area for detection of large samples of objects and/or maximizing the fraction of sky over which reference imaging is available. For example, it will enable prompt discovery of a uniform and all-sky sample of galaxy/group/cluster-scale lenses that will underpin LSST strong-lensing science. This white paper complements submissions from DESC, SLSC, and TVSSC, that discuss kilonova, GW, and strong lensing., Comment: A white paper on the LSST Cadence; submitted in November 2018; 10 pages

Satellite galaxies in galaxy clusters represent a significant fraction of the global galaxy population. Because of the unusual dense environment of clusters, their evolution is driven by different mechanisms than the ones affecting field or central galaxies. Understanding the different interactions they are subject to, and how they are influenced by them, is therefore an important step towards explaining the global picture of galaxy evolution. In this paper, we use the publicly-available high resolution hydrodynamical simulation Illustris-1 to study satellite galaxies in the three most massive host haloes (with masses $M_{200} > 10^{14}\,h^{-1}\rm{M}_{\odot}$) at $z=0$. We measure the Stellar-to-Halo Mass Relation (hereafter SHMR) of the galaxies, and find that for satellites it is shifted towards lower halo masses compared to the SHMR of central galaxies. We provide simple fitting functions for both the central and satellite SHMR. To explain the shift between the two, we follow the satellite galaxies since their time of accretion into the clusters, and quantify the impact of dark matter stripping and star formation. We find that subhaloes start losing their dark matter as soon as they get closer than $\sim 1.5\times R_{\rm{vir}}$ to the centre of their host, and that up to 80\% of their dark matter content gets stripped during infall. On the other hand, star formation quenching appears to be delayed, and galaxies continue to form stars for a few Gyr after accretion. The combination of these two effects impacts the ratio of stellar to dark matter mass which varies drastically during infall, from 0.03 to 0.3., Comment: 14 pages, 12 figures

Strong gravitational lensing by clusters of galaxies probes the mass distribution at the core of each cluster and magnifies the universe behind it. MACS J0417.5-1154 at z=0.443 is one of the most massive clusters known based on weak lensing, X-ray, and Sunyaev-Zel'dovich analyses. Here we compute a strong lens model of MACS J0417 based on Hubble Space Telescope imaging observations collected, in part, by the Reionization Lensing Cluster Survey (RELICS), and recently reported spectroscopic redshifts from the MUSE instrument on the Very Large Telescope (VLT). We measure an Einstein radius of ThetaE=36'' at z = 9 and a mass projected within 200 kpc of M(200 kpc) = 1.78+0.01-0.03x10**14Msol. Using this model, we measure a ratio between the mass attributed to cluster-member galaxy halos and the main cluster halo of order 1:100. We assess the probability to detect magnified high-redshift galaxies in the field of this cluster, both for comparison with RELICS HST results and as a prediction for the James Webb Space Telescope (JWST) Guaranteed Time Observations upcoming for this cluster. Our lensing analysis indicates that this cluster has similar lensing strength to other clusters in the RELICS program. Our lensing analysis predicts a detection of at least a few z~6-8 galaxies behind this cluster, at odds with a recent analysis that yielded no such candidates in this field. Reliable strong lensing models are crucial for accurately predicting the intrinsic properties of lensed galaxies. As part of the RELICS program, our strong lensing model produced with the Lenstool parametric method is publicly available through the Mikulski Archive for Space Telescopes (MAST)., Comment: 20 pages, 8 figures. Accepted to ApJ

We present a multi-wavelength analysis of the core of the massive galaxy cluster MACS\,J0417.5-1154 ($z = 0.441$; MACS\;J0417). Our analysis takes advantage of VLT/MUSE observations which allow the spectroscopic confirmation of three strongly-lensed systems. One of these, nick-named \emph{The Doughnut}, consists of three complete images of a complex ring galaxy at $z = 0.8718$ and a fourth, partial and radial image close to the Brightest Cluster Galaxy (BCG) only discernible thanks to its strong [OII] line emission. The best-fit mass model (rms of 0.38\arcsec) yields a two-dimensional enclosed mass of $M({\rm R < 200\,kpc}) = (1.77\pm0.03)\times10^{14}\,\msun$ and almost perfect alignment between the peaks of the BCG light and the dark matter of ($0.5\pm0.5$)\arcsec . Our finding that a significant misalignment results when the radial image of \emph{The Doughnut} is omitted serves as an important caveat for studies of BCG-dark matter offsets in galaxy clusters. Using \emph{Chandra} data to map the intra-cluster gas, we observe an offset between the gas and dark-matter peaks of ($1.7\pm0.5$)\arcsec, and excellent alignment of the X-ray peak with the location of optical emission line associated with the BCG. We interpret all observational evidence in the framework of on-going merger activity, noting specifically that the coincidence between the gas peak and the peak of blue light from the BCG may be evidence of dense, cold gas leading to direct star formation. We use the surface area $\sigma_{\mu}$ above a given magnification factor $\mu$ as a metric to estimate the lensing power of MACS\,J0417. We obtain $\sigma(\mu > 3) = 0.22$\,arcmin$^2$ confirming MACS\,J0417 as an efficient gravitational lens. Finally, we discuss the differences between our mass model and Mahler et al. (2018)., Comment: 15 pages, 6 figures, 8 tables, accepted for publication in MNRAS