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Galaxy clusters as gravitational lenses play a unique role in astrophysics and cosmology: they permit mapping the dark matter distribution on a range of scales; they reveal the properties of high and intermediate redshift background galaxies that would otherwise be unreachable with telescopes; they constrain the particle nature of dark matter and are a powerful probe of global cosmological parameters, like the Hubble constant. In this review we summarize the current status of cluster lensing observations and the insights they provide, and offer a glimpse into the capabilities that ongoing, and the upcoming next generation of telescopes and surveys will deliver. While many open questions remain, cluster lensing promises to remain at the forefront of discoveries in astrophysics and cosmology.

A massive galaxy cluster can serve as a magnifying glass for distant stellar populations, as strong gravitational lensing magnifies background galaxies and exposes details that are otherwise undetectable. In time-domain astronomy, imaging programmes with a short cadence are able to detect rapidly evolving transients, previously unseen by surveys designed for slowly evolving supernovae. Here, we describe two unusual transient events discovered in a Hubble Space Telescope programme that combined these techniques with high-cadence imaging on a field with a strong-lensing galaxy cluster. These transients were faster and fainter than any supernovae, but substantially more luminous than a classical nova. We find that they can be explained as separate eruptions of a luminous blue variable star or a recurrent nova, or as an unrelated pair of stellar microlensing events. To distinguish between these hypotheses will require clarification of the cluster lens models, along with more high-cadence imaging of the field that could detect related transient episodes. This discovery suggests that the intersection of strong lensing with high-cadence transient surveys may be a fruitful path for future astrophysical transient studies. Two unusual transient events, discovered by Hubble behind a strong-lensing galaxy cluster, can be explained as separate eruptions of a luminous blue variable star or a recurrent nova, or as an unrelated pair of stellar microlensing events.

We release fully reduced spectra obtained with NIRSpec onboard JWST as part of the GLASS-JWST Early Release Science Program and a follow-up Director's Discretionary Time program 2756. From these 263 spectra of 245 unique sources, acquired with low (\\(R =30-300\\)) and high dispersion (\\(R\\sim2700\\)) gratings, we derive redshifts for 200 unique sources in the redshift range \\(z=0-10\\). We describe the sample selection and characterize its high completeness as a function of redshift and apparent magnitude. Comparison with independent estimates based on different methods and instruments shows that the redshifts are accurate, with 80\\% differing less than 0.005. We stack the GLASS-JWST spectra to produce the first high-resolution (\\(R \\sim 2700\\)) JWST spectral template extending in the rest frame wavelength from 2000~\\AA\\ to 20, 000~\\AA. Catalogs, reduced spectra, and template are made publicly available to the community.

Weak gravitational lensing is considered to be one of the most powerful tools to study the mass and the mass distribution of galaxy clusters. However, the mass-sheet degeneracy transformation has limited its success. We present a novel method for a cluster mass reconstruction, which combines weak and strong lensing information on common scales and can as a consequence break the mass-sheet degeneracy. We extend the weak lensing formalism to the inner parts of the cluster, use redshift information of background sources and combine these with the constraints from multiple image systems. We apply the method to N-body simulations as well as to strong and weak lensing ground-based multi-colour data of RX J1347–1145, the most X-ray luminous cluster known to date. If the redshift measurements of background sources (for strong and weak lensing) and the identification of the multiple-image system are correct, we estimate the enclosed cluster mass within $360\\: {\\rm h}^{-1}\\mbox{kpc}$ to $M(<360\\: {\\rm h}^{-1}\\mbox{kpc})= (1.2 \\pm 0.3) \\times 10^{15} M_{\\odot}$. With higher resolution (e.g. HST) imaging data, reliable multiple imaging information could be obtained and the reconstruction further improved.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally-lensed galaxy (\\(\\rm \\mu=17-21\\)) at redshift 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is overall a typical compact and UV-faint (\\(\\rm M_{UV}=-17.8\\)) Lyman-\\(\\alpha\\) emitter; yet, the large magnification allows the detailed characterisation of structures on sub-galactic (down to few parsec) scales. Prominent optical \\(\\rm H\\alpha\\), \\(\\rm H\\beta\\) and [OIII]\\(\\lambda\\lambda4959,5007\\) lines are spatially resolved with the high spectral resolution grating (G395H, R~2700), with large equivalent widths, EW(\\(\\rm H\\beta\\)+[OIII])\\(\\gtrsim1000\\) \\AA, and elevated ionising photon production efficiencies \\(\\rm log(\\xi_{ion}/erg^{-1}Hz)=25.2-25.7\\). NIRCam deep imaging reveals the presence of compact rest-UV bright regions along with individual star clusters of sizes \\(\\rm R_{eff}=3-8~pc\\) and masses \\(\\rm M\\sim2\\cdot10^5-5\\cdot10^{6}~M_\\odot\\) These clusters are characterised by steep UV slopes, \\(\\rm\\beta_{UV}\\lesssim-2.5\\), in some cases associated with a dearth of line emission, indicating possible leaking of the ionising radiation, as also supported by a Lyman-\\(\\rm \\alpha\\) emission peaking at \\(\\rm \\sim100~km~s^{-1}\\) from the systemic redshift. While the entire system is characterised by low-metallicity, \\(\\sim0.1~Z_\\odot\\), the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with \\(\\rm Z\\lesssim2\\%~Z_\\odot\\), barely detected in NIRCam imaging; this region is displaced by \\(\\rm >200~pc\\) from one of the UV brightest structures of the system, and it would have been too faint to detect if not for the large magnification of the system.

Modern data empower observers to describe galaxies as the spatially and biographically complex objects they are. We illustrate this through case studies of four, \\(z\\sim1.3\\) systems based on deep, spatially resolved, 17-band + G102 + G141 Hubble Space Telescope grism spectrophotometry. Using full spectrum rest-UV/-optical continuum fitting, we characterize these galaxies' observed \\(\\sim\\)kpc-scale structures and star formation rates (SFRs) and reconstruct their history over the age of the universe. The sample's diversity---passive to vigorously starforming; stellar masses \\(\\log M_*/M_\\odot=10.5\\) to \\(11.2\\)---enables us to draw spatio-temporal inferences relevant to key areas of parameter space (Milky Way- to super-Andromeda-mass progenitors). Specifically, we find signs that bulge mass-fractions (\\(B/T\\)) and SF history shapes/spatial uniformity are linked, such that higher \\(B/T\\)s correlate with \"inside-out growth\" and central specific SFRs that peaked above the global average for all starforming galaxies at that epoch. Conversely, the system with the lowest \\(B/T\\) had a flat, spatially uniform SFH with normal peak activity. Both findings are consistent with models positing a feedback-driven connection between bulge formation and the switch from rising to falling SFRs (\"quenching\"). While sample size forces this conclusion to remain tentative, this work provides a proof-of-concept for future efforts to refine or refute it: JWST, WFIRST, and the 30-m class telescopes will routinely produce data amenable to this and more sophisticated analyses. These samples---spanning representative mass, redshift, SFR, and environmental regimes---will be ripe for converting into thousands of sub-galactic-scale empirical windows on what individual systems actually looked like in the past, ushering in a new dialog between observation and theory.

We study the projected spatial offset between the ultraviolet continuum and Ly\\(\\alpha\\) emission for 65 lensed and unlensed galaxies in the Epoch of Reionization (\\(5\\leq z\\leq7\\)), the first such study at these redshifts, in order to understand the potential for these offsets to confuse estimates of the Ly\\(\\alpha\\) properties of galaxies observed in slit spectroscopy. While we find that ~40% of galaxies in our sample show significant projected spatial offsets (\\(|\\Delta_{Ly\\alpha-UV}|\\)), we find a modest average offset of 0.61\\(\\pm\\)0.08 kpc. A small fraction of our sample, ~10%, exhibits offsets of 2-4 kpc, sizes that are larger than the effective radii of typical galaxies at these redshifts. An internal comparison and a comparison to studies at lower redshift yielded no significant evidence of evolution of \\(|\\Delta_{Ly\\alpha-UV}|\\) with redshift. In our own sample, UV-bright galaxies showed offsets a factor of three greater than their fainter counterparts, 0.89\\(\\pm\\)0.18 vs. 0.27\\(\\pm\\)0.05 kpc, respectively. We argue that offsets are likely not the result of merging processes, but are rather due to internal anisotropic processes resulting from stellar feedback facilitates Ly\\(\\alpha\\) fluorescence and/or backscattering from nearby or outflowing gas. The reduction in the Ly\\(\\alpha\\) flux due to offset effects for various observational setups was quantified through mock observations of simple simulations. It was found that the loss of Ly\\(\\alpha\\) photons for galaxies with average offsets is not, if corrected for, a limiting factor for all but the narrowest slit widths (<0.4''). However, for the largest offsets, if such offsets are mostly perpendicular to the slit major axis, slit losses were found to be extremely severe in cases where slit widths of \\(\\leq\\)1'' were employed, such as those planned for James Webb Space Telescope/NIRSpec observations. (abridged)

Features are often the basic unit of development for a very large software system and represent long-term efforts, spanning up to several years from inception to actual use. Developing an experiment to monitor (by means of sampling) such lengthy processes requires a great deal of care in order to minimize casts and to maximize benefits. Just as prototyping is often a necessary auxiliary step in a large-scale, long-term development effort, so, too, is prototyping a necessary step in the development of a large-scale, long-term process monitoring experiment. Therefore, we have prototyped our experiment using a representative process and reconstructed data from a large and rich feature development. This approach has yielded three interesting sets of results. First, we reconstructed a 30-month time diary for the lead engineer of a feature composed of both hardware and software. These data represent the daily state (where the lead engineer spent the majority of his time) for a complete cycle of the development process. Second, we found that we needed to modify our experimental design. Our initial set of states did not represent the data as well as we had hoped. This is exemplified by the fact that the \"Other\" category is too large. Finally, the data provide evidence for both a waterfall view and an interactive, cyclic view of software development. We conclude that the prototyping effort is a necessary part of developing and installing any large-scale process monitoring experiment.< >

We present new results from our search for z~7 galaxies from deep spectroscopic observations of candidate z-dropouts in the CANDELS fields. Despite the extremely low flux limits achieved by our sensitive observations, only 2 galaxies have robust redshift identifications, one from its Lyalpha emission line at z=6.65, the other from its Lyman-break, i.e. the continuum discontinuity at the Lyalpha wavelength consistent with a redshift 6.42, but with no emission line. In addition, for 23 galaxies we present deep limits in the Lyalpha EW derived from the non detections in ultra-deep observations. Using this new data as well as previous samples, we assemble a total of 68 candidate z~7 galaxies with deep spectroscopic observations, of which 12 have a line detection. With this much enlarged sample we can place solid constraints on the declining fraction of Ly\\(\\alpha\\) emission in z~7 Lyman break galaxies compared to z~6, both for bright and faint galaxies. Applying a simple analytical model, we show that the present data favor a patchy reionization process rather than a smooth one.

We present a strong and weak lensing reconstruction of the massive cluster Abell 2744, the first cluster for which deep Hubble Frontier Field (HFF) images and spectroscopy from the Grism Lens-Amplified Survey from Space (GLASS) are available. By performing a targeted search for emission lines in multiply imaged sources using the GLASS spectra, we obtain 5 high-confidence spectroscopic redshifts and 2 tentative ones. We confirm 1 strongly lensed system by detecting the same emission lines in all 3 multiple images. We also search for additional line emitters blindly and use the full GLASS spectroscopic catalog to test reliability of photometric redshifts for faint line emitters. We see a reasonable agreement between our photometric and spectroscopic redshift measurements, when including nebular emission in photometric redshift estimations. We introduce a stringent procedure to identify only secure multiple image sets based on colors, morphology, and spectroscopy. By combining 7 multiple image systems with secure spectroscopic redshifts (at 5 distinct redshift planes) with 18 multiple image systems with secure photometric redshifts, we reconstruct the gravitational potential of the cluster pixellated on an adaptive grid, using a total of 72 images. The resulting mass map is compared with a stellar mass map obtained from the deep Spitzer Frontier Fields data to study the relative distribution of stars and dark matter in the cluster. We find that the stellar to total mass ratio varies substantially across the cluster field, suggesting that stars do not trace exactly the total mass in this interacting system. The maps of convergence, shear, and magnification are made available in the standard HFF format.