While the identification of a worthy topic or challenge is fundamental to the goal of conducting cutting-edge research, another ingredient is indispensable in the field of observational astronomy: the best possible target. Indeed, the ability to choose from a significant number of extremely powerful gravitational lenses was central to the success of the Frontier Fields project. We here briefly review the surveys that provided this crucial pool of targets, before focusing on the results of ongoing work to identify similarly extreme (if not more extreme) systems at higher redshift for the exploration of a yet more distant extragalactic frontier – with JWST and other, evermore ambitious missions.

Chandra has deeply observed the clusters and parallel fields in four Frontier Fields. These observations allow us to dramatically improve our understanding of cluster mergers by comparing the detailed mapping of the hot cluster gas with high resolution mass maps and by identifying merger shocks and cold fronts. In merging clusters, relativistic particles can be re-accelerated to produce radio relics. A comparison of lensing maps and Chandra images allows us to determine the cluster morphology and dynamical state and if there are offsets between the dark matter and the hot gas.

Abell 2744 was the first HFF cluster completed. It displays a fascinating complexity in its distribution of dark and luminous matter, which led to its nickname of the Pandora cluster. In late 2014 we obtained a deep (110 ks) observation of this cluster with XMM-Newton, with the aim of making a detailed comparison between the optical, X-ray and lensing properties of this system. The new X-ray observation unveiled the presence of three hot gas filaments extending on scales of several Mpc and connected to the cluster core. The X-ray structures coincide spatially with the distribution of galaxies and dark matter and provide strong evidence for the existence of the elusive warm-hot intergalactic medium (WHIM). The new observation also reveals the complexity of the thermodynamic structure of the cluster core and a probable shock front associated with the radio relic located 1 Mpc NW of the cluster core.

Strong gravitational magnification in the core of lensing clusters allows to probe the faint-end of the galaxy luminosity function up to very high redshift. In particular, the Frontier Fields have allowed us to identify a large number of faint dropouts and constrain the Lyman-break luminosity function at z~5-7. I present here the results of an ongoing program with MUSE, a new integral field spectrograph on the Very Large Telescope having a large field of view (1 arcmin2), to confirm these candidate high redshift dropouts through Lyman-α emission and identify additional emitters with high equivalent width, fainter than the depth of the Frontier Fields Hubble images. Combined with similar deep exposures taken with MUSE in blank fields, this gives us the best opportunity to probe the Lyman-α luminosity function over a wide range in luminosity.

The Grism Lens-Amplified Survey From Space (GLASS) is a 140 orbit spectroscopic survey of 10 massive galaxy clusters, including the six Hubble Frontier Fields. GLASS has observed the cluster cores with the HST-WFC3 G102 and G141 grisms providing a wide wavelength coverage in the near-infrared from roughly 0.8–1.7μm. The parallel fields were observed through the optical ACS G800L grism. Taking advantage of the lensing magnification of the clusters, GLASS reaches intrinsic spectroscopic 1σ flux limits of roughly 10-18erg/s/cm2 and improved spatial resolution for lensed sources behind the clusters. These features are particularly useful for the three main science drivers of GLASS which are, I) exploring the universe at the epoch of reionization, II) describe how metals cycle in and out of galaxies, and III) asses the environmental dependence of galaxy evolution. The former two benefit highly from the improved depth and increased resolution provided by the cluster lensing. Apart from the main science drivers, a slew of ancillary science has been enabled by the survey, including improving cluster lens modeling and searches for supernovae. Here we present the survey and the GLASS data releases, which are continuously being made available to the community through https://archive.stsci.edu/prepds/glass/. For further information we refer to Schmidt et al. (2014), Treu et al. (2015), and http://glass.physics.ucsb.edu.

The Grism Lens-Amplified Survey from Space (GLASS) has obtained slitless near-infrared spectroscopy of 10 galaxy clusters selected for their strong lensing properties, including all six Hubble Frontier Fields. Slitless grism spectra are ideal for mapping emission lines such as [O ii], [O iii], and Hα at z=1–3. The combination of strong gravitational lensing and Hubble's diffraction limit provides excellent sensitivity with spatial resolution as fine as 100 pc for highly magnified sources, and ~500 pc for less magnified sources near the edge of the field of view. The GLASS survey represents the largest spectroscopic sample with such high resolution at z > 1. GLASS and Hubble Frontier Field data provide the distribution of stellar mass, star formation, gas-phase metallicity, and other aspects of the physical structure of high redshift galaxies, reaching stellar masses as low as ~107 M⊙ at z=2. I discuss precise measurements of these physical properties and implications for galaxy evolution.

We present new emission line identifications and improve the lensing reconstruction of the mass distribution of galaxy cluster Abell 2744 using the Grism Lens-Amplified Survey from Space (GLASS) spectroscopy and the Hubble Frontier Fields (HFF) imaging. We performed blind and targeted searches for faint line emitters on all objects, including the arc sample, within the field of view (FoV) of GLASS prime pointings. We report 55 high quality spectroscopic redshifts, 5 of which are for arc images. We also present an extensive analysis based on the HFF photometry, measuring the colors and photometric redshifts of all objects within the FoV, and comparing the spectroscopic and photometric redshift estimates. In order to improve the lens model of Abell 2744, we develop a rigorous algorithm to screen arc images, based on their colors and morphology, and selecting the most reliable ones to use. As a result, 25 systems (corresponding to 72 images) pass the screening process and are used to reconstruct the gravitational potential of the cluster pixellated on an adaptive mesh. The resulting total mass distribution is compared with a stellar mass map obtained from the Spitzer Frontier Fields data in order to study the relative distribution of stars and dark matter in the cluster.

Probing the central mass distribution of massive galaxy clusters is an important step towards mapping the overall distribution of their dark matter content. Thanks to gravitational lensing and the appearance of multiple images, we can constrain the inner region of galaxy clusters with a high precision. The Frontier Fields (FF) provide us with the deepest HST data ever in such clusters. Currently, most multiple-image systems are found by eye, yet in the FF, we expect hundreds to exist. Thus, in order to deal with such huge amounts of data, we need to develop an automated detection method. I present a new tool to perform this task, MISE (Multiple Image SEarcher), a program which identifies multiple images by combining their specific properties. MISE allows us to confirm or reject multiple images identified visually, but also detect new multiple-image candidates in MACS0416 and A2744, giving us additional constraints on the mass distribution in these clusters. A spectroscopic follow-up of these candidates is currently underway with MUSE.

We have combined the performances of the VLT/MUSE spectrograph together with the power of gravitational lensing by Frontier Fields galaxy clusters to offer a unique magnified view of the distant universe. The large field of view over a wide optical wavelength domain enables redshift measurements of numerous lensed galaxies in the cluster core. Spectroscopically-confirmed multiple-imaged systems are further used as strong constraints to improve the cluster mass model. Here, we focus on the galaxy cluster MACSJ0416.1-2403 and compare the revised magnification map with results from previous analysis.

Studies of high-redshift galaxies behind the cores of mass clusters require the correction of gravitational lensing effects. We present our approach to estimate shapes, magnitudes, and the selection effect of high-redshift galaxies in the image plane, which allows us to include not only lensing magnifications but also lensing distortions and image multiplications. For this purpose we construct new mass models for the Frontier Fields clusters using the public software glafic. We present some results on faint-end slopes of the luminosity function and the size evolution of high-redshift galaxies from the analysis of Frontier Fields clusters.

We reconstruct the projected mass distribution of a massive merging HFF cluster MACSJ0416 using the genetic algorithm based free-form technique called Grale. The reconstructions are constrained by 149 lensed images identified by Jauzac et al. using HFF data. No information about cluster galaxies or light is used, which makes our reconstruction unique in this regard. Using visual inspection of the maps, as well as galaxy-mass correlation functions we conclude that overall light does follow mass. Furthermore, the fact that brighter galaxies are more strongly clustered with mass is an important confirmation of the standard biasing scenario in galaxy clusters. On the smallest scales, ≲ few arcseconds the resolution afforded by 149 images is still not sufficient to confirm or rule out galaxy-mass offsets of the kind observed in ACO 3827. We also compare the mass maps of MACSJ0416 obtained by three different groups: Grale, and two parametric LENSTOOL reconstructions from the CATS and Sharon/Johnson teams. Overall, the three agree well; one interesting discrepancy between Grale and LENSTOOL galaxy-mass correlation functions occurs on scales of tens of kpc and may suggest that cluster galaxies are more biased tracers of mass than parametric methods generally assume.

The Frontier Fields cluster MACS J0416.1-2403 with its extensive imaging and spectroscopic data sets provides a great opportunity to study the mass distribution of the galaxy cluster and members, the high-redshift Universe and cosmology. By taking advantage of the observations in the 16 Hubble Space Telescope imaging bands of the Cluster Lensing And Supernova survey with Hubble (CLASH) survey and our large spectroscopic follow-up program with the VIsible Multi-Object Spectrograph (VIMOS) on the Very Large Telescope (VLT), we have been able to identify and obtain the spectroscopic redshifts of 10 important strong lensing systems in this cluster. Furthermore, we have selected and modeled the mass distribution of ~200 candidate cluster members residing in the inner regions of the cluster. We present the results on the model-predicted central mass profile and subhalo population, which are detailed in Grillo et al. (2015). Work is underway to quantify the effects of line-of-sight structures. These are essential elements to make progress in our understanding of the dark matter distribution in massive galaxy clusters and of the distant Universe within the current Frontier Fields initiative and before the advent of the James Webb Space Telescope.

We investigate how varying the number of multiple image constraints and the available redshift information can influence the systematic errors of strong lens models, specifically, the image predictability, mass distribution, and magnifications of background sources. This work will not only inform upon Frontier Field science, but also for work on the growing collection of strong lensing galaxy clusters, most of which are less massive and are capable of lensing a handful of galaxies.

Driven by the unprecedented wealth of high quality data that is accumulating for the Frontier Fields, they are becoming some of the best-studied strong lensing clusters to date, and probably the next few years. As will be discussed intensively in this focus meeting, the FF prove transformative for many fields: from studies of the high redshift Universe, to the assembly and structure of the clusters themselves. The FF data and the extensive collaborative effort around this program will also allow us to examine and improve upon current lens modeling techniques. Strong lensing is a powerful tool for mass reconstruction of the cores of galaxy clusters of all scales, providing an estimate of the total (dark and seen) projected mass density distribution out to 0.5 Mpc. Though SL mass may be biased by contribution from structures along the line of sight, its strength is that it is relatively insensitive to assumptions on cluster baryon astrophysics and dynamical state. Like the Frontier Fields clusters, the most “famous” strong lensing clusters are at the high mass end; they lens dozens of background sources into multiple images, providing ample lensing constraints. In this talk, I will focus on how we can leverage what we learn from modeling the FF clusters in strong lensing studies of the hundreds of clusters that will be discovered in upcoming surveys. In typical clusters, unlike the Frontier Fields, the Bullet Cluster and A1689, we observe only one to a handful of background sources, and have limited lensing constraints. I will describe the limitations that such a configuration imposes on strong lens modeling, highlight measurements that are robust to the richness of lensing evidence, and address the sources of uncertainty and what sort of information can help reduce those uncertainties. This category of lensing clusters is most relevant to the wide cluster surveys of the future.

Flexion is the second order weak gravitational lensing effect responsible for the arclike appearance of sources. It is highly sensitive to dark matter substructure and can greatly increase the resolution of mass maps, but it is very hard to measure. We present an automated flexion measurement pipeline for Hubble Space Telescope data and a preliminary application to the Frontier Fields cluster MACSJ0416.1-2403.

We present the first study of the spatial distribution of star formation in z ~ 0.5 cluster galaxies. The analysis is based on data taken with the Wide Field Camera 3 as part of the Grism Lens-Amplified Survey from Space (GLASS). We illustrate the methodology by focusing on two clusters (MACS0717.5+3745 and MACS1423.8+2404) with different morphologies (one relaxed and one merging) and use foreground and background galaxies as field control sample. The cluster+field sample consists of 42 galaxies with stellar masses in the range 108-1011M⊙, and star formation rates in the range 1-20 M⊙yr−1. In both environments, Hα is more extended than the rest-frame UV continuum in 60% of the cases, consistent with diffuse star formation and inside out growth. The Hα emission appears more extended in cluster galaxies than in the field, pointing perhaps to ionized gas being stripped and/or star formation being enhanced at large radii. The peak of the Hα emission and that of the continuum are offset by less than 1 kpc. We investigate trends with the hot gas density as traced by the X-ray emission, and with the surface mass density as inferred from gravitational lens models and find no conclusive results. The diversity of morphologies and sizes observed in Hα illustrates the complexity of the environmental process that regulate star formation.

The wealth of strong lensing features observed in the Frontier Fields clusters offers insights on the nature of dark energy. The large number of multiple-images systems with redshifts allows to simultaneously estimate the lens model parameters and the cosmological parameters involved in the distances calculations. In particular for the ΛCDM model, it is possible to estimate the matter density Ωm and the dark energy equations parameters wX. In this talk, I will present recent analyses of systematic errors based on Frontier Fields observed and simulated data.

We present z > 9 candidates identified in Hubble Frontier Fields imaging of the first four clusters and blank parallel fields (two-thirds of the complete program). Based on the deeper Hubble imaging, we revise the redshift estimate of the CLASH z ~ 9.6 candidate MACS1149-JD to z = 9.2+0.2-0.5 (95% C.L.). We identify a new possible fainter z ~ 9 companion 3” away (~1 kpc in the source plane). And we discover a new z ~ 9.2 candidate in the MACS1149 parallel field. Combined with previously published candidates at z ~ 9.8 and 9.1 (in A2744 and its parallel field, respectively), these five z > 9 candidates fall below our published expectation of 8 – 47 at this stage in the program. We attribute some of this shortfall to incompleteness, which we have yet to quantify. At z ~ 8 (7.5 – 8.5), we detect 26 candidates down to F160W H < 28.7 AB, roughly one-third the ~82 we expect. If our z > 9 incompleteness is similar (~68%), our results would support the sharp drop in z > 9 number counts claimed by some (but not all) previous works and supported by several (but not all) theoretical models. Properly quantifying our incompleteness will require adding simulated high-redshift galaxies into the images and testing our recovery rate. Additionally, incorporating the deep Spitzer imaging into our analysis could potentially significantly improve our identification of z > 9 candidates by rejecting low-redshift (z ~ 2) interlopers. Data from the full Frontier Fields program will provide strong evidence for or against accelerated evolution and a sharp drop in the cosmic star formation rate density at z > 9.

Understanding when and how the first galaxies formed and what sources reionized the universe are key goals of extragalactic astronomy. Thanks to deep surveys with the powerful WFC3/IR camera on the HST, the observational frontier of galaxy build-up now lies at only ~450 Myr after the Big Bang, at redshifts z ~10-12. In combination with deep data from Spitzer/IRAC we can now probe the evolution of the stellar mass density over 96% of cosmic history. However, detecting and characterizing galaxies at these early epochs is challenging even for HST and the sample sizes at the earliest redshifts are still very small. The Hubble Frontier Fields provide a prime new dataset to improve upon our current, sparse sampling of the UV luminosity function at z>8 from blank fields to answer some of the most pressing open questions. For instance, even the evolution of the cosmic star-formation rate density at z>8 is still debated. While our measurements based on blank field data indicate that galaxies with SFR>0.7 Msol/yr disappear quickly from the cosmic record between z~8 and z~10, other previous results, e.g., from the CLASH survey favor a more moderate decline. Here, we briefly review the recent progress in studying galaxy build-up out to z~10 from the combined blank field and existing Frontier Field datasets and discuss their implications for primordial galaxy formation and cosmic reionization.

With the combination of deep Hubble Space Telescope imaging and magnification due to gravitational lensing, the Hubble Frontier Fields program offers an unprecedented opportunity to study the faint end of the luminosity function at the highest redshifts. Unfortunately, the region of the field that benefits most from this magnification (the immediate vicinity of the critical line) suffers from a high abundance of bright foreground galaxies and high background due to intracluster light. To overcome these difficulties, some method of modelling and subtracting the foreground light is required. Here, I present results using wavelet decomposition to subtract large-scale structures in the clusters, which significantly increases the effective depth of the images, and crucially opens up the most magnified regions of the clusters.