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Exceptionally deep observations of the distant universe with the Hubble Space Telescope have consistently pushed the frontiers of human knowledge. How deep can we go? What are the faintest and most distant galaxies we can see with the Hubble Space Telescope now, before the launch of the James Webb Space Telescope? This is the challenge taken up by the Frontier Fields, a director's discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combines the power of HST with the natural gravitational telescopes of high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. I will review the original goals of the Frontier Fields program and its progress over the last several years. In addition to pushing forward the study of the most distant galaxies, the Frontier Fields have been transformative in the study of galaxy clusters and their lensing properties. Finally, I will discuss the prospects for studying galaxies at cosmic dawn with JWST, extremely large ground-based telescopes, and future space missions over the next decade and beyond.

At local, galaxy properties are well known to be clearly different in different environments. However, it is still an open question how this environment-dependent trend has been shaped. We present the results of our investigation about the evolution of star-formation properties of galaxies over a wide redshift range, from z ~ 2 to z ~ 0.5, focusing its dependence on their stellar mass and environment (Lee et al. 2015). In the UKIDSS/UDS region, covering ~2800 square arcmin, we estimated photometric redshifts and stellar population properties, such as stellar masses and star-formation rates, using the deep optical and near-infrared data available in this field. Then, we identified galaxy cluster candidates within the given redshift range. Through the analysis and comparison of star-formation (SF) properties of galaxies in clusters and in field, we found interesting results regarding the evolution of SF properties of galaxies: (1) regardless of redshifts, stellar mass is a key parameter controlling quenching of star formation in galaxies; (2) At z < 1, environmental effects become important at quenching star formation regardless of stellar mass of galaxies; and (3) However, the result of the environmental quenching is prominent only for low mass galaxies (M* < 1010 M⊙) since the star formation in most of high mass galaxies are already quenched at z > 1.

We study the photometric properties and sizes of the reionization-epoch galaxies in high-resolution Simba cosmological hydrodynamical simulations with box sizes of \\([25,50]~h^{-1}\\)Mpc. Assuming various attenuation laws, we compute photometry by extincting each star particle's spectrum using the line-of-sight gas metal column density. The predicted ultraviolet luminosity function (UVLF) generally agrees with observations at \\(z=6\\), owing to a partial cancellation between the high metallicities of the simulated galaxies and lower dust-to-metal ratios. The simulated \\(z=8\\) UVLF is low compared to observations, likely owing to excessive dust extinction. Simba predicts UV continuum slopes (\\(\\beta\\)) in agreement with the \\(z=6\\) observations, with the best agreement obtained using a Calzetti extinction law. Interestingly, the gas-phase mass-metallicity relation in Simba is higher at \\(z\\sim 6\\) than at \\(z\\sim 2\\), suggesting that rapid early enrichment (and dust growth) might be necessary to match the observed \\(\\beta\\). We find that \\(\\beta\\) is more sensitive to the dust extinction law than the UVLF. By generating mock James Webb Space Telescope (JWST) images and analysing in a manner similar to observations, we show that Simba's galaxy size-luminosity relation well reproduces the current \\(z=6\\) Hubble observations. Unlike observations at lower redshifts, Simba predicts similar rest-UV and rest-optical sizes of \\(z=6\\) galaxies, owing to weak age gradients and dust extinction in star-forming regions counteract each other to weaken the color gradients within galaxies. These predictions will be testable with JWST.

We present an overview of the CANDELS Lyman-a Emission At Reionization (CLEAR) survey. CLEAR is a 130 orbit program of the Hubble Space Telescope using the Wide Field Camera 3 (WFC3) IR G102 grism. CLEAR targets 12 pointings divided between the GOODS-N and GOODS-S fields of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). Combined with existing spectroscopic data from other programs, the full CLEAR dataset includes spectroscopic imaging of these fields over 0.8-1.7 um. In this Paper, we describe the CLEAR survey, the survey strategy, the data acquisition, reduction, processing, and science products and catalogs released alongside this paper. The catalogs include emission line fluxes and redshifts derived from the combination of the photometry and grism spectroscopy for 6048 galaxies, primarily ranging from 0.2 < z < 3. We also provide an overview of CLEAR science goals and results. In conjunction with this Paper we provide links to electronic versions of the data products, including 1D + 2D extracted spectra and emission line maps.

We examine the dynamical properties of interacting galaxies and the properties of shocked gas produced as a result of the interaction. We observed 22 galaxy mergers using the SparsePak IFU at Kitt Peak National Observatory (KPNO). The goal of the observations was to obtain the \\ha\\ velocity maps over the entire luminous parts of the galaxies including the faint tidal tails and to find extended shocks and outflows. Our sample consists of major and minor galaxy mergers with mass ratios \\(1<\\mu<8\\). We fit multiple kinematic components to the \\ha\\ and \\nii\\ emission lines, develop an MCMC code to robustly estimate the error of fit parameters, and use the F-test to determine the best number of kinematic components for each fiber. We use \\nii/\\ha\\ and velocity dispersion of components to separate star-forming (HII) regions from shocks. We use the kinematics of the \\ha\\ emission from HII regions and an automated modeling method to put the first-ever constraints on the encounter parameters of one of the observed systems. Besides, we roughly estimate the fraction of shocked \\ha\\ emission , \\(\\text{f}_\\text{shocked}\\), without taking extinction into account and examine the spatial distribution of shocks. We find that close galaxy pairs have, on average, a higher shock fraction than wide pairs, and coalesced mergers have the highest average \\(\\text{f}_\\text{shocked}\\). In addition, galaxy pairs with more equal mass ratio tend to have a higher \\(\\text{f}_\\text{shocked}\\). Combining the dynamical models from the literature and this work, we inspect trends between \\(\\text{f}_\\text{shocked}\\) and dynamical encounter parameters. Our findings are generally consistent with shocks being produced either by the direct collision of the ISM or by the chain of events provoked by the tidal impulse during the first passage.

We present JWST NIRSpec spectroscopy for 11 galaxy candidates with photometric redshifts of \\(z\\simeq9-13\\) and \\(M_{\\rm\\,UV} \\in[-21,-18]\\) newly identified in NIRCam images in the Cosmic Evolution Early Release Science (CEERS) Survey. We confirm emission line redshifts for 7 galaxies at \\(z=7.762-8.998\\) using spectra at \\(\\sim1-5\\mu\\)m either with the NIRSpec prism or its three medium resolution gratings. For \\(z\\simeq9\\) photometric candidates, we achieve a high confirmation rate of \\(\\simeq\\)90\\%, which validates the classical dropout selection from NIRCam photometry. No robust emission lines are identified in three galaxy candidates at \\(z>10\\), where the strong [OIII] and H\\(\\beta\\) lines would be redshifted beyond the wavelength range observed by NIRSpec, and the Lyman-\\(\\alpha\\) continuum break is not detected with the current sensitivity. Compared with HST-selected bright galaxies (\\(M_{\\rm\\,UV}\\simeq-22\\)) that are similarly spectroscopically confirmed at \\(z\\gtrsim8\\), these NIRCam-selected galaxies are characterized by lower star formation rates (SFR\\(\\simeq4\\,M_{\\odot}\\)~yr\\(^{-1}\\)) and lower stellar masses (\\(\\simeq10^{8}\\,M_{\\odot}\\)), but with higher [OIII]+H\\(\\beta\\) equivalent widths (\\(\\simeq\\)1100\\(Å\\)), and elevated production efficiency of ionizing photons (\\(\\log(\\xi_{\\rm\\,ion}/{\\rm\\,Hz\\,erg}^{-1})\\simeq25.8\\)) induced by young stellar populations (\\(<10\\)~Myrs) accounting for \\(\\simeq20\\%\\) of the galaxy mass, highlighting the key contribution of faint galaxies to cosmic reionization. Taking advantage of the homogeneous selection and sensitivity, we also investigate metallicity and ISM conditions with empirical calibrations using the [OIII]/H\\(\\beta\\) ratio. We find that galaxies at \\(z\\sim8-9\\) have higher SFRs and lower metallicities than galaxies at similar stellar masses at \\(z\\sim2-6\\), which is generally consistent with the current galaxy formation and evolution models.

We present an investigation into the first 500 Myr of galaxy evolution from the Cosmic Evolution Early Release Science (CEERS) survey. CEERS, one of 13 JWST ERS programs, targets galaxy formation from z~0.5 to z>10 using several imaging and spectroscopic modes. We make use of the first epoch of CEERS NIRCam imaging, spanning 35.5 sq. arcmin, to search for candidate galaxies at z>9. Following a detailed data reduction process implementing several custom steps to produce high-quality reduced images, we perform multi-band photometry across seven NIRCam broad and medium-band (and six Hubble broadband) filters focusing on robust colors and accurate total fluxes. We measure photometric redshifts and devise a robust set of selection criteria to identify a sample of 26 galaxy candidates at z~9-16. These objects are compact with a median half-light radius of ~0.5 kpc. We present an early estimate of the z~11 rest-frame ultraviolet (UV) luminosity function, finding that the number density of galaxies at M_UV ~ -20 appears to evolve very little from z~9 to z~11. We also find that the abundance (surface density [arcmin^-2]) of our candidates exceeds nearly all theoretical predictions. We explore potential implications, including that at z>10 star formation may be dominated by top-heavy initial mass functions, which would result in an increased ratio of UV light per unit halo mass, though a complete lack of dust attenuation and/or changing star-formation physics may also play a role. While spectroscopic confirmation of these sources is urgently required, our results suggest that the deeper views to come with JWST should yield prolific samples of ultra-high-redshift galaxies with which to further explore these conclusions.

The morphology-density relationship states that dense cosmic environments such as galaxy clusters have an overabundance of quiescent elliptical galaxies, but it is unclear at which redshift this relationship is first established. We study the morphology of 4 clusters with \\(1.2

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