We give an overview of the Grism Lens Amplified Survey from Space (GLASS), a large Hubble Space Telescope program aimed at obtaining grism spectroscopy of the fields of ten massive clusters of galaxies at redshift z = 0.308 − 0.686, including the Hubble Frontier Fields (HFF). The Wide Field Camera 3 yields near infrared spectra of the cluster cores, covering the wavelength range 0.81−1.69µm through grisms G102 and G141, while the Advanced Camera for Surveys in parallel mode provides G800L spectra of the infall regions of the clusters. The WFC3 spectra are taken at two almost orthogonal position angles in order to minimize the effects of confusion. After summarizing the scientific drivers of GLASS, we describe the sample selection as well as the observing strategy and data processing pipeline. We then utilize MACSJ0717.5+3745, a HFF cluster and the first one observed by GLASS, to illustrate the data quality and the high-level data products. Each spectrum brighter than H AB = 23 is visually inspected by at least two co-authors and a redshift is measured when sufficient information is present in the spectra. Furthermore, we conducted a thorough search for emission lines through all the GLASS WFC3 spectra with the aim of measuring redshifts for sources with continuum fainter than H AB = 23. We provide a catalog of 139 emission-line based spectroscopic redshifts for extragalactic sources, including three new redshifts of multiple image systems (one probable, two tentative). In addition to the data itself we also release software tools that are helpful to navigate the data.
We investigate the intracluster light (ICL) in the six Hubble Frontier Field clusters at 0.3 < z < 0.6 . We employ a new method, which is free from any functional form of the ICL profile, and exploit the unprecedented depth of this Hubble Space Telescope imaging to map the ICL’s diffuse light out to clustrocentric radii R ∼ 300 kpc ( μ ICL ∼ 27 mag arcsec−2). From these maps, we construct radial color and stellar mass profiles via SED fitting and find clear negative color gradients in all systems with increasing distance from the Brightest Cluster Galaxy (BCG). While this implies older/more metal-rich stellar components in the inner part of the ICL, we find that the ICL mostly consists of a ≲ 2 Gyr population, and plausibly originated with log M * / M ⊙ ≲ 10 cluster galaxies. Furthermore, we find that 10%–15% of the ICL’s mass at large radii ( ≳ 150 kpc) lies in a younger/bluer stellar population (∼1 Gyr), a phenomenon not seen in local samples. We attribute this light to the higher fraction of star-forming/(post-)starburst galaxies in clusters at z ∼ 0.5 . Ultimately, we find the ICL’s total mass to be log M * ICL / M ⊙ ∼ 11 –12, constituting 5%–20% of the clusters’ total stellar mass, or about half of the value at z ∼ 0 . The above implies distinct formation histories for the ICL and BCGs/other massive cluster galaxies; i.e., the ICL at this epoch is still being constructed rapidly ( ∼ 40 M ⊙ yr−1), while the BCGs have mostly completed their evolution. To be consistent with the ICL measurements of local massive clusters, such as Virgo, our data suggest mass acquisition mainly from quiescent cluster galaxies is the principal source of ICL material in the subsequent ∼5 Gyr of cosmic time.
The Grism Lens-Amplified Survey from Space (GLASS) is a Hubble Space Telescope (HST ) Large Program, which will obtain 140 orbits of grism spectroscopy of the core and infall regions of 10 galaxy clusters, selected to be among the very best cosmic telescopes. Extensive HST imaging is available from many sources including the CLASH and Frontier Fields programs. We introduce the survey by analyzing spectra of faint multiply-imaged galaxies and z 6 galaxy candidates obtained from the first seven orbits out of fourteen targeting the core of the Frontier Fields cluster MACSJ0717.5+3745. Using the G102 and G141 grisms to cover the wavelength range 0.8-1.7µm, we confirm 4 strongly lensed systems by detecting emission lines in each of the images. For the 9 z 6 galaxy candidates clear from contamination, we do not detect any emission lines down to a seven-orbit 1σ noise level of ∼5×10 −18 erg s −1 cm −2 . Taking lensing magnification into account, our flux sensitivity reaches ∼0.2-5×10 −18 erg s −1 cm −2 . These limits over an uninterrupted wavelength range rule out the possibility that the high-z galaxy candidates are instead strong line emitters at lower redshift. These results show that by means of careful modeling of the background -and with the assistance of lensing magnification -interesting flux limits can be reached for large numbers of objects, avoiding preselection and the wavelength restrictions inherent to ground-based multi-slit spectroscopy. These observations confirm the power of slitless HST spectroscopy even in fields as crowded as a cluster core.
We present the results of a first search for galaxy candidates at z ∼ 9–15 on deep seven-band NIRCam imaging acquired as part of the GLASS-James Webb Space Telescope (JWST) Early Release Science Program on a flanking field of the Frontier Fields cluster A2744. Candidates are selected via two different renditions of the Lyman-break technique, isolating objects at z ∼ 9–11, and z ∼ 9–15, respectively, supplemented by photometric redshifts obtained with two independent codes. We find five color-selected candidates at z > 9, plus one additional candidate with photometric redshift z phot ≥ 9. In particular, we identify two bright candidates at M UV ≃ −21 that are unambiguously placed at z ≃ 10.6 and z ≃ 12.2, respectively. The total number of galaxies discovered at z > 9 is in line with the predictions of a nonevolving luminosity function. The two bright ones at z > 10 are unexpected given the survey volume, although cosmic variance and small number statistics limits general conclusions. This first search demonstrates the unique power of JWST to discover galaxies at the high-redshift frontier. The candidates are ideal targets for spectroscopic follow-up in Cycle-2.
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