A lensed protocluster candidate at z=7.66z=7.66 identified in JWST observations of the galaxy cluster SMACS0723-7327

According to the current paradigm of galaxy formation, the first galaxies have been likely formed within large dark matter haloes. The fragmentation of these massive haloes led to the formation of galaxy protoclusters, which are usually composed of one to a few bright objects, surrounded by numerous fainter (and less massive) galaxies. These early structures could have played a major role in reionising the neutral hydrogen within the first billion years of the Universe; especially, if their number density is significant.Taking advantage of the unprecedented sensitivity reached by the \textit{James Webb Space Telescope (JWST)}, galaxy protoclusters can now be identified and studied in increasing numbers beyond $z\geq\$6. Characterising their contribution to the UV photon budget could supply new insights into the reionisation process. We analyse the first JWST dataset behind SMACS0723-7327 to search for protoclusters at $z\geq6$, combining the available spectroscopic and photometric data. We then compare our findings with semi-analytical models and simulations. In addition to two bright galaxies ($\leq$26.5 AB in F277W), separated by $\sim$11\arcsec and spectroscopically confirmed at $z_{spec}=7.66$, we identify 6 additional galaxies with similar colors in a $\theta\sim20$\arcsec radius around these (corresponding to R$\sim60-90$ kpc in the source plane). Using several methods, we estimate the mass of the dark matter halo of this protocluster, $\sim$3.3$\times$10$^{11}$M$_{\odot}$ accounting for magnification, consistent with various predictions. The physical properties of all protocluster members are also in excellent agreement with what has been previously found at lower redshifts: star-formation main sequence and protocluster size. This detection adds to just a few protoclusters currently known in the first billion years of the universe.Comment: 7 pages, 6 Figures. Accepted for publication in A&A Lette

Efficient survey design for finding high-redshift galaxies with JWST

Several large JWST blank field observing programs have not yet discovered the first galaxies expected to form at $15 \leq z \leq 20$. This has motivated the search for more effective survey strategies that will be able to effectively probe this redshift range. Here, we explore the use of gravitationally lensed cluster fields, that have historically been the most effective discovery tool with HST. In this paper, we analyze the effectiveness of the most massive galaxy clusters that provide the highest median magnification factor within a single JWST NIRCam module in uncovering this population. The results of exploiting these lensing clusters to break the $z > 15$ barrier are compared against the results from large area, blank field surveys such as JADES and CEERS in order to determine the most effective survey strategy for JWST. We report that the fields containing massive foreground galaxy clusters specifically chosen to occupy the largest fraction of a single NIRCam module with high magnification factors in the source plane, whilst containing all multiple images in the image plane within a single module provide the highest probability of both probing the $15 \leq z \leq 20$ regime, as well as discovering the highest redshift galaxy possible with JWST. We also find that using multiple massive clusters in exchange for shallower survey depths is a more time efficient method of probing the $z > 15$ regime.Comment: 12 pages, 5 figure

Unscrambling the lensed galaxies in JWST images behind SMACS0723

The first deep field images from the James Webb Space Telescope (JWST) of the galaxy cluster SMACS~J0723.3-7327 reveal a wealth of new lensed images at uncharted infrared wavelengths, with unprecedented depth and resolution. Here we securely identify 14 new sets of multiply imaged galaxies totalling 42 images, adding to the five sets of bright and multiply-imaged galaxies already known from Hubble Space Telescope data. We find examples of arcs crossing critical curves, allowing detailed community follow-up, such as JWST spectroscopy for precise redshift determinations, and measurements of the chemical abundances and of the detailed internal gas dynamics of very distant, young galaxies. One such arc contains compact knots of magnification $\mu\sim$750, and features a microlensed transient. We also detect an Einstein cross candidate only visible thanks to JWST's superb resolution. Our parametric lens model is available at https://www.dropbox.com/sh/gwup2lvks0jsqe5/AAC2RRSKce0aX-lIFCc9vhBXa?dl=0 , and will be regularly updated using additional spectroscopic redshifts. The model reproduces the multiple images to better than an rms of $0.5^{\prime \prime}$, and allows for accurate magnification estimates of high-redshift galaxies. The intracluster light extends beyond the cluster members, exhibiting large-scale features that suggest a significant past dynamical disturbance. This work represents a first taste of the enhanced power JWST will have for lensing-related science.Comment: Submitted to ApJ. 13 pages, 6 figure

A search for transients in the Reionization Lensing Cluster Survey (RELICS): Three new supernovae

The Reionization Cluster Survey (RELICS) imaged 41 galaxy clusters with the Hubble Space Telescope (HST), in order to detect lensed and high-redshift galaxies. Each cluster was imaged to about 26.5 AB mag in three optical and four near-infrared bands, taken in two distinct visits separated by varying time intervals. We make use of the multiple near-infrared epochs to search for transient sources in the cluster fields, with the primary motivation of building statistics for bright caustic crossing events in gravitational arcs. Over the whole sample, we do not find any significant ($\gtrsim5 \sigma$) caustic crossing events, in line with expectations from semi-analytic calculations but in contrast to what may be naively expected from previous detections of some bright events, or from deeper transient surveys that do find high rates of such events. Nevertheless, we find six prominent supernova (SN) candidates over the 41 fields: three of them were previously reported and three are new ones reported here for the first time. Out of the six candidates, four are likely core-collapse (CC) SNe -- three in cluster galaxies, and among which only one was known before, and one slightly behind the cluster at $z\sim0.6-0.7$. The other two are likely Ia -- both of them previously known, one probably in a cluster galaxy, and one behind it at $z\simeq2$. Our study supplies empirical bounds for the rate of caustic crossing events in galaxy cluster fields to typical HST magnitudes, and lays the groundwork for a future SN rate study.Comment: Accepted for publication in MNRAS. 10 pages, 3 figure

Two Lensed Star Candidates at z ≃ 4.8 behind the Galaxy Cluster MACS J0647.7+7015

We report the discovery of two extremely magnified lensed star candidates behind the galaxy cluster MACS J0647.7+015 using recent multiband James Webb Space Telescope (JWST) NIRCam observations. The star candidates are seen in a previously known, zphot ≃ 4.8 dropout giant arc that straddles the critical curve. The candidates lie near the expected critical curve position, but lack clear counter-images on the other side of it, suggesting these are possibly stars undergoing caustic crossings. We present revised lensing models for the cluster, including multiply imaged galaxies newly identified in the JWST data, and use them to estimate background macro-magnifications of at least ≳90 and ≳50 at the positions of the two candidates, respectively. With these values, we expect effective, caustic-crossing magnifications of ∼[103–105] for the two star candidates. The spectral energy distributions of the two candidates match well the spectra of B-type stars with best-fit surface temperatures of ∼10,000 K, and ∼12,000 K, respectively, and we show that such stars with masses ≳20 M⊙ and ≳50 M⊙, respectively, can become sufficiently magnified to be observable. We briefly discuss other alternative explanations and conclude that these objects are likely lensed stars, but also acknowledge that the less-magnified candidate may alternatively reside in a star cluster. These star candidates constitute the second highest-redshift examples to date after Earendel at zphot ≃ 6.2, establishing further the potential of studying extremely magnified stars at high redshifts with JWST. Planned future observations, including with NIRSpec, will enable a more detailed view of these candidates in the near future.A.K.M., A.Z., and L.J.F. acknowledge support from grant 2020750 from the United States-Israel Binational Science Foundation (BSF) and grant 2109066 from the United States National Science Foundation (NSF), and by the Ministry of Science & Technology, Israel. Y.J.-T. acknowledges financial support from the European Unionʼs Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 898633, the MSCA IF Extensions Program of the Spanish National Research Council (CSIC), and the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). E.Z. acknowl edges funding from the Swedish National Space Agency. J.M. D. acknowledges the support of projects PGC2018-101814-B 100 and MDM-2017-0765. B.W. acknowledges support from NASA under award No. 80GSFC21M0002. A.A. acknowledges support from the Swedish Research Council (Vetens kapsrådet project grants 2021–05559). R.A.B gratefully acknowledges support from the European Space Agency (ESA) Research Fellowship. M.B. acknowledges support from the Slovenian national research agency ARRS through grant N1-0238. P.D. acknowledges support from the NWO grant 016.VIDI.189.162 (“ODIN”) and from the European Commission’s and University of Groningen’s CO-FUND Rosalind Franklin program. G.M. acknowledges funding from the European Unionʼs Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. MARACHAS—DLV-896778. R.A.W. acknowledges support from NASA JWST Interdisciplinary Scientist grants NAG5-12460, NNX14AN10G and 80NSSC18K0200 from GSFC

JWST Spectroscopy of SN H0pe: Classification and Time Delays of a Triply-imaged Type Ia Supernova at z = 1.78

SN H0pe is a triply imaged supernova (SN) at redshift $z=1.78$ discovered using the James Webb Space Telescope (JWST). In order to classify the SN spectroscopically and measure the relative time delays of its three images (designated A, B, and C), we acquired NIRSpec follow-up spectroscopy spanning 0.6 to 5 microns. From the high signal-to-noise spectra of the two bright images B and C, we first classify the SN, whose spectra most closely match those of SN 1994D and SN 2013dy, as a Type Ia SN. We identify prominent blueshifted absorption features corresponding to Si II $\lambda6355$ and Ca II H $\lambda3970$ and K $\lambda3935$. We next measure the absolute phases of the three images from our spectra, which allows us to constrain their relative time delays. The absolute phases of the three images, determined by fitting the three spectra to Hsiao07 SN templates, are $6.5_{-1.8}^{+2.4}$d, $24.3_{-3.9}^{+3.9}$d, and $50.6_{-15.3}^{+16.1}$d for the brightest to faintest images. These correspond to relative time delays between Image A and Image B and between Image B and Image C of $-122.3_{-43.8}^{+43.7}$d and $49.3_{-14.7}^{+12.2}$d, respectively. The SALT3-NIR model yields phases and time delays consistent with these values. After unblinding, we additionally explored the effect of using Hsiao07 template spectra for simulations through eighty instead of sixty days past maximum, and found a small (11.5 and 1.0 days, respectively) yet statistically insignificant ($\sim$0.25$\sigma$ and $\sim$0.1$\sigma$) effect on the inferred image delays.Comment: 27 pages (including appendices), 11 figures, 13 supplemental figure

High-redshift Galaxy Candidates at z = 9-10 as Revealed by JWST Observations of WHL0137-08

We report the discovery of four galaxy candidates observed 450–600 Myr after the Big Bang with photometric redshifts between z ∼ 8.3 and 10.2 measured using James Webb Space Telescope (JWST) NIRCam imaging of the galaxy cluster WHL0137−08 observed in eight filters spanning 0.8–5.0 μm, plus nine Hubble Space Telescope filters spanning 0.4–1.7 μm. One candidate is gravitationally lensed with a magnification of μ ∼ 8, while the other three are located in a nearby NIRCam module with expected magnifications of μ 1.1. Using SED fitting, we estimate the stellar masses of these galaxies are typically in the range log M M = 8.3–8.7. All appear young, with mass-weighted ages < 0.15 mag, and specific star formation rates sSFR ∼0.25–10 Gyr−1 for most. One z ∼ 9 candidate is consistent with an ageBased on observations with the NASA/ESA/CSA James Webb Space Telescope obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Incorpo rated, under NASA contract NAS5-03127. Support for Program number JWST-GO-02282 was provided through a grant from the STScI under NASA contract NAS5-03127. The data described here may be obtained from the MAST archive at doi:10.17909/cqfq-5n80. Also based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA under NASA contract NAS5-26555. The HST observations are associated with programs HST-GO-14096, HST-GO-15842, and HST-GO 16668. Cloud-based data processing and file storage for this work is provided by the AWS Cloud Credits for Research program. The Cosmic Dawn Center is funded by the Danish National Research Foundation (DNRF) under grant #140. A.Z. and L.F. acknowledge support by grant No. 2020750 from the United States–Israel Binational Science Foundation (BSF) and grant No. 2109066 from the United States National Science Foundation (NSF), and by the Ministry of Science & Technology, Israel

Are JWST/NIRCam Color Gradients in the Lensed z = 2.3 Dusty Star-forming Galaxy El Anzuelo Due to Central Dust Attenuation or Inside-out Galaxy Growth?

Gradients in the mass-to-light ratio of distant galaxies impede our ability to characterize their size and compactness. The long-wavelength filters of JWST?s NIRCam offer a significant step forward. For galaxies at Cosmic Noon (z ? 2), this regime corresponds to the rest-frame near-infrared, which is less biased toward young stars and captures emission from the bulk of a galaxy?s stellar population. We present an initial analysis of an extraordinary lensed dusty star-forming galaxy at z = 2.3 behind the El Gordo cluster (z = 0.87), named El Anzuelo (?The Fishhook?) after its partial Einstein-ring morphology. The far-UV to near-IR spectral energy distribution suggests an intrinsic star formation rate of 81 yr 2 7 M 1 - ?+ - and dust attenuation AV ? 1.6, in line with other DSFGs on the star-forming main sequence. We develop a parametric lens model to reconstruct the source plane structure of dust imaged by the Atacama Large Millimeter/submillimeter Array, far-UV to optical light from Hubble, and near-IR imaging with 8 filters of JWST/NIRCam, as part of the Prime Extragalactic Areas for Reionization and Lensing Science program. The source-plane half-light radius is remarkably consistent from ?1 to 4.5 ?m, despite a clear color gradient where the inferred galaxy center is redder than the outskirts. We interpret this to be the result of both a radially decreasing gradient in attenuation and substantial spatial offsets between UV- and IR-emitting components. A spatial decomposition of the SED reveals modestly suppressed star formation in the inner kiloparsec, which suggests that we are witnessing the early stages of inside-out quenching

The JWST PEARLS View of the El Gordo galaxy cluster and of the structure it magnifies

We dedicate this study to the memory of Jill Bechtold, scholar and mentor, who with her great patience and investment in undergraduate and graduate education set many of us onto a career path in astronomy. We thank Sergey Cherkis for useful conversations and the anonymous referee for suggestions that improved the manuscript. B.L.F. obtained student support through a Faculty Challenge Grant for Increasing Access to Undergraduate Research and the Arthur L. and Lee G. Herbst Endowment for Innovation and the Science Deanʼs Innovation and Education Fund, both obtained at the University of Arizona. R.A.W. was funded by NASA JWST Interdisciplinary Scientist grants NAG5-12460, NNX14AN10G, and 80GNSSC18K0200 from NASA Goddard Space Flight Center. The BGU lensing group, L.J.F., and A.Z., acknowledge support by grant 2020750 from the United States–Israel Binational Science Foundation (BSF), grant 2109066 from the United States National Science Foundation (NSF), and the Ministry of Science & Technology, Israel. K.I.C. acknowledges funding from the Netherlands Research School for Astronomy (NOVA) and also from the Dutch Research Council (NWO), through the award of the Vici Grant VI.C.212.036. We thank the JWST Project at NASA GSFC and JWST Program at NASA HQ for their many-decades-long dedication to making the JWST mission a success. We especially thank Tony Roman, the JWST scheduling group, and Mission Operations Center staff at STScI for their continued dedicated support to getting the JWST observations scheduled. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with JWST program 1176. This work is also based on observations made with the NASA/ESA Hubble Space Telescope (HST). The data were obtained from the Barbara A. Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute (STScI), which is operated by the Associationof Universities for Research in Astronomy (AURA) Inc., under NASA contract NAS 5-26555 for HST. This research has made use of data obtained from the Chandra Data Archive and software provided by the Chandra X-ray Center (CXC) in the application package CIAO

Two lensed star candidates at z≃4.8z\simeq4.8 behind the galaxy cluster MACS J0647.7+7015

We report the discovery of two extremely magnified lensed star candidates behind the galaxy cluster MACS J0647.7+7015, in recent multi-band James Webb Space Telescope (JWST) NIRCam observations. The candidates are seen in a previously known, $z_{phot}\simeq4.8$ dropout giant arc that straddles the critical curve. The candidates lie near the expected critical curve position but lack clear counter images on the other side of it, suggesting these are possibly stars undergoing caustic crossings. We present revised lensing models for the cluster, including multiply imaged galaxies newly identified in the JWST data, and use them to estimate a background macro-magnification of at least $\gtrsim90$ and $\gtrsim50$ at the positions of the two candidates, respectively. With these values, we expect effective, caustic-crossing magnifications of $10^4-10^5$ for the two star candidates. The Spectral Energy Distributions (SEDs) of the two candidates match well spectra of B-type stars with best-fit surface temperatures of $\sim10,000$ K, and $\sim12,000$ K, respectively, and we show that such stars with masses $\gtrsim20$ M$_{\odot}$ and $\gtrsim50$ M$_{\odot}$, respectively, can become sufficiently magnified to be observed. We briefly discuss other alternative explanations and conclude these are likely lensed stars, but also acknowledge that the less magnified candidate may instead be or reside in a star cluster. These star candidates constitute the second highest-redshift examples to date after Earendel at $z_{phot}\simeq6.2$, establishing further the potential of studying extremely magnified stars to high redshifts with the JWST. Planned visits including NIRSpec observations will enable a more detailed view of the candidates already in the near future.Comment: 12 pages, 5 figures, 2 tables. Fixed Fig 3. comments are welcom