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Abstract We present stellar atmosphere modeling of JWST NIRCam photometry of nine highly magnified individual stars in a single galaxy at redshiftz= 0.94 known as the Warhol arc, which is strongly lensed by the galaxy cluster MACS J0416. Seven of these transients were identified by Yan et al. The nine sources are likely red supergiants with temperaturesT_eff ≈ 4000 K. We present new long-slit spectroscopy of the Warhol arc acquired with Keck I telescope and the Large Binocular Telescope, and use these data to help constrain the arc’s oxygen abundance to be $12+\log (\mathrm{O}/\mathrm{H})=8.45\pm 0.08$. A microlensing simulation is performed on synthetic stellar populations using a range of stellar metallicities and initial mass function (IMF) slopes. The temperature distribution of the simulated detectable stars is sensitive to the choice of stellar metallicity, and setting the stellar metallicity equal to the arc’s nebular metallicity ( $\log (Z_{*}/Z_{\odot })=-0.24$) produces a simulated temperature distribution that is consistent with the observations, while lower stellar metallicities ( $\log (Z_{*}/Z_{\odot })<\text{–}0.75$) produce simulated temperatures that are inconsistent with the observations. The expected detection rate is strongly anticorrelated with the IMF slope forα > 1.2. For the canonical IMF slopeα = 2.35, the simulation yields expected transient detection rates that agree with the observed detection rates in the Hubble Space Telescope Flashlights filters, but overpredicts the detection rate by a factor of ∼3–12 (<2σtension) in the JWST filters. The simulated detection rate is sensitive to the choice of stellar metallicity, with lower metallicities ( $\log (Z_{*}/Z_{\odot })<\text{–}0.75$) yielding a significantly lower simulated detection rate that further reduces the modest tension with the observations. 

Abstract Galaxy-cluster gravitational lenses enable the study of faint galaxies even at large lookback times, and, recently, time-delay constraints on the Hubble constant. There have been few tests, however, of lens model predictions adjacent to the critical curve (≲8″) where the magnification is greatest. In a companion paper, we use the GLAFIC lens model to constrain the BalmerL–σrelation for Hiiregions in a galaxy at redshiftz= 1.49 strongly lensed by the MACS J1149 galaxy cluster. Here we perform a detailed comparison between the predictions of 10 cluster lens models that employ multiple modeling assumptions with our measurements of 11 magnified, giant Hiiregions. We find that that the models predict magnifications an average factor of 6.2 smaller, a ∼2σtension, than that inferred from the Hiiregions under the assumption that they follow the low-redshiftL–σrelation. To evaluate the possibility that the lens model magnifications are strongly biased, we next consider the flux ratios among knots in three images of Sp1149, and find that these are consistent with model predictions. Moreover, while the mass-sheet degeneracy could in principle account for a factor of ∼6 discrepancy in magnification, the value ofH₀inferred from SN Refsdal’s time delay would become implausibly small. We conclude that the lens models are not likely to be highly biased, and that instead the Hiiregions in Sp1149 are substantially more luminous than the low-redshift BalmerL–σrelation predicts. 

Abstract We present the most detailed spectrum of intracluster light (ICL) in an individual cluster to date, the relaxed system RX J2129.7+0005, atz∼ 0.234. Using 15 broadband, deep images observed with the Hubble Space Telescope and JWST in the optical and the infrared, plus deep integral field spectroscopy from MUSE, we computed a total of 3696 ICL maps spanning the spectral range ∼0.4−5μm with our algorithm CICLE, a method that is extremely well suited to analyzing large samples of data in a fully automated way. We used both parametric and nonparametric approaches to fit the spectral energy distribution of the ICL and infer its physical properties, yielding a stellar mass ${\log }_{10}\left(M_{*}/M_{\odot }\right)$between 11.5 and 12.7 and an average age between 9.7 and 10.5 Gyr, from CIGALE and Prospector results. This implies that the ICL in RX J2129.7+0005 is, on average, older than that of disturbed clusters, suggesting that the contribution from different stellar populations to the ICL is at play depending on the cluster’s dynamical state. Coupled with X-ray observations of the hot gas distribution, we confirm the relaxed state of RX J2129.7+0005, showing clear signs of sloshing after a last major merger with a high-mass-ratio satellite that could have happened ∼6.6 Gyr ago in a relatively radial orbit. The presence of substructure in the ICL, such as shells, clouds with different densities and a certain degree of boxyness, and a clump, supports this scenario. 

Abstract The first James Webb Space Telescope (JWST) Near InfraRed Camera imaging in the field of the galaxy cluster PLCK G165.7+67.0 (z= 0.35) uncovered a Type Ia supernova (SN Ia) atz= 1.78, called “SN H0pe.” Three different images of this one SN were detected as a result of strong gravitational lensing, each one traversing a different path in spacetime, thereby inducing a relative delay in the arrival of each image. Follow-up JWST observations of all three SN images enabled photometric and rare spectroscopic measurements of the two relative time delays. Following strict blinding protocols which oversaw a live unblinding and regulated postunblinding changes, these two measured time delays were compared to the predictions of seven independently constructed cluster lens models to measure a value for the Hubble constant,H₀ =  71.8 + 9.2 − 8.1 km s⁻¹Mpc⁻¹. The range of admissibleH₀values predicted across the lens models limits further precision, reflecting the well-known degeneracies between lens model constraints and time delays. It has long been theorized that a way forward is to leverage a standard candle, but this has not been realized until now. For the first time, the lens models are evaluated by their agreement with the SN absolute magnifications, breaking degeneracies and producing our best estimate,H₀ =  ${75.7}_{-5.5}^{+8.1}$km s⁻¹Mpc⁻¹. This is the first precise measurement ofH₀from a multiply imaged SN Ia and only the second from any multiply imaged SN. 

Abstract Our understanding of galaxy properties and evolution is contingent on knowing the initial mass function (IMF), and yet to date the IMF is constrained only to local galaxies. Individual stars are now becoming routinely detected at cosmological distances, where luminous stars such as supergiants in background galaxies strongly lensed by galaxy clusters are temporarily further magnified by huge factors (up to 10⁴) by intracluster stars, thus being detected as transients. The detection rate of these events depends on the abundance of luminous stars in the background galaxy and is thus sensitive to the IMF and the star formation history (SFH), especially for the blue supergiants detected as transients in the rest-frame ultraviolet/optical filters. As a proof of concept, we use simple SFH and IMF models constrained by spectral energy distributions (SEDs) to see how well we can predict the Hubble Space Telescope and James Webb Space Telescope transient detection rate in a lensed arc dubbed “Spock” (z= 1.0054). We find that demanding a simultaneous fit of the SED and the transient detection rate places constraints on the IMF, independent of the assumed simple SFH model. We conclude that our likelihood analysis indicates that the data definitively prefers the “Spock” galaxy to have a Salpeter IMF (α = 2.35) rather than a top-heavy IMF (α = 1)—which is thought to be the case in the early universe—with no clear excess of supergiants above the standard IMF. 

We present a new analysis of the rest-frame ultraviolet (UV) and optical spectra of a sample of three z > 8 galaxies discovered behind the gravitational lensing cluster RX J2129.4+0009. We combine these observations with z > 7.5 galaxies from the literature, for which similar measurements are available. As already pointed out in other studies, the high [O iii]λ5007/[O ii]λ3727 ratios (O32) and steep UV continuum slopes (β) are consistent with the values observed for low-redshift Lyman continuum emitters, suggesting that such galaxies contribute to the ionizing budget of the intergalactic medium. We construct a logistic regression model to estimate the probability of a galaxy being a Lyman continuum emitter based on the measured MUV, β, and O32. Using this probability and the UV luminosity function, we construct an empirical model that estimates the contribution of high-redshift galaxies to reionization. The preferred scenario in our analysis shows that at z ∼ 8, the average escape fraction of the galaxy population [i.e. including both LyC emitters (LCEs) and non-emitters] varies with MUV, with intermediate UV luminosity (−19 < MUV < −16) galaxies having larger escape fraction. Galaxies with faint UV luminosity (−16 < MUV < −13.5) contribute most of the ionizing photons. The relative contribution of faint versus bright galaxies depends on redshift, with the intermediate UV galaxies becoming more important over time. UV bright galaxies, although more likely to be LCEs at a given log(O32) and β, contribute the least of the total ionizing photon budget. 

Abstract SN H0pe is a triply imaged supernova (SN) at redshiftz= 1.78 discovered using the James Webb Space Telescope. 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 0.6–5μm. 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 Siiiλ6355 and CaiiHλ3970 and Kλ3935. We next measure the absolute phases of the three images from our spectra, which allow 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 ${6.5}_{-1.8}^{+2.4}$days, ${24.3}_{-3.9}^{+3.9}$days, and ${50.6}_{-15.3}^{+16.1}$days 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 $-{122.3}_{-43.8}^{+43.7}$days and ${49.3}_{-14.7}^{+12.2}$days, 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 80 days instead of 60 days past maximum, and found a small (11.5 and 1.0 days, respectively) yet statistically insignificant (∼0.25σand ∼0.1σ) effect on the inferred image delays. 

Strong gravitational magnification enables the detection of faint background sources and allows researchers to resolve their internal structures and even identify individual stars in distant galaxies. Highly magnified individual stars are useful in various applications, including studies of stellar populations in distant galaxies and constraining dark matter structures in the lensing plane. However, these applications have been hampered by the small number of individual stars observed, as typically one or a few stars are identified from each distant galaxy. Here, we report the discovery of more than 40 microlensed stars in a single galaxy behind Abell 370 at redshift of 0.725 (dubbed ‘the Dragon arc’) when the Universe was half of its current age, using James Webb Space Telescope observations with the time-domain technique. These events were found near the expected lensing critical curves, suggesting that these are magnified stars that appear as transients from intracluster stellar microlenses. Through multi-wavelength photometry, we constrained their stellar types and found that many of them are consistent with red giants or supergiants magnified by factors of hundreds. This finding reveals a high occurrence of microlensing events in the Dragon arc and demonstrates that time-domain observations by the James Webb Space Telescope could lead to the possibility of conducting statistical studies of high-redshift stars.