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On 2022 July 8, NASA shared (https://www.nasa.gov/feature/goddard/2022/nasa-shares-list-of-cosmic-targets-for-webb-telescope-s-first-images) the list of five public showcase targets that have been observed with the new James Webb Space Telescope (JWST) and whose data—at the time of writing—are expected to be released to the public around Tuesday, July 12. One of these targets is the galaxy cluster SMACS J0723.3−7327 (z = 0.39), which acts as a gravitational lens and was recently imaged with the Hubble Space Telescope (HST) in the framework of the Reionization Lensing Cluster Survey (RELICS). To facilitate studies by the community with the upcoming JWST data, we publish here a strong-lensing model for SMACS J0723.3−7327—including mass density and magnification maps. We identify five multiple-image families in the HST imaging. For three of them, system membership and redshift are secured by public spectroscopic data. For the remaining two systems, we rely on robust photometric redshift estimates. We use the Light-Traces-Mass lens modeling method, which complements the parametric models already available in the RELICS repository and elsewhere and thus helps span a representative range of solutions. The new model published here can be accessed on the RELICS website at MAST. It will be interesting to examine which properties of the mass models change and improve, and by how much, when the JWST data are incorporated.

A Type Ia supernova (SN) at z = 1.78 was discovered in James Webb Space Telescope Near Infrared Camera imaging of the galaxy cluster PLCK G165.7+67.0 (G165; z = 0.35). The SN is situated 1.5–2 kpc from the host-galaxy nucleus and appears in three different locations as a result of gravitational lensing by G165. These data can yield a value for Hubble’s constant using time delays from this multiply imaged SN Ia that we call “SN H0pe.” Over the cluster, we identified 21 image multiplicities, confirmed five of them using the Near-Infrared Spectrograph, and constructed a new lens model that gives a total mass within 600 kpc of (2.6 ± 0.3) × 1014 M ⊙. The photometry uncovered a galaxy overdensity coincident with the SN host galaxy. NIRSpec confirmed six member galaxies, four of which surround the SN host galaxy with relative velocity ≲900 km s−1 and projected physical extent ≲33 kpc. This compact galaxy group is dominated by the SN host galaxy, which has a stellar mass of (5.0 ± 0.1) × 1011 M ⊙. The group members have specific star formation rates of 2–260 Gyr−1 derived from the Hα-line fluxes corrected for stellar absorption, dust extinction, and slit losses. Another group centered on a strongly lensed dusty star-forming galaxy is at z = 2.24. The total (unobscured and obscured) SFR of this second galaxy group is estimated to be (≳ 100 M ⊙ yr−1), which translates to a supernova rate of ∼1 SNe yr−1, suggesting that regular monitoring of this cluster may yield additional SNe.

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) at z = 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−1 Mpc−1. The range of admissible H0 values 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−1 Mpc−1. This is the first precise measurement of H0 from a multiply imaged SN Ia and only the second from any multiply imaged SN.

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 totaling 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 a pair of compact knots that are magnified by a factor of hundreds, 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 through the following link (https://www.dropbox.com/sh/gwup2lvks0jsqe5/AAC2RRSKce0aX-lIFCc9vhBXa?dl=0) and will be regularly updated using additional spectroscopic redshifts. The model is constrained by 16 of these sets of multiply imaged galaxies, three of which have spectroscopic redshifts, and reproduces the multiple images to better than an rms of 0.″5, allowing 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.

Dark matter subhalos with extended profiles and density cores, and globular star clusters of mass 106–108 M ⊙ that live near the critical curves in galaxy cluster lenses can potentially be detected through their lensing magnification of stars in background galaxies. In this work, we study the effect such subhalos have on lensed images, and compare to the case of more well-studied microlensing by stars and black holes near critical curves. We find that the cluster density gradient and the extended mass distribution of subhalos are important in determining image properties. Both lead to an asymmetry between the image properties on the positive- and negative-parity sides of the cluster that is more pronounced than in the case of microlensing. For example, on the negative-parity side, subhalos with cores larger than about 50 pc do not generate any images with magnification above ∼100 outside of the immediate vicinity of the cluster critical curve. We discuss these factors using analytical and numerical analysis, and exploit them to identify observable signatures of subhalos: Subhalos create pixel-to-pixel flux variations of ≳0.1 mag on the positive-parity side of clusters. These pixels tend to cluster around (otherwise invisible) subhalos. Unlike in the case of microlensing, signatures of subhalo lensing can be found up to 1″ away from the critical curves of massive clusters.

Mass estimates of black holes (BHs) in the centers of active galactic nuclei (AGNs) often rely on the radius–luminosity relation. However, this relation, usually probed by reverberation mapping (RM), is poorly constrained in the high-luminosity and high-redshift ends due to the very long expected RM lag times. Multiply imaged AGNs may offer a unique opportunity to explore the radius–luminosity relation at these ends. In addition to comprising several magnified images enabling a more efficient light-curve sampling, the time delay between multiple images of strongly lensed quasars can also aid in making such RM measurements feasible on reasonable timescales: if the strong-lensing time delay is, for example, of the order of the expected RM time lag, changes in the emission lines in the leading image can be observed around the same time as the changes in the continuum in the trailing image. In this work we probe the typical time-delay distribution in galaxy-cluster lenses and estimate the number of both high-mass (∼109−1010 M ⊙) and high-redshift (z ≳ 4−12) quasars that are expected to be strongly lensed by clusters. We find that up to several tens of thousands of M BH ∼ 106–108 M ⊙ broad-line AGNs at z > 4 should be multiply imaged by galaxy clusters and detectable with JWST, hundreds with Euclid, and several thousand with the Roman Space Telescope, across the whole sky. These could supply an important calibration for the BH mass scaling in the early Universe.

SN H0pe is a triply imaged supernova (SN) at redshift z = 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 0.6–5 μm. 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 λ6355 and Ca ii H λ3970 and K λ3935. 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 (∼0.25σ and ∼0.1σ) effect on the inferred image delays.

Many microlensed stars discovered by JWST closely follow the winding critical curve of A370 along the “Dragon Arc” with mAB > 26.5, which we show comprises asymptotic giant branch stars microlensed by the observed level of diffuse cluster stars, corresponding to ≃1% of the dark matter density. Most events appear along the inner edge of the critical curve, following an asymmetric band of width ≃4.5 kpc that is skewed by −0.7 ± 0.2 kpc. This asymmetry, we argue, follows from the parity difference in caustic structure inherent to microlensing that extends to higher magnification in the negative parity regions. This parity difference predicts a modest net shift of −0.04 kpc to the inside of the cluster critical curve within a narrower band of ≃1.4 kpc than observed. Adding cold-dark-matter-like subhalos of 106−8 M⊙ doubles the width, but detections are predicted to favor the outside of the critical curve, where the subhalos generate local Einstein rings, and subhalos inside the critical curve depress the magnification, reducing microlensing. Instead, the density perturbations of “wave dark matter” as a Bose–Einstein condensate (ψDM) can generate a wide band of corrugated critical curves with a large negative asymmetry. We find that a de Broglie wavelength of ≃10 pc reproduces the observed width of 4.5 kpc, with a negative skewness ≃−0.6 kpc, like the data, corresponding to a boson mass of ≃10−22 eV, in agreement with dwarf galaxy dynamical estimates. Independently, we also find clear asymmetry in the Jupiter Arc, with 12 microlensed stars lying along the inside of the critical curve, like the Dragon Arc.

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.

Massive galaxy clusters act as prominent strong lenses. Due to a combination of observational biases, cluster evolution, and lensing efficiency, most of the known cluster lenses lie typically at zl ∼ 0.2–0.7, with only a few prominent examples at higher redshifts. Here we report the first strong-lensing analysis of the massive galaxy cluster SPT-CL J0546-5345 at a redshift zl = 1.07. This cluster was first detected through the Sunyaev–Zel’dovich effect, with a high estimated mass for its redshift of M200,c = (7.95 ± 0.92) × 1014 M⊙. Using recent JWST/NIRCam and archival Hubble Space Telescope imaging, we identify at least 10 secure and 6 candidate sets of multiply imaged background galaxies, which we use to constrain the mass distribution in the cluster. We derive effective Einstein radii of θE = 18 .″ 1 ± 1 .″ 8 for a source at zs = 3 and θE = 27 .″ 9 ± 2 .″ 8 for a source at zs = 9. The total projected mass within a 200 kpc radius around the strong-lensing region is M (<200 kpc) = (1.9 ± 0.3) × 1014 M⊙. While our results rely on photometric redshifts warranting spectroscopic follow-up, this central mass resembles that of the Hubble Frontier Fields clusters—although SPT-CL J0546-5345 is observed when the Universe was ∼3–4 Gyr younger. Amongst the multiply imaged sources, we identify a hyperbolic-umbilic-like configuration, and, thanks to its point-like morphology, a possible active galactic nucleus (AGN). If confirmed spectroscopically, it will add to just a handful of other quasars and AGN known to be multiply lensed by galaxy clusters.