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ABSTRACT We explore for the first time the possibility of detecting lensed star transients in galaxy–galaxy strong lensing systems upon repeated, deep imaging using the James Webb Space Telescope (JWST). Our calculation predicts that the extremely high recent star formation rate of $${\sim} 140\ {\rm M}_{\odot } \, \textrm {yr}^{-1}$$ over the last 50 Myr (not accounting for image multiplicity) in the ‘Cosmic Horseshoe’ lensed system ($z = 2.381$) generates many young, bright stars, of which their large abundance is expected to lead to a detection rate of $${\sim} 60$$ transients per pointing in JWST observations with a $$5\sigma$$ limiting magnitude of $${\sim} 29\, m_{\rm AB}$$. With the high expected detection rate and little room for uncertainty for the lens model compared with cluster lenses, our result suggests that the Cosmic Horseshoe could be an excellent tool to test the nature of dark matter based on the spatial distribution of transients, and can be used to constrain axion mass if dark matter is constituted of ultra-light axions. We also argue that the large distance modulus of $${\sim} 46.5\,$$ mag at $$z \approx 2.4$$ can act as a filter to screen out less massive stars as transients and allow one to better constrain the high-mass end of the stellar initial mass function based on the transient detection rate. Follow-up JWST observations of the Cosmic Horseshoe would allow one to better probe the nature of dark matter and the star formation properties, such as the initial mass function at the cosmic noon, via lensed star transients. 

The extreme magnification from galaxy clusters and microlenses therein allows for the detection of individual, luminous stars in lensed galaxies as transient events, and hence provides a valuable window into the high-mass stellar population inz > 1 galaxies. As these bright stars can only be formed at specific ages, the relative abundance of transient events at blue (B) and red (R) optical and near-infrared wavelengths (B/Rratio) can provide insights into the recent star formation history of galaxies that are not well constrained by their spectral energy distributions (SEDs). Here, we forward model the transient detection rates in an idealized mock scenario to find that theB/Rratio of strongly lensedz > 1 galaxies decreases quickly with increasing age. This ratio has a moderate sensitivity to metallicity and a comparatively low sensitivity to dust attenuation, with no significant dependency on the stellar initial mass function. Fitting model stellar populations to either the SED orB/Rratio alone of the “Warhol” arc (z = 0.94), we find that neither a simple single starburst nor more complex star formation can simultaneously reproduce both constraints. We then demonstrate that a best-fit model constrained by both theB/Rratio and SED requires a star formation rate that has varied quite dramatically over the past ∼50 Myr, for which the total stellar mass formed over this time is a factor of 10 (with 2 − 3σsignificance) different from the best-fit models to the SED alone. Our work shows that the transientB/Rratio can be used as an additional powerful constraint on the recent star formation history of higher-redshift galaxies in future works that are strongly lensed by galaxy clusters. 

Abstract Massive stars at cosmological distances can be individually detected during transient microlensing events, when gravitational lensing magnifications may exceedμ ≈ 1000. Nine such sources were identified in JWST NIRCam imaging of a single galaxy at redshiftz= 0.94 known as the “Warhol arc,” which is mirror imaged by the galaxy cluster MACS J0416.1−2403. Here we present the discovery of two coincident and well-characterized microlensing events at the same location followed by a third event observed in a single filter approximately 18 months later. The events can be explained by microlensing of a binary star system consisting of a red supergiant (T ≈ 4000 K) and a B-type (T ≳ 13,000 K) companion star. The timescale of the coincident microlensing events constrains the estimated projected source-plane size to tens of astronomical units. The most likely binary configurations consistent with the observational constraints on the temperature and luminosity of each star are stars with initial masses $M{1}_{\mathrm{init}}=23.6_{-4.3}^{+5.3}$M_⊙and an initial mass ratio between the two stars close to unity. A kinematic model that reproduces the observed light curves in all filters gives a relatively small transverse velocity of ∼50 km s⁻¹. This requires the dominant velocity component of several hundreds of kilometers per second to be roughly parallel to the microcaustic. An alternative possibility would be that the three microlensing events correspond to unrelated stars crossing distinct microcaustics, but this would imply a highly elevated rate of events at their common position, even though no underlying knot is present at the location. 

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. 

Highly magnified stars (μ>100) are now routinely identified as transient events at cosmological distances thanks to microlensing by intra-cluster stars near the critical curves of galaxy clusters. Using theJames WebbSpace Telescope (JWST) in combination with theHubbleSpace Telescope, we outline here an analytical framework that is applied to the Warhol arc (atz= 0.94) in the MACS 0416 galaxy cluster (atz= 0.396), where over a dozen microlensed stars have been detected to date. This method is general and can be applied to other lensed arcs. Within this lensed galaxy, we fit the spatially resolved spectral energy distribution spanned by eight JWST-NIRCam filters combined with three ACS filters, for accurate lensed star predictions in 2D. With this tool we can generate 2D maps of microlensed stars for well-resolved arcs in general, incorporating wavelength dependence and limiting apparent magnitude. These maps can be directly compared with planned cadenced campaigns from JWST and Hubble, offering a means to constrain the initial mass function and the level of dark matter substructure. 

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. 

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. 

Dark matter subhalos with extended profiles and density cores, and globular star clusters of mass 10⁶–10⁸M_⊙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. 

Recent observations of caustic-crossing galaxies at redshift 0.7 ≲ z ≲ 1 show a wealth of transient events. Most of them are believed to be microlensing events of highly magnified stars. Earlier work predicts such events should be common near the critical curves (CCs) of galaxy clusters (“near region”), but some are found relatively far away from these CCs (“far region”). We consider the possibility that substructure on milliarcsecond scales (few parsecs in the lens plane) is boosting the microlensing signal in the far region. We study the combined magnification from the macrolens, millilenses, and microlenses (“3M lensing”), when the macromodel magnification is relatively low (common in the far region). After considering realistic populations of millilenses and microlenses, we conclude that the enhanced microlensing rate around millilenses is not sufficient to explain the high fraction of observed events in the far region. Instead, we find that the shape of the luminosity function (LF) of the lensed stars combined with the amount of substructure in the lens plane determines the number of microlensing events found near and far from the CC. By measuringβ(the exponent of the adopted power law LF,dN/dL = ϕ(L)∝(1/L)^β), and the number density of microlensing events at each location, one can create a pseudoimage of the underlying distribution of mass on small scales. We identify two regimes: (i) positive-imaging regime whereβ > 2 and the number density of events is greater around substructures, and (ii) negative-imaging regime whereβ < 2 and the number density of microlensing events is reduced around substructures. This technique opens a new window to map the distribution of dark-matter substructure down to ∼10³ M_⊙. We study the particular case of seven microlensing events found in the Flashlights program in the Dragon arc (z = 0.725). A population of supergiant stars having a steep LF withβ = 2.55_−0.56^+0.72fits the distribution of these events in the far and near regions. We also find that the new microlensing events from JWST observations in this arc imply a surface mass density substructure of Σ_∗= 54M_⊙pc⁻², consistent with the expected population of stars from the intracluster medium. We identify a small region of high density of microlensing events, and interpret it as evidence of a possible invisible substructure, for which we derive a mass of ∼1.3 × 10⁸ M_⊙(within its Einstein radius) in the galaxy cluster. 

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.