arXiv subjects

Jose M. Diego

Publications and source records attributed to Jose M. Diego.

At least 19 recordsLinked to original sources

arXiv2026

VENUS: Strong Lens Mass Models and Einstein Radii of 60 Galaxy Clusters

We present the strong lens mass modeling result of 60 galaxy clusters targeted by Vast Exploration for Nascent, Unexplored Sources (VENUS), the James Webb Space Telescope (JWST) multi-epoch Treasury program. We obtain refined mass models for the 60 clusters with the glafic software by fitting positions of $\sim 4500$ multiple images, $\sim 1150$ of which are newly identified multiple images with help of multi-band JWST images. Each cluster contains typically $\sim 50$ multiple images, thanks to the selection of target clusters based on their high lensing strengths. We validate our models with the Jackknife method. The median Einstein radius is $\sim 30''$ and the distribution of the Einstein radii extends out to $\sim 60''$ for the source redshift of 9, which confirms that their lensing strengths are indeed among the highest known to date. For the source redshift of 9, we identify MACS J0717.5+3745, MACS J0600.1$-$2008, and PLCK G287.0+32.9 as clusters with the largest Einstein radii as well as the largest areas of the high-magnification ($|μ|>10$) regions. We find that the observed Einstein radii of VENUS clusters are fully compatible with the standard cold dark matter model.

astro-ph.CO↗

arXiv2026

SN Helios: A Multiply Imaged Type II Supernova Opening Time-Delay Cosmography beyond redshift of 3

We report the discovery and spectroscopic confirmation of SN Helios, a strongly lensed, multiply imaged Type II supernova (SN) at $z=3.34$ behind the galaxy cluster RXC J0018.5+1626. The transient was identified as a red F150W-dropout source (F277W = 26.1 mag) in JWST/NIRCam imaging taken on 2026 July 3 for the VENUS survey, with no counterpart in 2024 December imaging. Director's Discretionary Time observations on 2026 August 17 added second-epoch NIRCam imaging and a deep NIRSpec prism spectrum. The spectrum shows a strong, spectrally resolved H$α$ emission line (FWHM $\approx 7900$ km/s) with a P-Cygni absorption blueshifted by 8800 km/s, together with corresponding H$β$ and He I $λ$10830 features, establishing a hydrogen-rich photosphere at $z=3.34$. Template fits classify SN Helios as a Type II SN, and light-curve comparisons with well-studied local SNe II yield a phase of $27.5\pm2.8$ rest-frame days past B-band maximum. Independent cluster lens models magnify the observed image by $μ\approx 10$-20 and predict another image within $\simeq 2$-6 yr, offering a rare opportunity for a time-delay measurement of $H_{0}$ with a source at $z>3$. The host galaxy is undetected in deep rest-frame ultraviolet and optical imaging, in emission lines, and in ALMA 2 mm dust continuum, leaving a heavily obscured counterpart unlikely. Its de-lensed $M_{\rm UV}\gtrsim -13$ places it far below blank-field limits and adds support to an elevated core-collapse rate per unit star formation in ultra-faint galaxies. Without the multi-epoch data, SN Helios would masquerade as a convincing $z\approx 16$ Lyman-break galaxy, with galaxy-template fits strongly disfavoring any low-redshift solution even with multiple medium bands. Its spectrum thus provides an empirical SED template with which high-redshift searches can vet similar dropout candidates.

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arXiv2026

(Monty Python and the) HoliGRALE: A Hybrid GRALE Lens Inversion Methodology Diagnostically Evaluated with Synthetic and Real Data

We present HoliGRALE (Hybrid, only light included, GRAvitational LEnsing), a hybrid lens modeling extension of the freeform method GRALE. HoliGRALE incorporates physically motivated parametric components within the freeform genetic algorithm GRALE. Using three synthetic clusters from IllustrisTNG, each of varying complexity, we evaluate its performance according to its accuracy in reconstructing the density distribution, image positions, magnifications, and time delays. We find that HoliGRALE generally outperforms GRALE on all fronts, and shows greater accuracy in regions containing a high image number density. We estimate the error budget in projected density to be $<5\%$ for simple, few component clusters, and $\sim5-10\%$ for complex, multi-component clusters in the regions constrained by observed images. We then use HoliGRALE to model three lenses: RX J2129.7$+$0005, MACS J0138.0$-$2155, and Abell S1063. In each case, HoliGRALE is able to accurately reconstruct observed magnifications and time delays. In its predictions, HoliGRALE is found to be more precise than GRALE. We particularly highlight the predicted time delay for the next arriving image of SN Requiem to be $4346.61^{+490.27}_{-818.36}$ days ($\approx$ April 2026 -- October 2029), consistent with previous work, albeit with less precision. In Abell S1063, we report the reconstruction of a main dark matter halo core radius of $89.9\pm33.2$ kpc, in excellent agreement with past work and useful in testing theories of dark matter. Our results emphasize the expected error budget of HoliGRALE, which should be mentioned and scrutinized in future applications of HoliGRALE for other lens systems.

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arXiv2026

JWST evidence for a sharp "Cosmic Daybreak" at z = 15

Luminous young galaxies have been uncovered with relative ease by JWST, extending to z=14.5, so it is puzzling that deeper spectroscopy of fainter candidates now finds only interlopers. This redshift `ìmpasse" is underscored by the measured stellar ages of these high-z galaxies, which we show converge to zero by z=15, with a marked absence of earlier star-formation. Taken literally, such a late transition from the Dark Ages to luminous galaxies is unlike the gradual Cosmic Dawn of standard LCDM, but does confirm a key prediction of Wave Dark Matter, $ψ$DM, as a Bose-Einstein condensate. The de Broglie wave pressure resists gravity until a substantial Jeans mass of $4\times 10^9M_\odot$ is overcome at z$=$15, corresponding to a light boson $m_ψ=2.2_{-0.3}^{+0.4} \times 10^{-22}$eV, and similar to independent estimates from lensing anomalies and dwarf galaxies. Furthermore, the substantial luminosities of the highest redshift galaxies appear to converge to the initial Jeans scale of $ψ$DM, whereas LCDM predictions extend to lower luminosities and larger ages than observed. These contrasting predictions can be definitively tested as JWST observations accumulate, with diametric implications for Dark Matter as heavy particles beyond the Standard Model or ultra-light bosons motivated by the String Axiverse.

astro-ph.CO↗

arXiv2026

Observing population III stars in the pristine region

The detection of the long-sought Population III (Pop III) stars is, perhaps, one of the most intriguing missing links in modern astronomy. Caustic crossing events (CCE) -- extreme magnification at the critical curves of galaxy clusters of individual stars at cosmological distances -- are postulated to allow for the direct detection of individual Pop III stars. However, in the usual CCE regime, the abundant existence of intracluster stars as microlenses limits the maximum magnification attainable, thus limiting the theoretical detection of luminous stars to relatively low redshifts, significantly smaller than the redshifts where Pop III stars are expected to exist. Here in this letter, we propose to observe Pop III stellar populations as CCEs in the pristine regions of merging clusters. Pristine regions are defined as areas in the lens where the intracluster light, and hence the corresponding stelalr microlenses, is exceptionally low. In thes regions, the low density of microlenses allows background stellar populations to achieve magnification closer to the theoretical limit (whitout microlenses). We show that the maximum magnification in the pristine region scales inversely with the surface mass density of stellar microlenses. Adopting Pop III isochrones, regular James-Webb $\sim29\,$mag observations are sufficient to detect individual Pop III stars in pristine regions, offering a unique observational strategy to detect Pop III stars with the incoming cluster observations. The intermediate regions between high-redshift merging clusters (such as El Gordo) are naturally devoid of microlenses and constitute exceptional targets for the search of Pop III stars.

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arXiv2026

PEARLS: NuSTAR and XMM-Newton Extragalactic Survey of the JWST North Ecliptic Pole Time-Domain Field IV: X-Ray Variability Analysis

NuSTAR and XMM-Newton have observed the James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-Domain Field (TDF) for almost five contiguous years starting in 2019. In that time, the NEP X-ray survey has accumulated 3.5 Ms and 228 ks of quasi-simultaneous NuSTAR and XMM-Newton observations, respectively. This paper presents variability results for the 112 NuSTAR and 453 XMM-Newton sources detected in this field, based solely on the X-ray photometric data. Four NuSTAR sources and 11 XMM-Newton sources varied in at least one band at >=99% confidence. The sources with redshift measurements show a relationship between luminosity and variability with 74% of variable sources brighter than 5x the sensitivity limit of the survey. This is supported by about 1/3 of sources with more than 400 counts detected being variable and only 4 sources with fewer counts showing variability. Variability timescales are not well determined, but variability amplitude tends to be larger on longer timescales.

astro-ph.HE↗

arXiv2026

Spatially Resolved Physical Properties of Young Star Clusters and Star-forming Clumps in the Brightest z>6 Galaxy, the Strongly Lensed Cosmic Spear at z=6.2

We present spatially resolved analysis of stellar populations in the brightest $z>6$ galaxy known to date (AB mag 23), the strongly lensed MACS0308$-$zD1 (dubbed the ``Cosmic Spear'') at $z_{\rm spec}=6.2$. New JWST NIRCam imaging and high-resolution NIRSpec IFU spectroscopy span the rest-frame ultraviolet to optical. The NIRCam imaging reveals bright star-forming clumps and a tail consisting of three distinct, extremely compact star clusters that are multiply-imaged by gravitational lensing. The star clusters have delensed effective radii of $R_{\rm{eff}} \lesssim 8$ pc, stellar masses of $M_{*} \sim 10^{6}-10^{7}\,M_{\odot}$, and high stellar mass surface densities of $Σ_{*} \gtrsim 2\times 10^{4}\,M_{\odot}~\rm{pc}^{-2}$. While their stellar populations are very young ($\sim 6-11$ Myr), their dynamical ages exceed unity, consistent with the clusters being gravitationally bound systems. Placing the star clusters in the size vs.~stellar mass density plane, we find they occupy a region similar to other high-redshift star clusters within galaxies observed recently with JWST, being significantly more massive and denser than local star clusters. Spatially resolved analysis of the brightest clump reveals a compact, intensely star-forming core. The ionizing photon production efficiency ($ξ_{\rm{ion}}$) is slightly suppressed in this central region, potentially indicating a locally elevated Lyman continuum escape fraction facilitated by feedback-driven channels.

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arXiv2026

Extragalactic microlensing through Ultra Diffuse Galaxies

Stellar microlensing is a powerful method to constrain compact dark matter models, uncover binary stars, and exoplanets during caustic crossing events. At cosmological distances, {\it James-Webb Space Telescope} ({\it JWST}) is routinely detecting microlensed giant stars in highly magnified galaxies behind massive lensing clusters. Here, we explore for the first time microlensing in modest redshift galaxies commonly seen through local Ultra Diffuse Galaxies (UDGs). Using the UDG NGC1052-DF2 as a proof-of-concept, we find that detecting microlensing events through UDGs is possible. Euclid is ideal for identifying samples of low-redshift star-forming galaxies seen through local galaxies for deeper cadenced follow-up, where our zeroth-order calculation estimates that $\mathcal{O}(1-10)$ events per year are expected over the whole sky under the monitoring of LSST, assuming all UDGs share similar distance and stellar content as NGC1052-DF2. Such measurements can help settle the DM issue for UDG galaxies by providing a stellar mass estimate weighted more by the low end of the stellar initial mass function (IMF) than standard stellar synthesis estimates. Note, lower efficiency is estimated for {\it JWST} of $\sim 5.6\times10^{-2}\,\textrm{yr}^{-1}$ over its five background galaxies is expected for typical {\it JWST}$\sim29$\,mag visits, and a low Vera Rubin Legacy Survey of Space and Time (LSST) detection rate of$\sim 0.03\,\textrm{yr}^{-1}$ such that NGC1052-DF2 might not be a prime target given its lack of low-redshift background galaxies.

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arXiv2026

Constraints on the Pop III Sky Surface Brightness from High-Redshift Caustic Transients in MACS0416

Population III (Pop III) stars are hypothetical zero-metallicity stellar structures formed from primordial hydrogen and helium. They are theorized to span a wide mass range, extending to several 100 solar masses, and may have played important roles in nucleosynthesis and reionization. Their expected fluxes are far below JWST NIRCam detection limits, making direct observation unlikely. However, extreme magnification near the caustics of massive foreground galaxy clusters may enable their detection. We search for overlooked high-redshift (7 <= z <= 17) caustic transits in the lensing cluster MACS J0416.1-2403. Using three observations spanning 126 days, we create difference images to identify potential candidates. Critical curves for sources at 7 <= z <= 17, derived from a strong-lensing model, guided visual inspection of three difference-images. No additional transits were found. The longer caustics in our model sweep a larger source-plane area, increasing the probability of detecting an event. Combining this increased statistical sensitivity with deeper imaging, we establish a fainter limit for the unresolved stellar population at z >= 7. From this null result, we constrain the 100 solar mass 2 micron Pop III sky surface brightness to >= 32.8 +/- 0.6 mag arcsec^-2. Modeling the non-detection as a Poisson process gives a posterior mean caustic-transit rate of 0.29 cluster^-1 yr^-1 and a 95 percent upper credible limit of lambda_95 = 0.86 cluster^-1 yr^-1. The inferred rate remains consistent with the adopted fiducial Pop III caustic-transit model and provides an empirical benchmark for future multi-epoch monitoring campaigns. This search exploits the fact that individual stars projected close to source-plane caustics can be briefly magnified far beyond their unlensed fluxes. These constraints provide a direct test of the abundance of luminous Pop III stars at early cosmic times.

astro-ph.CO↗

arXiv2026

SN 2022riv in RX J2129: Discovery, Spectroscopic Classification, and Microlensing of a Strongly Lensed Type Ia Supernova from JWST and HST Observations

The multiply imaged SN 2022riv was discovered through a search of galaxy cluster fields as part of a Hubble Space Telescope (HST) SNAP program to find highly magnified stars. The supernova (SN) was detected in the image corresponding to the longest time delay of a galaxy at redshift $z=1.522$ strongly lensed by the foreground galaxy cluster RX J2129.7+0005. Follow up James Webb Space Telescope (JWST) NIRSpec G140M and PRISM spectroscopy yields a Type Ia SN classification. Using the SALT3-NIR light-curve fitter, we obtain a cosmology-independent measurement of the magnification of $5.35\pm1.01$ for the last-to-arrive image of the SN, with multiple SALT SN spectral time-series models yielding consistent constraints. The last-to-arrive image of SN 2022riv we detect appeared adjacent to the brightest cluster galaxy (BCG) at a location with an exceptionally high stellar mass density ($\sim 1-2$ dex higher than that of SN Refsdal), where microlensing is expected to introduce a 20-50% modulation of the magnification. Analyzing six independent lens models of the cluster, we find that four predict the magnification with much greater precision ($p < 0.05$) than would be expected by random chance, given the large effect anticipated from microlensing. Five models yield magnifications of roughly $4-7$ (within $1σ$) prior to accounting for microlensing, whereas HoliGRALE favors a significantly higher value of $15.39 \pm 0.85$. After incorporating nominal microlensing, the HoliGRALE prediction is within $1σ$ tension with our measurement. A companion paper (Dalrymple et al.) will present constraints on the relative time delay of the image that arrived earlier.

astro-ph.CO↗

arXiv2026

Gamma Rays from ALP-Photon Conversion and Inverse Compton Reprocessing in Neutron Star Magnetospheres

Exploring axion-like particle (ALP) signatures from neutron stars (NSs) in the \emph{Fermi}-LAT energy range remains largely unexplored. Neutron stars with exceptionally strong magnetic fields, such as magnetars and pulsars with magnetar-like magnetic fields, provide particularly promising environments for ALP--photon conversion. Magnetars are characterized by surface magnetic fields as large as $B_0\sim(10^{14}$--$10^{15})\,\mathrm{G}$; however, despite their extreme magnetic fields, no steady magnetar emission has been firmly detected in the \emph{Fermi}-LAT energy range, with high-energy activity generally associated with rare flaring episodes. In this work, we investigate ALP production in the interiors of different classes of NSs and the subsequent conversion of ALPs into photons in their magnetospheres. The ALP emissivity is determined by the stellar density and temperature $T$, while the conversion probability is enhanced by the strong magnetic fields surrounding the star. We further account for photon propagation through the Galactic magnetic field, which can provide an additional contribution to the observable photon flux. We investigate the resulting gamma-ray signatures and assess whether ALP-induced emission from NS magnetospheres could be detectable at energies $E\gtrsim100,\mathrm{MeV}$ in the \emph{Fermi}-LAT band. In addition, we consider if the reprocessing of the magnetospheric photons through inverse Compton scattering can shift part of the emission to higher energies and provide an additional observational signature. We use the resulting fluxes to derive constraints from existing gamma-ray observations and to estimate the sensitivity of future MeV--GeV observations, taking COSI as a representative example.

astro-ph.HE↗

arXiv2026

Mirror images of lensed star clusters with mismatched spectral energy distributions: A possible signature of top-heavy stellar initial mass functions and extreme stars in high-redshift star clusters

Strongly lensed star clusters have recently been detected up to redshift $z\approx 10$ in galaxy cluster fields using the James Webb Space Telescope (JWST). When pairs of mirror images of such star clusters appear across the lensing critical curve, it is usually assumed that both images will display identical spectral energy distributions (SEDs). However, this assumption may be invalidated in the presence of gravitational microlensing from stars or other compact objects in the lens, since microlensing will affect the SED contribution from bright stars within the star cluster independently in the two mirror images. Here, we explore under what circumstances mismatched mirror-image SEDs are likely to be observable, and argue that SED differences detectable in JWST observations of lensing-cluster fields will be limited to star clusters of mass $< 10^5\ M_\odot$ and ages $\lesssim 5$ Myr. The probability of severely mismatched mirror-image SEDs increases if the stellar initial mass function is very top-heavy and extends to stellar masses $\gg 100\ M_\odot$, as has been suggested to be the case for Population III stars. The prevalence of lensed star clusters with highly discrepant mirror-image SEDs could therefore serve as a probe of very massive stars and extreme stellar populations in the early Universe.

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arXiv2026

A comprehensive separation of dark matter and baryonic mass components in galaxy clusters I: Mass constraints from Abell S1063

In this two-part series, we present a multi-probe mass modelling method for massive galaxy clusters, designed to disentangle the contributions of individual mass components (Dark matter, intra-cluster gas, stellar masses). In this first paper, we focus on recovering the mass constraint datasets required for the modelling approach introduced in the second paper. Specifically, we measure the light distribution, stellar mass, and kinematics of the cluster members, the brightest cluster galaxy (BCG), and the intra-cluster light (ICL) in Abell S1063. To that end, we developed a new method to extract the light profiles of the cluster members, BCG, and ICL, while accounting for contamination from nearby foreground and background galaxies in \textsc{Hubble Space Telescope} (HST) imaging. We obtained light profiles for $289$ cluster members using a dual Pseudo-Isothermal Elliptical (dPIE) model based on the HST F160W filter, while the BCG \& ICL is modelled as a single component using a multi-Gaussian expansion. To estimate stellar masses and velocity dispersions, we rely on multi-band HST photometry and \textsc{VLT/MUSE} integral field spectroscopy, respectively. Stellar masses are derived using three different spectral energy distribution (SED) models. We measure the line-of-sight velocity dispersions of the cluster members at their half-light radii, as determined from their light profiles, while for the BCG \& ICL components, we use elliptical annular apertures. Thanks to these measurements, we will be able to constrain the cluster stellar mass content, which is detailed in the second paper of the series. We publicly release these measurements with intermediary data products.

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arXiv2026

A comprehensive separation of dark matter and baryonic mass components in galaxy clusters II: an overview of the mass distribution in Abell S1063

In the first paper of this series, we derived mass constraints on the total mass and the baryonic components of the galaxy cluster Abell S1063. The main focus was to recover stellar masses and kinematics for cluster members, the brightest cluster galaxy (BCG) and the intra-cluster light (ICL). In this second paper, we introduce a multi-probe mass modelling approach that incorporates constraints on both the total mass and the individual baryonic components. We obtain comprehensive mass models of Abell S1063, in which the dark matter distribution is disentangled from the baryonic mass at both cluster and galaxy scales. The best-fitting mass model achieves an RMS of $0.50"$ on the multiple image positions. The kinematic profiles of the BCG \& ICL, as well as the X-ray surface brightness of the intra-cluster gas, are accurately reproduced within observational uncertainties. However, a $35~\mathrm{km/s}$ scatter is required for the cluster member line-of-sight dispersions. This method yields the most complex parametric mass model with consistency among almost all available mass constraints. We find a $1σ$ agreement between the inferred stellar-to-subhalo mass relation and that predicted by large-scale cosmological simulations. The ICL stellar mass derived from our model is consistent with estimates from stellar population modelling. We present the first multi-probe mass modelling method capable of disentangling the dark matter from the baryonic mass distributions in massive galaxy clusters. Its results, such as the stellar-to-subhalo mass relation or the distribution of each mass component, can be directly compared to hydrodynamical cosmological simulations such as illustrisTNG.

astro-ph.CO↗

arXiv2026

VENUS: A Strongly Lensed Clumpy Galaxy at $z\sim11-12$ behind the Galaxy Cluster MACS J0257.1-2325

We present the discovery of a strongly lensed galaxy at $z\sim11-12$, dubbed the ``Misty Moons'', identified in the JWST Treasury Survey, Vast Exploration for Nascent, Unexplored Sources (VENUS). The Misty Moons is gravitationally lensed by the galaxy cluster MACS J0257.1-2325 at $z=0.505$, and has five multiple images suggested by two independent lensing models. Two of the five images, ID1 and ID2 ($μ\sim 20-30$), are very bright (F200W$\sim26$ AB mag) and exhibit blue SEDs with prominent Ly$α$ breaks. In the source plane, the Misty Moons is a sub-$L^*$ galaxy ($M_{\rm UV}\sim-18.0$ mag) resolved into multiple stellar clumps, each of which has an effective radius of $r_\mathrm{eff}\sim 10-70$ pc and a stellar mass of $\sim10^7\ M_\odot$. These clumps dominate the stellar mass budget of the Misty Moons ($\gtrsim80\%$), similar to other high-$z$ clumps, which suggests a highly clustered mode of star formation in the early Universe, unlike seen in local dwarf galaxies. We convolve the source-plane image with the JWST/NIRCam point-spread function to produce a mock NIRCam image of the Misty Moons without lensing magnification, and find that the intrinsic galaxy has a radial surface-brightness profile comparable to those of $z\gtrsim10$ faint galaxies, such as JADES-GS-z13-0 and JADES-GS-z14-1, indicating that the Misty Moons represents a typical $z\gtrsim10$ faint galaxy. The Misty Moons, a lensed galaxy with resolved internal structures, provides an ideal laboratory for exploring the early stages of galaxy formation at $z\gtrsim10$.

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arXiv2026

Two Exciting High-redshift Galaxy Candidates Turn Out to Be Two Exciting Ultra-cool Brown Dwarfs

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts $z>10$ and as high as $z=14$. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at $z>15$ may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright $z\sim 14$ galaxies should be visible at redshifts as high as $z\sim 20-30$, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures ${272\mbox{--}351\mbox{K}}$ and ${445\mbox{--}525\mbox{K}}$ (using ATMO2020 and Sonora Elf Owl models) and distances of $\sim 150\mbox{--}650\mbox{pc}$. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken $\sim 1$ year later, we also detect their proper motions of $(49 \pm 8)\,\mbox{mas/yr}$ and $(24 \pm 3)\,\mbox{mas/yr}$, further indicating that at least some F277W and F356W dropouts are sub-stellar cold Milky Way objects such as brown dwarfs.

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arXiv2026

Kinematic Mapping of Giant Arcs: A New Method to Locate Lensing Critical Curves

The vicinity of lensing critical curves features highly magnified portions of lensed galaxies. Accurate knowledge of the location and shape of the critical curve will be useful for understanding the nature of highly magnified stellar sources near critical curves and for revealing sub-galactic dark matter structures within the lens. In galaxy-cluster lenses, however, prediction of critical curves can be uncertain due to complexity in global mass modeling. We explore and validate a kinematics-based method for locating the critical curve. This method leverages the continuous line-of-sight velocity profile of the lensed galaxy mapped through integral field spectroscopy of emission lines, and combines an agnostic local lens model and a disk rotation model. Applying our method to a highly magnified region of the Dragon Arc in the Abell 370 cluster lensing field using archival VLT/MUSE IFU mapping of the H$β$ line, we constrain the critical curve to an uncertainty band with a half-width of 0.23" ($1σ$). This result reveals locations of recently detected extremely magnified stars biased toward the negative-parity side of the critical curve, as predicted for intracluster microlensing. With future JWST/NIRSpec IFU mapping of the H$α$ line at SNR $\simeq$ 10 (20), uncertainty could improve to 0.12" (0.08"). A measurement of this type with sufficiently small uncertainty may reveal small-scale wiggles in the shape of the critical curve, which can arise from the lensing perturbation of sub-galactic dark matter substructure. Our approach is generally applicable to caustic-crossing giant arcs and can be incorporated into global lens modeling.

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arXiv2026

The Subversive Role of Excessive External Shear in Concealing Lensing Anomalies

To best reproduce observed multiply-lensed lensed images, lens models usually incorporate shear attributed to objects unrelated to the lensing galaxy (i.e., external shear): whether it be neighbouring galaxies not explicitly included in the lens model or other cosmic structures along the sightline. When constrained solely by the positions of image counterparts, such lens models, even those utilising simple ellipsoidal mass distributions, can satisfactorily -- if not near perfectly -- reproduce the observed image positions, but often leave significant differences in flux ratios between the predicted and observed images. For the narrow-line regions (NLRs) of quasars, which are too large to be affected by micro-lensing from stars in the lensing galaxy, the flux ratio anomalies thus left are commonly attributed to small-scale structures (sub-structures) in Dark Matter associated with the lensing galaxy. Here, we show that external shear can always resolve, among the quadruply-lensed quasar NLRs studied, position anomalies in lens models constrained solely by the observed image positions, and in addition reduce although not fully resolving flux ratio anomalies when constrained by both the observed image positions and flux ratios -- provided, usually, that the external shear incorporated have strengths that far exceed (as is the common practise) those typically inferred from weak lensing along general sightlines (i.e., cosmic shear). Our work highlights the subversive role of excessive external shear in concealing lensing anomalies, undermining inferences on the characteristics of Dark Matter sub-structures -- and, correspondingly, the nature (mass and temperature) of the Dark Matter particle -- when not sensibly incorporated into lens models.

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