arXiv subjects

Lukas J. Furtak

Publications and source records attributed to Lukas J. Furtak.

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

GLIMPSE-DDT spectroscopic properties of faint-end galaxies at $z\sim6$: Towards first metal enrichment, dust production, and ionizing photon production

Ultra-faint galaxies at high-$z$ are fundamental elements of the early galaxy assembly, and spectroscopic characterization of this population is essential to understand the earliest galaxy evolution. Leveraging the ultra-deep JWST/NIRCam and NIRSpec observations of a gravitational lensing field of Abell S1063, taken as part of the GLIMPSE survey, we present spectroscopic properties of 16 galaxies fainter than $M_{\rm UV}=-17$ mag, including the metallicity, dust attenuation, and the ionizing photon production efficiency. The emission lines are generally quite strong, roughly half of which cannot be replicated with standard stellar populations and require an extreme ionizing source. We also identify relatively strong [OIII] emission lines from all sample galaxies, which indicates that the low-mass end of the mass-metallicity relation is extended down to $M_\star\sim10^6\ M_\odot$ at $z\sim6$. The strong [OIII] line detection from the lowest-mass galaxy among the sample ($M_\star\sim10^{5.6}\ M_\odot$) stands in contrast to recent reports of extremely metal-poor galaxy candidates at similar mass and redshift, suggesting that there could be two distinct pathways of the earliest metal enrichment as simulations have predicted. Interestingly, we detect both dust attenuation and galactic outflow in one of the sample galaxies with $M_\star=10^{6.6}\ M_\odot$ at $z=5.5$. All the dust, metal, and outflow contents in this galaxy can be consistently explained by supernovae (SNe), indicative of the key roles of SNe in the earliest galaxy assembly such as dust production, metal enrichment, stellar feedback, and baryon cycle.

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arXiv2026

A sub-100 pc view at z~5 of a multiply imaged massive quiescent galaxy

James Webb Space Telescope surveys reveal many rapidly assembled galaxies in the first billion years. Simulations struggle to reproduce their rapid star formation and abrupt cessation, known as quenching. Testing these models is difficult because high-redshift galaxies are compact and appear unresolved. We present VENUS-CLJ0152-QG1, a massive post-starburst galaxy at redshift 5.18. A foreground cluster lenses it into three images, yielding a physical resolution of about 70 parsecs, enough to probe quenching processes on sub-kiloparsec scales. We find a half-light radius twice as large as expected, a central stellar mass density three times lower than field analogs, and off-center ionized gas emission. These features suggest it quenched without compaction or an obvious active galactic nucleus, a departure from unlensed high-redshift galaxies that lensing makes visible.

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arXiv2026

Little Red Dots with Broad H$β$ Lines Reveal the Black Hole Mass Function at 7<z<9.5

We present a systematic search for broad H$β$ emission at $7<z<9.5$ in 21,770 JWST/NIRSpec PRISM spectra drawn from four blank fields and six lensing clusters with photometry spanning 1942 arcmin$^2$. We find a sample of 20 sources with statistically significant broad-H$β$ emission, including 13 new sources. Our entire sample has colors and morphologies consistent with little red dots (LRDs), compared to only 37\% of broad-line emitters at $3.5<z<6$. Interpreting the broad-lines as indicators of Broad-line Active Galactic Nuclei (BLAGN), we infer black hole masses of $4.2\times10^{6}-2.1\times10^{8}$\,$M_{\odot}$. We calculate completeness corrections and construct the black hole mass function at $z\sim8$. The normalization of the black hole mass function rises by $\approx1.0$~dex from $z\sim8-5$, consistent with average black hole growth at $f_{\textrm{Edd}}\approx0.1$. Comparing with cosmological simulations, the $z\sim8$ black hole mass function requires heavy ($10^5$\,$M_{\odot}$) seeds or super-Eddington accretion onto light or intermediate mass seeds to match its normalization. Finally, we infer bolometric luminosities for the sample and construct a bolometric luminosity function. We integrate over this luminosity function to derive a black hole accretion density (BHAD) of $4.8^{+5.8}_{-2.6}\times10^{-7}~M_{\odot}~\textrm{yr}^{-1}~\textrm{Mpc}^{-3}$, showing broad agreement with previous photometric work, but disagreement with previous X-ray models, reinforcing the idea that BLAGN are more common in the early universe than previously expected.

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arXiv2026

A GLIMPSE of Dark Matter: JWST Constraints on Early Structure Formation

While we have a solid understanding of how dark matter (DM) behaves during recombination ($z\sim 10^3$) and in the late universe ($z\lesssim$ a few), much less is known about intermediate times, when the first galaxies formed. These galaxies provide a powerful handle on DM through structure formation, as halos have to exist to host luminous sources. Here we use recent determinations of the UV luminosity function from the James Webb Space Telescope (JWST) GLIMPSE lensing program, along with legacy Hubble and Subaru data, to test the small-scale behavior of DM during the epoch of reionization ($z\sim 7$). Marginalizing over astrophysical uncertainties, we find the matter power spectrum to be consistent with the cold-DM prediction down to sub-Mpc scales, with 1$σ$ uncertainties of 0.2 dex at $k=3-9 \, \rm Mpc^{-1}$ and 0.5 dex at $k=9-27 \, \rm Mpc^{-1}$, the smallest scales probed at these early times. For thermal-relic warm DM we constrain $m_{\rm WDM} > 3.2\, \rm keV$ (95% CL), independent of lower-$z$ probes. Our results test small-scale structure formation in a new redshift window and demonstrate the promise of JWST to constrain the nature of DM.

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arXiv2026

SN Ares: A Strongly Lensed Type IIb Supernova at $\mathbf{z=1.276}$ with a Predicted 70-Year Time Delay

We present the discovery of supernova (SN) Ares, a strongly lensed supernova at $z_{\rm spec}=1.276\pm0.002$ in a massive, triply imaged host galaxy behind the MACS J0308.9+2645 galaxy cluster. JWST/NIRSpec spectroscopy spanning the rest-frame optical to NIR of Ares reveals helium features at $5876\,\mathring{\mathrm{A}}$, $7065\,\mathring{\mathrm{A}}$, and $10830\,\mathring{\mathrm{A}}$, as well as H$α$ and Pa$β$. A comparison to local core-collapse SN spectra and light curves leads to a classification of SN Ares as a Type IIb supernova, the highest-redshift member of its class spectroscopically identified. A comparison to local SN IIb light curves provides a phase estimate for the spectrum of $34_{-9}^{+10}$ rest-frame days post-radioactive peak, providing a unique view into a high-redshift stripped-envelope supernova in its late-photospheric evolution. A lens model of the cluster constructed using recent JWST data indicates that SN Ares is the first image to arrive and will reappear in two future images near the center of the cluster plane in $\gtrsim 70$ years, with predicted magnifications of $\sim5\times$ and $\sim1.5\times$. Planned JWST/NIRSpec Multi-Object Spectroscopy of the cluster will allow for more detailed modeling of the lensed system for tighter constraints on the time delay and magnifications, as well as a template spectrum at the location of SN Ares.

astro-ph.HE↗

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

The BUFFALO Survey : The Subhalo Mass Function for Abell 2744 with Strong+Weak Gravitational Lensing

We present a combined strong and weak gravitational lensing mass model of the massive merging galaxy cluster Abell 2744 (z = 0.308). The mass reconstruction combines strong-lensing constraints in the cluster core with weak-lensing measurements derived from deep imaging with the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST), enabling a reconstruction of the projected mass distribution from the core to the cluster outskirts. Using the resulting mass map, we identify cluster substructures and construct the subhalo mass function (SHMF) for Abell 2744. To interpret the observed substructure population, we apply identical detection methods to projected mass maps of massive galaxy clusters from the BAryons and HAloes of MAssive Systems (BAHAMAS) simulations. We observe a systematic excess of detected substructures in Abell 2744 compared to BAHAMAS. We interpret this excess as likely reflecting Abell 2744's exceptionally disturbed dynamical state, characterised by complex ongoing mergers, rather than tension with $Λ$Cold Dark Matter ($Λ$CDM) or self-interacting dark matter (SIDM) predictions. Although confirming this interpretation will require extending the analysis to a larger sample of clusters spanning a range of dynamical states.

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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

Balmer Absorption Series and Broad Metal Lines in Two Luminous Little Red Dots

Balmer absorption is common among little red dots (LRDs), but absorbers at or redward of systemic are rare, occurring in only $\sim10-15$\% of H$α$ absorbers. In this paper, we study two such exceptional cases with deep JWST/NIRSpec spectroscopy: 15 hr of high-resolution (G395H) observations of RUBIES-EGS-49140 (z=6.68), resolving the absorption in all four transitions from H$α$ through H$δ$, and medium-resolution spectroscopy (10 hr of G235M, 2 hr of G395M) of UNCOVER-A2744-45924 (z=4.46; 1.7x magnification). Both sources are among the optically reddest and most luminous LRDs known, and both show deep, near-systemic Balmer absorption troughs. We find two systematic trends along the Balmer series: the absorption centroids become more redshifted toward higher-order transitions, while the absorbed equivalent widths decline only weakly with increasing order, far less than expected from the atomic optical-depth ratios for a single attenuating screen. Ca\,{\sc{ii}}\,K is detected in absorption in both sources, whose offset follows the H$α$ trough rather than the more redshifted higher-order Balmer lines. We further report the detection of a broad base in [Ne\,{\sc{iii}}]\,$λ$3870, along with broad [O\,{\sc{iii}}]\,$λ$4364, [O\,{\sc{iii}}]\,$\lambda5008$, and He\,{\sc{i}}\,$\lambda5877,\lambda7067$, while He\,{\sc{ii}}\,$λ$4687 remains undetected or weak. Standard AGN photoionization models cannot reproduce the observed line ratios, whereas AGNs with high gas densities provide a consistent explanation, as also indicated by the anomalously high He\,{\sc{i}}\,$\lambda7067/\lambda5877$ ratio. A possible explanation for the relative strengths of the Balmer absorption lines could be a dense, optically thick medium whose re-emission modifies their apparent absorption strengths, while the velocity progression may arise from stratification in the absorbing gas.

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arXiv2026

Resolving 3 Exotic Hyperbolic-Umbilic Lensing Configurations in the "Cosmic Mantis": An Exploration of RXJ0437.1+0043 with JWST

We present an updated strong lensing model for the galaxy cluster RXJ0437.1+0043 (z=0.285) using new JWST NIRCam imaging in four pass-bands. The cluster field notably displays at least three separate gravitationally lensed galaxies in hyperbolic-umbilic (H-U) configurations, which in the JWST imaging show exquisitely resolved stellar clumps at scales as small as 0.04''. We identify 13 new systems of multiply-imaged lensed galaxies and determine the redshifts for two sources based on archival spectroscopic observations. We also identify over 50 individually resolved substructure clumps in the multiple images of eight galaxies and add them into the lens model, for a total of 275 positional lensing constraints between all 26 lensed sources used in the model. We investigate the impact of different modeling choices on our results. In the final lens model, we reproduce the observed multiple images with a precision of 0.23'' , and achieve 0.13'' specifically for the H-U systems. We estimate a total enclosed mass in the cluster's strong lensing regime of M(<110 kpc) ~ 8.2 x 10^13 Msun, reconstruct the source images of the most highly magnified galaxies, and measure time delays in the most magnified, morphologically detailed systems. The isotropic lensing effect from the H-U configurations enables future detailed studies of the properties of their source galaxies, and the number and spatial density of additional constraints improve the model's ability to detect dark matter substructures within the cluster core.

astro-ph.CO↗

arXiv2026

Caught Napping by JWST UNCOVER+MegaScience: Constraining bursty star formation histories and number densities of mini-quenched galaxies at redshifts 4-7

We explore the prevalence of mini-quenched or ``napping'' galaxies selected from spectroscopic and photometric samples in the UNCOVER/MegaScience survey. These galaxies are empirically identified by the presence of moderate Balmer breaks, weak emission lines ($EW(Hα) < 100$A) and relatively blue UV continua. We infer the star formation histories (SFHs) of our sample using flexible non-parametric models with Prospector optimized to capture recent episodes of bursty star formation and quenching, and find that they are uniquely identifiable in the SFR$_{10}$/SFR$_{100}$ parameter space moving towards (temporary) quiescence. We demonstrate that although spectroscopy is best able to identify rapidly declining SFRs, densely sampled medium-band photometry recover these key spectral features and thus robustly identify pure samples of this transient phase -- with imaging alone. We quantify the number density of napping galaxies at $z=4-7$ in the Abell 2744 lensing field, finding 8 spectroscopically confirmed nappers and 60 photometric candidates spanning log$_{10}$(M$_*$/M$_\odot$) $= 7.5-10$. We verify that the photometry alone can identify a pure sample of nappers, leveraging a smaller high signal-to-noise ratio spectroscopic sample. Consistent with previous studies, we find that nappers are most common at low stellar mass (log$_{10}$(M$_*$/M$_\odot$) $\sim9$). We see a hint that the number densities increase from $z\sim6$ to $z\sim4$, though our small sample is likely affected by cosmic variance. Our study demonstrates the increasing importance of stochastic star formation as a regulator of low-mass galaxy growth in the several hundred Myr after reionization, and offers a direct observational testbed for the strength and duty cycle of stellar feedback in cosmological simulations.

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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

An Optical Illusion: High Electron Densities Create Extremely Metal-Poor Galaxy Impostors

JWST has enabled the discovery of dozens of extremely metal-poor galaxies (EMPGs) with metallicities below $5\%\,Z_{\odot}$, representing a significant leap toward detecting the first galaxies without metals. However, accurate metallicity measurements require careful determination of physical conditions in the ionized gas. In this paper, we study four galaxies that appear to be EMPGs when analyzed using the direct $T_e$ method with the common low-density assumption ($n_e=10^3\,{\rm cm}^{-3}$). To test whether their metal-poor status holds when accurately measuring the density, we apply the direct method with a self-consistent determination of electron temperature ($T_e$) and density ($n_e$) in the high-ionization zone using the [OIII]$λ$5008, [OIII]$λ$4364, and [OIII]$λ$1666 lines, using JWST/NIRSpec data from the SPURS program in the Abell 2744 lensed field. We find that three out of four galaxies in our sample have extremely high electron densities ($n_e \sim 10^{5}-10^{6}\,{\rm cm^{-3}}$), which suppress the [OIII]$λ$5008 line and lead to underestimates of metallicity by up to $\sim1.1\,$dex when this density is not accounted for, and have true metallicities of 12+log(O/H) $\sim7.3-8.2$. Failure to account for high densities can therefore lead to systematic misclassification of metal-poor galaxies and biased conclusions about early chemical enrichment.

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arXiv2026

Little Red Dots host Black Hole Stars: A unified family of gas-reddened AGN revealed by JWST/NIRSpec spectroscopy

We use the DAWN JWST Archive to construct and characterise a sample of 146 little red dots (LRDs) across 2.0<z<9.3, selecting all sources with v-shaped UV-optical continua from NIRSpec/PRISM spectra and compact morphologies in NIRCam/F444W imaging. We show that LRD continuum spectra are ubiquitously well described by modified blackbodies across ~$0.4-1.0μ$m, with typical T~5000K or $λ_{peak}$~$0.65μ$m across 2 dex in luminosity, and a tail toward T~2000K. LRDs therefore trace a locus in the Hertzsprung-Russell diagram that is directly analogous to stars on the Hayashi track, strongly supporting the picture that LRDs are AGN embedded in optically-thick dense gas envelopes. Hotter LRDs with $λ_{peak}<0.65μ$m typically have strong Balmer breaks, redder UV slopes and high optical luminosities; other LRDs show weak or no Balmer breaks, and wide variety in $β_{UV}$ and $L_{5100}$. Crucially, we demonstrate that the UV-optical continuum shapes and luminosities are strongly linked to the $Hα,\ Hβ$, [OIII] and OI line properties. There is a tight linear relation between the H$α$ and optical continuum luminosities, as well as H$α$ and OI$_{8446}$, indicating that Balmer, OI and optical emission must primarily be powered by the same source. The Balmer decrement increases strongly toward higher $L_{Hα}$, $L_{5100}$ and Balmer break strength, providing key evidence for luminosity-dependent effects of collisional (de-)excitation and resonant scattering in the gaseous envelopes. In contrast, we show that [OIII] emission likely originates from star-forming host galaxies, and that its strong correlation with Balmer break strength arises naturally from variation in the AGN-to-host ratio among the LRD population. Our work presents an empirical description of the nature and structure of LRDs, defining a new benchmark for ongoing LRD model developments.

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arXiv2026

A GLIMPSE of the 99%: a census of the faintest galaxies during the epoch of reionization and its implications for galaxy formation models

We present a comprehensive study of the galaxy UV luminosity function (UVLF) at $z=6-9$ leveraging deep JWST observations from the GLIMPSE survey. Thanks to gravitational lensing, we probe the UVLF to an unprecedented depth of $M_{\text{UV}} = -12$ mag, approximately three magnitudes deeper than previous robust constraints. Our UVLF determination incorporates a rigorous end-to-end uncertainty framework, including statistical and systematic lensing uncertainties. We find that the $z \sim 7$ UVLF continues to rise steeply with a faint-end slope of $α= -1.98_{-0.05}^{+0.06}$. Crucially, our data show no clear evidence of a turnover down to \muv $= -12.3$. The persistence of this faint population provides stringent constraints on galaxy formation models and cosmological simulations that predict an early flattening of the luminosity function due to radiative feedback or star-formation thresholds. Furthermore, post-JWST models specifically calibrated to match the UV-bright excess at $z > 10$ generally fail to reproduce the observed evolution toward lower redshifts and fainter magnitudes, highlighting a significant tension in our current understanding of early galaxy assembly. We derive a comoving ionizing emissivity at $z=7$ of log($n_{\mathrm ion}$ / s$^{-1}$ Mpc$^{-3}$) $\approx 50.85$, which suggests that faint galaxies dominate the ionizing budget, providing enough photons to maintain reionization even in a highly clumped IGM ($C_{\text{HII}} = 5$). As our detection of faint galaxies effectively rules out a luminosity function truncation at $M_{\text{UV}} \geq -15$, these results emphasize the need to either accurately characterize the ionizing properties of the global, low-mass galaxy population at $z > 6$, or to refine physical models of intergalactic medium clumping and its redshift evolution to maintain consistency with the observed reionization timeline.

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arXiv2026

Everything Every Band All at Once I: A Global Morphology Catalog in Abell 2744 based on UNCOVER/MegaScience

We present spectrally-resolved structural parameter measurements of 28,274 sources from the legacy lensing field of Abell 2744, quantifying global structures from observed $0.7 μm - 4.8 μm$ and spanning rest-frame UV to NIR at $R\sim15$. These measurements are made on imaging mosaics mainly from the UNCOVER/MegaScience survey, including 20 JWST NIRCam broad and medium bands. We perform single-component Sérsic fitting to these galaxies using \texttt{pysersic}, a Bayesian structural fitting tool, to infer their structural parameters and associated random uncertainties from the posterior distributions. Through various quality evaluation criteria, we infer robust structural parameters among $> 85\%$ of the selected $\rm SNR>10$ sources. For each galaxy with reliable sizes in at least two bands and a high quality redshift, we fit its observed size as a function of wavelength and infer rest-frame UV, optical, and near-infrared sizes where applicable. By performing injection-recovery tests on simulated galaxy cutouts in selected bands, we establish that our structural parameter measurements achieve fractional error $< 10 -20\%$ above $\rm SNR>10$. With this paper, all raw structural measurements and fitted rest-frame sizes are quality-flagged, cataloged, and released to the community. Finally, we demonstrate that this catalog enables the structural study of galaxies over an unprecedentedly wide parameter space of redshift ($0.3<z<8$), stellar mass ($\rm 10^{7}\, M_{\odot}<M_{*} <10^{11.5}\, M_{\odot}$), and rest-frame optical size ($\rm 100 \,pc<R_{e}<10\,kpc$), after correcting for lensing magnification.

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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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