arXiv subjects

Yoshinobu Fudamoto

Publications and source records attributed to Yoshinobu Fudamoto.

At least 19 recordsLinked to original sources

arXiv2026

Dust at cosmic dawn: unveiling the onset of dust and obscured star formation at $z\gtrsim8$

Although less than 1 percent of the baryonic mass budget of a galaxy, cosmic dust profoundly alters the multi-wavelength appearance of galaxies across cosmic time. The (sub)millimeter wavelength regime is uniquely capable of weighing these dust reservoirs, and revealing the total extent of galaxy evolution that remains hidden behind them. With current facilities, however, doing so at $z > 8$ is largely out of reach. ALMA2040, a next-generation (sub)millimeter interferometer, is set to change this. Through an order-of-magnitude increase in sensitivity compared to ALMA, ALMA2040 is a transformational facility that will be able to routinely detect dust emission from galaxies at $z > 8$ and, through simultaneous multi-band characterization, accurately measure their dust masses, temperatures, and infrared luminosities. Combined with ancillary metallicity measurements -- including from the far-infrared lines detected by ALMA2040 itself -- this will establish the relative roles of supernovae and grain growth in building up dust in the earliest galaxies. Simultaneously, ALMA2040 will measure the fraction of star formation that is obscured by dust at cosmic dawn, and thereby the extent to which rest-UV and optical surveys miss early galaxy build-up. Without a 10$\times$ upgrade to ALMA -- comparable to the leap from the VLT to the ELT -- we will not be able to capitalize on the momentous advances enabled by JWST, and will remain blind to the cold and obscured component of galaxy formation in the $z > 8$ Universe.

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arXiv2026

Sleeping or Dead? Probing the Quenching Mechanism of JWST Dormant Galaxies at z > 3 with ALMA

JWST has revealed a population of "dormant'' galaxies at z>3 with strong Balmer breaks and weak emission lines, whose star formation appears to have declined only recently. Whether these galaxies are undergoing a lasting transition toward quiescence or merely experiencing a temporary suppression of star formation remains unclear. We investigate their molecular gas content using archival ALMA observations of [CII]158um and CO rotational transitions. Starting from all publicly available JWST/NIRSpec prism observations and using joint spectrophotometric fitting, we construct a parent sample at z>3 and log(Mstar/Msun)>9.5 in the ALMA-accessible COSMOS, UDS, and GOODS-South fields. Our final sample comprises 7 dormant galaxies and 15 quiescent galaxies with ALMA constraints on their molecular gas content (mostly 3 sigma upper limits). We find that dormant galaxies exhibit systematically lower star formation rates averaged over the past 10Myr (SFR10Myr) than over the past 100Myr (SFR100Myr), supporting a recent rapid decline in their star formation. For the ~70% of the sample with sufficiently constraining ALMA observations, their molecular gas fractions lie at least ~0.3-2dex below those expected for main-sequence galaxies. Moreover, the molecular gas fractions of dormant galaxies show no systematic difference from those of quiescent galaxies in our sample. This indicates that dormant galaxies have already reached a gas-poor state shortly after quenching and favors a scenario in which the rapid shutdown of star formation is associated with depletion or removal of the molecular gas reservoir, rather than primarily with suppressed star formation efficiency while retaining a substantial molecular gas reservoir. Our results suggest that, if no substantial gas replenishment occurs in the future, dormant galaxies are more consistent with becoming "dead'' than with being temporarily "sleeping''.

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arXiv2026

Has JWST Seen Everything? Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at $z \simeq 6 - 9$ with ALMA2040

JWST/NIRCam has recovered the stellar counterparts of many ALMA-detected, formerly HST-dark galaxies, yet ALMA has started identifying dusty star-forming galaxies (DSFGs) at $z>6$ that remain extremely faint or undetected even with NIRCam. These "NIRCam-dark" DSFGs may trace a missing rapid-growth phase connecting UV-luminous galaxies at $z>10$ to massive quiescent galaxies at $z\gtrsim4$. So far, the small, heterogeneously selected NIRCam-dark sample is insufficient to determine their abundance or role in the evolution of massive galaxies. We show that a blind 1.3-mm continuum survey with ALMA2040 can provide the first high-significance census of this elusive NIRCam-dark DSFG population across $z\simeq 6 - 9$. We conduct sensitivity and survey-speed calculations requiring a $10σ$ detection of a NIRCam-dark DSFG with a dust-obscured star-formation rate of $60\,M_\odot\,{\rm yr}^{-1}$ at $z = 8$. At a like-for-like 7.5-GHz normalization, ALMA2040 will reach the required 1.3-mm RMS of $21\,μ$Jy beam$^{-1}$ about 90 times faster than current ALMA. With 100 hr on-source, ALMA2040 can cover an area of $\sim0.5$ deg$^2$ and detect $\langle N \rangle\sim16$ NIRCam-dark DSFGs over $7.5 10$ and the rapid emergence of massive quiescent systems at $z\gtrsim 4$.

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arXiv2026

The dawn of the first galaxies

Understanding how the first galaxies assembled their stellar mass at cosmic dawn is a central frontier of modern astrophysics. JWST has now revealed an unexpected population of ultraviolet-bright galaxies at z>10, whose intense emission points towards a rapid stellar mass growth that challenges current models of early galaxy assembly. Yet, the cold gas fueling this growth remains hidden: current ALMA sensitivity restricts routine detection to only the brightest ionized-gas tracers such as [OIII] 88$μ$m, while [CII] 158$μ$m, a key coolant and gas-mass proxy of the neutral interstellar medium (ISM), is undetected beyond $z\sim8$. This leaves fundamental questions unanswered: how quickly primeval galaxies gathered cold gas and turned it into stars; what physical conditions (density, temperature, radiation field) characterized their ISM; how fast metals and dust built up; and how feedback regulated early gas assembly. Only a transformational sub-millimeter facility like ALMA2040 can address these questions, enabling systematic surveys of [CII] in the first galaxies and providing a representative census of gas fractions and depletion times. A simulated galaxy at $z=11.3$ shows that ALMA2040 not only will detect, but also spatially resolve its merging cold-gas structure in [CII] on $\sim$200 pc scales in 5 hours (below 100 pc in 10 hours). ALMA2040 will also spectrally resolve multiple lines simultaneously, akin to integral-field spectroscopy, providing the key diagnostics needed to break density-metallicity-radiation-field degeneracies and map the multi-phase ISM of z>10 galaxies. The combination of dramatically increased line sensitivity and wide instantaneous bandwidth will push the search for the first galaxies out to the earliest cosmic times, with [OIII] detectable up to $z\sim20$. ALMA2040 will thus be transformative for understanding how the first galaxies assembled at cosmic dawn.

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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

GREX-PLUS Science Book v2

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA strategic L-class mission to be launched in the 2030s. Its primary science goals are two-fold: galaxy formation and evolution, and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a telescope with a 1 m primary mirror aperture cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2--8 $μ$m wavelength band and a high-resolution spectrometer with a wavelength resolution of 30,000 in the 10--18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high-resolution spectrometer aims to identify the location of the water ``snowline'' in protoplanetary disks. Both instruments will provide unique datasets for a broad range of scientific topics, including galaxy mass assembly, the origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy of exoplanet atmospheres, planetary atmospheres in the Solar System, and so on. This document is the second version of a collection of scientific themes that can be achieved with GREX-PLUS. Each section in Chapters~2 and 3 is based on presentations at several GREX-PLUS Science Workshops.

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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

PRISMS. GHZ1. A standard tale of galaxy evolution with atypical ionizing conditions at $z=9.878$

[Abridged] The first operating years of the James Webb Space Telescope (JWST) have revealed a surprising number of $z \gtrsim 10$ UV-bright objects, many of them characterized by their compactness, anomalous chemical abundance patterns and complex ISM physics (e.g. N-enhanced systems). To assess whether these objects are a phase of early galaxy evolution or a peculiar population, it is essential to understand the physics of systems without anomalous abundance patterns in this epoch. We present a detailed analysis of GHZ1 ($z=9.878$), a moderately ($μ= 1.715$) lensed UV-bright ($M_{UV} = -20.07$) galaxy in the Abell2744 field, characterized by an extended size ($R_{e} \sim 410$ pc). These properties, which contrast with the properties of many of the N-enhanced galaxies, make GHZ1 an excellent laboratory to assess what are the major drivers of early galaxy evolution and the origin of the differences. By means of JWST spectroscopic observations from NIRSpec (PRISM-CLEAR configuration) and the MIR/LRS, we analyzed the full UV and optical emission from GHZ1. We used a spectro-photometric SED fitting to characterize the stellar evolution and the emission line spectra to constrain its chemical enrichment. We find that GHZ1 is among the most massive systems at $z\sim 10$ (log(M$_{\star}$ [M$_{\odot}$]) = 9.17$_{-0.26}^{+0.27}$), with a moderate amount of dust attenuation ($A_{V} = 0.21_{-0.12}^{+0.12}$~mag) and ongoing star formation (with a median SFR $= 8.75_{-1.72}^{+1.71}$ M$_{\odot}\cdot$yr$^{-1}$ over the last 10 Myr). Its chemical enrichment patterns (12+log(O/H) = $7.81_{-0.11}^{+0.12}$, log(C/O) = $-0.66_{-0.13}^{+0.13}$, log(N/O)$< -1.04$) are consistent with a standard evolution dominated by primary production from massive stars. We also derive an H$α$/H$β$ Balmer ratio below Case B predictions. We speculate that [...].

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arXiv2026

The Roman eXtreme Deep Field (RXDF)

The Roman eXtreme Deep Field (RXDF) program is one of the five General Astrophysics Survey (GAS) programs approved for observing time with the Nancy Grace Roman Space Telescope in Cycles 1 and 2. It has been allocated 386.41 hours to carry out an imaging survey to AB = 30 mag (5-sigma) over ~140x larger area than the Hubble eXtreme Deep Field (HXDF) full-depth area (ACS+WFC3/IR). The RXDF will cover the full Roman wavelength range with 7 bands, reaching AB = 30 mag in RZYJH, 29 mag in F, and 28 mag in K, over a full-depth area of 678.75 arcmin^2 embedded in a total area of 1,243 arcmin^2, and far exceeding the depths of the Roman Core Community Surveys (CCS). The RXDF is within the Euclid Ultra Deep Field (EUDF) near the North Ecliptic Pole (NEP), a strategic long-term field for generational space facilities, with a wealth of multi-wavelength data including extensive coverage from the James Webb Space Telescope (JWST) NEXUS Treasury program. The observations will cover 3 epochs at a 1-year cadence, each epoch divided into 3 sub-epochs ~10 days apart, enabling time-domain studies on time baselines from ~10 days to over ~2 years. The RXDF is uniquely positioned to address critical questions in reionization, large scale structure (LSS), growth of supermassive black holes (SMBHs), little red dots (LRDs), and high-z supernovae (SNe); the volumes probed by HST+JWST are too small at these extreme depths, and even the deepest CCS tiers are too shallow. In addition to our key objectives, a wealth of additional science will be enabled by engaging the community with our rapidly released datasets, revolutionizing a wide range of science for a lasting legacy. This short document, which is converted from the approved RXDF proposal, aims to provide the community with a summary of the program.

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arXiv2026

REBELS-25: multi-phase morphology and kinematics at z = 7.31

We present a spatially resolved, multi-wavelength study of the massive, star-forming galaxy REBELS-25 at a redshift of $z=7.31$. We combine new high-resolution ALMA [O III]88$μ$m observations with resolution-matched [C II], dust continuum, and JWST/NIRSpec IFU spectroscopy, providing a $\lesssim1$ kpc view of the morphology, interstellar medium (ISM) conditions, and multi-tracer gas kinematics of one of the most mature galaxies known in the reionisation era. We find differing morphologies from the rest-frame UV to far-infrared (FIR) emission, with the UV and optical emission appearing clumpy and irregular, whereas the FIR emission is well-described by near-exponential disc profiles, with [C II] being the most extended. Comparing the resolved UV and FIR emission, we find that obscured star formation contributes $\sim55$-$98$% of the total star formation rate across the galaxy, demonstrating that dust obscuration strongly shapes the observed UV and optical morphology. Resolved ionisation-line diagnostics show no significant variation across the source at $\sim1$ kpc resolution, consistent with broadly similar ISM conditions among the identified regions. Kinematic modelling reveals that both the warm ionised gas, traced by [O III]88$μ$m, and the colder neutral gas, traced by [C II], share the same large-scale rotating structure and are dynamically cold, with ratios of ordered-to-random motion, $V/σ$, of $\sim11$ and 4.5, respectively, although we find evidence for non-circular motions that are not well-described by a simple rotating disc. Overall, these results further support a picture in which REBELS-25 hosts a dusty, chemically enriched, and dynamically cold ISM already in place at $z=7.31$.

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arXiv2026

RIOJA. Environmental Effects on Stellar Populations and Ionized Gas in a Protocluster at $z=7.88$

Protoclusters in the epoch of reionization provide key laboratories for investigating how environment shapes early galaxy formation and evolution, and may also have contributed to cosmic reionization. We analyze 23 member galaxies of A2744-z7p9OD, a protocluster at $z=7.88$, using JWST/NIRCam and NIRSpec to investigate their stellar population properties, rest-frame UV sizes, and ionized-gas properties. We also quantify the internal structure of A2744-z7p9OD using the projected distance to the most massive galaxy ($D_{\rm YD4}$), and to the nearest neighbor ($D_{\rm nei}$), as global and local environmental indicators, respectively. Stellar mass, SFR on a 100 Myr timescale, dust attenuation, and galaxy size show significant correlations ($p<0.05$) with $D_{\rm YD4}$, but not with $D_{\rm nei}$, suggesting that these properties are primarily linked to the global protocluster structure. The member galaxies also show a large galaxy-to-galaxy variation in R23 ($=\log{(([\mathrm{O}\text{\textsc{iii}}]λ\lambda4960,5008\rmÅ+[\mathrm{O}\text{\textsc{ii}}]λ\lambda3727,3730\rmÅ)/\rm{H}β)}$), implying inhomogeneous chemical enrichment in the protocluster environment. O32 ($=\log{([\mathrm{O}\text{\textsc{iii}}]\lambda5008\rmÅ/[\mathrm{O}\text{\textsc{ii}}]λ\lambda3727,3730\rmÅ)}$) correlates with $D_{\rm YD4}$, indicating that the core region is characterized by low-ionization gas. Together with the non-detection of Ly$α$ emission, the possible neutral-gas reservoir traced by ALMA [C{\sc ii}]~$158μ$m emission, and evidence for high-column-density neutral hydrogen in the core, this suggests a neutral-gas-rich protocluster core where the current escape of ionizing photons may be suppressed, even in a overdense environment during the EoR.

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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

Breathing Fire: Hot Dust in the Big Three Dragons at z = 7.15

We present new Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 ($λ_\mathrm{obs} = 0.45\,\mathrm{mm}$) and 4 ($λ_\mathrm{obs} = 2.2\,\mathrm{mm}$) observations towards the Big Three Dragons, a pair of merging Lyman-break galaxies (LBGs) at $z=7.15$. The system was previously detected in ALMA Bands 6, 7 and 8 ($λ_\mathrm{obs} = 0.73 - 1.32\,\mathrm{mm}$), which, combined with our new observations, allows us to robustly constrain its dust temperature and obscured SFR. The unresolved Band 4 observations yield a $3.5σ$ detection, and the $0.4''$ Band 9 observations detect the Eastern and Western LBGs at $3.7$ and $3.5σ$, respectively. Through modified blackbody fitting, we infer a global dust temperature of $T_d = 78_{-23}^{+35}\,\mathrm{K}$ for the system, which implies a high IR luminosity of $\log(L_\mathrm{IR}/L_\odot) = 12.32_{-0.41}^{+0.43}$. This makes the Big Three Dragons one of the most IR-luminous systems known at $z>7$, with a total $\mathrm{SFR}_\mathrm{UV+IR} = 267_{-153}^{+418}\,M_\odot\,\mathrm{yr}^{-1}$ that is almost completely obscured ($f_\mathrm{obs} = 0.94_{-0.09}^{+0.04}$). Using resolved ALMA observations in Bands 6, 8 and 9, we confirm both LBGs have hot dust temperatures ($T_d \approx 67 - 84\,\mathrm{K}$) and correspondingly high obscured fractions ($f_\mathrm{obs} \approx 0.88 -0.95$). We find the Western LBG to fall $\sim1\,\mathrm{dex}$ above the canonical IRX-$β_\mathrm{UV}$ relation, suggesting patchy dust obscuration. The compact Eastern component, on the other hand, is consistent with a Calzetti- or SMC-like dust screen within the uncertainties. Together with the similarly hot dust temperature recently reported for the $z=8.31$ galaxy MACS0416-Y1, our results suggest a non-negligible fraction of star formation at the bright end of the UV luminosity function is highly dust-obscured, even at $z\gtrsim7$.

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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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arXiv2026

A Compact Proto-group at $z \sim 5$: A Massive Galaxy Caught in Formation

We report the discovery of SCGG-z5, a compact galaxy proto-group at $z = 4.97$ in the MACS0416 field, identified from the SAPPHIRES Early Data Release. Six members are spectroscopically confirmed via H$α$ emission, spanning $4.96 \leq z_{\rm spec} \leq 4.98$ within a projected diameter of $\sim16$ pkpc. Spectral energy distribution (SED) fitting yields individual stellar masses $8.4 \leq \log(M_*/M_{\odot}) \leq 9.8$, a total group stellar mass of $\log(M_*/M_{\odot}) = 10.07 \pm 0.04$; three of the six members lie above or on the star-forming main sequence at $z \sim 5$, by up to $0.5$ dex. Pixel-by-pixel analysis reveals diverse resolved radial star-formation profiles: three members show declining specific SFR radial profiles and outward-rising stellar age gradients, consistent with inside-out stellar mass growth, while the most massive member shows a tentative inverted sSFR profile suggestive of reduced central star formation. The line-of-sight velocity dispersion over all six members is $σ_v = 375^{+55}_{-195}$ km s$^{-1}$. The projected mass estimator yields $\log(M_{\rm PM}/M_{\odot}) \approx 12.30^{+0.30}_{-0.25}$, consistent with a dark-matter-dominated group halo. EAGLE simulations of structurally similar groups predict full coalescence by $z \sim 3$--$4$, with the merged remnant reaching $\log(M_*/M_{\odot}) > 11$ by $z \sim 1$, consistent with SCGG-z5 representing a rare pre-coalescence phase of early massive galaxy formation, possibly tracing the assembly of a future brightest group or cluster galaxy.

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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

FINER: development of the wideband millimeter-wave receiver system and preparations for first light on the Large Millimeter Telescope

The recent discovery of an excess of luminous galaxies in the early Universe necessitates sensitive and wideband millimeter spectroscopy to understand their rapid growth. To address this, we present the development of the Far-Infrared Nebular Emission Receiver (FINER) for the Large Millimeter Telescope (LMT). The FINER frontend comprises two receivers covering 120-350 GHz (corresponding to ALMA Bands 4+5 and 6+7). The warm optics are designed to enable simultaneous two-band observations. Combined with the 10.24-GHz-wide digital spectrometer array, the system aims to deliver an instantaneous bandwidth approximately five times wider than current ALMA capabilities. We report that the 210-350 GHz receiver has already achieved commissioning-level performance, with sideband rejection further enhanced by the digital sideband separation technique. With installation expected in 2026, we discuss parallel preparations, including integrated testing and commissioning plans for first-look targets.

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