Seiji Fujimoto

We present a 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 _spec = 6.2. New JWST NIRCam imaging and high-resolution NIRSpec integral field unit (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 _eff ≲ 8 pc, stellar masses of M _* ∼ 10 ⁶ –10 ⁷ M _⊙ , and high stellar mass surface densities of Σ _* ≳ 2 × 10 ⁴ M _⊙ pc ⁻² . While their stellar populations are very young (∼6–11 Myr), their dynamical ages exceed unity, consistent with the clusters being gravitationally bound systems. Placing the star clusters in the size versus 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 _ion ) is slightly suppressed in this central region, potentially indicating a locally elevated Lyman continuum escape fraction facilitated by feedback-driven channels.

We present JWST/NIRSpec PRISM and G395M grating spectroscopy for AzTEC-1, a massive submillimeter bright galaxy at z = 4.34 in the COSMOS extragalactic field. The PRISM spectrum reveals strong H α , a significant Balmer break, and no H β detection, indicating a 100–400 Myr old stellar population and high dust attenuation. Baldwin–Phillips–Terlevich (BPT) line ratios indicate the presence of an active galactic nucleus (AGN). Decomposing narrow- and broad-line components, we recover broad, blueshifted H α with a velocity offset of 1245 km s ⁻¹ from the systemic narrow-line velocity and with FWHM ≈ 2500 km s ⁻¹ . AzTEC-1’s smooth morphology and stellar age is suggestive of a past merger-induced starburst period that would have brought in a second supermassive black hole (SMBH), raising the possibility for a binary SMBH system. In this scenario, we assume that the lower-mass black hole (BH) hosts a broad-line region orbiting a quiescent primary. Evidence for an extended outflow is not found in the 2D spectrum, NIRCam imaging, resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of dust continuum, or CO, [C ii ] _{157 μ m} , and [N ii ] _{2 05 μ m} kinematics. AzTEC-1’s high central gas-mass surface density and dynamically unstable gas disk indicate that massive gas clouds external to the candidate binary SMBH’s orbit might have played a role in stalling infall from O ( 10 Myr) to O ( 100 Myr) through dynamical torques, which has been theorized to occur in the nuclei of massive galaxies like AzTEC-1. If the SMBH binary is confirmed, AzTEC-1 would be an excellent laboratory into the astrophysics driving low-frequency gravitational-wave detections.

We present the discovery of a strongly lensed galaxy at z ∼ 11–12, dubbed the “Misty Moons,” identified in the JWST Treasury Survey, Vast Exploration for Nascent, Unexplored Sources. 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 lens models. Two of the five images, ID1 and ID2 ( μ ∼ 20–30), are very bright (F200W∼26 AB mag) and exhibit blue SEDs with prominent Ly α breaks. In the source plane, the Misty Moons is a sub- L ^* galaxy ( M _UV ∼ −18.0 mag) resolved into multiple stellar clumps, each of which has an effective radius of r _eff ∼ 10–70 pc and a stellar mass of ∼10 ⁷ M _⊙ . These clumps dominate the stellar mass budget of the Misty Moons (≳80%), 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 ≳ 10 faint galaxies, such as JADES-GS-z13-0 and JADES-GS-z14-1, indicating that the Misty Moons represents a typical z ≳ 10 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 ≳ 10.

We present a panchromatic optical-millimeter characterization of GNz7q, a recently identified X-ray-weak, rapidly growing red quasar embedded within a dusty starburst galaxy at z = 7.1899, using the full suite of JWST/NIRCam, NIRSpec, Mid-Infrared Instrument (MIRI), and archival NOEMA observations. Our deep NIRSpec/G395M spectroscopy reveals unambiguous broad Balmer emission (FWHM = 2221 ± 20 km s ⁻¹ ), confirming a super-Eddington accreting black hole (BH) ( λ _Edd = 2.7 ± 0.4) with a mass of log ( M BH / M ⊙ ) = 7.55 ± 0.34 , using accretion-rate-corrected BH mass estimators. After subtracting the point source, we robustly detect stellar emission from the host galaxy across multiple NIRCam and MIRI filters. Our joint morphological-spectral analysis yields a stellar mass of log ( M * / M ⊙ ) = 10.5 ± 0.4 and an intense star formation rate of SFR = 330 ± 97 M _⊙ yr ⁻¹ , confirming the host as a massive, dusty starburst galaxy. We find that GNz7q lies on the local M _BH – M _* relation ( M _BH / M _* ≃ 0.001) and is well positioned to evolve into the locus of massive Sloan Digital Sky Survey (SDSS) quasars with log ( M BH / M ⊙ ) ≈ 9 and M _* ≈ 10 ¹¹ M _⊙ at z ∼ 6, owing to its remarkably rapid growth in both the BH and its host galaxy. This stands in stark contrast to many recently reported JWST active galactic nucleus populations at similar redshifts, including the little red dots, whose weak or undetected star formation makes it difficult for them to grow into the massive galaxies hosting SDSS-like quasars. These results suggest that GNz7q marks a rare, pivotal phase of early BH–galaxy coevolution, plausibly providing a crucial direct pathway to the supermassive BH systems within the first billion years of the Universe.

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 ∼ 14 galaxies should be visible at redshifts as high as z ∼ 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 272–351 K and and 445–525 K (using ATMO2020 and Sonora Elf Owl models) and distances of ∼150–650 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 ∼1 yr later, we also detect their proper motions of (49 ± 8) mas yr ⁻¹ and (24 ± 3) mas yr ⁻¹ , further indicating that at least some F277W and F356W dropouts are substellar cold Milky Way objects such as brown dwarfs.

We present the first sample of 218 high-redshift ( z > 0.5) star clusters, detected with JWST/NIRCam in 78 magnified galaxies from different galaxy cluster fields. The majority of the systems (~60%) are observed in the very deep NIRCam observations of the cluster AbellS1063 (GLIMPSE program), showing the power that deep observations, combined with lensing, has to reveal these primordial stellar structures. We performed simultaneous size–flux estimates in all available NIRCam filters and spectral energy distribution (SED) fitting analysis to recover star-cluster physical properties. All star-cluster candidates have very high magnification (µ>10). Star clusters and clumps show similar ages and redshift distributions, although noticeable differences are seen in their masses, sizes and stellar surface densities inherent to the lack of resolution in the latter group. We reconstructed the formation redshift of star clusters and find that the vast majority of the observed star clusters show young ages (z < 4). A small sample of CN star clusters are about 1 Gyr old; these potential globular clusters formed well within cosmic reionization. Star clusters have stellar densities in the range of 10 ² to 10 ⁶ M _⊙ /pc ² , with median values around 10 ⁴ M _⊙ /pc ² . Their sizes and densities better overlap with those of nuclear star clusters in the local Universe. These intrinsic properties make high-z star clusters a viable channel to grow intermediate-mass black holes. We used Bayesian inference to make the first direct measurement of the starcluster mass function at z > 1 based on a subsample of 60 star clusters younger than 100 Myr and with masses above 2 × 10 ⁶ M _⊙ . The star cluster mass function is well described by a power law with a slope of β _50% = −1.89 _−0.12 ^+0.13 , suggesting that a power-law -2 function might already be in place in the distant Universe.

Highly magnified stars at cosmological distances ( z ≳ 0.7) become detectable thanks to microlensing by intracluster stars near the critical curves of galaxy clusters. Multi-epoch photometric campaigns targeting caustic crossing galaxies magnified by massive galaxy clusters enable the detection of these objects as transient events. Such stars provide unique opportunities to study stellar populations at early cosmic times, probe the nature of dark matter, reveal small-scale structure in the cluster, and improve lens models. To date, only a few dozen high-redshift stars have been reported, with a single lensed galaxy, the Dragon, holding the current record of 44 detections. These numbers, however, remain insufficient to exploit their full potential. In this paper, owing to the inclusion of new observations, we report the identification of more than 100 magnified stellar events in the Dragon, behind the massive galaxy cluster Abell 370. The relatively low redshift of the Dragon ( z ≈ 0.725) facilitates the detection of its most massive stars. Using imaging data from three different cycles (2022–2024) with the James Webb Space Telescope, we applied a time-domain technique to identify flux variations associated with caustic-crossing events. From the spatial distribution of stellar events, we constrained the high-end slope of the stellar luminosity function, finding β = 2.18 _−0.30 ^+0.20 . Alternatively, assuming a fixed slope, we constrained the microlens surface mass density. In addition, we examined the parity asymmetry of the detected caustic-crossing events, a proposed probe of wave dark matter, and found that it remains present in our fiducial lens model. We also used the events to trace the regions of highest magnification, offering an alternative way to map the system critical curves.

Obscured active galactic nuclei (AGNs) are often invoked to explain the rapid emergence of young quasars at high redshift and are crucial for building a complete census of AGN activity and black hole growth. The advent of the James Webb Space Telescope (JWST) extends the discovery space for obscured AGNs into the mid-IR with unprecedented precision through reprocessed dust emission. In this work, we use deep JWST Mid-Infrared Instrument (MIRI) imaging from the MIRI Early Obscured-AGN Wide Survey (MEOW), together with existing JWST Near Infrared Camera, spectroscopic, and Hubble Space Telescope imaging data, to identify a previously unrecognized population of obscured AGNs out to z ≈ 6. Using spectral energy distribution modeling of the MIRI-detected sources, we identify 883 AGNs over an area of ~ 131 arcmin 2 and construct the AGN bolometric luminosity function, including both obscured and unobscured sources, across 5 redshift bins. We find an excess in AGN abundance relative to UV-selected AGN luminosity functions, indicating a substantial obscured population missed by optical/UV surveys, with the inferred obscured fraction increasing with redshift and reaching ∼98–99% in our highest-redshift bin, 4.5 < z < 6. We also find higher AGN abundances and obscured fractions than X-ray-based studies, consistent with a previously unrecognized population of heavily obscured, Compton-thick AGN revealed by mid-IR selection. These results suggest that a large fraction of supermassive black hole growth at early times occurs during heavily obscured phases largely inaccessible at other wavelengths.