Hollis B. Akins

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.

Brown dwarfs are unique laboratories for studying the physics of cool atmospheres, the low-mass end of star formation, and the transition between stars and giant planets. Their spectra encode the effects of temperature, gravity, metallicity, clouds, and disequilibrium chemistry, but identifying and characterizing the coldest brown dwarfs remains challenging, particularly at faint magnitudes in deep extragalactic surveys. We aim to establish the nature and atmospheric properties of COSMOS2025 ID 639188, hereafter Shabgard, a compact source previously selected as a brown-dwarf candidate from COSMOS-Web photometry on the basis of its point-source morphology and T/Y-dwarf-like NIRCam colors. In this work, we characterize this source using serendipitous JWST/NIRCam F444W wide-field slitless spectroscopy from COSMOS-3D. We extract its local-background-subtracted, profile-weighted 4.05–4.85 μ m spectrum, combine it with COSMOS2025 NIRCam photometry, compare the data with six atmosphere-model families, and measure its motion using multi-epoch NIRCam imaging. The LOWZ atmosphere-grid fit, based on principal component analysis (PCA) interpolation and Markov Chain Monte Carlo (MCMC) sampling, gives T eff = 718 . 6 − 10.2 + 10.6 K and [ M / H ] = − 1.0 0 − 0.08 + 0.07 , favoring a cool, subsolar-metallicity atmosphere characteristic of the late-T/Y transition. A fixed-atmosphere molecular-opacity test strongly favors a contribution from CH ₄ to the observed 4–5 μ m spectral shape. The restricted wavelength coverage, however, prevents a molecule-by-molecule abundance determination. Model-dependent physical estimates span approximately 23–52 M _Jup and 140–180 pc. We measure a proper motion of ∣ μ ∣ = 89.2 ± 0.4 mas yr ⁻¹ , while the LOWZ MCMC flux scaling gives a distance of 176 . 2 − 17.8 + 18.4 pc, supporting the interpretation of Shabgard as a Galactic brown dwarf. Together, these results support its classification as a cool, metal-poor brown dwarf and demonstrate the ability of JWST deep-field surveys to discover and characterize faint Galactic substellar objects.

The launch of the James Webb Space Telescope (JWST) has enabled the discovery of a small but increasing sample of high-redshift core-collapse supernovae (CC SNe), which provide new tests of massive star evolution in the early Universe. In this study, we report the analysis of SN 2023aeaf discovered in COSMOS-Web survey observations, which at z = 3.195 has one of the highest SN spectroscopic redshifts to date. Using two epochs of JWST photometry separated by ∼1 month in the rest frame, we photometrically classify SN 2023aeaf by comparing the JWST photometry to spectrophotometric CC SN and Type Ia (SN Ia) models and UV observations of SNe from the Swift telescope, finding that SN 2023aeaf is highly likely to be a Type II SN. A spectrum of the SN+host galaxy was also obtained ∼30 rest-frame days after discovery but shows no clearly identifiable SN features, with H α emission from the host potentially masking emission from the SN. Although the limited photometric coverage prevents strong constraints on the explosion properties, we find that the data are most consistent with a ∼12 M _⊙ progenitor with ∼0.5 M _⊙ of circumstellar material. We next use the host-galaxy spectrum and photometry to model the host spectral energy distribution (SED) using the Prospector Bayesian inference framework. We find that the host is a star-forming galaxy with an sSFR of log 10 ( sSFR / yr − 1 ) = − 10.1 7 − 0.10 + 0.13 , a stellar mass of log ( M ⋆ / M ⊙ ) = 9.0 4 − 0.04 + 0.03 , and a gas-phase metallicity of 12 + log 10 ( O / H ) = 7.82 ± 0.02 . SN 2023aeaf joins a growing sample of early Universe CC SNe with high luminosities, dense circumstellar material (CSM), and low-metallicity environments.

We present high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) [C ii ] 158 μ m observations and JWST/NIRCam+MIRI imaging of MAMBO-9, a pair of optically dark, dusty star-forming galaxies at z = 5.85. MAMBO-9 is among the most massive, gas-rich, and actively star-forming galaxies at this epoch, when the Universe was less than 1 Gyr old. The new 400 pc resolution [C ii ] observations reveal velocity gradients in both objects; we estimate the dynamical masses and find a relative mass ratio of 1:5. The kinematics of both objects suggest both rotation and strong tidal interaction, suggesting that the pair has already experienced a close encounter. Indeed, the new JWST imaging reveals a continuous bridge of moderately dust-obscured material between the two. We perform spatially resolved spectral energy distribution (SED) fitting using the high-resolution ALMA+JWST imaging, finding that the majority of the recent star formation is concentrated in extremely obscured ( A _V > 10) clouds, while the majority of the rest-optical light (stellar continuum and H α emission) is emergent from moderately to highly obscured ( A _V ∼ 1–5) regions on the outskirts. Combining our new stellar and dynamical mass measurements with previous CO observations, we find that the mass budget of MAMBO-9 requires a CO-to-H ₂ conversion factor ( α _CO ) of order unity (∼1–2), indicative of a highly metal-enriched interstellar medium. Finally, we show that MAMBO-9 resides in a large overdensity spanning the PRIMER-COSMOS field, with 39 galaxies spectroscopically confirmed within ∼25 cMpc. With a total baryonic mass of ∼10 ¹¹ M _⊙ , MAMBO-9 can be considered a prototype of massive galaxy formation and a likely progenitor of the brightest cluster galaxies in the lower-redshift Universe.

We present a parametric strong-lensing model for the galaxy cluster MACS J1931.8-2635 ( z _l = 0.35), accompanying the detection of the spectroscopically confirmed SN Eos at z = 5.13. We identify 10 new multiple-image systems in recent VENUS JWST/NIRCam imaging, so that the model is constrained with a total of 19 robust multiple-image systems—nine of which also have a spectroscopic redshift. For the point-like source corresponding to SN Eos, our model predicts a total of five images, with the observed radial image pair having a similar magnification of μ ≃ 25–30 and a small time delay of . ″ 44. We present the lens-modeling results, including newly identified systems such as a triply imaged, grand-design spiral galaxy candidate at z ≃ 3.6 5 − 0.09 + 0.04 , and discuss the potential of using high-redshift lensed SNe for cosmography.

The detection of strong Balmer breaks and absorption features in Little Red Dots (LRDs) suggests they host active galactic nuclei embedded within dense gas envelopes, potentially powered by super-Eddington accretion. We present GLIMPSE-17775, a luminous ( L _bol ∼ 10 ⁴⁵ erg s ⁻¹ ) LRD at z = 3.501 behind Abell S1063 ( μ ∼ 2), observed with deep JWST/NIRCam and a ∼20 hr (80 hr delensed) NIRSpec G395M spectrum. The data reveal over 40 emission and absorption features, including a rich forest of low-ionization Fe ii lines and numerous broad hydrogen recombination transitions. We use this depth to test the dense-gas interpretation through five independent diagnostics. Nearly all permitted lines show exponential wings with consistent FWHM, a signature of Thomson scattering requiring n _e ≳ 10 ⁸ cm ⁻³ . Adopting this width yields M _BH ∼ 10 ^6.7 M _⊙ , a factor of 10 lower than Gaussian fits, and λ _Edd ∼ 1.8. Additional diagnostics support the same picture: a pronounced Balmer break ( f _{ν ,4050} / f _{ν ,3670} = 2.0 ± 0.1), enhanced He i λ 7065 and λ 10830 with P-Cygni absorption, Bowen-fluorescent O i λ 8446– λ 11290 emission requiring Ly β pumping, and 16 Fe ii lines matching fluorescence models. These features indicate a dense ( n ∼ 10 ⁸ cm ⁻³ ), partially ionized cocoon where scattering and fluorescence dominate line formation, providing strong evidence that at least some LRDs are powered by super-Eddington black hole growth in the early Universe.

Cosmological simulations suggest that various galaxy properties depend on their location within the cosmic web. Yet direct observational evidence of the dependence of star formation activity on distance to filaments remains scarce and is missing at z We investigate how starburst, main-sequence (MS), and quenched galaxies are distributed with respect to cosmic web filaments and how this distribution evolves with redshift. We first used the cosmological hydrodynamical simulation to predict the redshift evolution of the mean distance to the closest filament from z=3 to z=0 for different galaxy populations, after removing stellar-mass dependencies. We then measured the corresponding signal in the ̧osmos field, using ̧osmostwo and ̧osmosweb data, where accurate photometric redshifts enable a reconstruction of the projected cosmic web from z=2 to z=0.5, and starbursts were identified through far-infrared spectral energy distribution fitting. In agreement with the results from starburst galaxies are found closer to filaments at z>1 and at larger distances at z<1, MS galaxies occupy intermediate environments with little evolution, and quenched galaxies show progressively shorter distances to filaments towards low redshift, with a crossing between starburst and MS populations around z ∼ 1. In ̧osmosweb, the relative evolution in the average distance to filaments between starburst and MS galaxies is detected at a significance level of at least 5σ. We show that a minimal toy model in which the only environmental ingredient is the specific star formation rate-filament distance modulation measured in simulations is sufficient to reproduce the observed differential evolution of the average filament distance between starburst and MS galaxies. These results show evidence for a link between the large-scale environment and the star formation activity of galaxies, as predicted by simulations, from z=2 down to z=0.5.