Caitlin M. Casey

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 analyze the rest-frame optical (∼8000 Å) morphologies and star formation activity of Ly α emitters (LAEs) at redshifts 2.4, 3.1, and 4.5, identified in the One-hundred-deg ² DECam Imaging in Narrowband (ODIN) survey. To compare their physical properties with those of other galaxies, we construct a comparison sample of typical star-forming galaxies (SFGs) at similar redshifts from the COSMOS2025 catalog. Using James Webb Space Telescope NIRCam images from the COSMOS-Web survey, we measure the rest-frame optical sizes and Sérsic indices. We first examine their size–mass relations and find that LAEs at all three redshifts have smaller sizes than typical SFGs, with the size difference decreasing at higher redshifts. We also find that LAEs tend to have larger Sérsic indices at z = 2.4 and 3.1 than typical SFGs, but the difference becomes weaker at z = 4.5. These trends are qualitatively reproduced in the Horizon Run 5 cosmological hydrodynamical simulation. We then investigate star formation activity and find that LAEs exhibit higher star formation rates (SFRs) than typical SFGs at all redshifts considered. Finally, we examine the connection between Ly α emission and galaxy structure, finding that the rest-frame equivalent width (REW) of the Ly α emission line is negatively and positively correlated with size and Sérsic index, respectively. In addition, we find a strong positive correlation between Ly α REW and the ratio of the instantaneous SFR to that averaged over the last 100 Myr (i.e., SFR _inst /SFR _100Myr ). These results suggest the compact and starbursting nature of LAEs, and provide important constraints on the physical mechanisms responsible for Ly α photon escape from galaxies.

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.

The superb sensitivity and angular resolution of the next-generation radio telescopes with combined frequency coverage of approximately over 3 orders of magnitude (100 MHz–100 GHz) will sample the radio and far-infrared (FIR) spectral energy distribution (SED) of galaxies and revolutionize the galaxy formation study at the epoch of reionization and beyond. We present a prospect of observing the radio–FIR continuum SEDs of galaxies in the redshift of up to z ≈ 20 based on an ensemble of the simulated “energy balanced” panchromatic SED (from UV to FIR) extended to the radio. For “realistic” populations of UV star-forming galaxies and dusty star-forming galaxies (DSFGs), we simulate their SEDs by accounting for the CMB effect and the radio-IR correlation. The flux density evolution of the UV-bright star-forming galaxies and the DSFGs at the selected observing frequencies covered by the current (ALMA) and next generation (SKA and ngVLA) radio-millimeter telescopes, suggest that massive galaxies ( M _* ≳ 10 ¹⁰ M _⊙ ) are detectable at any redshift (0 < z < 20) in high frequency ( ν > 90GHz). In particular, when operating, the ngVLA high-frequency (≈100 GHz) band is capable of detecting galaxies with M _* ≳ 10 ⁹ M _⊙ almost independently from redshift and the SKA low-frequency observing window (≲1 GHz) has sufficient sensitivity to detect M _* ≳ 10 ¹⁰ M _⊙ DSFGs up to the epoch of reionization ( z = 5 ∼ 7). We also show that the brightness of anomalous microwave emission (AME) in the galaxy SED is insignificant if the galaxies are beyond the local Universe (e.g., z ≳ 0.1).

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.

We present the Parallel Application of Slitless Spectroscopy to Analyze Galaxy Evolution (PASSAGE) spectroscopic redshift catalog in the COSMOS field. PASSAGE is a JWST Cycle 1 Near Infrared Imager and Slitless Spectrograph (or NIRISS) wide-field slitless spectroscopy pure-parallel survey, obtaining near-infrared spectra of thousands of extragalactic sources. Fifteen out of 63 PASSAGE fields fall within the Hubble Space Telescope COSMOS footprint, of which 11 overlap with COSMOS-Web, a JWST treasury survey providing additional space-based photometry. We present our custom line-finding algorithm and visual inspection effort used to identify emission lines and derive the spectroscopic redshifts for line-emitting sources in PASSAGE. The line-finding algorithm identifies between ∼200 and 950 line-emitting candidates per field, of which typically 47% were identified as true emission lines post visual inspection. We identify 2183 emission-line sources at 0.08 ≲ z ≲ 4.7, 1896 of which have available COSMOS photometric redshifts. We find excellent redshift agreement between the COSMOS photometric redshifts and the PASSAGE spectroscopic redshifts for strong (signal-to-noise ratio > 5), multi-line-emitting sources. This agreement weakens for PASSAGE single-line emitters with ambiguous identities. These single-line emitters are likely misidentified around 18% of the time based on comparisons to photometric redshifts. We derive stellar masses using PASSAGE photometry and spectroscopic redshifts, in broad agreement with existing COSMOS-Web stellar masses, but with some discrepancy driven by redshift disagreements. We publicly release this spectroscopic redshift catalog, which will enable community-led science in prime extragalactic fields and serve as a crucial dataset for validating Euclid and Roman spectroscopy.

We present a reconstruction of the large-scale structure, using the James Webb Space Telescope (JWST) COSMOS-Web program to trace environmentally driven galaxy evolution up to z ∼ 7. We apply a weighted kernel density estimation method to 164,000 galaxies with robust photometric redshifts. We find that stellar mass has a positive correlation with density at all redshifts, which is stronger for quiescent galaxies (QGs) at z ≲ 2.5, while at higher redshifts (2.5 ≲ z ≲ 5.5) this trend is confined to extreme overdense environments, consistent with early mass assembly in protoclusters. The star formation rate (SFR) shows a negative trend with density for QGs at z ≲ 1.2, reversing at z ≳ 1.8, while star-forming galaxies (SFGs) show a mild positive correlation up to z ∼ 5.5. The specific SFR remains nearly flat for SFGs and declines with density for QGs at z ≲ 1.2. Moreover, mass and environmental quenching efficiencies show that mass-driven processes dominate at z ≳ 2.5, that the two processes act with comparable strength between 0.8 ≲ z ≲ 2.5, and that environmental quenching becomes stronger for low-mass galaxies ( M _⋆ ≲ 10 ¹⁰ M _⊙ ) at z ≲ 0.8. These findings reveal that large-scale structure drives galaxy evolution by enhancing early mass assembly in dense regions and increasingly suppressing star formation in low-mass systems at later times, establishing the environmental role of the cosmic web across cosmic history. COSMOS-Web, the largest JWST survey, provides accurate and deep photometric redshifts, reaching 80% mass completeness at log M ⋆ / M ⊙ ∼ 8.7 at z ∼ 7, enabling the first view of how environments shaped galaxy evolution from the Epoch of Reionization to the present day.