Andreas L. Faisst

While stellar population synthesis (SPS) models have been widely used for spectral analysis in optical wavelengths, their characteristics remain uncertain in the near-infrared (NIR) due to a relative lack of observed NIR spectra. The spectrophotometric data from SPHEREx are well-suited for investigating the performance of SPS models in the NIR, thanks to its wide wavelength coverage over 0.75–5.0 μ m. In this work, we compare the observed SPHEREx data of SDSS compact galaxies, including 2726 non-emission-line galaxies and 1163 emission-line galaxies, to the NIR SEDs predicted from the full spectrum fitting of SDSS optical spectra. We use four different SPS models that extend into the NIR: E-MILES, Bruzual & Charlot (BC03), Charlot & Bruzual (CB19), and FSPS. We find that all four models tend to overpredict the stellar continuum at 2.4–5 μ m by 0.1–0.3 mag. This trend is particularly prominent for intermediate-age stellar populations (∼1–5 Gyr), suggesting a systematic bias in the NIR SED predictions of current SPS models. For stellar populations older than 5 Gyr, E-MILES shows relatively smaller offsets at 3.8–5 μ m compared to other models. Meanwhile, for emission-line galaxies, the SPS models underestimate the SED by up to ∼0.5 mag at longer wavelengths due to the contribution of nonstellar emission. Overall, these results highlight the necessity of refining the NIR stellar spectral features in SPS models, such as emissions from thermally pulsating asymptotic giant branch stars or molecular absorptions from cool stars.

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.

Context. A complete overview of the stellar, gas and dust contents of galaxies is key to understanding their assembly at early times. However, an estimation of molecular and atomic gas reservoirs at high redshift relies on various indirect tracers, while robust dust mass measurements require multiband far-infrared continuum observations. Aims. We present a census of the full baryonic content of the main sequence star-forming galaxy HZ10 at z = 5.65, a unique case study where all the necessary tracers are available. Methods. We used new ALMA Band 10 ( λ _rest ≈ 50 μm) and Band 4 (≈300 μm) observations toward HZ10, combined with data previously taken in ALMA Bands 6 through 9 (≈70 − 200 μm) to constrain its dust properties. We complete the baryonic picture using archival high-resolution [C II ] observations that provide both a dynamical mass and molecular and atomic gas mass estimates, a JVLA CO(2-1)-based molecular gas mass, and JWST metallicity and stellar mass measurements. Results. We detected continuum emission from HZ10 in Bands 10 and 4 at the 3.4 − 4.0 σ level. We measured a dust temperature of T _dust = 37 ⁺⁶ ₋₅ K and dust mass of log( M _dust / M _⊙ ) = 8.0 ± 0.1. By leveraging the dynamical constraints, we were able to infer its total gas budget. We find that commonly used [C II ]-to-H ₂ and [C II ]-to-HI conversions might overpredict the gas mass relative to the dynamical mass. For this reason, we derived a [C II ]-to-total-ISM mass (atomic + molecular) conversion factor. For HZ10, this corresponds to α _[CII] ^ISM = 38 ⁺²⁸ ₋₂₆ M _⊙ L _⊙ ⁻¹ . We also found that HZ10 falls below the local scaling relation between the dust-to-gas ratio and metallicity, suggesting inefficient ISM dust growth. Conclusions. These results demonstrate a powerful synergy between ALMA and JWST in disentangling the baryonic components of early galaxies, paving the way for future studies of larger samples to dynamically calibrate [C II ]-to-gas mass conversion factors and further unravel the pathways of early dust buildup.

The SPHEREx near-infrared space telescope is an all-sky spectroscopic survey mission launched on 2025 March 12 UTC. In addition to providing the community with a spectral database applicable to a wide range of investigations, it is optimized to address three core science goals: to survey the large-scale structure of the Universe for signatures of non-Gaussianity during inflation, to conduct intensity mapping studies of the extragalactic background light for probing the history of galaxy evolution, and to survey the plane of the Milky Way for the prevalence and distribution of water and other biogenic ices. Each of these science goals imposes unique requirements on the performance of the instrument. We detail the design and testing strategies and report the performance results for the full instrument test campaign, ranging from component-level screening to in-orbit tests during the commissioning phase. The instrument, currently operating in full science survey mode, meets all of its driving requirements including optical performance, point-source sensitivity, thermal stability, and correlated noise minimization.

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 report on the spectroscopic confirmation of overdense regions of massive quiescent galaxies (QGs) in the early Universe with JWST/NIRSpec. Based on data from the DeepDive NIRSpec program and archival data from the Dawn JWST Archive, we confirm three QGs in the vicinity of Jekyll and Hyde, a pair of massive QG and a dusty star-forming galaxy, at z = 3.71 and two QGs around SXDS-27434 at z = 4.01. According to the analysis of galaxy number density with photometric redshifts, Jekyll and Hyde (SXDS-27434) are in an overdense region, where the number density of galaxies is three (four) times higher than the average in the COSMOS (SXDS) field. SED fitting suggests that most of the QGs follow similar star formation histories and have consistent formation and quenching epochs. The same trend is observed in other protoclusters hosting QGs that were already identified by ground-based telescopes, indicating that the large-scale environment plays an important role in the formation of QGs. In addition, JWST spectra reveal a broad H α emission line from SXDS-27434 and faint emission lines from the other three QGs, which are identified as AGN-driven based on their emission line ratios. The overdensity is also reproduced by the Illustris TNG300 simulation at z = 3.71, in which the member QGs also have similar quenching epochs. These results are consistent with a scenario in which the large-scale structure enhances merger activity and/or gas accretion and triggers AGN feedback, thereby driving simultaneous formation and quenching of QGs in the overdensity.

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.

Dust is a key component of galaxies that regulates their thermal balance and, consequently, star formation and the buildup of stellar mass. Polycyclic aromatic hydrocarbons (PAHs) are responsible for reprocessing the radiative energy of galaxies, thus tracing dust evolution. Using JWST/MIRI Medium Resolution Spectroscopy (MRS), we present the first resolved view of the PAHs and mid-infrared emission lines in a sample of eight z ≈ 1 galaxies. A key novelty is our ability to directly map PAH emission near the end of cosmic noon at JWST’s limits. The sample is selected to be on the star-forming main sequence with stellar masses M _⋆ = 10 ^10.6−11.2 M _⊙ and infrared luminosities L _IR = 10 ^11.5−11.9 L _⊙ . Two of them contain active galactic nuclei (AGN), and two are interacting systems. We detect and quantify primary PAH emissions from 3.3 to 11.3 μ m throughout the galaxies, alongside atomic fine-structure lines (Ar, Ne, and Fe), Br α , and H ₂ rotational transitions. Through PAH ratio diagnostics and comparison to theoretical models, we qualitatively probe the physical properties of PAH molecules, i.e., size and charge. The AGN and mergers in our sample exhibit a higher fraction of neutral PAHs, possibly related to high radiation intensity and/or shocks, as suggested by increased atomic and H ₂ line ratios. Leveraging the integral field units data, we find that the grain sizes of the centrally located PAHs tend to be larger and less ionized than those in the outskirts of the galaxies. Finally, we compare our results to observations of PAHs in local and similar-redshift galaxies, revealing a potential evolutionary trend when controlling L _IR .

Aims. We investigate the mechanisms regulating the escape of Lyman-alpha (Ly α ) photons in the star-forming galaxy CRISTAL-02 at redshift z ≈ 5.3. Previous studies revealed a diffused and extended Ly α emitting gas (radius ∼16.5 kpc) and a possible [C II ] outflow. The galaxy has been identified with five distinct clumps, two of which have been suggested as potential active galactic nuclei (AGN) candidates (Clump A and B hereafter). We study how AGN or star formation activities influence the spatial distribution and visibility of Ly α emission. Methods. Using integral field spectroscopic Ly α data from VLT/MUSE and JWST/NIRSpec integral field unit (IFU) observations of H α and Hβ, complemented by NIRCam UV imaging, we constructed spatially matched emission-line maps. We derived flux, emission-line ratio, and extinction maps as well as spatially resolved measurements of the Ly α escape fraction and ionizing photon production efficiency. Results. We find that the Ly α emission extends preferentially along the [C II ] outflow direction, while the H α and UV emission trace the galactic disk. Two compact regions (Clumps A and B) exhibit contrasting properties: Clump A, associated with lower dust attenuation, shows enhanced Ly α /H α ratios and a higher Ly α escape fraction, whereas Clump B shows stronger H α and UV emission but suppressed Ly α . The results indicate that Ly α escape can be influenced by AGN or star formation-driven outflows, dust attenuation, and local gas conditions. Conclusions. This study provides one of the first spatially resolved measurements of the Ly α escape fraction and ionizing photon production efficiency in distant systems, and it highlights the role of feedback-driven gas clearing and anisotropic outflows in shaping the observed Ly α properties. The current observational evidence is most consistent with the presence of an outflow, while the mechanism driving it remains uncertain, as the available data do not uniquely distinguish between AGN and star formation-driven feedback. Future spatially resolved multiwavelength IFU observations combining Ly α , H α , and [C II ] emission will be essential to disentangling these scenarios and establishing the dominant feedback mechanism.

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.