While bursty star formation in low-mass galaxies has been observed in local populations and reproduced in simulations, the dormant phase of the burst cycle has not been well studied beyond the local Universe due to observational limitations. We present a unique sample of 43 JWST PRISM spectra of low-mass galaxies ( M _⋆ < 10 ^9.5 M _⊙ ) at cosmic noon (1 < z < 3), uniformly selected on F200W magnitude and precise photometric redshifts enabled by 20-band JWST photometry from the UNCOVER and MegaScience surveys. The spectra reveal numerous strong Balmer breaks, which are negatively correlated with the galaxies’ H α equivalent width. By comparing these observations to synthetic samples of spectra generated using a simple parameterization of bursty star formation histories, we show that star formation in low-mass galaxies at cosmic noon is likely dominated by burst cycles with long timescales (≳100 Myr) and large deviations below the star-forming main sequence (≳0.8 dex). Our results suggest that galaxies in this population—at least those within our detection limits—should not be classified solely by their current star formation rates, but instead viewed as a unified population undergoing dynamic movement above and below the star-forming main sequence. The derived constraints demonstrate that long-timescale fluctuations are important for this class of galaxies, indicating that galaxy-scale gas cycles—rather than molecular-cloud-scale stochasticity—are the primary regulators of star formation variability in low-mass galaxies at cosmic noon.

JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, z>10, where few such sources were expected. Here we present the most distant example of this class yet – MoM-z14, a luminous (M_UV=-20.2) source in the COSMOS field at z_spec=14.44^+0.02_-0.02 that expands the observational frontier to a mere 280 million years after the Big Bang. The redshift is confirmed with NIRSpec/PRISM spectroscopy through a sharp Lyman-α break and ∼3σ detections of five rest-UV emission lines. The number density of bright z_spec∼14-15 sources implied by our "Mirage or Miracle" survey spanning ∼350 arcmin^2 is >100× larger (182^+329_-105×) than pre-JWST consensus models. The high EWs of UV lines (∼15-35 Å) signal a rising star-formation history, with a ∼10× increase in the last 5 Myr (SFR_5Myr/SFR_50Myr=9.9^+3.0_-5.8). The source is extremely compact (circularized r_e = 74^+15_-12 pc), and yet elongated (b/a=0.25^+0.11_-0.06), suggesting an AGN is not the dominant source of UV light. The steep UV slope (β=-2.5^+0.2_-0.2) implies negligible dust attenuation and a young stellar population. The absence of a strong damping wing provides tentative evidence that the immediate surroundings of MoM-z14 may be partially ionized at a redshift where virtually every reionization model predicts a ∼100% neutral fraction. The nitrogen emission and highly super-solar [N/C]>1 hint at an abundance pattern similar to local globular clusters that may have once hosted luminous supermassive stars. Since this abundance pattern is also common among the most ancient stars born in the Milky Way, we may be directly witnessing the formation of such stars in dense clusters, connecting galaxy evolution across the entire sweep of cosmic time.

We report the discovery of two z similar to 12 galaxy candidates with unusually red UV slopes (beta(UV) greater than or similar to -1.5), and probe the origin of such colors at cosmic dawn. From Prospector fits to the UNCOVER/MegaScience dataset-deep JWST/NIRCam imaging of A2744 in 20 broad- and medium bands-we identify several new z > 10 galaxies. Medium-band data improve redshift estimates, revealing two lensed (mu similar to 3.3) z similar to 12 galaxies in a close pair with beta(UV) greater than or similar to -1.5 at an UV absolute magnitude of M-UV similar to -19 mag, lying away from typical scatter on previously known M-UV-beta(UV) relations. Spectral energy distribution fitting with Prospector, Bagpipes, and EAZY support their high-z nature, with probability of low-z interlopers of p(z < 7) < 10%. The potential low-z interlopers are z similar to 3 quiescent galaxies (QGs), but unexpected to be detected at the given field of view unless z similar to 3 QG stellar mass function has a strong turn up at log M-*[M-circle dot]similar to 9 . Unlike typical blue high-redshift candidates (beta(UV) less than or similar to -2.0), these red slopes require either dust or nebular continuum reddening. The dust scenario implies A(V) similar to 0.8 mag, which is larger than theoretical predictions, but is consistent with a dust-to-stellar mass ratio ( log M-dust/M-*similar to-3 ). The nebular scenario demands dense gas ( logn(H)[cm(-3)]similar to 4.0 ) around hot stars ( log T-eff[K]similar to 4.9 ). Spectroscopic follow-up is essential to determine their true nature and reveal missing galaxies at the cosmic dawn.

New populations of red active galactic nuclei (known as “little red dots”) discovered by JWST exhibit remarkable spectral energy distributions. Leveraging X-ray through far-infrared observations of two of the most luminous known little red dots, we directly measure their bolometric luminosities. We find evidence that more than half of the bolometric luminosity likely emerges in the rest-frame optical, with L _bol / L _5100 = 5, roughly half the value for “standard” active galactic nuclei. Meanwhile, the X-ray emitting corona, UV-emitting blackbody, and reprocessed mid to far-infrared emission are all considerably subdominant, assuming that the far-infrared luminosity is well below current measured limits. We present new bolometric corrections that dramatically lower inferred bolometric luminosities by a factor of 10 compared to published values in the literature. These bolometric corrections are in accord with expectations from models in which gas absorption and reprocessing are responsible for the red rest-frame optical colors of little red dots. We discuss how this lowered luminosity scale suggests a lower mass scale for the population by at least an order of magnitude (e.g., ∼10 ^5 –10 ^7 M _⊙ black holes, and ∼10 ^8 M _⊙ galaxies), alleviating tensions with clustering, overmassive black holes, and the integrated black hole mass density in the Universe.

Globular clusters (GCs) are some of the oldest bound structures in the Universe, holding clues to the earliest epochs of star formation and galaxy assembly. However, accurate age measurements of ancient clusters are challenging due to the age-metallicity degeneracy. Here, we report the discovery of 36 compact stellar systems within the 'Relic', a massive, quiescent galaxy at z=2.53. The Relic resides in an overdensity behind the Abell 2744 cluster, with a prominent tidal tail extending towards two low-mass companions. Using deep data from the UNCOVER/MegaScience JWST Surveys, we find that clusters formed in age intervals ranging from 8 Myr up to ∼2 Gyr, suggesting a rich formation history starting at z∼10. While the cluster-based star formation history is broadly consistent with the high past star formation rates derived from the diffuse host galaxy light, one potential discrepancy is a tentative ∼2-3× higher rate in the cluster population for the past Gyr. Taken together with the spatial distribution and low inferred metallicities of these young-to-intermediate age clusters, we may be seeing direct evidence for the accretion of star clusters in addition to their early in situ formation. The cluster masses are high, ∼10^6-10^7 M_⊙, which may explain why we are able to detect them around this likely post-merger galaxy. Overall, the Relic clusters are consistent with being precursors of the most-massive present-day GCs. This unique laboratory enables the first connection between long-lived, high-redshift clusters and local stellar populations, offering insights into the early stages of GC evolution and the broader processes of galaxy assembly.

The physical processes that led to the formation of billion-solar-mass black holes within the first 700 million years of cosmic time, a period known as cosmic dawn, remain a puzzle1. Several theoretical scenarios have been proposed to seed and rapidly grow black holes2-4, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that exhibits singular properties: among the largest hydrogen Balmer breaks reported at any redshift, broad multi-peaked Hβ emission, and Balmer line absorption in several transitions. We model this source as an enshrouded black hole in which the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free envelope around a supermassive black hole5,6. This source may provide evidence of an early black hole embedded in dense gas-a theoretical configuration proposed to rapidly grow black holes by super-Eddington accretion7,8. Radiation from the black hole seems to dominate almost all observed light, leaving limited room for contribution from its host galaxy. If the source merged with its brighter neighbour, it would resemble the recently discovered 'little red dots' with perplexing spectral energy distributions9-11. The redness of the black hole is due to gas, not dust12,13, and scattering, not kinematics, gives rise to the complex line shapes and luminosities-black hole masses of these sources may therefore be overestimated by orders of magnitude.

Dusty star-forming galaxies (DSFGs) have long been suspected to serve as the missing evolutionary bridge between the star-forming and quiescent phases of massive galaxy evolution. With the combined power of JWST and the Atacama Large Millimeter/submillimeter Array (ALMA), it is now possible to use high-resolution imaging at rest-frame ultraviolet (UV), optical, near-infrared (NIR), and submillimeter wavelengths to study the multiwavelength morphologies tracing both the stellar populations and dust during this key phase. We present the joint analysis of JWST/NIRCam imaging in GOODS-S and millimeter dust emission traced by ALMA for a sample of 33 galaxies at z = 1.5–5.5 selected from the 1.1 mm GOODS-ALMA 2.0 survey, and compare the morphologies of this population to mass- and redshift-selected samples of field star-forming and quiescent galaxies. The 1.1 mm selected sample is morphologically distinct from other similarly massive star-forming galaxies; we find a steeper size-wavelength gradient from 1.5 to 4.4 μ m, with a more dramatic decrease in size toward longer wavelengths. While the rest-NIR surface brightness profiles of the 1.1 mm selected galaxies are brighter in the inner regions relative to the field star-forming population, they are remarkably similar to the quiescent population. These morphological differences could suggest that DSFGs, unlike more typical star-forming galaxies, have already built up stellar mass in a severely dust-obscured core, leading to extended and clumpy morphologies at rest-UV and rest-optical wavelengths and more compact emission in the rest-NIR that is co-spatial with dust. If the bulge is already established, we speculate that millimeter-selected galaxies may imminently evolve to join their quiescent descendants.

James Webb Space Telescope (JWST) has revealed temporarily quenched and ultraviolet-luminous galaxies in the early Universe, suggesting enhanced star formation stochasticity. Verifying this hypothesis is critical yet challenging. Outshining, wherein light from young stars dominates the spectral energy distribution, represents perhaps the greatest challenge in inferring the formation histories of unresolved galaxies. In this paper, we take a simple model of burstiness and show that state-of-the-art inference methods with flexible star formation histories (SFHs) and neutral priors, while recovering average star formation rates (SFRs; ∼0.1 dex median offset), fail to recover the complexities of fluctuations on tens of Myr timescales, and typically underestimate masses in bursty systems (∼0.15 dex). Surprisingly, detailed SFH recovery is still sensitive to priors even when data quality is optimal, e.g., including high signal-to-noise (20 pixel ^−1 ) spectroscopy with wide coverage (rest-frame 0.12–1.06 μ m). Crucially, however, refitting the same data with a prior correctly encoding the bursty expectation eliminates these biases: median offsets in mass and SFRs decrease to ∼0.04 dex and ∼0.05 dex, respectively. Under the assumption that current population burstiness predicts past SFH, the solution to outshining in modeling statistical samples is empirically measuring recent galaxy SFHs with population modeling. A prototype is H α /UV: while helpful, it is insufficient to constrain the expected complex burstiness. To this end, we introduce a more complete, quantitative population-level approach and demonstrate that it promises to recover the typical amplitude, timescale, and slope of the recent SFH to high accuracy. This approach thus has the strong potential to solve outshining using observations from JWST.

The James Webb Space Telescope (JWST) is unveiling the rest-frame near-IR structure of galaxies. We measure the evolution with redshift of the rest-frame optical and near-IR Sérsic index (n), and examine the dependence on stellar mass and star-formation activity across the redshift range 0.5≤ z≤2.5. We infer rest-frame near-IR Sérsic profiles for ≈ 15.000 galaxies in publicly available NIRCam imaging mosaics from the COSMOS-Web and PRIMER surveys. We augment these with rest-frame optical Sérsic indices, previously measured from HST imaging mosaics. The median Sérsic index evolves slowly or not at all with redshift, except for very high-mass galaxies (M_⋆ > 10^11 M_⊙), which show an increase from n≈ 2.5 to n≈ 4 at z<1. High-mass galaxies have higher n than lower-mass galaxies (M_⋆=10^9.5 M_⊙) at all redshifts, with a stronger dependence in the rest-frame near-IR than in the rest-frame optical at z>1. This wavelength dependence is caused by star-forming galaxies that have lower optical than near-IR n at z>1 (but not at z<1). Both at optical and near-IR wavelengths, star-forming galaxies have lower n than quiescent galaxies, fortifying the connection between star-formation activity and radial stellar mass distribution. At z>1 the median near-IR n varies strongly with star formation activity, but not with stellar mass. The scatter in near-IR n is higher in the green valley (0.25 dex) than on the star-forming sequence and among quiescent galaxies (0.18 dex) – this trend is not seen in the optical because dust and young stars contribute to the variety in optical light profiles. Our newly measured rest-frame near-IR radial light profiles motivate future comparisons with radial stellar mass profiles of simulated galaxies as a stringent constraint on processes that govern galaxy formation.

Tracking the cold molecular gas contents of galaxies is critical to understand the interplay between star formation and galaxy growth across cosmic time. Observations of the long-wavelength dust continuum, a proxy for the cold gas, are widely used in the high-redshift community because of their ease and efficiency. These measurements rely on the assumption of a molecular gas-to-dust mass ratio, typically taken to be δ _GDR ≈ 100 in massive, metal-rich systems. We present Atacama Large Millimeter/submillimeter Array observations of the 870 μ m dust continuum in a sample of five massive quiescent galaxies at z ∼ 1 with existing detections of CO(2–1). We find surprisingly weak dust emission, falling a factor of ≳0.4–0.8 dex below the typical correlation between CO and continuum luminosity. We interpret this dust deficiency as evidence for unusually high δ _GDR in these galaxies, which we calculate to range from 300 to at least 1200. Our results and other observations from the literature are generally compatible with predictions from the SIMBA cosmological simulation that dust is preferentially destroyed in quiescent galaxies. Ultimately, we conclude that the dust continuum is a highly unreliable tracer of the molecular gas in high-redshift quiescent galaxies. As a consequence, we may know much less about the cold gas contents of this population than previously thought.

The physical nature of little red dots (LRDs), a population of compact red galaxies revealed by JWST, remains unclear. Photometric samples were constructed from varying selection criteria with limited spectroscopic follow-up available to test intrinsic spectral shapes and the prevalence of broad emission lines. We used the RUBIES survey, a large spectroscopic program with wide color-morphology coverage and homogeneous data quality, to systematically analyze the emission-line kinematics, spectral shapes, and morphologies of similar to 1500 galaxies at z > 3.1. We identified broad Balmer lines via a novel fitting approach that simultaneously models NIRSpec/PRISM and G395M spectra, yielding 80 broad-line sources with 28 (35%) at z > 6. A large subpopulation naturally emerged from the broad Balmer line sources, with 36 exhibiting v-shaped UV-to-optical continua and a dominant point source component in the rest-optical; we define these as spectroscopic LRDs, constituting the largest such sample to date. Strikingly, the spectroscopic LRD population is largely recovered when either a broad line or rest-optical point source is required in combination with a v-shaped continuum, suggesting an inherent link between these three defining characteristics. We compared the spectroscopic LRD sample to published photometric searches. Although these selections have high accuracy, 80%-95% down to F444W < 26.5, only 50%-80% of the RUBIES LRDs were photometrically identified, depending on the selection criteria used. The remainder were missed due to a mixture of faint rest-UV photometry, comparatively blue rest-optical colors, or highly uncertain photometric redshifts. Our findings highlight that well-selected spectroscopic campaigns are essential for robust LRD identification, while photometric criteria require refinement to capture the full population.

We report the discovery of an ultra-massive grand-design red spiral galaxy, named Zh & uacute;l & oacute;ng (Torch Dragon), at z(phot) = 5.2(-0.2)(+0.3) in the JWST PANORAMIC survey; it is the most distant bulge+disk galaxy candidate with spiral arms known to date. Zh & uacute;l & oacute;ng displays an extraordinary combination of properties: (1) a classical bulge centered in a large, face-on exponential stellar disk (half-light radius of R-e = 3.7 +/- 0.1 kpc) with spiral arms extending across 19 kpc; (2) a clear transition from the red, quiescent core (F150W - F444W = 3.1 mag) with a high stellar mass surface density (log(Sigma M-star/M-circle dot kpc(-2)) = 9.91(-0.09)(+0.11) to the star-forming outer regions, as revealed by spatially resolved spectral energy distribution analysis, which indicates significant inside-out galaxy growth; (3) an extremely high stellar mass for its redshift, with log(M-star/M-circle dot) = 11.03(-0.08)(+0.10), which is comparable to the Milky Way's mass, and an implied baryon-to-star conversion efficiency (& varepsilon; similar to 0.3) that is 1.5 times higher than even the most efficient galaxies at later epochs; and (4) despite an active disk, a relatively modest overall star formation rate (SFR = 66(-46)(+89) M(circle dot)yr(-1)), which is > 0.5 dex below the star formation main sequence at z similar to 5.2 and > 10 times lower than ultra-massive dusty galaxies at z = 5 - 6. Zh & uacute;l & oacute;ng shows that mature galaxies emerged much earlier than previously believed, in the first billion years after the Big Bang, through rapid galaxy formation and morphological evolution. Our finding offers key constraints for models of massive galaxy formation and the origin of spiral structures in the early Universe.

We explore the physical properties of five massive quiescent galaxies at z ∼ 2.5, revealing the presence of nonnegligible dust reservoirs. JWST NIRSpec observations were obtained for each target, finding no significant line emission; multiple star formation tracers independently place upper limits between 0.1 and 10 M ⊙ yr −1 . Spectral energy distribution modeling with Prospector infers stellar masses of log 10 [ M / M ⊙ ] ∼ 10 − 11 and stellar-mass-weighted ages between 1 and 2 Gyr. The inferred mass-weighted effective radii ( r eff ∼ 0.4–1.4 kpc) and inner 1 kpc stellar surface densities ( log 10 [ Σ < 1 kpc / M ⊙ kpc 2 ] ≳ 9 ) are typical of quiescent galaxies at z ≳ 2. The galaxies predominately display negative color gradients (redder core and bluer outskirts); for one galaxy, this effect results from a dusty core. Unlike local quiescent galaxies, we identify significant reddening in these typical cosmic noon passive galaxies; all but one require A V ≳ 0.4. This finding is in qualitative agreement with previous studies, but our deep 20-band NIRCam imaging is able to significantly suppress the dust–age degeneracy and confidently determine that these galaxies are reddened. We speculate about the physical effects that may drive the decline in dust content in quiescent galaxies over cosmic time.

In its first two years of operation, the James Webb Space Telescope has enabled the discovery of a surprising number of UV-bright galaxies at z∼10-14. Their number density is still relatively uncertain, due to cosmic variance effects, and the limited survey area with deep imaging. Here, we combine pure parallel imaging from the PANORAMIC survey with data from legacy fields to constrain the bright end (M_ UV<-18.5) of the UV luminosity function (UVLF) over 0.28deg^2 of NIRCam imaging in 6 or more filters, and along 35 independent lines of sight. Using conservative color selections, we compile robust dropout samples at z∼10, z∼13, and z∼17, and identify 16 new candidates from PANORAMIC. Our inferred UVLFs at z∼10 are consistent with literature results and we confirm the high abundance of galaxies at the bright end (M_ UV≲-21) with better number statistics. We find somewhat lower number densities at z∼13 compared to previous studies, and no robust candidates at z∼17, indicating a rapid evolution of the galaxy population from z∼10-17. The improved upper limits at z∼17 imply that the cosmic UV luminosity density drops by at least a factor ∼50 from z∼10 to z∼17. Comparing our results to models proposed to explain the abundance of UV-bright galaxies at z≳10, we conclude that a modest increase in the star formation efficiency, or in the burstiness of star formation, a more top-heavy initial mass function, a lack of dust attenuation, or a combination of these effects at z≳10, is sufficient to match our observational constraints.

The origin of the rest-optical emission of compact, red, high-redshift sources known as little red dots (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam, and MIRI observations from the RUBIES and PRIMER programs of The Cliff: a bright LRD at z = 3.55 with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad hydrogen (H alpha FWHM similar to 1500 km s(-1)) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size (M-* similar to 10(10.5) M-circle dot, r(e) similar to 40 pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and H alpha absorption line are instead most plausibly explained by a black hole star (BH*) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH* models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas.

JWST has revealed an abundance of compact, red objects at z ≈ 5–8 dubbed “little red dots” (LRDs), whose SEDs display a faint blue UV continuum followed by a steep rise in the optical. Despite extensive study of their characteristic V-shaped SEDs, the nature of LRDs remains unknown. We present a new analysis of the NIRSpec/PRISM spectrum of A2744-QSO1, a triply imaged LRD at z = 7.04 from the UNCOVER survey. The spectrum shows a strong Balmer break and broad Balmer emission lines, both of which are difficult to explain with models invoking exclusively AGN or stellar contributions. Our fiducial model decomposes the spectrum into a post-starburst galaxy dominating the UV-optical continuum and a reddened AGN being subdominant at all wavelengths and contributing at a level of ∼20%. However, this model infers a stellar mass of M _⋆ ≈ 4 × 10 ^9 M _⊙ within a radius of r _e < 30 pc, driving its central density to the highest among observations to date. This high central density could be explained if A2744-QSO1 is the early-forming core of a modern-day massive elliptical galaxy that later puffed up via the inside-out growth channel. The models also necessitate an unusually steep dust extinction law to preserve the strong break strength, though this steepness may be explained by a deficit of large dust grains. It is also probable that these challenges reflect our ignorance of A2744-QSO1's true nature. Future variability and reverberation mapping studies could help disentangle the galaxy and AGN contribution to the continuum, and deeper redder observations could also unveil the dust properties in LRDs.

Over the past decades, a population of galaxies invisible in optical/near-infrared (NIR), but bright at longer wavelengths, have been identified through color selections. These so-called optically faint/dark galaxies are considered to be massive quiescent galaxies or highly dust-attenuated galaxies. Having the entire galaxy obscured by dust, however, is likely an extreme case of the much more common occurrence of optically thin and thick absorption coexisting in the same system. With the power of JWST imaging, we are able to spatially resolve massive galaxies at z similar to 3, accurately model their spectral energy distributions, and identify candidate optically thick substructures. We target galaxies with log(M-*/M-circle dot)> 10.3 and 2.5 < z < 3.5, and get 486 galaxies in Cosmic Evolution Early Release Science Survey and Public Release Imaging for Extragalactic Research fields. Based on excess NIR luminosity, we identify 162 galaxies (similar to 33% of the parent sample) as candidate hosts of optically thick substructures. We then carry out spatially resolved spectral energy distribution modeling to explore the physical properties of those dark substructures and estimate the amount of optically thick obscuration. We find that optically thick dust is ubiquitous in normal massive galaxies with a wide variety of star formation rate (SFR) and morphology. 10%-20% of the stellar mass/SFR are unaccounted for in our selected galaxies, and the fraction is insensitive to stellar mass or SFR. The dark substructures are generally dustier than the rest of the galaxies and are irregularly distributed, arguing against an obscured active galactic nucleus as the source of the NIR excess. A correlation between the obscured luminosity and the presence of a recent starburst in the past less than or similar to 100 Myr is also observed.

Among the most puzzling early discoveries of JWST are “little red dots” (LRDs), compact red sources that host broad Balmer emission lines, and in many cases exhibit a “V-shaped” change in slope in the rest-optical. The physical properties of LRDs currently have order-of-magnitude uncertainties, because models to explain the continuum of these sources differ immensely. Here, we leverage the complete selection of red sources in the RUBIES program, supplemented with public PRISM spectra, to study the origin of this V shape. By fitting a broken power law with a flexible inflection point, we find that a large fraction of red H α emitters at 2 < z < 6 exhibit a strong change in slope, and that all strong inflections appear associated with the Balmer limit (0.3645 μ m). Using a simple model of a reddened active galactic nucleus (AGN) with an unobscured scattered-light component, we demonstrate that the observed V shape in LRDs is unlikely to occur at any specific wavelength if the entire continuum is dominated by light from a power-law AGN continuum. In contrast, models with an intrinsic feature at the Balmer limit, such as those that are dominated by an evolved stellar population, can produce the observed spectral shapes, provided that a reddened component picks up sufficiently redward of the break. While no model can comfortably explain the full LRD spectral energy distribution, the common inflection location suggests that a single component consistently dominates the rest-frame UV optical in LRDs, and that this component is associated with T ∼ 10 ^4 K hydrogen.

We present an overview of the MINERVA survey, a 259.8 hour (prime) and 127 hour (parallel) Cycle 4 treasury program on the James Webb Space Telescope (JWST). MINERVA is obtaining 8 filter NIRCam medium band imaging (F140M, F162M, F182M, F210M, F250M, F300M, F360M, F460M) and 2 filter MIRI imaging (F1280W, F1500W) in four of the five CANDELS Extragalactic fields: UDS, COSMOS, AEGIS and GOODS-N. These fields were previously observed in Cycle 1 with 7 - 9 NIRCam filters by the PRIMER, CEERS and JADES programs. MINERVA reaches a 5σ depth of 28.1 mag in F300M and covers ∼ 542 arcmin^2, increasing the area of existing JWST medium-band coverage in at least 8 bands by ∼ 7×. The MIRI imaging reaches a 5σ depth of 23.9 mag in F1280W and covers ∼ 275 arcmin^2 in at least 2 MIRI filters. When combined with existing imaging, these data will provide a photometric catalog with 20-26 JWST filters (depending on field) and 26-35 filters total, including HST. This paper presents a detailed breakdown of the filter coverage, exposure times, and field layout relative to previous observations, as well as an overview of the primary science goals of the project. These include uncovering the physics of enigmatic sources hiding in current broadband catalogs, improving systematics on stellar mass functions and number densities by factors of ≳ 3, and resolved mapping of stellar mass and star formation at 1 < z < 6. When complete, MINERVA will become an integral part of the treasury deep field imaging datasets, significantly improving population studies with well-understood completeness, robust photometric redshifts, stellar masses, and sizes, and facilitating spectroscopic follow up for decades to come.

We present the redshift evolution of radial color gradients (in rest-frame U - V and V - J) for galaxies in the range 0.5< z <2.5 and investigate their origin and dependence on stellar mass. We select ∼ 10,200 galaxies with stellar masses M_⋆>10^9.5 M_⊙ from publicly available JWST/NIRCam-selected catalogs. Using 2D Sérsic profile fits to account for PSF broadening, we perform spatially resolved SED fitting on HST and JWST/NIRCam photometry retrieving accurate rest-frame U - V and V - J color gradients within 2R_e, F444W. Star-forming galaxies generally exhibit negative V - J color gradients that are strongly mass and redshift dependent. For massive star-forming galaxies (M_⋆>10^10.5 M_⊙) at z>1.5 V - J colors are ≈ 0.5 mag redder within the effective radius than outside, on average. We find that, at all redshifts and across the entire stellar mass range, V - J gradients strongly correlate with global attenuation (A_V), suggesting that they predominantly trace dust attenuation gradients. Edge-on galaxies are redder and have stronger gradients at all z, although the correlation weakens at higher z. The U - V and V - J color gradients in the quiescent galaxy population, in contrast, are weakly negative (from ≈ -0.1 to ≈- 0.2 mag), though significant, and show little or no dependence on stellar mass, redshift or axis ratio. The implication is that quiescent galaxies must be largely transparent, with low A_V, and color gradients mostly attributable to stellar population gradients.