We constrain the nebular attenuation curve and investigate dust geometry in star-forming galaxies at cosmic noon using photometric medium-band emission line measurements. We measure H-alpha emission line fluxes for a sample of 209 star-forming galaxies at 1.2<z<2.4 in MegaScience/UNCOVER with stellar masses spanning 7.85<log_10(M_*/M_⊙)<11.0. For 66 of these galaxies, we also measure a Pa-beta flux. We find that the Pa-beta/H-alpha line ratio increases strongly with stellar mass and star-formation rate (SFR) across our full mass range, indicating that more massive galaxies are dustier. We compare our results with a mass-, SFR-, and redshift-matched sample of galaxies from the MOSDEF survey with spectroscopic measurements of H-alpha/H-beta, finding that a shallow Reddy et al. (2025) nebular attenuation curve is more consistent with our observations than the typically assumed Cardelli et al. (1989) attenuation curve, especially for massive galaxies. This shallow attenuation curve could be explained by low dust covering fractions in star-forming regions. Through comparison to other studies, we show that assuming this shallower attenuation curve can increase the inferred A_Halpha,neb by up to 1 magnitude at high masses. We observe no trend between A_Halpha,neb and axis ratio, indicating that nebular attenuation is likely localized to small clumps. Altogether, our results strongly suggest that dust geometry is patchy and non-uniform, especially in massive galaxies. Our results highlight the ability of JWST medium bands to probe emission lines for large samples of galaxies, and statistically constrain dust properties in upcoming large programs.

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.

Galaxies at Cosmic Noon ( z ∼ 2–3) are characterized by rapid star formation that will lead to significant metal enrichment in the interstellar medium (ISM). While much observational evidence suggests that these galaxies are chemically distinct from those in the local Universe, directly measuring the ISM chemistry in large samples of high- z galaxies is only now possible with the observational capabilities of JWST. In this first key paper of the CECILIA program, we present the direct-method physical conditions and multielement abundances in 20 galaxies at Cosmic Noon. Using a combination of archival Keck/MOSFIRE and new ∼30 hr NIRSpec spectroscopy, we measure multiple electron gas densities and the temperature structure from the O ^+ and S ^2+ ions. We find that n _e [O ii ] and n _e [S ii ] are comparable but elevated with respect to n _e in local star-forming galaxies, and the simultaneous T _e [O ii ] and T _e [S iii ] generally agree with photoionization model T _e scaling relations. The O abundances in the CECILIA galaxies range from 12 + log(O/H) = 7.76 to 8.81 (12%–131% solar O/H), representing some of the highest direct-method metallicities and lowest T _e ( T _e [O ii ] ≈ 6500 K) measured with JWST to date. The CECILIA galaxies exhibit significantly subsolar S/O and Ar/O, a signature of predominant enrichment from core-collapse supernovae. The N/O–O/H trends in the CECILIA galaxies generally agree with the abundance trends in local nebulae, but the large scatter in N/O could be sensitive to the star formation history. The CECILIA observations demonstrate that exceptionally deep JWST spectroscopy can unveil the multielement ISM abundance patterns in typical high- z galaxies.

While the size-mass relation provides insight into the structural evolution of galaxies, the data available and methods employed have hindered our ability to study a detailed and comprehensive description of this key relation across cosmic history. The first paper in this series presents a morphology catalog based on 20 band JWST data in the field of Abell 2744. In this paper we utilize this catalog to measure the size-mass relation from 0.510, we discuss three distinct phases: Rapid growth at z>5, growth that mimics dark matter halos at 5< z <1 and a late plateau at 0.51. Our results imply that quiescent galaxies are smaller than their star-forming counterparts only at around log M_*/M_⊙ = 10; the two populations have similar sizes at lower and higher masses.

We present the number density of massive ( log ( M * / M ⊙ ) > 10.3 ) quiescent galaxies at 2 < z < 5 using JWST NIRSpec PRISM spectra. This work relies on spectra from RUBIES, which provides excellent data quality and an unparalleled, well-defined targeting strategy to robustly infer physical properties and number densities. We identify quiescent galaxy candidates within RUBIES through principal component analysis and construct a final sample using star formation histories derived from spectrophotometric fitting of the NIRSpec PRISM spectra and NIRCam photometry. By inverting the RUBIES selection function, we correct for survey incompleteness and calculate the number density of massive quiescent galaxies at these redshifts, providing the most complete spectroscopic estimates prior to cosmic noon to date. We find that early massive quiescent galaxies are surprisingly common (≳10 −5 Mpc −3 by 4 < z < 5), which is consistent with previous studies based on JWST photometry alone and/or in smaller survey areas. We compare our number densities with predictions from six state-of-the-art cosmological galaxy formation simulations. At z > 3, most simulations fail to produce enough massive quiescent galaxies, suggesting the treatment of feedback and/or the channels for early efficient formation are incomplete in most galaxy evolution models.

We use the DAWN JWST Archive to construct and characterise a sample of 116 little red dots (LRDs) across 2.3<z<9.3, selecting all sources with v-shaped UV-optical continua from NIRSpec/PRISM spectra and compact morphologies in NIRCam/F444W imaging. We show that LRD continuum spectra are ubiquitously well described by modified blackbodies across 0.4-1.0μm, with typical T 5000K or λ_peak 0.65μm across 2 dex in luminosity, and a tail toward T 2000K. LRDs therefore trace a locus in the Hertzsprung-Russell diagram that is directly analogous to stars on the Hayashi track, strongly supporting the picture that LRDs are AGN embedded in thermalised dense gas envelopes in approximate hydrostatic equilibrium. Hotter LRDs with λ_peak<0.65μm typically have strong Balmer breaks, redder UV slopes and high optical luminosities; other LRDs show weak or no Balmer breaks, and wide variety in β_UV and L_5100. Crucially, we demonstrate that the UV-optical continuum shapes and luminosities are strongly linked to the Hα, Hβ, [OIII] and OI line properties. There is a tight linear relation between the Hα and optical continuum luminosities, as well as Hα and OI_8446, indicating that Balmer, OI and optical emission must primarily be powered by the same source. The Balmer decrement increases strongly toward higher L_Hα, L_5100 and Balmer break strength, providing key evidence for luminosity-dependent effects of collisional (de-)excitation and resonant scattering in the gaseous envelopes. In contrast, we show that [OIII] emission likely originates from star-forming host galaxies, and that its strong correlation with Balmer break strength arises naturally from variation in the AGN-to-host ratio. Our work presents an empirical description of the nature and structure of LRDs, defining a new benchmark for ongoing LRD model developments.

Little red dots (LRDs) are an abundant population of compact high-redshift sources with red rest-frame optical continua, discovered by the James Webb Space Telescope (JWST). Their red colors and power sources have been attributed either to dust reddening of standard hot accretion disks or to intrinsically cool thermal emission from dense hydrogen envelopes, in both cases surrounding accreting supermassive black holes. These scenarios predict order-of-magnitude differences in emission temperature but have lacked decisive temperature diagnostics. Here we report a prominent absorption feature at rest-frame ∼ 1.4 μm in two out of four LRDs at z ∼ 2 with high signal-to-noise JWST spectra, among the coolest from a large LRD sample. The feature matches the shape and wavelength of the water absorption band seen in cool stars. Atmosphere models require T ≲ 3000 K to reproduce it, confirming unambiguously the presence of a cool, dense gas component contributing 20-30% to the emergent continuum. A composite model reproduces both the absorption and the rest-frame optical-to-infrared continuum shape and suggests a temperature range (∼2000 K - 4000 K) rather than a single blackbody predicted by some gas envelope models. Molecular absorption demonstrates that the red continua of some LRDs are intrinsic rather than dust-reddened, implying order-of-magnitude lower bolometric luminosities and black-hole masses, and providing a new diagnostic of the emitting gas.

Observations of the early Universe (z > 4) with the James Webb Space Telescope reveal galaxy populations with a wide range of intrinsic luminosities and colors. Bursty star formation histories (SFHs), characterized by short-term fluctuations in the star formation rate (SFR), may explain this diversity, but constraining burst timescales and amplitudes in individual galaxies is challenging due to degeneracies and sensitivity limits. We introduce a population-level simulation-based inference framework that recovers the power and timescales of SFR fluctuations by forward-modeling galaxy populations and distributions of rest-UV to rest-optical spectral features sensitive to star formation timescales. We adopt a stochastic SFH model based on a power spectral density formalism spanning 1 Myr-10 Gyr. Using simulated samples of N=500 galaxies at z 4 with typical JWST/NIRSpec uncertainties, we demonstrate that: (i) the power of SFR fluctuations can be measured with sufficient precision to distinguish between simulations (e.g., FIRE-2-like vs. Illustris-like populations at >99

Galaxy profile fitting is a ubiquitous technique that provides the backbone for photometric and morphological measurements in modern extragalactic surveys. A recent innovation in profile fitting algorithms is to render, or create, the model profile in Fourier space, which aims to provide faster and more accurate results. However, the most common parameterization, the Sérsic profile, has no closed form Fourier transform, requiring the use of computationally expensive approximations. In this paper our goal is to develop an emulator to mimic the radial Fourier transform of the Sérsic profile, for use in profile fitting. We first numerically compute the radial Fourier transform and demonstrate that it varies smoothly as a function of the Sérsic index and k, the spatial frequency coordinate. Using this set of numerically calculated transforms as a training set, we use symbolic regression to discover an equation which approximates its behavior. This ensures the emulator will be based on computationally efficient and differentiable building blocks. We implement this novel rendering method in the pysersic profile fitter, and ensure it is accurate by conducting both injection-recovery tests using model galaxy profiles and applying multiple rendering methods to a real sample of galaxies in HSC-SSP imaging. Crucially, the Fourier emulator rendering technique enables measurements of morphological parameters of galaxies 2.5 times faster than standard methods with minimal loss in accuracy. This increased performance while maintaining accuracy is a step that ensures these tools can continue to scale with the ever-increasing flow of incoming data.

The discovery of a population of massive, ancient quiescent galaxies within the first 2 Gyr of the Universe's history has led to significant tensions with models of galaxy formation. However, these analyses are often based on slit spectroscopy, which typically captures only the center-most region of these galaxies and, crucially, assumes these cores are representative of the entire galaxy. To illustrate the varying stellar populations present throughout these galaxies, we present an analysis of color gradients in four z>3, log(M_⋆/M_⊙)>11 quiescent galaxies which previous works have argued are in tension with models. Using medium-band photometry from MINERVA JWST observations, we measure resolved photometry in a series of elliptical annuli out to 0.7^'' (∼4 R_e). We find negative color gradients in three galaxies, and for the most extreme color gradient (Δ(U-V)/ΔR=-0.126±0.030 mag kpc^-1), we find the stellar mass is 0.1 dex lower when compared to photometry measured within NIRSpec slits. In the limiting case where these color gradients are entirely driven by age, we find lessened tensions with extreme value statistics models out to z∼9.5, though different stellar population modeling choices also contribute significantly. Ultimately, these findings highlight the need for integral field unit spectroscopy. Spatially-resolved spectra can provide the evidence needed to break the age–dust–metallicity degeneracy, and reliably separate the effects of the observed color gradients from the effects of different physical modeling assumptions on the formation histories of these galaxies.

We present spectrally-resolved structural parameter measurements of 29,608 sources from the legacy lensing field of Abell 2744, quantifying global structures from observed 0.7 μm - 4.8 μm and spanning rest-frame UV to NIR at R∼15. These measurements are made on imaging mosaics mainly from the UNCOVER/MegaScience survey, including 20 JWST NIRCam broad and medium bands. We perform single-component Sérsic fitting to these galaxies using , a Bayesian structural fitting tool, to infer their structural parameters and associated random uncertainties from the posterior distributions. Through various quality evaluation criteria, we infer robust structural parameters among > 90% of the selected SNR>10 sources. For each galaxy with reliable sizes in at least two bands and a high-quality redshift, we fit its observed size as a function of wavelength and infer rest-frame UV, optical, and near-infrared sizes where applicable. By performing injection-recovery tests on simulated galaxy cutouts in selected bands, we establish that our structural parameter measurements achieve fractional error < 10 -20% above SNR>10. With this paper, all raw structural measurements and fitted rest-frame sizes are quality-flagged, cataloged, and released to the community. Finally, we demonstrate that this catalog enables the structural study of galaxies over an unprecedentedly wide parameter space of redshift (0.3

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 correlation between galaxy stellar mass and gas-phase metallicity, known as the mass-metallicity relation (MZR), gives key insights into the processes that govern galaxy evolution. However, unquantified observational and selection biases can result in systematic errors in attempts to recover the intrinsic MZR, particularly at higher redshifts. We characterize the MZR at z similar to 3-6 within a fully Bayesian framework using JWST/NIRSpec spectra of 191 galaxies from the RUBIES survey. We forward model the observed mass-metallicity surface using prospector-generated spectra to account for two selection biases: the survey selection function and the success in observing high signal-to-noise ratio emission lines. We demonstrate that the RUBIES selection function, based on F444W magnitude and F150W - F444W color, has a negligible effect on our measured MZR. A correct treatment of the non-Gaussian metallicity uncertainties from strong-line calibrations lowers the derived MZR normalization by 0.2 dex and flattens the slope by similar to 20%; forward modeling the effect of emission line observability steepens the slope by similar to 15%. Both of these biases must be taken into account in order to properly measure the intrinsic MZR. This novel forward-modeling process motivates careful consideration of selection functions in future surveys, and paves the way for robust, high-redshift chemical enrichment studies that trace the evolution of the MZR across cosmic time.

Galactic outflows play a significant role in regulating galaxy evolution. Scaling relations between stellar mass (M_∗), star formation (SFR), and outflow properties have been extensively studied at low redshifts (z<1) but less so beyond z ∼ 2. We construct a joint sample of 387 galaxies from z ∼ 0-9, including 98 new Cosmic Noon galaxies from the Keck Baryonic Structure Survey and the Keck Lyman Continuum Spectroscopic Survey. Using high signal-to-noise emission lines (SNR > 50 for Hα or [OIII] λ5007) from Keck/MOSFIRE spectra, we detect warm-ionized outflows by decomposing lines into narrow and broad components. With the joint sample, we explore the redshift evolution of outflow scaling relations. On average, outflows at Cosmic Noon have higher maximum velocities than those at low-z by up to a factor of 3 for a fixed M_∗, SFR, or SFR surface density. They also have higher mass outflow rates for a fixed M_∗. Despite faster and stronger outflows, there is no evolution in the mass loading factor for a fixed M_∗. We find evidence for galactic fountains, as the majority of outflowing gas is recycled at a radius of ∼0.03 R_vir. We also constrain the overall outflow occurrence rate in our galaxy sample to be at least 30% when taking galaxy orientation and outflow geometry into account. By analyzing the largest sample of warm-ionized outflows at Cosmic Noon to date and compiling large galaxy samples across all redshifts, we present a comprehensive analysis of galactic outflows throughout cosmic time.

We present first results from the JWST LEGGOS Survey (LEnsing and Galaxy Growth: Observing Substructures), aimed at studying the physics of clumpy star formation and quenching in eight lensed galaxies at z∼2–4. LEGGOS combines multiple Cycle 2 JWST GO programs (GO 4125, GO 3843) and Cycle 1 archival data, and utilizes strong gravitational lensing with NIRCam imaging and NIRSpec integral-field spectroscopy. LEGGOS targets UV-bright, highly magnified systems to resolve ∼10–200 pc regions in both rest-frame optical continuum and nebular emission. This overview paper describes the survey design, data reduction and calibration strategy, and science-quality data products, and highlights early examples demonstrating how spectroscopy breaks key degeneracies inherent to photometry-only clump studies, including identifying recent quenching in previously-thought UV star forming galaxies. We introduce a uniform analysis framework that jointly models lensing reconstruction, multi-band photometry, and integral field spectroscopy to disentangle multiple stellar populations within individual clumps and their surrounding diffuse regions. Using maps of Balmer recombination lines and key emission line diagnostic ratios, we connect star formation histories, dust attenuation, and nebular conditions on sub-kpc scales – LEGGOS galaxies range from uniform metallicities across the whole galaxy, to having higher clump metallicities and harder ionization conditions relative to diffuse regions. The full survey dataset, with simultaneous flux and morphology constraints on clumpy source-plane regions, and a flexible spectrophotometric SPS modeling approach, provides a direct bridge between parsec-scale star formation physics and galaxy assembly at and beyond cosmic noon, offering a robust and efficient means of resolving star formation in the first galaxies.

We explore the prevalence of mini-quenched or “napping” galaxies selected from spectroscopic and photometric samples in the UNCOVER/MegaScience survey. These galaxies are empirically identified by the presence of moderate Balmer breaks, weak emission lines (EW(Hα) < 100A) and relatively blue UV continua. We infer the star formation histories (SFHs) of our sample using flexible non-parametric models with Prospector optimized to capture recent episodes of bursty star formation and quenching, and find that they are uniquely identifiable in the SFR_10/SFR_100 parameter space moving towards (temporary) quiescence. We demonstrate that although spectroscopy is best able to identify rapidly declining SFRs, densely sampled medium-band photometry recover these key spectral features and thus robustly identify pure samples of this transient phase – with imaging alone. We quantify the number density of napping galaxies at z=4-7 in the Abell 2744 lensing field, finding 8 spectroscopically confirmed nappers and 60 photometric candidates spanning log_10(M_*/M_⊙) = 7.5-10. We verify that the photometry alone can identify a pure sample of nappers, leveraging a smaller high signal-to-noise ratio spectroscopic sample. Consistent with previous studies, we find that nappers are most common at low stellar mass (log_10(M_*/M_⊙) ∼9). We see a hint that the number densities increase from z∼6 to z∼4, though our small sample is likely affected by cosmic variance. Our study demonstrates the increasing importance of stochastic star formation as a regulator of low-mass galaxy growth in the several hundred Myr after reionization, and offers a direct observational testbed for the strength and duty cycle of stellar feedback in cosmological simulations.

The James Webb Space Telescope has unveiled an abundant population of potential active galactic nuclei (AGN) at high redshift (z greater than or similar to 4) known as little red dots (LRDs), which are likely hosted in relatively low-mass galaxies. However, previous theoretical models have highlighted the difficulty in continuously feeding massive black holes in the central regions of bursty, high-redshift galaxies because of repeated gas evacuation by stellar feedback. We analyze galaxies in high-redshift FIRE-2 simulations to understand whether they are capable of hosting the observed abundant population of high-redshift AGN. We use a gravitational torque-driven accretion (GTDA) model and a simple freefall accretion model to derive black hole accretion rates and construct predicted AGN bolometric luminosity functions for z = 5-7. The GTDA model and the freefall model with black holes accreting less than or similar to 1% of their central gas supply (<100 pc) per freefall time predict AGN abundances that are more than sufficient to explain the most recent LRD observations. The fiducial models, in fact, overpredict the number of low-luminosity AGN as compared with observations. We explore possible resolutions of this tension. A plausible, though likely not unique, scenario for alleviating the AGN overpredictions, which also provides a good match to the host-galaxy UV luminosity distribution, suggests that LRDs are super-Eddington-accreting, Eddington-luminosity-limited, M-BH greater than or similar to 2 & times; 10(5)M(circle dot) black holes residing in M-star greater than or similar to 2 & times; 10(7)M(circle dot) galaxies. We show that, under simple assumptions, mock observations of such sources can reproduce key observed LRD characteristics.

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 ∼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.

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.

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.