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.

James Webb Space Telescope (JWST) observations have revealed that massive galaxies formed and evolved faster than predicted by galaxy formation models, with many having already assembled a large mass in stars approximately 12 billion years ago. However, masses of distant galaxies are uncertain, as they assume a distribution of stellar birth masses (the initial mass function (IMF)) similar to that in the Milky Way. Specifically, the contribution from low-mass stars, which make up the bulk of stellar mass, is not directly observed but inferred on the basis of an extrapolation of the Milky Way IMF. Here, we provide robust constraints on the low-mass IMF beyond the local Universe from full-spectrum models. Using ultra-deep spectra of nine massive quiescent galaxies at redshift z ≈ 0.7 from the JWST Initial Mass Function of Early Red NIRSpec Objects program, extended to bluer wavelengths with deep Very Large Telescope Large Early Galaxy Astrophysics Census spectra, we find that the most massive galaxies have excess low-mass stars. Remarkably, our oldest galaxy (formation redshift, zform > 5) has the most bottom-heavy IMF. This galaxy may be a descendant of JWST’s ‘impossibly early’ galaxies, implying that the latter may have had similarly bottom-heavy IMFs, increasing their masses by a factor of approximately 4 ± 1. Our findings may thus amplify the tension with galaxy formation models. JWST spectra show that distant massive galaxies contain far more low-mass stars than expected, quadrupling the overall galaxy mass and challenging galaxy formation models.

It is widely believed that the ultraviolet background produced during the epoch of reionization conspires against the formation of low-mass galaxies. Indeed, this mechanism is often invoked as part of the solution to the so-called “missing satellites problem.” In this paper we employ FIREbox , a large-volume cosmological simulation based on the Feedback In Realistic Environments physics model, to characterize the mechanisms governing galaxy ignition in the postreionization era. By carefully matching recently ignited halos (with stellar ages below 100 Myr at the time of selection) to halos that failed to form any stars, we conclude that the presence of cold dense gas and halo concentration helps incite the process of galaxy formation. Concretely, we find that 100% of recently ignited halos experience cold dense gas enhancements relative to their matched failed counterparts. Likewise, approximately 83% display enhancements in both cold dense gas and Navarro–Frenk–White concentration ( c _NFW ), while the remaining ∼17% exhibit enhanced cold dense gas content and suppressed c _NFW values. Lastly, our simulation suggests that galaxy ignition can occur as late as z = 2, potentially allowing us to observationally catch this process “in the act” in the foreseeable future.

Little Red Dots (LRDs) are compact sources with broad Balmer lines, Balmer breaks, anomalous UV emission, rising red continuum, and uncertain origin. We use FIRE cosmological simulations, 3D dust radiative transfer, and synthetic emission-line data cubes to test whether ultra-compact early galaxies can reproduce LRD-like observables without invoking AGN. In progenitors of present-day group halos (M_ halo > 10^13.5 M_⊙), we identify transient phases at z ≈ 4-8 lasting ∼ 150-400 Myr in which strong dissipative inflows build massive (M_⋆∼ 10^8.5-10^10.5 M_⊙), UV-bright (-23 ≲ M_ UV≲ -20), ultra-compact (R_ eff < 300 pc) stellar cores with extreme circular velocity (V_ circ > 500 km s^-1) and consistent with several LRD properties: strong Balmer breaks (F_ν(4200 Å)/F_ν(3500 Å) ∼ 2); blue UV beta slopes (β_ UV≈ -1.25); dust masses; ALMA non-detections; and Balmer-line widths up to ∼ 1500 km s^-1 broadened by galaxy-scale dynamics. However, stellar emission and host-galaxy kinematics alone do not reproduce the red rest-optical continuum, more extreme Balmer breaks (≳ 2.5) and line widths (≳ 2000 km s^-1), or the broad-Balmer/narrow-forbidden-line signature of broad-line AGN. The same ultra-compact conditions efficiently fuel central BHs, suggesting a hybrid stellar+AGN scenario in which compact stars explain the UV continuum, Balmer break, and intermediate line widths while AGN supply the red optical continuum and more extreme line properties. With halo masses M_ halo∼ 10^11-12.5 M_⊙ and comoving abundance ∼ 2 × 10^-5 cMpc^-3 (for ∼ 20% duty-cycle at z ≈ 4-8), ultra-compact galaxies can contribute to the massive, bright LRD population.

Recent observations from the James Webb Space Telescope have revealed unexpectedly luminous galaxies, exhibiting stellar masses and luminosities significantly higher than predicted by theoretical models at Cosmic Dawn. In this study, we present a suite of cosmological zoomed-in simulations targeting high-redshift (z >= 10) galaxies with dark matter halo masses in the range 1010-1011M circle dot at z = 10, using state-of-the-art galaxy formation simulation codes (Enzo, Ramses, Changa, Gadget-3, Gadget-4, and Gizmo). This study aims to evaluate the convergence of the participating codes and their reproducibility of high-redshift galaxies with the galaxy formation model calibrated at relatively low redshift, without additional physics for high-redshift environments. The subgrid physics follows the AGORA CosmoRun framework, with adjustments to resolution and initial conditions to emulate similar physical environments in the early Universe. The participating codes show consistent results for key galaxy properties (e.g., stellar mass), but also reveal notable differences (e.g., metallicity), indicating that galaxy properties at high redshifts are highly sensitive to the feedback implementation of the simulation. Massive halos (Mhalo >= 5 & times; 1010 M circle dot at z = 10) succeed in reproducing observed stellar masses, metallicities, and UV luminosities at 10 <= z <= 12 without requiring additional subgrid physics, but tend to underpredict those properties at higher redshift. We also find that varying the dust-to-metal ratio modestly affects UV luminosity of simulated galaxies, whereas the absence of dust significantly enhances it. In future work, higher-resolution simulations will be conducted to better understand the formation and evolution of galaxies at Cosmic Dawn.

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 use reduced-mass eigentensors to quantify the 3D ellipsoidal shape evolution of 13 Milky Way-mass galaxies simulated using zoom simulations with FIRE-2 physics; all but one form discs at z = 0 . We find that all of our Milky Way progenitors go through phases when they are elongated. They often oscillate between spheroidal and elongated shapes in the early Universe over billion-year time-scales, with similar to 25-45 per cent of the population having elongated luminosity-weighted shapes at any given time at z = 0.5-8.5 . In contrast, all stellar populations in our z = 0 Milky Way analogues are symmetric about their minor axes at z = 0 , even though the old and intermediate-age stellar populations were often arranged in the shape of elongated pickles or triaxial spheroids at the time they formed meaning these populations changed shape significantly over time. During their transient elongated phases, our galaxies have anisotropic velocity dispersion ellipsoids directed along their spatial major axis; however, their shapes do not correlate with their dark matter fraction nor with the shapes and orientations of their underlying dark matter haloes. We find that when treated as a population, the fraction of our galaxy progenitors that are elongated at z > 0 . 5 is roughly consistent with what is observed for systems of the same mass and redshift. Our results suggest that observed elongated galaxies seen in the early Universe with James Webb Space Telescope and Hubble Space Telescope are not stable structures, but rather transitory phases that are nevertheless statistically common. Some of these observed objects may evolve into Milky Way-like galaxies at z = 0 .

Stellar streams from disrupted globular clusters are excellent probes of dark matter (DM) subhalos. Observed Milky Way streams display a remarkable diversity of features: spurs, gaps, kinks, cocoons, and density variations, many attributed to subhalo encounters. But how much of this diversity arises from the host itself? We simulate ∼15,000 globular cluster streams across four Milky Way-mass halos from the FIRE-2 cosmological simulations, evolved in basis function expansion potentials capturing the evolving disk, halo, and large-scale structure while excluding small-scale perturbers such as DM subhalos and giant molecular clouds. We find that roughly three quarters of streams develop complex features from the host potential, such as spurs, kinks, and cocoon-like envelopes. Even the smoothest streams exhibit 10–25% width variation along their track and host overdensities and gaps at scales of ∼2^∘, squarely in the 1^∘–5^∘ range predicted for subhalo-induced gaps. Pericentric distance is the primary predictor of stream morphology, with ∼15 kpc separating smooth from disturbed streams and circular orbits beyond ∼20 kpc producing the smoothest streams. Only ∼70 out of ∼15,000 streams are free of detectable wiggles in the track at any scale. Analogs to observed features, such as the GD-1 spur and the ATLAS–Aliqa Uma kink, emerge even without the presence of subhalos. As next-generation surveys (LSST, Euclid, and Roman) resolve stream structure across hundreds of streams, the baseline established here, streams evolved without small-scale perturbers, becomes essential for extracting DM substructure constraints.

Next-generation radio telescopes will provide unprecedented data volumes of the neutral hydrogen (H i) distribution across cosmic time. The spatial and kinematic distribution of Hi is a biased tracer of the underlying matter field, and as such contains information on the distribution of dark matter over a wide range of scales. Extracting dark matter properties from Hi, however, is non-trivial because baryonic processes linked to galaxy formation significantly modify the H i distribution. Additionally, methods that use empirical relations, often calibrated via numerical simulations, do not use the full field-level information to model the complex relation between H-i and dark matter . We use the recently introduced EMBER-2 model to directly predict dark matter distributions from H( i )tracers over a wide redshift range, z = 0-6. After training on cosmological galaxy formation simulations run with FIRE-2, our method accurately recovers key statistics, including dark matter mass fractions, surface density profiles and cross-correlations, where the latter are reconstructed at an accuracy of 20 per cent down to scales of k = 100 h cMpc(-1) constituting a significant improvement over traditional approaches. The presented method may become a key ingredient in future inference pipelines as it can be readily integrated into downstream analysis tasks of radio surveys.

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.

HI-rich starless halos, should they exist, hold great promise for elucidating dark matter halo structure. Yet realizing this potential demands reliable theoretical predictions for their properties and abundances. Indeed, the recent identification of Cloud-9 as a strong HI-rich starless halo candidate in the nearby universe makes such predictions timely. This Letter examines HI-rich (M_HI≥ 10^6 M_⊙) starless (isolated/central) halos at z=0 across three cosmological simulations: FIREbox, Recal-EAGLE and NIVARIA-LG. All three successfully produce such objects, with M_HI extending up to ∼1–2 dex above Cloud-9, but with number densities that vary by a factor of ∼30. These populations span different regions of the M_HI–M_gas–M_200 space: NIVARIA-LG produces objects with higher M_HI and M_gas values (≳ 10^7 and ≳ 10^8 M_⊙), while FIREbox predicts they lie within extremely narrow ranges of M_gas∼(1.1–1.6)×10^8 M_⊙ and M_200∼(7.8–8.6)×10^9 M_⊙. Recal-EAGLE and NIVARIA-LG exhibit a strong M_gas-M_200 correlation, with similar slopes but different normalizations. The simulations predict numerical Cloud-9 analogs – though similarities in the shapes of their HI column-density profiles may be driven by FAST's modest beam; halving it already reveals differences. Collectively, these inter-simulation discrepancies make a compelling case for discoveries beyond Cloud-9: a statistical sample of well-resolved HI-rich starless halos is needed to discriminate amongst competing predictions.

We compare satellite quenched fractions across three cosmological simulation suites (FIREbox, the FIRE-2 zoom-ins, and IllustrisTNG50) and observational datasets from SAGA, ELVES, and the combined satellite population of the Milky Way and M31. To enable consistent comparisons, we select Milky Way-mass hosts with M_ halo = 10^11.9 - 10^12.2 M_⊙ and satellites with stellar masses of 10^7 - 10^10 M_⊙, applying uniform projected apertures and a common quenching definition. All three simulations reproduce the strong observed trend that lower-mass satellites are more likely to be quenched, closely matching the stellar-mass dependence seen in SAGA, ELVES, and the MW+M31 system. This agreement indicates that the mass dependence of satellite quenching is a robust outcome of contemporary galaxy formation models. Radial trends, however, show meaningful differences. SAGA and ELVES exhibit gently declining quenched fractions with projected distance, reflecting strong environmental quenching at small radii. TNG50 most closely matches this behavior, FIREbox, remains consistent with with a nearly flat trend within uncertainties, and the FIRE-2 zoom-ins show suppressed inner quenched fractions driven almost entirely by their paired MW-M31 hosts, which lack high-mass satellites and show strong radial segregation between star-forming and quenched systems. This environmental imprint suggests that host environment and assembly history can influence satellite quenching outcomes and may contribute to diversity across simulations. Overall, while the simulations consistently recover the stellar-mass dependence of quenching their radial trends vary, highlighting the influence of host-halo conditions and motivating deeper exploration of how host environments shape satellite quenching.

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.

Galaxy simulations have come a long way from the early days of simple N-body calculations, which considered only gravitational interactions, to the complex, multi-physics models used today. Beginning with initial conditions representative of the Universe shortly after the Big Bang, these modern simulations integrate the relevant physical processes involved in galaxy formation, such as gravity, gas dynamics, cooling, star formation, and feedback, while accounting for cosmic expansion and structure formation. This review provides an introductory overview of cosmological galaxy simulations, outlining the essential components and methods used to model the formation and evolution of galaxies on the computer. It also discusses common steps in the post-processing analysis, essential for extracting physical insights from these numerical experiments, along with basic tests to assess simulation validity and accuracy. Looking forward, next-generation simulations aim to push resolution boundaries, incorporate additional physical processes, and improve the robustness of the numerical models, promising to lead to a deeper understanding of how galaxies emerged and evolved over cosmic time.

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

Many poststarburst galaxies at z similar to 0.7 have been shown to retain substantial molecular gas reservoirs yet host low ongoing star formation, suggesting that the remaining gas may be inefficient at forming stars during the early postburst phase. We present new Atacama Large Millimeter/submillimeter Array CO(5-4) observations of nine gas-rich poststarburst galaxies at z similar to 0.7 from the Studying Quenching in Intermediate-z Galaxies: Gas, angu L -> ar momentum, and Evolution (SQuIGG L -> E) survey, providing a view of the molecular gas excitation in these systems. Combined with existing CO(2-1) data, we detect CO(5-4) in eight out of nine targets and find that most have moderate CO excitation with r52 equivalent to LCO(5-4)'/LCO(2-1)'approximate to 0.1-0.3 . These systems show no clear trend between r52 and either total or surface-density of star formation. Specifically, all objects have Sigma SFR similar to 0.01-1 M circle dot yr-1 kpc-2, consistent with compact, modest star formation, even when allowing for buried activity, as these galaxies decline from their peak. One object, J1448+1010, which has clear optical, mid-infrared, and radio indicators of an active galactic nucleus, is an outlier with r52 approximate to 0.6; its elevated excitation likely requires significant nonstellar heating, with a contribution from potentially obscured star formation. Together, most gas-rich SQuIGG L -> E poststarbursts have moderately excited molecular gas alongside little to modest star-forming activity, indicating that the remaining gas hosts relatively suppressed star formation efficiencies instead of strong buried starburst activity.

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.

The elevated UV luminosity functions (UVLFs) from recent James Webb Space Telescope (JWST) observations have challenged the viability of existing theoretical models. To address this, we use a semianalytical framework-which couples a physically motivated source model derived from radiative transfer hydrodynamic simulations of reionization with a Markov Chain Monte Carlo sampler-to perform a joint calibration to JWST galaxy surveys (UVLF, phi(UV), and UV luminosity density, rho(UV)) and reionization-era observables (ionizing emissivity, N-ion , neutral hydrogen fraction, xHI, and Thomson optical depth, tau). We find that models with weak feedback and a higher contribution from faint galaxies reproduce the reionization observables but struggle to match the elevated JWST UVLF at z > 9. In contrast, models with stronger feedback (i.e., rapid redshift evolution) and a higher contribution from bright galaxies successfully reproduce JWST UVLF at z >= 10 but overestimate the bright end at z < 9. The strong-feedback model constrained by the JWST UVLF predicts a more gradual and extended reionization history, as opposed to the sudden reionization seen in the weak-feedback models. This extended nature of reionization (z similar to 16-6) yields an optical depth consistent (at 2 sigma) with the cosmic microwave background (CMB) constraint, thereby alleviating the photon budget crisis. In both scenarios, reionization is complete by z similar to 6, consistent with current data. Our analysis highlights the importance of accurately modeling feedback and ionizing emissivities from different source populations during the first billion years after the Big Bang.

The “near–far” approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ( z ≳ 6), beyond the current James Webb Space Telescope detection limits ( M _UV ≳ −15). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to ∼50% of the total stellar-mass budget of the proto-MW/LG at z = 6−9. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of ∼2–3 and reduces the halo-to-halo variation in the slope by ∼20%–40%. This enables robust constraints down to at least the resolution limit of the simulations, near M _⋆ ∼ 10 ^5 M _⊙ or M _UV ∼ −10 at z ≳ 6. We also show that “fossil-record” reconstructions—which assume each present-day system descends from a single reionization-era progenitor—are sensitive to the stellar-mass/UV-magnitude thresholds, which introduces bias in the inferred slopes at the low-mass/faint end. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with M _UV ≲ −15 to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction (∼50%) of streams with M _⋆ ≳ 10 ^6 M _⊙ at z = 0 should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.

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