arXiv subjects

Nikko J. Cleri

Publications and source records attributed to Nikko J. Cleri.

At least 19 recordsLinked to original sources

arXiv2026

RUBIES: The Evolution of the Ionization Parameter from 0 < z < 9

The dimensionless ionization parameter, U=q/c, where q is the ratio of the local ionizing photon flux to the local hydrogen density, is a key metric to parameterize nebular conditions. Prior to JWST, the rest-frame optical emission lines and their ratios which trace the ionization parameter (e.g., O32=[OIII]/[OII]) were inaccessible at high redshifts. Here we quantify, for the first time, the evolution of the ionization parameter in galaxies across the last 13 billion years of cosmic time by comparing JWST/NIRSpec PRISM and G395M spectroscopy of 434 galaxies at 3<z<9 from the RUBIES survey with z<3 samples from SDSS, LEGA-C, and KBSS. We leverage a large suite of photoionization models to infer U from [OIII] and [OII]. We find that U increases with redshift and specific star formation rate (sSFR), and decreases with stellar mass. Crucially, and in contrast to previous linear best-fit calibrations, our inference results in a systematic uncertainty in logU of ~0.3 dex at zero measurement uncertainty due to the wide range of models that predict the same O32 ratio without informative priors. We compare to SPHINX20 and LUMEN simulations and find that the simulated galaxies exhibit higher O32 ratios at fixed redshift and stellar mass compared to RUBIES observations. Finally, we combine the predictive power of observed and inferred quantities with multivariate relations to estimate U from redshift, stellar mass, and sSFR for use where O32 is not available. We find that U increases at fixed stellar mass and sSFR by a factor of ~4 from z=2 to z=6, demonstrating that the redshift evolution encapsulates physics beyond that traced by stellar mass and sSFR alone. Finally, we show that a toy model with the first order assumption that HII region volume is proportional to galaxy volume can explain the excess redshift dependence of logU as being consistent with observed evolution in galaxy sizes.

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arXiv2026

Balmer Absorption Series and Broad Metal Lines in Two Luminous Little Red Dots

Balmer absorption is common among little red dots (LRDs), but absorbers at or redward of systemic are rare, occurring in only $\sim10-15$\% of H$α$ absorbers. In this paper, we study two such exceptional cases with deep JWST/NIRSpec spectroscopy: 15 hr of high-resolution (G395H) observations of RUBIES-EGS-49140 (z=6.68), resolving the absorption in all four transitions from H$α$ through H$δ$, and medium-resolution spectroscopy (10 hr of G235M, 2 hr of G395M) of UNCOVER-A2744-45924 (z=4.46; 1.7x magnification). Both sources are among the optically reddest and most luminous LRDs known, and both show deep, near-systemic Balmer absorption troughs. We find two systematic trends along the Balmer series: the absorption centroids become more redshifted toward higher-order transitions, while the absorbed equivalent widths decline only weakly with increasing order, far less than expected from the atomic optical-depth ratios for a single attenuating screen. Ca\,{\sc{ii}}\,K is detected in absorption in both sources, whose offset follows the H$α$ trough rather than the more redshifted higher-order Balmer lines. We further report the detection of a broad base in [Ne\,{\sc{iii}}]\,$λ$3870, along with broad [O\,{\sc{iii}}]\,$λ$4364, [O\,{\sc{iii}}]\,$\lambda5008$, and He\,{\sc{i}}\,$\lambda5877,\lambda7067$, while He\,{\sc{ii}}\,$λ$4687 remains undetected or weak. Standard AGN photoionization models cannot reproduce the observed line ratios, whereas AGNs with high gas densities provide a consistent explanation, as also indicated by the anomalously high He\,{\sc{i}}\,$\lambda7067/\lambda5877$ ratio. A possible explanation for the relative strengths of the Balmer absorption lines could be a dense, optically thick medium whose re-emission modifies their apparent absorption strengths, while the velocity progression may arise from stratification in the absorbing gas.

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arXiv2026

Quiescent Galaxies at $z > 2$ from CAPERS spectroscopy and their Number densities

Recent JWST observations have revealed an overabundance of quiescent galaxies at high redshift. Photometric methods, such as photometry-derived sSFRs, $UVJ$ colors, and synthetic $ugi_s$ colors, are commonly used to identify quiescent galaxies, but their completeness and purity remain poorly constrained. Here we present a spectroscopically selected sample of 19 quiescent galaxies at $2 \leq z \leq 4$ from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS). They are selected with $M_\ast > 10^{9.5} M_\odot$ and a specific star formation rate below 20% of the inverse cosmic age. These galaxies formed 50% of their stellar mass at $z \sim 3.0$--4.6 and quenched rapidly on timescales of $\sim0.1$--0.2 Gyr. These quenching timescales are comparable to those of other high-redshift QGs but shorter than those at lower redshift. Using the spectroscopic sample as a benchmark, we assess commonly adopted photometric QG selection methods. We find completeness of 63--89% and purities of only 40--63%, with strong emission-line galaxies constituting the dominant source of contamination. Applying completeness and purity corrections calibrated from the spectroscopic sample, we measure the number density of quiescent galaxies at $2 \leq z \leq 5$. The resulting number densities exceed the predictions of most current galaxy-formation simulations at $2<z<4$, with the largest discrepancy at $M_\ast \geq 10^{10.5} M_\odot$. This discrepancy suggests that current cosmological models may not assemble and quench massive galaxies sufficiently early or efficiently.

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arXiv2026

The quasi-star model for Little Red Dots: potential and challenges

(Abridged) Little Red Dots (LRDs) are a class of sources discovered by JWST observationally defined by a "V-shaped" rest-frame UV-Optical SED, a compact or unresolved morphology, and for having, frequently, broad hydrogen emission lines. Among various models, those involving a quasi-star interpret LRDs as an intermediate stage in the evolution of a super-massive black hole (SMBH) seed into a classic AGN. In this paper, we employ the radiative-transfer code \texttt{Cloudy} to study whether this model is able to reproduce the spectral features commonly observed in LRDs. The model consists of an accreting SMBH ($M_{\rm BH}\sim10^{5-6} \ M_\odot$) surrounded by a convective layer where a black-body (BB) spectrum with $T\sim5000 \ {\rm K}$ and $L\sim10^{44.4} \ {\rm erg \ s}^{-1}$ is produced. This BB is then reprocessed by a concentric thick ($ΔR\sim1000 \ {\rm AU}$) shell of dense ($n_{\rm H}\sim10^{11} \ {\rm cm}^{-3}$) gas partially ionised by thermal collisions. The emerging radiation is further reprocessed by a diffuse clumpy medium surrounding the quasi-star. We fit this model to JWST/NIRSpec spectra of LRDs from the literature, deriving the main physical parameters and the SMBH masses. Once coupled with the UV emission from a host galaxy, this model is able to reproduce the shape of the UV-to-NIR continuum, including the presence of a Balmer break, as well as the luminosity of the hydrogen emission lines. However, this quasi-star model does not natively account for the presence of broad helium lines and for the possible presence of hot dust, needing additional components to match these observables. Our main result is to show how some LRDs can be modeled as quasi-stars, highlighting that a significant degeneracy exists among different LRD models. This has important consequences for our understanding of the mechanisms driving black hole growth in the early Universe.

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arXiv2026

A Population of Little Red Dot-like Quasars in SDSS

Compact and red sources in the high redshift ($z\sim5$) Universe, known as "Little Red Dots" (LRDs), are among JWST's most intriguing discoveries. These sources have broad Balmer emission lines, weak X-ray emission, and unique spectral energy distributions (SEDs) poorly fit by either stellar or AGN templates. Local LRD-like objects allow for detailed studies of the underlying physical processes with archival multi-wavelength datasets unavailable in the high-$z$ Universe. We show that the SDSS $ugriz$ filters at $z\approx0.4, 0.8$ overlap well with the JWST filters used to select LRDs at $z\sim5$. We use SDSS quasars to define a sample of $\sim1300$ Local Red Dots (LoRDs) which share the same photometric colors of LRDs. A subset of the LoRD sample selected to have V-shaped continua ($N=244$) show prominent higher-order Balmer absorption features and [NeV]$λ$3426 emission, both of which would likely be missed in JWST/PRISM observations given the low spectral resolution. A composite SED of the LoRDs differs from a typical quasar SED in the rest-frame UV/optical, but the two agree with each other in the NIR. The LoRD SED matches well with a stack of LRDs and can be modeled either as a reddened AGN combined with a host galaxy, or as a reddened AGN combined with a host galaxy and a cool blackbody. Interestingly, the LoRDs are X-ray detected at a rate comparable to typical quasars. However, the probability that LoRDs and typical quasars would go undetected, if subject to the LRD X-ray upper limits, is $>50\%$.

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arXiv2026

The THRILS Factor: Investigating the properties of Little Red Dots (LRDs) at 3<z<6 with JWST/NIRSpec

JWST has uncovered a class of objects called LRDs, whose nature is still widely debated. In this work, we present a comprehensive spectroscopic analysis of nine LRDs in the Extended Groth Strip (EGS) field studied as part of THRILS and C3PO, both JWST Cycle 3 programs. These targets, photometrically selected based on their compact red appearance, are observed with deep spectroscopic exposures ($\geq8$ hours), enabling robust detections of broad Balmer lines, He I emission, and other spectral features characteristic of AGN activity. Using the [SII] $λ\lambda6716,6731$ doublet, we find electron densities ($n_e$) between $2.33 < \log (n_e) < 2.97 \ \mathrm{cm^{-3}}$, comparable to those in narrow line regions (NLR) of local AGN and high-$z$ galaxies. The spectroscopic depth further enables detailed characterization of broad Balmer line profiles. We fit both Gaussian and convolved exponential models to each source and find that five LRDs are statistically better described by the latter model. We measure optical depths $τ_{\rm sc} = 0.56-0.91$, scattering fractions $f_{\rm SC} = 0.40-0.82$ which correspond to column densities log(N$_e$) $\sim$ 23.93-24.14 cm$^{-2}$, and covering fractions $c_f = 0.43-0.59$. These results indicate a clumpier broad line region (BLR) geometry that deviates from conventional LRD models, which predict covering fractions close to unity. Furthermore, these Compton-thick gas columns may explain the X-ray weakness of LRDs. We also find that THRILS LRDs are narrow-line dominated compared to literature AGN-dominated LRDs, and show that exponential profile fitting corrects for systematic overestimation of black hole masses from Gaussian-based measurements.

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arXiv2026

A Spectral Framework for Testing the Quasi-Star Hypothesis in Little Red Dots I: Weighing LRDs by Their Super-Eddington Luminosity Ratios---No Signs of Overmassive Black Holes

We present a spectral test of the quasi-star hypothesis for Little Red Dots (LRDs) whereby a black hole grows inside a stellar-like envelope. We use \texttt{Prospector} to fit host galaxies and \texttt{TLUSTY} photospheres to four LRDs that show strong molecular or atomic absorption. We approximate each object's electron-scattered Eddington luminosity ratio, $ϕ\equiv κ_{\rm es}σT_{\rm eff}^{4}/(gc)$, and we use \texttt{MESA-QUEST} to simulate their envelopes. All four are super-Eddington at $ϕ= 4.0$--$299$, although the largest value sits at the edge of our atmosphere grid and beyond our simulations. We derive envelope masses between $700$--$27{,}000\,M_\odot$, where quasi-star theory requires the black hole to be less than a third of that. GN-28074 falls five decades below its published virial mass estimate, alleviating the overmassive black hole problem. Their black holes double in mass every $\sim0.05$--$3.5$~Myr and can produce intermediate-mass black holes in $\lesssim30$~Myr. The two fitted components of the water-absorbing object WIDE-EGS-2974 share a single $ϕ$, consistent with our hypothesis that the absorption originates in an extended atmosphere. We then extend our measurements to 82 archival LRDs, finding super-Eddington photospheres throughout the population that launch winds at $1$--$3\times$ their own escape speeds, which implies that LRDs evolve from being massive, continuum-driven sources to more eruptive winds as they mature and shed their outer envelopes. We thus constrain the Eddington ratios and masses of LRDs, and, for the first time, show self-consistently that quasi-stars may be the central engines powering LRDs.

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arXiv2026

LEGGOS: A Shocking Lack of Evidence for Shocks at sub-kiloparsec Scales at 2 < z < 4

Here we present the first systematic search for shocks in six gravitationally lensed galaxies at 2.37 < z < 3.625 with JWST/NIRSpec integral field spectroscopy from the LEnsing and Galaxy Growth: Observing Substructures (LEGGOS) survey. We employ diagnostics that utilize the fluxes and kinematics of shock-sensitive rest-frame optical emission lines H\b{eta}, [O iii] λ5008, [O i] λ6302, Hα, [N ii] λ6585, and [S ii] λλ6718, 6733. We find that, on pixel, clump, and galaxy-integrated scales, the LEGGOS spectra show minimal if any evidence for shocks. The image plane pixels are < 8% within the shock regions of the rest-frame optical line ratio diagnostics for any individual galaxy, and the shock-identified pixels do not show a coherent spatial structure. We also leverage MAPPINGS V shock models to infer shock velocities from the observed emission lines, and find that the distributions of inferred shock velocities are inconsistent with those expected for shock-dominated gas. Altogether, none of these methods provide significant evidence for shocks in the six LEGGOS sources. We conclude by discussing the implications of the lack of evidence for shocks in the broader context of galaxy evolution at cosmic noon and earlier epochs.

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arXiv2026

LEGGOS: Direct abundances of N, O, Ne, S, and Ar in five lensed galaxies at Cosmic Noon

We present direct $T_e$ chemical abundances in five strongly lensed galaxies at Cosmic Noon ($2.481 \leq z \leq 3.625$) using JWST/NIRSpec data from the LEGGOS survey. We measure gas-phase abundances of N, O, S, and Ar in all five galaxies, and Ne in two galaxies. Three galaxies have electron temperature constraints from multiple different ionization zones, and we find that these are broadly consistent with temperature scaling relations observed in both local and high-$z$ galaxies. We find a range of oxygen abundances $8.04 \leq 12+\log(\text{O/H}) \leq 8.79$ (22 -- 126\% $Z_{\odot}$). The ratios of N/O and Ne/O are consistent with trends observed in local galaxies. We do not observe any evidence for significant N enhancement in our sample, though the youngest galaxy in our sample (SGAS-J1050) has a mildly elevated $\log(\text{N/O}) = 1.20 \pm 0.08$, which we suggest may be driven by a population of young massive stars. The ratios of S/O and Ar/O are generally sub-solar, similar to trends observed in other high-$z$ galaxies. We find that the S/O and Ar/O abundances are sub-solar, consistent with enrichment from core-collapse supernovae (CCSNe). Modeling of the star formation histories of the LEGGOS galaxies supports CCSNe enrichment, as each galaxy shows a recent period of star formation lasting $\lesssim 100$ Myr, indicating that type Ia supernovae would not yet have had enough time to contribute significantly to the gas-phase abundances of these galaxies.

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arXiv2026

C3PO: A Lyα Emitting Galaxy at $z \sim 9$

We present deep, $>10$ hr, JWST NIRSpec G140M and G395M observations of a previously known galaxy C3PO-8447 at $z=8.99781$. Our new NIRSpec data from the Carbon-3 plus Oxygen (C3PO) survey detect Ly$α$ emission with an escape fraction of $1.5\pm0.3\%$ and a velocity offset of $210\pm80$ km s$^{-1}$, making C3PO-8447 the third Ly$α$ emitting galaxy confirmed at $z\gtrsim 9$. The galaxy has a low stellar mass of $2.2\pm0.1\times 10^7\ M_\odot$, a young stellar age of $1.6\pm0.2$ Myr, and a low oxygen abundance of $12+\log(\mathrm{O/H}) = 7.50\pm0.08$. UV diagnostic diagrams incorporating the C IV, O III], and C III] emission lines indicate that the ionization of C3PO-8447 is dominated by the recent starburst. The low Ly$α$ escape fraction of C3PO-8447 implies that C3PO-8447 resides in a small ionized bubble with radius $R \sim 0.07$ to 0.20 pMpc, which can be produced by C3PO-8447 alone. We find no evidence for either a large-scale galaxy overdensity around C3PO-8447 or a significant contribution from nearby galaxies to its ionizing photon budget. Taken together, these results suggest that C3PO-8447 likely represents an ionized bubble powered by a single low-mass, vigorously star-forming galaxy during the early stages of cosmic reionization.

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arXiv2026

Skyfire: A Spectroscopic Census of Little Red Dots and Broad-Line AGN in the CEERS Field

We present the Skyfire program, a 21-hour Cycle 3 JWST/NIRSpec survey with the G395M medium-resolution grating covering five pointings in the Extended Groth Strip. The survey is designed to carry out a systematic census of faint, broad-line AGN candidates with a range of rest-optical colors identified at z > 3 by the Cosmic Evolution Early Release Science (CEERS) Survey. Our primary targets include photometrically-selected Little Red Dots (LRDs), blue extreme emission line galaxies (EELGs), and X-ray-detected AGN. We present spectroscopic redshifts for 178 sources observed by Skyfire, as well as a catalog of 34 sources with broad emission lines in the redshift range 2.7 < z < 6.5. Our broad-line sample includes 18 LRDs, which brings the spectroscopic completeness of LRDs with $β_{\rm opt}>-0.02$ in the CEERS field to 73%. We explore the prevalence of broad emission lines in photometrically-selected LRDs as a function of their rest-frame continuum slope and observed color distributions. We find the broad-line detection fraction in LRDs remains high at relatively blue rest-optical colors and extends smoothly into the bluer regime occupied by Little Blue Dots (LBDs). We discuss the implications of this finding for LRD-LBD unification scenarios. We also find that only 18% (3/17) of EELGs selected primarily for their high-equivalent-width emission lines and compact morphologies exhibit broad emission lines, suggesting these criteria alone are poor predictors of broad-line activity. We present a revised set of LRD selection criteria that captures bluer sources by extending down to $β_{\rm opt}=-0.52$. Using this new threshold, we find that $80.9^{+4.6}_{-7.5}\%$ of photometrically-selected LRDs brighter than 26.5 in F444W show broad emission lines and that LRDs make up 54% of the overall broad-line population identified in the CEERS field.

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arXiv2026

The [Ne V] AGN Diagnostic in SDSS-IV/eBOSS: An Anticorrelation Between Ionization State and AGN Luminosity in Low-Redshift Coronal-Line Galaxies

Traditional narrow-line diagnostics fail at high redshift, where low metallicity drives star-forming galaxies and Active Galactic Nuclei (AGN) into overlapping regions of the Baldwin-Phillips-Terlevich (BPT) diagram. Coronal lines, with ionization potentials exceeding 100 eV, offer a robust alternative: they cannot be produced by normal stellar populations, arise almost exclusively from AGN or fast shocks, and are insensitive to abundance evolution. Among these, the [Ne V] doublet is the brightest optical coronal line tracer of AGN activity. Existing coronal line catalogs contain only about 1750 objects, too few to statistically assess the completeness and purity of [Ne V]-selected AGN samples. We identify 33,817 galaxies with [Ne V] S/N > 5 at z = 0.147-1.12 in the Sloan Digital Sky Survey IV extended Baryon Oscillations Spectroscopic Survey (SDSS-IV/eBOSS), exceeding the combined literature total by more than an order of magnitude. [Ne V] preferentially selects rapidly accreting, relatively unobscured AGN; only 41.5% of BPT AGN show detectable emission. By stacking spectra that lack [Ne V] on a grid of black hole mass and [O III] luminosity, we recover [Ne V] in the majority of bins, implying that non-detections reflect limited survey sensitivity rather than a deficit of coronal emission. We find [Ne V]/[Ne III] is only weakly correlated with [O III]/[O II]. Galaxies with high [Ne V]/[Ne III] for their [O III]/[O II] ratio have lower Eddington ratios and are more likely classified as LIERs or Composites. The [Ne V]/[O III] ratio shows a strong anticorrelation with [O III] luminosity, with the most extreme coronal-line strengths found among the lowest-luminosity AGN, in analogy with the accretion states of stellar-mass black holes. Comparison with a sample of z=2-9 [Ne V]-detected galaxies suggests some, but not all, high-redshift AGN follow the same relations.

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arXiv2026

Little Red Dots host Black Hole Stars: A unified family of gas-reddened AGN revealed by JWST/NIRSpec spectroscopy

We use the DAWN JWST Archive to construct and characterise a sample of 146 little red dots (LRDs) across 2.0<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 ~$0.4-1.0μ$m, with typical T~5000K or $λ_{peak}$~$0.65μ$m 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 optically-thick dense gas envelopes. Hotter LRDs with $λ_{peak}<0.65μ$m 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 H$α$ and optical continuum luminosities, as well as H$α$ 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 among the LRD population. Our work presents an empirical description of the nature and structure of LRDs, defining a new benchmark for ongoing LRD model developments.

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arXiv2026

Ripples in the OCEANS: Broad Line Variability of Little Red Dots

Little Red Dots (LRDs) are a unique class of compact, red sources discovered in the JWST extragalactic deep fields. Determining if they are indeed powered by accreting supermassive black holes (SMBHs) is one of the main drivers of the intense study of these objects. Evidence for variability in these objects provides a direct test for the active galactic nucleus (AGN) nature of their central engine. In this study, we present a variability analysis of 6 LRDs observed by the $R \sim 2700$ OCEANS survey and leverage archival $R \sim 1000$ spectroscopic data from the CEERS and RUBIES surveys. We report marginal detections of $\rm Hα$ broad-line (BL) variability in the LRDs OCEANS-100424/RUBIES-42232 (27\% variability at 2.1$σ$ significance) and OCEANS-35829/RUBIES-49140 (GlimmIr/Irony; 50\% variability at 1.5$σ$ significance). The other 4 LRDs in our sample do not show evidence for BL variability, with a 1$σ$ upper limit of $4.8 \% - 30\%$ variability between their epochs of observations. We also find no evidence ($<1σ$) for continuum variability in our LRD sample. We compare our results to a sample of SDSS-RM quasars to determine the probability of our broad $\rm Hα$ variability detections. We find that the probability of reproducing 2 variable and 4 nonvariable quasars is $4.71\%$, corresponding to $\sim 2 σ$ departure from typical quasar variability. The detection of BL $\rm Hα$ variability in 2 LRDs provides some evidence for the AGN nature of these objects as opposed to pure scattering models.

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arXiv2026

Implications of Broad [O III] 4364 and UV Line Emission in Two Little Red Dots at z ~ 7 - 8

We present deep, NIRSpec G140M and G395M spectroscopy of Little Red Dots (LRDs) at z = 6.68 and z = 8.35. Both LRDs show broad Balmer and [O III] $λ$4364 emission. The broad [O III] $λ$4364 lines have FWHM ~1000 km/s, about 1/3 that of the H$β$ lines. Assuming gas temperatures T ~ 15,000 - 25,000 K, the [O III] $λ$4364/[O III] $λ$5008 ratios of the broad lines yield high gas densities, log n/cm^-3 = 6.3 to 7.9, 3-10$\times$ higher than those in broad-line regions of low-redshift quasars. If the broad-lines trace virial motions, it is evidence for metal-enhanced gas clouds, ~1-10~pc from the LRD engine. Both LRDs show narrow [C III] $λ$1907 + C III] $λ$1909, and O III] $λλ$1661,1666. The C III] ratios yield narrow-line gas densities, log n/cm^-3 = 4.2-5.2, similar to those in other star-forming galaxies. The line equivalent widths, EW(O III]), EW(C III]), are at, or exceed, limits expected for stellar populations, likely requiring an additional ionizing source. The LRDs also have [O III] $λ$4364/H$γ$ ratios that favor ionization from an accretion disk, possibly combined with stars. Both LRDs show nitrogen enhancement based on detections of N III] $λ$1746 or N IV] $λ$1486, which may imply rapid, recent star-formation. These results favor a scenario where the LRD gas envelopes are highly stratified, having high-density clouds with a non-unity covering factors and a complex geometry, such that ionizing radiation from the LRD accretion disk, combined with that from star-forming regions, produce the nebular emission features.

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arXiv2026

Hunting Wandering 3<z<8 Black Holes: Spatial Offsets in Ionization Ratio and Continuum Emission

The early growth and assembly of supermassive black holes (SMBHs) remain key topics of interest in galaxy evolution. One of the scenarios predicted by theoretical models is that frequent minor mergers and asymmetric gas inflows may cause SMBHs to temporarily reside off-center within their host galaxies in the early universe. To observationally test this scenario, we investigate whether spatially offset ionization signatures-which may be indicative of active galactic nuclei (AGN)-can be identified. Using JWST NIRSpec PRISM spectroscopy from the Cosmic Evolution Early Release Science (CEERS) survey, we analyze the 2D spectra of 90 high-redshift galaxies (3 < z < 8), including two known broad-line AGN. By measuring key emission lines such as Hα, Hβ, [OIII]λ5007, [NeIII]λ3868, and [OII]λλ3727, 3729 we derive spatial flux ratio profiles, and focus on [OIII]/Hβ as a tracer of high-ionization mechanisms that may indicate AGN activity. We identify 26 galaxies (~30% of the sample) with significant localized peaks in [OIII]/Hβ. Out of these 26 galaxies, 12 sources (~46%) exhibit significant spatial offsets between the peak [OIII]/Hβ ratio and the stellar continuum center. Six of these sources show the highest amount (> 1.5) pixel spatial offsets. This spatial offset between ionization structure and stellar centers offers a promising avenue to probe early SMBH evolution and its connection to galaxy formation.

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arXiv2026

LEGGOS I: The JWST LEGGOS Survey -- LEnsing and Galaxy Growth: Observing Substructures -- Unpacks the Nature of Clumpy Star Formation and Quenching in Gravitationally Lensed Galaxies beyond Cosmic Noon

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 $z\sim2$--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 $\sim$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.

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arXiv2026

Deep Spectroscopic Follow-Up of Maisie's Galaxy -- A Typical Galaxy in the Early Universe

The first several years of JWST observations have yielded surprisingly large numbers of bright $z>10$ galaxies, with follow-up spectroscopy of many of these sources implying extreme star formation activity and/or AGN content. Here, we present a combination of two deep Cycle 3 NIRSpec G395M programs, totaling over 19 hours of exposure time, plus MIRI/LRS observations for one such high-redshift source: Maisie's Galaxy. We provide an updated redshift measurement of $z = 11.408 \pm 0.005$ for this source. Measurements of the [OII] doublet in these data yield an electron density ($n_e = 108.56^{+873.9}_{-35.37}$) and a star-formation rate (SFR$_{[OII]} = 1.3 \pm 0.35$), placing it along the star-formation main sequence (SFMS) and indicating that this is a much more typical, rather than extreme, source in the early Universe. We also report fluxes for the [OIII]$λ$5008 and [NeIII]$λ$3869 lines that provide us with a $\log$(Ne3O2) $= -0.219 \pm 0.145$ and a $\log$(O32) $=0.724 \pm 0.191$. We estimate the metallicity ($Z/Z_{\odot} = 0.17 \pm 0.05$) and ionization parameter ($\log$(U) $= -2.26 \pm 0.13$) from the Ne3O2 ratio. We place this galaxy in the context of other $z>10$ sources with similar line detections and compare the results to those obtained from SED fitting. The results suggest that we should go deeper with our observations to better understand the average galaxy population at these early times.

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