arXiv subjects

Karl Glazebrook

Publications and source records attributed to Karl Glazebrook.

At least 37 records · Page 2Linked to original sources

arXiv2026

Early Results from GLASS-JWST. XXVI. Spatially Resolved Star Formation and Balmer Decrements at $1.1<z<2.3$ from NIRISS Slitless Spectroscopy

Using JWST/NIRISS slitless spectroscopy, we present spatially resolved Balmer decrement measurements for 79 galaxies at $1.1 < z < 2.3$, which are gravitationally lensed by the foreground cluster Abell 2744. By stacking $\mathrm{H}α$ and $\mathrm{H}β$ emission maps in bins of stellar mass and redshift, we derive radial profiles of nebular dust attenuation and dust-corrected star formation rate (SFR). We find tentative evidence that the radial gradients of dust attenuation toward $\mathrm{H}α$ ($\rm A(\mathrm{H}α)$) vary with both redshift and stellar mass. At lower redshifts ($z = 1.10$--$1.53$), low-mass galaxies ($\rm 7.0<log(M_*/M_\odot)\leq8.5$) exhibit steeper $\rm A(Hα)$ gradients than higher-mass galaxies ($\rm 9.5<log(M_*/M_\odot)\leq11.0$), while the latter maintain detectable dust attenuation out to larger galactocentric radii. Galaxies at higher redshifts ($z = 1.76$--$2.29$) show lower attenuation levels. At fixed galactocentric radius, galaxies in the low-redshift bin generally exhibit higher dust attenuation than those at high redshifts, consistent with an increase in dust content toward later cosmic times. Dust-corrected SFR profiles in massive systems at lower redshifts are more spatially extended than those at higher redshifts, consistent with inside-out disk growth at $z\lesssim1.5$. These results suggest possible differences in attenuation properties across stellar mass and redshift bins, and demonstrate the power of gravitational lensing to probe internal structures in faint galaxies at sub-kiloparsec resolution.

astro-ph.GA↗

arXiv2026

MSA-3D: Connecting the Chemical and Kinematic Structures of Galaxies at $z \sim 1$

We investigate the connection between ionized gas kinematics and gas-phase metallicity gradients in 21 star-forming galaxies at $0.5 < z < 1.7$ from the MSA-3D survey, using spatially resolved JWST/NIRSpec slit-stepping observations. Galaxy kinematics are characterized by the ratio of rotational velocity to intrinsic velocity dispersion, $v/σ$, measured at $1.5\,R_e$, where $R_e$ is the effective radius. We find that dynamically hotter disks exhibit systematically flatter metallicity gradients, with a moderate anti-correlation between metallicity gradient and $v/σ$ (Pearson $r=-0.43$, $p=0.05$) and a linear fit yields a slope of $\sim 0.005$ dex per dex in $v/σ$, weaker than the dependence on stellar mass. A significantly stronger anti-correlation is observed with $R_e/σ$, interpreted as a proxy for the radial mixing timescale ($r=-0.59$, $p=0.005$), indicating that cumulative radial mixing more directly regulates chemical stratification. The metallicity gradients in our sample are uniformly shallow, indicating that efficient turbulent mixing in kinematically settled disks regulates the chemical structure of typical star-forming galaxies at $z\sim1$.

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arXiv2026

Revisiting the mass metallicity relation and the fundamental metallicity relation of dwarf galaxies at cosmic noon with NIRISS

We extend the stellar-mass gas-phase metallicity relation (MZR) at $z = 1.1-3.4$ down to the extremely low-mass regime using 183 galaxies with $\log(M_*/M_\odot) = 6.3-10.2$, based on deep JWST/NIRISS slitless spectroscopy from the NGDEEP program. The derived MZR is in excellent agreement with our previous result from 50 galaxies in the GLASS-JWST sample, underscoring the robustness and universality of this relation. Together, these datasets constitute the largest sample of dwarf galaxies yet obtained with NIRISS. The observed MZR slope, $β\simeq0.24\pm0.03$, remains constant across nearly four orders of magnitude in stellar mass. Analytical modeling of the metal-loading factor of outflows ($ζ_\textrm{out}$) indicates that, at $M_*\lesssim10^8M_\odot$, $ζ_\textrm{out}$ becomes progressively less dominant than the gas fraction ($μ_\textrm{gas}$) in regulating the MZR slope. Using this enlarged NIRISS sample, we further test the existence of the fundamental metallicity relation (FMR). We find no robust evidence for an additional SFR dependence beyond the MZR, nor any reduction in metallicity scatter when SFR is included. Examination of systematic uncertainties in \oh, $M_*$ and SFR suggests that the MZR slope ($β\sim0.22$) is robust, and that different assumptions about the strong-line calibrations or star-formation history (SFH) of the galaxies change the slope by less than 1-$σ$. At the current depth of the NIRISS data, evidence for an FMR among high-redshift dwarf galaxies remains inconclusive, highlighting the need for larger samples, and deeper observations.

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arXiv2026

The AURORA Survey: The Nebular Attenuation Curve of a Galaxy at z=4.41 from Ultraviolet to Near-Infrared Wavelengths

We use JWST/NIRSpec observations from the Assembly of Ultradeep Rest-optical Observations Revealing Astrophysics (AURORA) survey to constrain the shape of the nebular attenuation curve of a star-forming galaxy at z=4.41, GOODSN-17940. We utilize 11 unblended HI recombination lines to derive the attenuation curve spanning optical to near-infrared wavelengths (3751-9550 Å). We then leverage a high-S/N spectroscopic detection of the rest-frame ultraviolet continuum in combination with rest-UV photometric measurements to constrain the shape of the curve at ultraviolet wavelengths. While this UV constraint is predominantly based on stellar emission, the large measured equivalent widths of H$α$ and H$β$ indicate that GOODSN-17940 is dominated by an extremely young stellar population <10 Myr in age such that the UV stellar continuum experiences the same attenuation as the nebular emission. The resulting combined nebular attenuation curve spans 1400-9550 Å and has a shape that deviates significantly from commonly assumed dust curves in high-redshift studies. Relative to the Milky Way, SMC, and Calzetti curves, the new curve has a steeper slope at long wavelengths ($λ>5000$ Å) while displaying a similar slope across blue-optical wavelengths ($λ=3750-5000$ Å). In the ultraviolet, the new curve is shallower than the SMC and Calzetti curves and displays no significant 2175 Å bump. This work demonstrates that the most commonly assumed dust curves are not appropriate for all high-redshift galaxies. These results highlight the ability to derive nebular attenuation curves for individual high-redshift sources with deep JWST/NIRSpec spectroscopy, thereby improving the accuracy of physical properties inferred from nebular emission lines.

astro-ph.GA↗

arXiv2026

The AURORA Survey: High-Redshift Empirical Metallicity Calibrations from Electron Temperature Measurements at z=2-10

We present detections of auroral emission lines of [OIII], [OII], [SIII], and [SII] in deep JWST/NIRSpec spectroscopy for 41 star-forming galaxies at $z=1.4-7.2$ from the AURORA survey. We combine these new observations with 98 star-forming galaxies at $z=1.3-10.6$ with detected auroral lines drawn from the literature to form a sample of 139 high-redshift galaxies with robust electron temperature and direct-method oxygen abundance determinations. This sample notably covers a wider dynamic range in metallicity than previous work, spanning $0.02-0.9$~Z$_\odot$. We calibrate empirical relations between 19 emission-line ratios and oxygen abundance, providing a robust tool set to infer accurate gas-phase metallicities of high-redshift galaxies when auroral lines are not detected. While calibrations based on lines of $α$ elements (O, Ne, S, Ar) appear reliable, we find significant scatter in calibrations involving lines of N driven by a high dispersion in N/O at fixed O/H, suggesting that N-based line ratios are less reliable tracers of the oxygen abundance at high redshift. These new high-redshift calibrations are notably offset from those based on typical $z\sim0$ galaxy and HII region samples, and are better matched by samples of extreme local galaxies that are analogs of high-redshift sources. The new metallicity calibrations presented in this work pave the way for robust studies of galaxy chemical evolution in the early Universe, leading to a better understanding of baryon cycling and galaxy formation from Cosmic Noon through the Epoch of Reionization.

astro-ph.GA↗

arXiv2026

A Census of Double-Peaked Lyman-alpha Emitters in MAGPI: Classification, Global Characteristics, and Spatially Resolved Properties

Double-peaked Ly$α$ profiles provide critical insights into gas kinematics and the distribution of neutral hydrogen (HI) from the interstellar to the intergalactic medium (ISM to IGM), and serve as valuable diagnostics of ionising Lyman continuum (LyC) photon escape. We present a study of the global and spatially resolved properties of double-peaked Ly$α$ emitters (LAEs) based on VLT/MUSE data from the MAGPI survey. From a parent sample of 417 LAEs at z = 2.9 - 6.6 in the first 35 fields, we identify 108 double-peaked LAEs using an automated peak classification technique. We measure a double-peak fraction of $\sim37\%$ at $z < 4$, decreasing to $\sim14\%$ at $z > 4$, likely due to enhanced IGM attenuation. Approximately $17\%$ of the double-peaked LAEs are blue-dominated, possibly tracing gas inflows, though backscattering remains a viable alternative for sources without systemic redshift. The blue-to-total flux ratio exhibits a luminosity dependence: fainter lines generally show higher blue flux. We find a narrowing of the red peak at $z > 4$, despite the presence of the blue peak, indicating intrinsic galaxy evolution rather than IGM attenuation. Several LAEs exhibit residual flux in the absorption trough, with normalised trough flux anticorrelating with peak separation, reflecting variations in HI column density. We further investigate spatially resolved properties of ten red-dominated LAEs with extended Ly$α$ halos. Despite azimuthal variations, both the blue-to-total flux ratio and normalised trough flux density increase with radius, while peak separation decreases. The red peak asymmetry shows only minor radial changes. These trends are consistent with variations in shell outflow velocity and HI column density across the halos. Based on peak separation, red peak asymmetry, and residual trough flux, we identify five LAEs as strong LyC-leaker candidates.

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arXiv2026

The Structure and Evolution of LRDs: Insights from JWST NIRSpec Medium and High Resolution Spectroscopy at $z\sim4$

We present an analysis of medium/high-resolution JWST/NIRSpec spectra for 11 LRDs at $z \sim 4$. By decomposing the broad and narrow components of the Balmer emission lines, we investigate the connection between line emission and UV/optical continua for the LRD population. We find that the broad H$α$ luminosity strongly correlates with the optical continuum (but not with the UV), indicating a common AGN origin for both. In contrast, the [O III] line strength is correlated with the UV continuum rather than the optical. Using the width and luminosity of the broad H$α$ line, we estimate central black hole masses of $10^6-10^8 M_{\odot}$ accreting at high Eddington ratios, consistent with an early ($λ_{\rm Edd} \sim 0.6$), rapid-growth phase of AGN evolution. Assuming a constant mass accretion rate in the framework of slim-disk models, we infer growth timescales of $\sim 10^5-10^7\rm yr$, and suggest LRDs may evolve into narrow-line Seyfert 1 galaxies. Upper limits from our spectra indicate that LRDs exhibit intrinsically weak optical Fe II emission compared to typical AGN. To simultaneously account for the inferred broad-line region size and observed luminosity, we propose a "Clumpy Envelope" model in which the optical emission arises from an extended, clumpy gas with a characteristic radius of tens of light-days. The diversity in observed optical continuum shapes can be explained by radial temperature gradients and self-absorption effects within this structure. Our results demonstrate the power of JWST high-resolution spectroscopy in probing the central engines and physical nature of the LRD population.

astro-ph.GA↗

arXiv2026

Differences between emission and absorption tracers of spatially resolved outflows in clumpy z ~ 0.1 star-forming galaxies

We present spatially resolved Keck/LRIS spectroscopy of three clumpy star-forming galaxies at $z\sim0.1$, comparing outflow properties traced by H$α$ and Mg II emission with those probed by Mg II and Na I D absorption. Outflow velocities measured using Mg II absorption ($\langle v_{\rm out} \rangle = -560 \pm 30$~\kms) are consistently higher than those traced by H$α$ emission ($\langle v_{\rm out} \rangle = -124 \pm 3$~\kms) across $\sim$5 kpc$^{2}$ regions. Despite this offset, the correlation between $v_{\rm out}$ and galaxy properties, such as SFR and $Σ_{\rm SFR}$, show similar slopes for both tracers, with Mg II absorption systematically offset by $\sim 0.4$ dex. In two galaxies, Mg II emission is also detected, yielding velocities consistent with H$α$. In one galaxy we also detect outflows in Na I D absorption and find similar velocities as Mg II in absorption, which leads to a $\sim$0.4 dex higher Na I D outflow velocities compared to those measured in emission. Our spatially resolved results are consistent with those found for galactic-scale measurements, implying the outflow relationships are similar from the sales of $\sim$1-2 kpc to global measurements. Combined with literature measurements, these results suggest that the offset in velocities is driven not by ionisation state, but rather by the systematics associated to how absorption and emission measures trace the gas density.

astro-ph.GA↗

arXiv2026

The GLASS-JWST Early Release Science Program. V. H$α$ luminosity functions at $z\sim1.3$ and $z\sim2.0$

We present H$α$ luminosity function (LF) measurements at redshifts $z\sim1.3$ and $z\sim2.0$ using JWST NIRISS grism data from the GLASS-JWST survey. Based on emission lines spectroscopically identified in the F115W, F150W and F200W filters, we select 99 H$α$ emitters. Through detailed effective volume and completeness analysis for each source, we construct the H$α$ LF in two redshift bins. Thanks to the sensitivity of NIRISS WFSS and gravitational lensing magnification, our sample reaches intrinsic H$α$ luminosities $\sim$10 times deeper than previous grism surveys, down to $L_{\rm Hα}\sim10^{40.5}~\rm erg~s^{-1}$ at $z\sim1.3$ and $L_{\rm Hα}\sim10^{40.9}~\rm erg~s^{-1}$ at $z\sim2.0$ with completeness larger than 0.8, corresponding to star formation rates of 0.4 and 1.0 $M_{\odot}~\rm yr^{-1}$, respectively. We robustly constrain the faint-end slope of the H$α$ luminosity function to be $-1.50^{+0.14}_{-0.08}$ at $z\sim1.3$ and $-1.60^{+0.17}_{-0.09}$ at $z\sim2.0$ after considering the cosmic variance of $\sim 20\%$, consistent with previous estimations. The emission-line samples presented here will enable further detailed studies of galaxy properties including metallicities. We find a negligible contribution from bright active galactic nuclei in our sample. We estimate integrated cosmic star formation rate densities of $0.097^{+0.015}_{-0.016}~M_{\odot}~\rm yr^{-1}~Mpc^{-3}$ at $z\sim1.3$ and $0.129^{+0.025}_{-0.030}~M_{\odot}~\rm yr^{-1}~Mpc^{-3}$ at $z\sim2.0$. The methodology presented here can be readily applicable to other JWST slitless spectroscopic datasets and future wide-field slitless surveys, including those from Euclid, Roman, and the Chinese Space Station Telescope.

astro-ph.GA↗

arXiv2026

Discovery of Ancient Globular Cluster Candidates in the Relic, a Quiescent Galaxy at $z=2.5$

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 $\sim2$~Gyr, suggesting a rich formation history starting at $z\sim10$. 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 $\sim2-3\times$ 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, $\sim10^6-10^7~M_{\odot}$, 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.

astro-ph.GA↗

arXiv2026

On the unique evolutionary mechanisms of massive quiescent galaxies in the epoch of reionisation

We investigate the evolutionary histories of a population of high mass, high redshift, quiescent galaxies in the cosmohydrodynamical simulation Thesan, studying the characteristic properties of their haloes and environments over the epoch of reionisation. Thesan employs a modified version of the Arepo moving-mesh code utilised in IllustrisTNG, which incorporates on-the-fly radiative transfer to couple haloes and galaxies with the evolving radiation field. Thesan exhibits nine massive quiescent galaxies at $z=5.5$, in a $(95.5 \text{cMpc})^3$ volume, with no counterpart in IllustrisTNG. A numerical issue in the simulation reduces AGN feedback efficiency by a factor of 25 while enhancing accretion rates, creating a regime of suppressed feedback. We find their stellar mass assembles rapidly through smooth halo accretion in dense environments, particularly from massive neighbouring structures, while their early-forming haloes develop fast-growing potential wells hosting massive black holes. This suppressed feedback allows prolonged black hole growth before eventual kinetic-mode quenching, providing insight into galaxy evolution under weakened AGN regulation. We find that megaparsec-scale overdensities and halo masses continue growing after quenching, suggesting these galaxies will reside in some of the largest haloes and densest regions of space by $z=6$. With massive quiescent galaxies found in JWST data, the identification of such galaxies in Thesan enables isolation of halo and environmental conditions most conducive to their evolution under this suppressed feedback regime, guiding future deep surveys and N-body simulation studies of analogous systems.

astro-ph.GA↗

arXiv2026

Most Strong Lensing Deflectors in the AGEL Survey Are in Group and Cluster Environments

The environments of deflectors in strong lensing systems affect our ability to test cosmological models and constrain evolutionary properties of galaxies. Here we measure the deflector scale (Einstein mass) and deflector environment (halo mass) of 89 spectroscopically confirmed strong lenses in the ASTRO3D Galaxy Evolution With Lenses (AGEL) survey. We classify deflector scale by measuring $θ_{\rm{E}}$ to determine the mass enclosed by the Einstein radius, $M(<θ_{\rm{E}})$. We quantify deflector environment by using photometric redshifts to determine the galaxy surface density to the fifth-nearest neighbor $Σ_5(z)$. We find that 47.2% of our deflectors are embedded in cluster environments, whereas only 9.0% have cluster-scale Einstein radii (masses). We measure a weak correlation ($r = 0.38$) between Einstein mass and $Σ_5(z)$, suggesting that the assumption of single galaxy-scale deflectors in lens modeling is overly-simplified. We hypothesize that the weak correlation results from galaxy-scale bias in the original AGEL selection and the observational challenge of detecting faint arcs with large Einstein radii. Comparing number densities, $N_{\rm{gal}}$, between AGEL and control fields, we find that AGEL deflectors are in systematically denser environments. Our study provides a method to identify strong lenses as a function of deflector environment and approximate the impact of large-scale environment in lens modeling. We provide the measured lensing parameters for our 89 AGEL systems as well as $z_{\rm{phot}}$ and $r$-mag (AB) maps of the line-of-sight.

astro-ph.GA↗

arXiv2026

Taking a Break at Cosmic Noon: Continuum-selected Low-mass Galaxies Require Long Burst Cycles

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 ($M_\star < 10^{9.5}\,M_\odot$) 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 parametrization 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 ($\gtrsim 100$ Myr) and large deviations below the star-forming main sequence ($\gtrsim 0.8$ 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.

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arXiv2026

The Carousel Lens II: Cosmological Constraints with GIGA-Lens

The nature of dark matter and dark energy are among the central questions in cosmology. Strong gravitational lenses with multiple source planes provide a geometric probe of cosmology: the ratio of deflection angles at different redshifts depends only on angular-diameter distances, constraining the matter density $Ω_m$ and the dark energy equation of state $w$. However, constraints from this technique have historically lagged behind those from the CMB, SNe Ia, and BAO. In this work, we present new cosmological constraints from the Carousel Lens, a cluster-scale lens with more than 40 extended images from 11 spectroscopically confirmed sources. Its relaxed core and rich set of extended images behind the main halo make it particularly suitable for cosmological inference. Using the GIGA-Lens pipeline, we construct a pixel-level lens model including six HST-detected sources and four mass components. From this model, we obtain $w$CDM constraints of $Ω_m = 0.34^{+0.16}_{-0.13}$ and $w = -1.31^{+0.35}_{-0.32}$ from the Carousel Lens alone, accounting for both statistical and systematic uncertainties. We further project that including four additional known higher-redshift sources, assuming similar fractional uncertainties, could improve the constraining power by ~80%, bringing the precision close to that of the CMB and SNe Ia. For an evolving dark energy model ($w_0w_a$CDM), the Carousel Lens alone yields constraints comparable to the CMB, providing an independent and complementary probe alongside SN Ia and BAO. While currently systematic uncertainties dominate, which we quantify through simulations, our results demonstrate that relaxed multi-source-plane cluster lenses can deliver competitive cosmological constraints. Further improvements are expected from reductions in systematics and from incorporating higher-redshift sources (known and new) with high-resolution imaging.

astro-ph.CO↗

arXiv2026

Feeding the Dead: Neutral Gas Inflow in a Long-Quenched Ancient Massive Galaxy at $z\sim 2.7$ Observed with JWST/NIRSpec

We report the spectroscopic detection of neutral gas inflow into a massive ($M_* \simeq 4\times 10^{10} M_\odot$) quiescent galaxy observed at $z_{\rm{spec}} = 2.6576$ with JWST. From the redshifted absorption of the NaI doublet at $λλ5890, 5896 $ Ang, we estimate an inflow velocity $v=278^{+79}_{-79}$ km s$^{-1}$ and a column density $\log(N_{NaI}/\rm{cm^2}) = 13.02^{+0.03}_{-0.03}$. We derive the inflowing mass of the gas $M_{in} = 1.6^{+0.1}_{-0.1} \times 10^8 M_\odot$ and rate $\dot{M}_{in} = 19^{+6}_{-7} \, M_\odot \, \rm{yr}^{-1}$. The presence of several surrounding galaxies suggests that the galaxy may be accreting gas from nearby companions. However, we cannot confirm it with current data and the intergalactic medium or cosmic filaments are also viable sources of the inflowing gas. Despite the ongoing inflow, the galaxy remains quiescent, with an upper limit to the star formation rate of $0.2 \, M_\odot \, \rm{yr}^{-1}$. Moreover, its star formation history suggests that the galaxy has remained quiescent during the past $\sim1$ Gyr, with half of its stars formed by redshift $z_{50}=11^{+18}_{-3}$. We discuss that the inflow is not massive, dense, or long-lived enough to ignite significant star formation, or it is fueling low-level AGN activity instead. This is direct evidence that quiescent galaxies can accrete cold gas after their quenching while keeping their star formation subdued. Follow-up observations with JWST and ALMA will be needed to constraint the nature of the inflowing gas.

astro-ph.GA↗

arXiv2026

MAMMOTH-LyC: Investigating the Role of Galaxy Mergers in a Strong Lyman Continuum Leaker at $z=2.39$

The MAMMOTH-LyC survey is a cycle 30 Hubble Space Telescope (HST) medium program obtaining 18-orbit-deep WFC3/UVIS F225W imaging in two massive galaxy protocluster fields at $z\sim2.2$. We introduce this survey by reporting the discovery of J1244-LyC1, a strong Lyman continuum (LyC) leaker at $z = 2.39$, exhibiting clear merger signatures. J1244-LyC1 has a highly significant ($10σ$) LyC detection, corresponding to an absolute escape fraction of $f_{\mathrm{esc}} \! =\!36\%\pm4\%$ ($1σ$). The LyC emission is spatially resolved into multiple peaks that coincide with the system's disturbed morphology, confirming genuine multi-site LyC leakage. With a stellar mass of $10^{10.2}{M_\odot}$, J1244-LyC1 is both the first confirmed high-redshift LyC-leaking merger and the most massive LyC emitter known to date. We interpret J1244-LyC1 as a merger-driven starburst system in which tidal interactions have disrupted the interstellar medium, creating multiple low-column-density pathways that facilitate LyC escape. This discovery provides the first direct evidence of spatially resolved LyC escape in a merging system, offering new insight into the potential role of major mergers in driving the cosmic reionization.

astro-ph.GA↗

arXiv2026

Water absorption confirms cool atmospheres in two little red dots

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 $\sim 1.4 \, μ\mathrm{m}$ in two out of four LRDs at $z \sim 2$ 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 $T \lesssim 3000\, \mathrm{K}$ 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 ($\sim2000\, \mathrm{K} - 4000 \, \mathrm{K}$) 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.

astro-ph.GA↗

arXiv2026

UNCOVER/MegaScience Finds Uniform and Highly Bursty Star Formation at 3 < z < 9, consistent with the High-Redshift UV Luminosity Function

Star formation timescales are key to understanding fundamental physics like feedback mechanisms, as well as the abundance of bright galaxies at $z>10$. We investigate galaxy star formation histories (SFHs) and their evolution across $z\sim3$--9 by measuring the line-to-UV ratio (\rline) and line equivalent width (EW) of \hanii\ and \oiiihb\ directly from UNCOVER/MegaScience spectro-photometry without relying on a specific SFH or nebular line modeling. Our photometric measurements recover \rline\ and EW to $<10\%$ systematic accuracy compared to spectroscopy. This allows us to construct a large mass- (and flux-) complete sample and quantitatively examine how \rline\ evolves with redshift and stellar mass. We find that the intrinsic scatter in \rline\ does not significantly evolve with redshift across $3 10$. This suggests that no significant evolution in star formation burstiness is required to explain the abundance of UV-bright galaxies at high redshift.

astro-ph.GA↗