arXiv subjects

Gabriel Brammer

Publications and source records attributed to Gabriel Brammer.

At least 91 records · Page 5Linked to original sources

arXiv2025

A GLIMPSE of Intermediate Mass Black holes in the epoch of reionization: Witnessing the Descendants of Direct Collapse?

JWST has revealed an abundance of supermassive black holes (BHs) in the early Universe, and yet the lowest mass seed black holes that gave rise to these populations remain elusive. Here we present a systematic search for broad-line Active Galactic Nuclei (AGNs) in some of the faintest high-$z$ galaxies surveyed yet by combining ultra-deep JWST/NIRSpec G395M spectroscopy with the strong lensing aid in Abell S1063. By employing the profile of the [OIII]$λ5007$ emission lines as a template for narrow-line components and carefully cross-validating with mock observations, we identify a sample of ten broad-line AGNs at $4.5<z<7.0$ (eight secure, two tentative). The inferred BH masses from the broad H$α$ line explore the intermediate BH mass regime down to $\sim 10^{5.5}\,M_\odot$. The stellar mass ($M_*$) is estimated with a galaxy+AGN composite model, and we find the BH to stellar mass ratio spans down to $M_{\rm BH}/M_*\lesssim 0.1\%$, unveiling populations on the empirical $M_{\rm BH}-M*$ relation observed in the local universe. We also derive the black hole mass function and investigate its low-mass end at this epoch. While we confirm the agreement of our results with previous studies at $M_{\rm BH}\gtrsim10^{6.5}M_{\odot}$, we find the mass range of $\sim 10^{5.5}\,M_\odot$ features an enhanced abundance with respect to the extrapolated best-fit Schechter function. Comparison with theoretical models suggests that a possible origin for this enhanced abundance is the direct-collapse BH formation, supporting the scenario that the direct collapse of massive gas clouds is a significant pathway for the earliest supermassive BHs.

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arXiv2025

Early galaxy evolution: The complex interstellar medium distribution of the z~7 galaxy A1689-zD1

We observed the gravitationally lensed ($μ= 9.6\pm0.19$) galaxy A1689-zD1 at $z = 7.1$ in bands 3, 6, and 8 of the Atacama Large Millimeter/submillimeter Array. These high-resolution observations ($\approx 200$ pc) enabled us to separate the source into five components in the [CII] 158$μ$m and [OIII] 88$mu$m emission lines within a projected distance of 2 kpc. Even though these components appear to vary strongly from one another in both their line, continuum, and optical characteristics, the assembly of components do not show ordered rotation and appear consistent with simulations of a galaxy system undergoing the process of assembly. The total dynamical mass of the galaxy ($2\times10^{10}$ M$_\odot$) is an order of magnitude larger than the spectrally estimated stellar mass, suggesting a near-complete optical obscuration of the bulk of the stellar component. Comparing the line ratios as well as the line properties to other properties such as the star formation rate, we find that A1689-zD1 is consistent with the relations derived from local star-forming galaxies. Even though A1689-zD1 lies on local star formation scaling relations and has a high dust and stellar mass estimate, the kinematics suggest it is in an early assembly stage, which could lead to it becoming a disk galaxy at a later stage.

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arXiv2025

The Light Echo of a High-Redshift Quasar mapped with Lyman-$α$ Tomography

Ultra-violet (UV) radiation from accreting black holes ionizes the intergalactic gas around early quasars, carving out highly ionized bubbles in their surroundings. Any changes in a quasar's luminosity are therefore predicted to produce outward-propagating ionization gradients, affecting the Lyman-$α$ (Ly$α$) absorption opacity near the quasar's systemic redshift. This "proximity effect" is well-documented in rest-UV quasar spectra but only provides a one-dimensional probe along our line-of-sight. Here we present deep spectroscopic observations with the James Webb Space Telescope (JWST) of galaxies in the background of a super-luminous quasar at $z_{\rm QSO}\approx6.3$, which reveal the quasar's "light echo" with Ly$α$ tomography in the transverse direction. This transverse proximity effect is detected for the first time towards multiple galaxy sightlines, allowing us to map the extent and geometry of the quasar's ionization cone. We obtain constraints on the orientation and inclination of the cone, as well as an upper limit on the obscured solid angle fraction of $f_{\rm obsc}<91\%$. Additionally, we find a timescale of the quasar's UV radiation of $t_{\rm QSO}=10^{5.6^{+0.1}_{-0.3}}$ years, which is significantly shorter than would be required to build up the central supermassive black hole (SMBH) with conventional growth models, but is consistent with independent measurements of the quasars' duty cycle. Our inferred obscured fraction disfavors a scenario where short quasar lifetimes can be explained exclusively by geometric obscuration, and instead supports the idea that radiatively inefficient accretion or growth in initially heavily enshrouded cocoons plays a pivotal role in early SMBH growth. Our results pave the way for novel studies of quasars' ionizing geometries and radiative histories at early cosmic times.

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arXiv2025

What you see is what you get: empirically measured bolometric luminosities of Little Red Dots

New populations of red active galactic nuclei (known as ``Little Red Dots'') discovered by JWST exhibit remarkable spectral energy distributions. Leveraging X-ray through far-infrared observations of two of the most luminous known Little Red Dots, we directly their bolometric luminosities. We find evidence that more than half of the bolometric luminosity likely emerges in the rest-frame optical, with $L_{\rm bol}/L_{5100} = 5$, roughly half the value for ``standard'' Active Galactic Nuclei. Meanwhile, the X-ray emitting corona, UV-emitting black-body, and reprocessed mid to far-infrared emission are all considerably sub-dominant, assuming that the far-infrared luminosity is well below current measured limits. We present new bolometric corrections that dramatically lower inferred bolometric luminosities by a factor of ten compared to published values in the literature. These bolometric corrections are in accord with expectations from models in which gas absorption and reprocessing are responsible for the red rest-frame optical colors of Little Red Dots. We discuss how this lowered luminosity scale suggests a lower mass scale for the population by at least an order of magnitude {\bf (e.g., $\sim 10^5-10^7~{\rm M_{\odot}}$ black holes, and $\sim 10^8~{\rm M_{\odot}}$ galaxies)}, alleviating tensions with clustering, overmassive black holes, and the integrated black hole mass density in the Universe.

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arXiv2025

UNCOVER/MegaScience: No Evidence of Environmental Quenching in a z$\sim$2.6 Proto-cluster

Environmental quenching -- where interactions with other galaxies and/or the intra-cluster medium (ICM) suppress star formation in low-mass galaxies -- has been well-established as the primary driver behind the formation of the red sequence for low-mass galaxies within clusters at low redshift ($z<1$). However, it remains unclear whether these mechanisms are active at higher-redshifts in proto-cluster environments that are not yet fully virialized. In large part, this regime has remained unexplored due to observational limitations; however, JWST has recently opened a new window into the role of environmental quenching on low-mass (log(M$_{\star}$/M$_{\odot}$$<$9.0) galaxies at cosmic noon ($2 < z < 3$). Here, we leverage the deep imaging and R$\sim$15 spectrophotometry enabled by the 20 band JWST/NIRCam data from the UNCOVER and MegaScience programs to examine environmental quenching in a newly discovered $z\approx2.58$ proto-cluster. We compare the star formation histories (SFHs) of 19 low-mass quiescent galaxies in the proto-cluster to a matched sample of 18 in the field, and find no significant differences. This similarity extends to galaxy sizes and quenched fractions, which also show no significant differences between the two environments across the full stellar mass range (8.5$<$log(M$_{\star}$/M$_{\odot}$$\leq$11.0). This indicates that the proto-cluster has not yet accelerated quenching relative to the field and is consistent with expectations that $z>2$ proto-clusters have yet to virialize and develop a dense enough environment required to efficiently quench low-mass galaxies.

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arXiv2025

Further Evidence for a Direct-Collapse Origin of the Supermassive Black Hole at the Center of the Infinity Galaxy

The z=1.14 $\infty$ galaxy consists of two ringed nuclei with an active supermassive black hole (SMBH) in between them. The system is likely the result of a nearly face-on collision between two disk galaxies with massive bulges. In van Dokkum et al. (2025) we suggested that the SMBH may have formed from shocked and compressed gas at the collision site, in a runaway gravitational collapse. Here we test this hypothesis using newly obtained JWST NIRSpec IFU observations. We first confirm that the system has a cloud of gas in between the nuclei that is photo-ionized by an AGN-like object near its center. Next, we constrain the origin of the SMBH from its radial velocity. If it formed in the cloud its velocity should be similar to the surrounding gas, whereas it would be offset if the SMBH had escaped from one of the nuclei or were associated with a faint galaxy. We find that the radial velocity of the SMBH is within $\sim 50$ km/s of that of the surrounding gas, as expected if the SMBH formed within the cloud. Unexpectedly, we find that both nuclei have active SMBHs as well, as inferred from very broad H$α$ emission with FWHM $\sim 3000$ km/s. This rules out scenarios where the central SMBH was ejected from one of the nuclei in a gravitational recoil. Taken together, these results strengthen the hypothesis that the object at the center of the $\infty$ galaxy is a newly formed SMBH.

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arXiv2025

The Structure and Formation Histories of Low-Mass Quiescent Galaxies in the Abell 2744 Cluster Environment

Low-mass quiescent galaxies are thought to predominantly reside in overdense regions, as environmental effects are often invoked to explain their shutdown of star formation. These longer-timescale quenching mechanisms - such as interactions with hot gas in the intracluster medium and dynamical encounters with other cluster galaxies - leave imprints on galaxy morphologies, emphasizing the importance of quantifying the structures of low-mass quiescent galaxies in galaxy clusters at $z<0.5$. Using spectrophotometric data from the UNCOVER and MegaScience programs, we present the first measurement of the quiescent size-mass relation between $7<\log(M_\star/M_\odot)<10$ using JWST imaging, based on a sample of 1531 galaxies in the $z=0.308$ Abell 2744 galaxy cluster. The resulting size-mass relation has a significantly higher scatter than similar-redshift field samples, despite comparable best-fit relations in both the dwarf and intermediate-mass regimes. Both "progenitor bias", where larger, diskier low-mass galaxies enter the cluster at later epochs, and a general expansion of galaxy structure from dynamical interactions could be at play. This evolutionary framework is further supported by the tentative evidence that older low-mass quiescent galaxies in the cluster are more spheroidal. The star-formation histories derived for our cluster sample imply their formation and quenching occurs relatively late, at $z<1.5$. In this scenario, the progenitor population would have disky axis-ratio distributions at cosmic noon, in agreement with recent observations. While this leaves ample time for dynamical interactions to maintain quiescence and drive the observed subsequent morphological evolution post-quenching, the data disfavors an onset of quenching due to the environment.

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arXiv2025

MINERVA: A NIRCam Medium Band and MIRI Imaging Survey to Unlock the Hidden Gems of the Distant Universe

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

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arXiv2025

A remarkable Ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a Little Red Dot at $z=3.5$

The origin of the rest-optical emission of compact, red, high-redshift sources known as `little red dots' (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs would constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam and MIRI observations from the RUBIES and PRIMER programs of The Cliff: a bright LRD at $z=3.55$ with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad Hydrogen (H$α \rm FWHM\sim1500$km/s) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation, or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size ($M_*\sim10^{10.5}\,M_\odot,\ r_{e}\sim40\,$pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and H$α$ absorption line are instead most plausibly explained by a `black hole star' (BH*) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH* models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense, massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas.

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arXiv2025

No [CII] or dust detection in two Little Red Dots at z$_{\rm spec}$ > 7

Little Red Dots (LRDs) are compact, point-like sources characterized by their red color and broad Balmer lines, which have been debated to be either dominated by active galactic nuclei (AGN) or dusty star-forming galaxies (DSFGs). Here we report two LRDs (ID9094 and ID2756) at z$_{\rm spec}$>7, recently discovered in the JWST FRESCO GOODS-North field. Both satisfy the "v-shape" colors and compactness criteria for LRDs and are identified as Type-I AGN candidates based on their broad H$β$ emission lines (full width at half maximum: 2280$\pm$490 km/s for ID9094 and 1070$\pm$240 km/s for ID2756) and narrow [OI] lines ($\sim$ 300-400 km/s). To investigate their nature, we conduct deep NOEMA follow-up observations targeting the [CII] 158${\rm μm}$ emission line and the 1.3 mm dust continuum. We do not detect [CII] or 1.3 mm continuum emission for either source. Notably, in the scenario that the two LRDs were DSFGs, we would expect significant detections: $>16σ$ for [CII] and $>3σ$ for the 1.3 mm continuum of ID9094, and $>5σ$ for [CII] of ID2756. Using the 3$σ$ upper limits of [CII] and 1.3 mm, we perform two analyses: (1) UV-to-FIR spectral energy distribution (SED) fitting with and without AGN components, and (2) comparison of their properties with the L$_{[CII]}$-SFR$_{tot}$ empirical relation. Both analyses are consistent with a scenario where AGN activity may contribute to the observed properties, though a dusty star-forming origin cannot be fully ruled out. Our results highlight the importance of far-infrared observations for studying LRDs, a regime that remains largely unexplored.

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arXiv2025

Breaking Through the Cosmic Fog: JWST/NIRSpec Constraints on Ionizing Photon Escape in Reionization-Era Galaxies

The escape fraction of Lyman continuum photons (fesc(LyC)) is the last key unknown in our understanding of cosmic reionization. Directly estimating the escape fraction (fesc) of ionizing photons in the epoch of reionization (EoR) is impossible, due to the opacity of the intergalactic medium (IGM). However, a high fesc leaves clear imprints in the spectrum of a galaxy, due to reduced nebular line and continuum emission, which also leads to bluer UV continuum slopes (betaUV). Here, we exploit the large archive of deep JWST/NIRSpec spectra from the DAWN JWST Archive to analyze over 1'400 galaxies at 5 < zspec < 10 and constrain their fesc based on SED fitting enhanced with a picket fence model. We identify 71 high-confidence sources with significant fesc based on Bayes factor analysis strongly favouring fesc > 0 over fesc = 0 solutions. We compare the characteristics of this high-escape subset against both the parent sample and established diagnostics including betaUV slope, O32, and SFR surface density (SigmaSFR). For the overall sample, we find that most sources have a low escape fraction (<1%), however, a small subset of sources seems to emit a large number of their ionizing photons into the IGM, such that the average fesc is found to be ~10%, as needed for galaxies to drive reionization. Although uncertainties remain regarding recent burstiness and the intrinsic stellar ionizing photon output at low metallicities, our results demonstrate the unique capability of JWST/NIRSpec to identify individual LyC leakers, measure average fesc and thus constrain the drivers of cosmic reionization.

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arXiv2025

CANUCS/Technicolor Data Release 1: Imaging, Photometry, Slit Spectroscopy, and Stellar Population Parameters

We present the first data release of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), a JWST Cycle 1 GTO program targeting 5 lensing clusters and flanking fields in parallel (Abell 370, MACS0416, MACS0417, MACS1149, MACS1423; survey area \tilda100 arcmin$^{2}$), with NIRCam imaging, NIRISS slitless spectroscopy, and NIRSpec prism multi-object spectroscopy. Fields centered on cluster cores include imaging in 8 bands from 0.9-4.4$μ$m, alongside continuous NIRISS coverage from 1.15-2$μ$m, while the NIRCam flanking fields provide 5 wide and 9 medium band filters for exceptional spectral sampling, all to \tilda29 mag$_{AB}$. We also present JWST in Technicolor, a Cycle 2 follow-up GO program targeting 3 CANUCS clusters (Abell 370, MACS0416, MACS1149). The Technicolor program adds NIRISS slitless spectroscopy in F090W to the cluster fields while adding 8 wide, medium, and narrow band filters to the flanking fields. This provides NIRCam imaging in all wide and medium band filters over \tilda30 arcmin$^{2}$. This paper describes our data reduction and photometry methodology. We release NIRCam, NIRISS, and HST imaging, PSFs, PSF-matched imaging, photometric catalogs, and photometric and spectroscopic redshifts. We provide lens models and stellar population parameters in up to 19 filters for \tilda53,000 galaxies in the cluster fields, and \tilda44,000 galaxies in up to 29 filters in the flanking fields. We further present 733 NIRSpec spectra and redshift measurements up to $z=10.8$. Comparing against our photometric redshifts, we find catastrophic outlier rates of only 4-7\% and scatter of $σ_{\rm NMAD}$ of 0.01-0.03.

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arXiv2025

The Infinity Galaxy: a Candidate Direct-Collapse Supermassive Black Hole Between Two Massive, Ringed Nuclei

We report the discovery of an unusual z=1.14 object, dubbed the $\infty$ galaxy, in JWST imaging of the COSMOS field. Its rest-frame near-IR light is dominated by two compact nuclei with stellar masses of $\sim 10^{11}$ Msun and a projected separation of 10 kpc. Both nuclei have a prominent ring or shell around them, giving the galaxy the appearance of a figure eight or an $\infty$ symbol. The morphology resembles that of the nearby system II Hz 4, where the head-on collision of two galaxies with parallel disks led to the formation of collisional rings around both of their bulges. Keck spectroscopy, VLA radio data, and Chandra X-ray data show that the $\infty$ galaxy hosts an actively accreting supermassive black hole (SMBH), with quasar-like radio and X-ray luminosity. Remarkably, the SMBH is not associated with either of the two nuclei, but is in between them in both position and radial velocity. Furthermore, from excess emission in the NIRCAM F150W filter we infer that the SMBH is embedded in an extended distribution of H$α$-emitting gas, with a rest-frame equivalent width ranging from 400 - 2000 Angstrom. The gas spans the entire width of the system and was likely shocked and compressed at the collision site, in a galaxy-scale equivalent of what happened in the bullet cluster. We suggest that the SMBH formed within this gas in the immediate aftermath of the collision, when it was dense and highly turbulent. If corroborated with simulations and follow-up JWST spectroscopy, this would demonstrate that `direct' SMBH formation by a runaway gravitational collapse is possible in extreme conditions.

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arXiv2025

JWST UNCOVERs the Optical Size - Stellar Mass Relation at $4<z<8$: Rapid Growth in the Sizes of Low Mass Galaxies in the First Billion Years of the Universe

We study the rest-frame optical and ultraviolet morphology of galaxies in the first billion years of the Universe. Using JWST data from the UNCOVER and MegaScience surveys targeting the lensing cluster Abell 2744 we present multi-band morphological measurements for a sample of 995 galaxies selected using 20-band NIRCam photometry and 35 using NIRSpec Prism spectroscopy over the redshift range of $4<z<8$. The wavelength-dependent morphology is measured using pysersic by simultaneously modeling the images in 6 NIRCam wide filters covering the rest-frame UV to optical. The joint modeling technique increases the precision of measured radii by 50\%. Galaxies in our sample show a wide range of Sersic indices, with no systematic difference between optical and UV morphology. We model the size-mass relation in a Bayesian manner using a continuity model to directly fit the redshift evolution while accounting for observational uncertainties. We find the average size of galaxies at $\log M_*/M_\odot=8.5$ grows rapidly, from 400 pc at $z=8$ to 830 pc at $z=4$. This is faster evolution than expected from power law scalings of the Hubble parameter or scale factor that describe well previous results at $z<2$. This suggests that different and/or much stronger processes affect low mass systems during the epoch of reionization. The measured logarithmic slope (0.25) and scatter (0.23 dex) are non-evolving. We discuss the remarkable consistency of the slope and scatter over cosmic time in the context of the galaxy-halo connection.

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arXiv2025

RUBIES: A Spectroscopic Census of Little Red Dots; All V-Shaped Point Sources Have Broad Lines

The physical nature of Little Red Dots (LRDs) - a population of compact, red galaxies revealed by JWST - remains unclear. Photometric samples are constructed from varying selection criteria with limited spectroscopic follow-up available to test intrinsic spectral shapes and prevalence of broad emission lines. We use the RUBIES survey, a large spectroscopic program with wide color-morphology coverage and homogeneous data quality, to systematically analyze the emission-line kinematics, spectral shapes, and morphologies of $\sim$1500 galaxies at $z > 3.1$. We identify broad Balmer lines via a novel fitting approach that simultaneously models NIRSpec/PRISM and G395M spectra, yielding 80 broad-line sources with 28 (35%) at $z > 6$. A large subpopulation naturally emerges from the broad Balmer line sources, with 36 exhibiting `v-shaped' UV-to-optical continua and a dominant point source component in the rest-optical; we define these as spectroscopic LRDs, constituting the largest such sample to date. Strikingly, the spectroscopic LRD population is largely recovered when either a broad line or rest-optical point source is required in combination with a v-shaped continuum, suggesting an inherent link between these three defining characteristics. We compare the spectroscopic LRD sample to published photometric searches. Although these selections have high accuracy, down to $\rm F444W<26.5$, only 50-62% of the RUBIES LRDs were previously identified. The remainder were missed due to a mixture of faint rest-UV photometry, comparatively blue rest-optical colors, or highly uncertain photometric redshifts. Our findings highlight that well-selected spectroscopic campaigns are essential for robust LRD identification, while photometric criteria require refinement to capture the full population.

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arXiv2025

Measuring Emission Lines with JWST-MegaScience Medium-Bands: A New Window into Dust and Star Formation at Cosmic Noon

We demonstrate the power of JWST-NIRCam medium-band photometry to measure emission line fluxes and study dust and star formation properties of galaxies at cosmic noon. In this work, we present photometric emission line measurements and spatially-resolved maps of H$α$ and Pa$β$ for a sample of 14 galaxies at $1.3\leq z\leq 2.4$, observed by the MegaScience medium-band survey and the UNCOVER deep spectroscopic survey. We measure line fluxes directly from the medium-band photometry and compare with spectroscopic measurements from UNCOVER. We find reasonable agreement between the photometric and spectroscopic emission line fluxes for both H$α$ and Pa$β$, with scatter $<0.15$ dex down to emission line equivalent widths of $10$Å. We also make a nebular dust measurement from the ratio Pa$β$ / H$α$, finding an average nebular A$_\mathrm{V}$ of 1.4. Our photometric A$_\mathrm{V}$ measurements show a slightly larger scatter of $0.5$ magnitudes when compared to spectroscopic measurements; however, this scatter may be partially caused by aperture effects. Finally, we produce spatially resolved maps of H$α$ emission, Pa$β$ emission, and stellar continuum. We find that offsets in H$α$ and Pa$β$ emission are common, especially for galaxies with the highest A$_\mathrm{V}$, indicating dusty sub-structures. Furthermore, the correlation between H$α$ and continuum emission decreases with increasing A$_\mathrm{V}$, suggesting that the dustiest objects have clumpy dust and star formation distributions. Our study demonstrates the power of medium-band photometry to directly probe emission line strengths, star formation, and dust attenuation for hundreds of galaxies in UNCOVER and thousands of galaxies in upcoming JWST medium-band surveys.

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arXiv2025

UNCOVERing the High-redshift AGN Population among Extreme UV Line Emitters

JWST has revealed diverse new populations of high-redshift ($z\sim4-11$) AGN and extreme star-forming galaxies that challenge current models. In this paper, we use rest-frame UV emission-line diagnostics to identify AGN candidates and other exceptional ionizing sources, complementing previous studies predominantly focused on broad-line AGN. From a parent sample of 205 $\mathrm{z_{spec}}>3$ UNCOVER galaxies with NIRSpec/PRISM follow-up, we identify 12 C IV, He II, and C III] emitters. Three of these galaxies also exhibit clear N III] and/or N IV] lines. Leveraging the combined rest-optical and UV coverage of PRISM, we limit the emission-line model space using the sample's [O III]/H$β$ distribution, significantly decreasing the overlap between AGN and star-formation models in the UV diagnostics. We then find that the five He II emitters are the strongest AGN candidates, with further support from two [Ne V] detections and one X-ray detection from Chandra. Our Balmer line fits also reveal one new broad-line AGN at z=6.87. We cannot robustly quantify the AGN fraction in this sample, but we note that close to 20% of $\mathrm{M_{*}>2\times10^{9}\,M_{\odot}}$ parent sample galaxies are AGN candidates. The lower-mass line emitters, which are consistent with both AGN and star-forming photoionization models, have more compact sizes and higher specific star formation rates than the parent sample. Higher-resolution and deeper data on these UV line emitters should provide much stronger constraints on the obscured AGN fraction at $z > 3$.

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arXiv2025

CANUCS: Constraining the MACS J0416.1-2403 Strong Lensing Model with JWST NIRISS, NIRSpec and NIRCam

(Abridged) The strong lensing models of galaxy clusters rely on the number and quality of multiple images with spectroscopic redshifts. We present an updated strong lensing model of MACS J0416.1-2403 with the largest sample of multiple images with spectroscopic redshifts in a galaxy cluster field to date. Furthermore, we aim to demonstrate the effectiveness of JWST, particularly its NIRISS camera, for strong lensing studies. We used NIRCam imaging and NIRSpec and NIRISS spectroscopy from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). The cluster mass model was constrained using Lenstool software. Our new dataset, which we used for constraining the lens model, comprises 303 secure multiple images with spectroscopic redshifts from 111 background sources and includes 95 systems with previously known MUSE redshift and 16 systems (with 46 multiple images) for which we obtained spectroscopic redshift for the first time using NIRISS and NIRSpec spectroscopy. Three of the spectroscopic systems were not identified by previous JWST studies. The total number of secure spectroscopic systems is more than 20% higher than in the previous strong lensing studies of this cluster. The model can reproduce multiple images with the root-mean-square distance of ~ 0.52''. We also provide a full catalogue with 415 multiple images, including less reliable candidates. In total, we provide 15 new multiple-image system candidates (with 38 multiple images) not reported in previous studies. Furthermore, we demonstrate the effectiveness of JWST, particularly NIRISS, for obtaining spectroscopic redshifts of multiple images. As NIRISS F115W, F150W, and F200W grism spectroscopy captures at least two of the [OII], [OIII] and H$α$ lines at 1 < z < 3 (a redshift range particularly relevant for strong lensing studies) without target pre-selection, it complements MUSE and NIRSpec observations extremely well.

astro-ph.CO↗