arXiv subjects

Pieter van Dokkum

Publications and source records attributed to Pieter van Dokkum.

At least 37 records · Page 2Linked to original sources

arXiv2026

JWST Confirmation of a Runaway Supermassive Black Hole via its Supersonic Bow Shock

We present JWST/NIRSpec IFU observations of a candidate runaway supermassive black hole at the tip of a 62 kpc-long linear feature at z=0.96. The JWST data show a sharp kinematic discontinuity at the tip, with a radial velocity change of $\approx 600$ km/s across 0.1'' (1 kpc). The velocity gradient, together with the projected post-shock flow velocity of $\approx 300$ km/s, is well described by a simple shock-compression model of a supersonic object, with a velocity of $v_{BH} = 954^{+110}_{-126}$ km/s and an inclination $i=29^{+6}_{-3}$ deg. The previously puzzling kinematics along the linear feature, with the observed radial velocity decreasing from $\approx 300$ km/s near the tip to $\approx 100$ km/s closer to the former host galaxy, are naturally explained as gradual downstream mixing of shocked gas with the circumgalactic medium through turbulent entrainment. The bow shock interpretation is further supported by the morphology of the gas at the tip of the wake and an analysis of the [OIII]/H$α$, [NII]/H$α$, [SII]/H$α$, and [SIII]/[SII] line ratios. The line ratios are consistent with fast radiative shocks and rapid cooling, with best-fit shock velocities that are in agreement with expectations from the black hole velocity and the shock geometry. Energy conservation over the lifetime of the wake suggests a SMBH mass of $M_{BH} \gtrsim 10^7$ M$_{\odot}$. These results confirm that the wake is powered by a supersonic runaway supermassive black hole, a long-predicted consequence of gravitational-wave recoil or multi-body ejection from galactic nuclei.

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arXiv2025

Kinematic Confirmation of a Remarkable Linear Trail of Galaxies in the NGC 1052 Field, Consistent with Formation in a High-Speed Bullet Dwarf Collision

A unique linear trail of diffuse galaxies was recently identified in the NGC 1052 field. This trail includes the remarkable, ultra-diffuse galaxies DF2 and DF4 which lack dark matter and host unusually luminous globular clusters. It has been proposed that the trail formed via a high-speed collision between two gas-rich dwarf galaxies. This scenario predicts that the trail galaxies are kinematically connected and follow a specific trend in radial velocity as a function of position, based on the known velocities and positions of DF2 and DF4. To test this hypothesis, we measured radial velocities for seven additional galaxies on the trail. While the galaxies' low surface brightnesses presented observational challenges, we employ several methods to obtain measurements for galaxies with effective surface brightnesses up to 28.6 mag arcsec$^{-2}$, including a narrow slit placed over globular clusters and a novel wide slit mode on Keck/LRIS, as well as a 'light bucket' mode on Keck/KCWI. We find that five of our seven targets follow the precise velocity trend predicted by DF2 and DF4, to a degree with just a 2% chance of randomly occurring. Moreover, the trail galaxies' radial velocities are significantly higher than those of the NGC 1052 group, setting it apart as a separate, kinematically connected system. Our findings support the theory that this trail of galaxies, including DF2 and DF4, formed together in a single event. A 'bullet dwarf' collision remains the only known explanation for all the unusual properties of DF2, DF4, and the associated trail of galaxies.

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arXiv2025

A first look at a complete view of spatially resolved star formation at 1<z<1.8 with JWST NGDEEP+FRESCO slitless spectroscopy

[abridged] The previously inaccessible star formation tracer Pa$α$ can now be spatially resolved by JWST NIRCam slitless spectroscopy in distant galaxies up to cosmic noon. In the first study of its kind, we combine JWST NGDEEP NIRISS and FRESCO NIRCam slitless spectroscopy to provide the first direct comparison of spatially resolved dust-obscured (traced by Pa$α$) versus unobscured (traced by H$α$) star formation across the main sequence. We stack Pa$α$ and H$α$ emission-line maps, along with stellar continuum images at both wavelengths of 31 galaxies at 1<z<1.8 in three bins of stellar mass. Surface brightness profiles are measured and equivalent width (EW) profiles computed. Increasing Pa$α$ and H$α$ EW profiles with galactocentric radius across all stellar masses probed provide direct evidence for the inside-out growth of galaxies both via dust-obscured and unobscured star formation for the first time. For galaxies on the main sequence, a weakly positive ($0.1\pm0.1$) Pa$α$/H$α$ line profile as a function of radius is found at $8.8\leqslant\mathrm{log}(M_{*}/\mathrm{M}_{\odot})<9.9$ with a negative ($-0.4\pm0.1$) Pa$α$/H$α$ line profile found at $9.9\leqslant\mathrm{log}(M_{*}/\mathrm{M}_{\odot})<11.0$. Low mass galaxies ($7.7\leqslant\mathrm{log}(M_{*}/\mathrm{M}_{\odot})<8.8$) with high sSFRs are found to have a negative ($-0.5\pm0.1$) Pa$α$/H$α$ line profile gradient. Our results demonstrate that while inside-out growth via star formation is ubiquitous across the main sequence just after cosmic noon, centrally concentrated dust attenuation is not. Along with other recent work in the literature, our findings motivate future studies of resolved SFR profiles in large samples of individual cosmic noon galaxies across the main sequence, to understand the intrinsic scatter in spatially resolved star formation.

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arXiv2025

Quantitative Morphology of Galactic Cirrus in Deep Optical Imaging

Imaging of optical Galactic cirrus, the spatially resolved form of diffuse Galactic light, provides important insights into the properties of the diffuse interstellar medium (ISM) in the Milky Way. While previous investigations have focused mainly on the intensity characteristics of optical cirrus, their morphological properties remain largely unexplored. In this study, we employ several complementary statistical approaches -- local intensity statistics, angular power spectrum / $Δ$-variance analysis, and wavelet scattering transform analysis -- to characterize the morphology of cirrus in deep optical imaging data. We place our investigation of optical cirrus into a multi-wavelength context by comparing the morphology of cirrus seen with the Dragonfly Telephoto Array to that seen with space-based facilities working at longer wavelengths (Herschel 250 $μm$, WISE 12 $μm$, and Planck radiance), as well as with structures seen in the DHIGLS HI column density map. Our statistical methods quantify the similarities and the differences of cirrus morphology in all these datasets. The morphology of cirrus at visible wavelengths resembles that of far-infrared cirrus more closely than that of mid-infrared cirrus; on small scales, anisotropies in the cosmic infrared background and systematics may lead to differences. Across all dust tracers, cirrus morphology can be well described by a power spectrum with a common power-law index $γ\sim-2.9$. We demonstrate quantitatively that optical cirrus exhibits filamentary, coherent structures across a broad range of angular scales. Our results offer promising avenues for linking the analysis of coherent structures in optical cirrus to the underlying physical processes in the ISM that shape them. Furthermore, we demonstrate that these morphological signatures can be leveraged to distinguish and disentangle cirrus from extragalactic light.

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arXiv2025

Fast Rotators at Cosmic Noon: Stellar Kinematics for 15 Quiescent Galaxies from JWST-SUSPENSE

We present spatially-resolved stellar kinematics of 15 massive ($M_*=10^{10.5-11.5}M_{\odot}$) quiescent galaxies at $z\sim1.2-2.3$ from the JWST-SUSPENSE program. This is the largest sample of spatially-resolved kinematic measurements of quiescent galaxies at cosmic noon to date. Our measurements are derived from ultra-deep NIRSpec/MSA stellar absorption line spectra, using a forward modelling approach that accounts for optics, source morphology, positioning, and data reduction effects. 10 out of 15 galaxies are orientated such that we can measure rotational support. Remarkably, all 10 galaxies show significant rotation ($V_{r_e}=117-345$km/s, $σ_0 = 180-387$km/s), and are classified as "fast rotators" from their spin parameter. The remaining galaxies are too misaligned with respect to the slit to constrain their rotational velocities. The widespread rotational support in our sample indicates that the process responsible for quenching star formation in early massive galaxies did not destroy rotating disc structures. When combined with other quiescent galaxy samples at $z\sim0.5-2.5$, we find a trend between rotational support and age, with younger quiescent galaxies being more rotationally supported. This age trend has also been found at $z\sim0$, and likely explains why our high-redshift galaxies show more rotational support compared to massive ETGs at $z\sim0$, which are, on average, older. Our kinematic modelling also enables us to calculate dynamical masses. These dynamical masses greatly exceed the stellar masses for our sample (median $M_{\text{dyn}}/M_*=2.7$); they even allow for the bottom-heavy IMF found in the cores of low-$z$ massive ellipticals. Altogether, our results support a scenario in which distant quiescent galaxies evolve into nearby massive ETGs, gradually building up their outskirts and simultaneously losing rotation, due to a series of (mostly minor) mergers.

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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

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

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

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

Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster

Candidate Dark Galaxy-2 (CDG-2) is a potential dark galaxy consisting of four globular clusters (GCs) in the Perseus cluster, first identified in Li et al. (2025) through a sophisticated statistical method. The method searched for over-densities of GCs from a \textit{Hubble Space Telescope} (\textit{HST}) survey targeting Perseus. Using the same \textit{HST} images and the new imaging data from the \textit{Euclid} survey, we report the detection of extremely faint but significant diffuse emission around the four GCs of CDG-2. We thus have exceptionally strong evidence that CDG-2 is a galaxy. This is the first galaxy detected purely through its GC population. Under the conservative assumption that the four GCs make up the entire GC population, preliminary analysis shows that CDG-2 has a total luminosity of $L_{V, \mathrm{gal}}= 6.2\pm{3.0} \times 10^6 L_{\odot}$ and a minimum GC luminosity of $L_{V, \mathrm{GC}}= 1.03\pm{0.2}\times 10^6 L_{\odot}$. Our results indicate that CDG-2 is one of the faintest galaxies having associated GCs, while at least $\sim 16.6\%$ of its light is contained in its GC population. This ratio is likely to be much higher ($\sim 33\%$) if CDG-2 has a canonical GC luminosity function (GCLF). In addition, if the previously observed GC-to-halo mass relations apply to CDG-2, it would have a minimum dark matter halo mass fraction of $99.94\%$ to $99.98\%$. If it has a canonical GCLF, then the dark matter halo mass fraction is $\gtrsim 99.99\%$. Therefore, CDG-2 may be the most GC dominated galaxy and potentially one of the most dark matter dominated galaxies ever discovered.

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arXiv2025

Silkscreen: Direct Measurements of Galaxy Distances from Survey Image Cutouts

With upcoming wide field surveys from the ground and space the number of known dwarf galaxies at $\lesssim 25$ Mpc is expected to dramatically increase. Insight into their nature and analyses of these systems' intrinsic properties will rely on reliable distance estimates. Currently employed techniques are limited in their widespread applicability, especially in the semi-resolved regime. In this work we turn to the rapidly growing field of simulation based inference to infer distances, and other physical properties, of dwarf galaxies directly from multi-band images. We introduce silkscreen: a code leveraging neural posterior estimation to infer the posterior distribution of parameters while simultaneously training a convolutional neural network such that inference is performed directly on the images. Utilizing this combination of machine learning and Bayesian inference, we demonstrate the method's ability to recover accurate distances from ground-based survey images for a set of nearby galaxies ($2 < D ({\rm Mpc)} < 12$) with measured SBF or TRGB distances. We discuss caveats of the current implementation along with future prospects, focusing on the goal of applying silkscreen to large upcoming surveys, like LSST. While the current implementation performs simulations and training on a per-galaxy basis, future implementations will aim to provide a broadly-trained model that can facilitate inference for new dwarf galaxies in a matter of seconds using only broadband cutouts. We focus here on dwarf galaxies, we note that this method can be generalized to more luminous systems as well.

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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

Investigating the Ultra-diffuse Galaxy NGC5846_UDG1 through the Kinematics of its Rich Globular Cluster System

Recent studies of ultra-diffuse galaxies (UDGs) have shown their globular cluster (GC) systems to be central in unveiling their remarkable properties and halo masses. Deep HST imaging revealed 54 GC candidates around the UDG NGC5846_UDG1 (UDG1), with a remarkable 13 per cent of the stellar light contained in the GC system. We present a kinematic analysis of UDG1's GC system from observations with the integral field spectrograph KCWI on the Keck II telescope. We measure recessional velocities for 19 GCs, confirming them as members of UDG1, giving a total of 20 confirmed GCs when combined with literature. Approximately 9 per cent of the stellar light are contained just in the confirmed GCs. We determine the GC system's velocity dispersion to be $σ_{\rm GC}$=29.8$^{+6.4}_{-4.9}$ km s$^{-1}$. We find that $σ_{\rm GC}$ increases with increasing magnitude, consistent with predictions for a GC system that evolved under the influence of dynamical friction. The GC system velocity dispersion is constant out to $\sim1R_{\rm eff}$. Using $σ_{\rm GC}$, we calculate $M_{\rm dyn}$=$2.09^{+1.00}_{-0.64}\times$10$^{9}$M$_{\odot}$ as the dynamical mass enclosed within $\sim$2.5 kpc. The dark matter halo mass suggested by the GC number-halo mass relationship agrees with our dynamical mass estimate, implying a halo more massive than suggested by common stellar mass-halo mass relationships. UDG1, being GC-rich with a massive halo, fits the picture of a failed galaxy.

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arXiv2025

DUALZ: Deep UNCOVER-ALMA Legacy High-Z Survey

We present the survey design and initial results of the ALMA Cycle 9 program of DUALZ, which aims to establish a joint ALMA and JWST public legacy field targeting the massive galaxy cluster Abell 2744. DUALZ features a contiguous $4'\times6'$ ALMA 30-GHz-wide mosaic in Band 6, covering areas of $μ>2$ down to a sensitivity of $σ=32.7~μ$Jy. Through a blind search, we identified 69 dust continuum sources at S/N $\gtrsim5.0$ with median redshift and intrinsic 1.2-mm flux of $z=2.30$ and $S_{\rm 1.2mm}^{\rm int}=0.24$~mJy. Of these, 27 have been spectroscopically confirmed, leveraged by the latest NIRSpec observations, while photometric redshift estimates are constrained by the comprehensive HST, NIRCam, and ALMA data for the remaining sources. With priors, we further identify a [CII]158 $μ$m line emitter at $z=6.3254\pm0.0004$, confirmed by the latest NIRSpec spectroscopy. The NIRCam counterparts of the 1.2-mm continuum exhibit undisturbed morphologies, denoted either by disk or spheroid, implying the triggers for the faint mm emission are less catastrophic than mergers. We have identified 8 HST-dark galaxies (F150W$>$27mag, F150W$-$F444W$>$2.3) and 2 JWST-dark (F444W$>$30mag) galaxy candidates among the ALMA continuum sources. The former includes face-on disk galaxies, hinting that substantial dust obscuration does not always result from inclination. We also detect a marginal dust emission from an X-ray-detected galaxy at $z_{\rm spec}=10.07$, suggesting an active co-evolution of the central black hole and its host. We assess the infrared luminosity function up to $z\sim10$ and find it consistent with predictions from galaxy formation models. To foster diverse scientific outcomes from the community, we publicly release reduced ALMA mosaic maps, cubes, and the source catalog.

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arXiv2025

A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that displays singular properties: among the largest Hydrogen Balmer breaks reported at any redshift, broad multi-peaked H$β$ emission, and Balmer line absorption in multiple transitions. We model this source as a "black hole star" (BH*) where the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free "atmosphere" around a supermassive black hole. This source may provide evidence of an early black hole embedded in dense gas -- a theoretical configuration proposed to rapidly grow black holes via super-Eddington accretion. Radiation from the BH* appears to dominate almost all observed light, leaving limited room for contribution from its host galaxy. We demonstrate that the recently discovered "Little Red Dots" (LRDs) with perplexing spectral energy distributions can be explained as BH*s embedded in relatively brighter host galaxies. This source provides evidence that black hole masses in the LRDs may be over-estimated by orders of magnitude -- the BH* is effectively dust-free contrary to the steep dust corrections applied while modeling LRDs, and the physics that gives rise to the complex line shapes and luminosities may deviate from assumptions underlying standard scaling relations.

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arXiv2025

UNCOVER: 404 Error -- Models Not Found for the Triply Imaged Little Red Dot A2744-QSO1

JWST has revealed an abundance of compact, red objects at $z\approx5-8$ dubbed "little red dots" (LRDs), whose SEDs display a faint blue UV continuum followed by a steep rise in the optical. Despite extensive study of their characteristic V-shaped SEDs, the nature of LRDs remains unknown. We present a new analysis of the NIRSpec/PRISM spectrum of A2744-QSO1, a triply imaged LRD at $z=7.04$ from the UNCOVER survey. The spectrum shows a strong Balmer break and broad Balmer emission lines, both of which are difficult to explain with models invoking exclusively AGN or stellar contributions. Our fiducial model decomposes the spectrum into a post-starburst galaxy dominating the UV-optical continuum and a reddened AGN being sub-dominant at all wavelength and contributing at $\sim20\%$ level. However, our most credible model infers a stellar mass of $M_\star\approx 4\times10^9\,\mathrm{M_\odot}$ within a radius of $r_\mathrm{e}<30\,$pc, driving its central density to the highest among observations to date. This high central density could be explained if A2744-QSO-1 is the early-forming core of a modern-day massive elliptical galaxy that later puffed up via the inside-out growth channel. The models also necessitate an unusually steep dust law to preserve the strong break strength, though this steepness may be explained by a deficit of large dust grains. It is also probable that these challenges reflect our ignorance of A2744-QSO1's true nature. Future variability and reverberation mapping studies could help disentangle the galaxy and AGN contribution to the continuum, and deeper redder observations could also unveil the dust properties in LRDs.

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arXiv2025

RUBIES Reveals a Massive Quiescent Galaxy at z=7.3

We report the spectroscopic discovery of a massive quiescent galaxy at $z_{\rm spec}=7.29\pm0.01$, just $\sim700\,$Myr after the Big Bang. RUBIES-UDS-QG-z7 was selected from public JWST/NIRCam and MIRI imaging from the PRIMER survey and observed with JWST/NIRSpec as part of RUBIES. The NIRSpec/PRISM spectrum reveals one of the strongest Balmer breaks observed thus far at $z>6$, no emission lines, but tentative Balmer and Ca absorption features, as well as a Lyman break. Simultaneous modeling of the NIRSpec/PRISM spectrum and NIRCam and MIRI photometry (spanning $0.9-18\,μm$) shows that the galaxy formed a stellar mass of log$(M_*/M_\odot)=10.23^{+0.04}_{-0.04}$ before $z\sim8$, and ceased forming stars $50-100\,$Myr prior to the time of observation, resulting in $\log(\rm{sSFR/Gyr}^{-1})<-1$. We measure a small physical size of $209_{-24}^{+33}\,{\rm pc}$, which implies a high stellar mass surface density within the effective radius of $\log(Σ_{*,\rm e}/M_\odot\,kpc^{-2})=10.85_{-0.12}^{+0.11}$ comparable to the highest densities measured in quiescent galaxies at $z\sim2-5$. The 3D stellar mass density profile of RUBIES-UDS-QG-z7 is remarkably similar to the central densities of local massive ellipticals, suggesting that at least some of their cores may have already been in place at $z>7$. The discovery of RUBIES-UDS-QG-z7 has strong implications for galaxy formation models: the estimated number density of quiescent galaxies at $z\sim7$ is $>100\times$ larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.

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arXiv2025

The UNCOVER Survey: First Release of Ultradeep JWST/NIRSpec PRISM spectra for ~700 galaxies from z~0.3-13 in Abell 2744

We present the design and observations of low resolution JWST/NIRSpec PRISM spectroscopy from the Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) Cycle 1 JWST Treasury program. Targets are selected using JWST/NIRCam photometry from UNCOVER and other programs, and cover a wide range of categories and redshifts to ensure the legacy value of the survey. These categories include the first galaxies at $z\gtrsim10$, faint galaxies during the Epoch of Reionization ($z\sim6-8$), high redshift AGN ($z\gtrsim6$), Population III star candidates, distant quiescent and dusty galaxies ($1\lesssim{}z\lesssim 6$), and filler galaxies sampling redshift--color--magnitude space from z~0.1-13. Seven NIRSpec MSA masks across the extended Abell 2744 cluster were observed, along with NIRCam parallel imaging in 8 filters (F090W, F115W, F150W, F200W, F277W, F356W, F410M, F444W, F480M) over a total area of ~26 arcmin$^2$, overlapping existing HST coverage from programs including the Hubble Frontier Fields and BUFFALO. We successfully observed 553 objects down to $m_{\mathrm{F444W}}\sim30\mathrm{AB}$, and by leveraging mask overlaps, we reach total on-target exposure times ranging from 2.4-16.7h. We demonstrate the success rate and distribution of confirmed redshifts, and also highlight the rich information revealed by these ultradeep spectra for a subset of our targets. An updated lens model of Abell 2744 is also presented, including 14 additional spectroscopic redshifts and finding a total cluster mass of $M_{\mathrm{SL}}=(2.1\pm0.3)\times10^{15}\,\mathrm{M}_{\odot}$. We publicly release reduced 1D and 2D spectra for all objects observed in Summer 2023 along with a spectroscopic redshift catalog and the updated lens model of the cluster (https://jwst-uncover.github.io/DR4.html).

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