arXiv subjects

Arianna S. Long

Publications and source records attributed to Arianna S. Long.

At least 19 recordsLinked to original sources

arXiv2026

CHAMPS: The COSMOS High-Redshift ALMA-MIRI Population Survey - A New Census of the Dusty Early Universe

Understanding how dust forms and evolves over cosmic time is a key open problem in extragalactic astrophysics. Galaxies in the early Universe were once thought to be mostly dust-poor, but recent discoveries of heavily dust-obscured galaxies near the Epoch of Reionization challenge models of early dust production and raise new questions about the true cosmic star formation rate density. In this paper, we present the COSMOS High-Redshift ALMA-MIRI Population Survey (CHAMPS), which sheds light on the dusty early universe at z>4. CHAMPS is a new 144h ALMA 1.2mm (band 6) large program mapping the combined JWST/NIRCam+MIRI footprint in the COSMOS field. It covers 0.2deg2 with a synthesized beam size of 1.1" and a sensitivity of 0.14mJy/beam. It is complemented, alongside JWST, by ancillary X-ray, UV, optical, sub-mm, and radio data. CHAMPS, optimized as a blind survey, detects above 5$σ$ approximately 385 individual dusty high-redshift sub-mm sources, 68 dusty AGN, and a dozen dusty galaxies at z>6. The galaxies are largely not detected in JWST/F150W, making them prototypical near-IR dark galaxies. We showcase three of the highest-redshift most dusty and massive candidate galaxies detected in CHAMPS with redshifts up to z=7.5. CHAMPS also enables stacks of thousands of galaxies grouped in bins of various physical parameters. Its sub-mm coverage of dust-attenuated JWST-identified galaxies provides robust constraints on dust masses and obscured star formation rates to study early dust formation and evolution. In addition, CHAMPS enables a blind search for luminous CO line emitters at z=0.4-2.7 and [CII]158$μ$m emitters at z=6.7.

astro-ph.GA↗

arXiv2026

CHAMPS: Data Reduction and Detection of ~3300 Dusty Galaxies at 1.2 mm

We present the reduction and source catalogs for the \textit{COSMOS High-$z$ ALMA-MIRI Population Survey} (CHAMPS), the largest Atacama Large (Sub)Millimeter Array (ALMA) blank field survey to date at 0.2\,deg$^2$. With observing frequency centered at $250\, $GHz ($1.2\,$mm) and bandwidth of 7.5 GHz, CHAMPS covers the JWST/NIRCam+MIRI footprint in the COSMOS field, taking advantage of the rich, multiwavelength ancillary datasets in COSMOS ranging from X-ray to radio. The majority of the CHAMPS survey (90$\%$) has depths at or below 200 $μ$Jy beam$^{-1}$ and with an average RMS of 146 $μ$Jy beam$^{-1}$. The reduced CHAMPS mosaic has a beam size of $θ_{\rm res} = 1.1'' \times 1.1''$. CHAMPS identifies sources via two detection methods, a blind search for high-SNR sources, and a prior-based search involving MIRI detections to identify faint sources. The blind search yields a total of 595 sources down to an SNR $\ge4.5σ$ while the prior catalog yields a total of 3162 source down to an SNR $\ge3σ$, leading to a full sample of 3355 unique objects detected across both catalogs. We identify a unique population of 385 SNR $\ge5σ$ sources across the two catalogs; after excluding 9 likely spurious edge detections we define a robust science sample of 376 sources above $\ge5σ$. These sources have a median redshift of $\langle z_{1.2 \text{ mm}}\rangle = 2.54^{+0.89}_{-1.69}$ based on optical/near-IR/mid-IR photometric redshifts and high-quality spectroscopic redshifts, and hold a median stellar mass of $\langle\log (\rm M_\star/M_\odot)\rangle = 10.79^{+0.42}_{-3.08}$. We have further identified 68/376 ($\sim18\%$) sources as candidate AGN through X-ray, UV-optical, infrared and radio selection techniques.

astro-ph.GA↗

arXiv2026

Surveying the Universe in 4D: Beating Cosmic Variance with Wide-Field Slitless Spectroscopy from HST, JWST, Euclid, Roman, and Beyond

We summarize strategies, lessons learned, and future directions from the Space Telescope Science Institute workshop Surveying the Universe in 4D: Beating Cosmic Variance with Wide-Field Slitless Spectroscopy from HST, JWST, Euclid, Roman, and Beyond, held August 24--28, 2026. The workshop examined scientific results, observational and data analysis challenges, extraction tools, and future opportunities. Discussions highlighted (1) the transformative potential of WFSS for the study of transient phenomena, galaxy evolution --both spatially-resolved and within the broader context of the cosmic web--, and rare populations and (2) the synergies among Euclid and Roman surveys, Rubin-LSST monitoring, JWST WFSS, and high-resolution integral-field observations. Participants identified advances in forward modeling and physics-informed machine learning as essential for addressing spectral overlap, crowded fields, and upcoming, very large data volumes. Realizing WFSS's full potential will require community-wide infrastructure, science-ready data products, accessible cloud-based analysis tools, and robust benchmarking of reduction pipelines. Crucially, participants called for systemic changes to properly recognize early-career researchers who invest significant efforts in pipeline, code, and calibration developments that enable WFSS science, and stressed that progress requires collaborative, multidisciplinary practices that optimize the participation and benefits of the next generation.

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arXiv2026

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

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

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arXiv2026

JWST Spectra Conclusively Show an Excess of Neutral Gas Outflows in Quiescent Galaxies at z=2-5

Galaxies exhibit a broad range of star formation activity, from actively star-forming to quiescent systems. Yet, the mechanisms responsible for the rapid shutdown and continued suppression of star formation remain poorly understood, particularly for the quiescent galaxies recently uncovered by JWST at $z>3$. Neutral gas outflows provide a direct tracer of gas removal and regulation in quiescent galaxies. We therefore present a NaID $λ\lambda5891,5897$ absorption--line stacking analysis of 274 galaxies at $z=2$--5 drawn from the DAWN JWST Archive (DJA) and the EMBER JWST program, using JWST/NIRSpec medium-resolution grating spectroscopy and NIRCam photometry. NaID absorption is detected in all quiescent galaxy stacks spanning $z=2$--5, providing the first statistical evidence that neutral gas outflows are a ubiquitous feature of quiescent galaxies in the early Universe. Compared to the non-quiescent galaxy stacks, the quiescent stacks show mass loading factors higher by 2--4 dex, with mass outflow rates $\dot{M}_{\rm out} \sim 9$--$30\,M_\odot\,{\rm yr}^{-1}$ elevated by $\sim0.5$--$1$ dex and outflow velocities $v_{\rm out} \sim 310$--$570\,{\rm km\,s}^{-1}$ higher by a factor of $\sim2$. The extreme mass loading factor values and declining star formation histories strongly indicate that these outflows cannot be driven by current star formation alone. In the statistically reliable quiescent redshift bins at $2 \leq z < 3$ and $3 \leq z < 4$, the elevated $[\mathrm{N\,II}]/\mathrm{H}α$ and $[\mathrm{O\,III}]/\mathrm{H}β$ ratios are consistent with a possible AGN contribution, suggesting that non-stellar feedback may play a role in sustaining quiescence in galaxies at these epochs.

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arXiv2026

Dust in the Average Galaxy: Attenuation, Emission, and Opacity from $0<z<7$

We present constraints on the dust emission and attenuation properties of galaxies across 0 10^{10.5}M_\odot$. We derive empirical relationships for the effective attenuation, dust temperature, fraction of star formation that is unobscured, and dust-to-stellar mass ratio as functions of redshift and stellar mass. We separate the first order effect of star/dust geometry from dust grain properties by combining constraints on the IR SED, UV SED, and dust mass surface density. Importantly, we measure over an order of magnitude decrease in $κ_{UV}/κ_{FIR}$--the ratio of dust mass absorption coefficients in the UV at 1600Å and FIR at 500$μ$m--from z~0 to z~7. A depressed $κ_{UV}/κ_{FIR}$ is consistent with a deficit of small dust grains, possibly attributable to the intense radiation fields of high-$z$ star formation; indeed, we find a redshift-invariant inverse relationship between $κ_{UV}/κ_{FIR}$ and $Σ_{SFR}$. Most evolution in the dust-to-stellar ratio is at $z<1$, the product of mild downward evolution in the dust-to-gas ratio combined with steep evolution in the gas-to-stellar ratio. The significant evolution and dynamic range of $κ_{UV}/κ_{FIR}$ and prevailing disconnect between the UV/optical and FIR regimes emphasize that direct dust constraints are irreplaceable for the majority of star-forming galaxies at z<7, not just the most extreme star-formers.

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arXiv2026

Observing Co-Located Neutral and Ionized Gas-Phase Iron Depletion in the Magellanic Clouds

Depletion is the observed phenomenon where gas-phase elemental abundances are reduced through accretion onto dust grains. We measure neutral gas-phase elemental abundances (S, Fe) in the Magellanic Clouds along 33 sightlines using high-resolution UV spectroscopy (HST/COS and HST/STIS), and compare them to ionized gas-phase abundances (S, Fe) adopted from the literature for six co-located H\,\textsc{ii} regions (with the furthest separation of $\lesssim3'$, 50 pc). Comparing S abundances show that S is minimally depleted in the H\,\textsc{ii} regions and surrounding diffuse ISM. However, we find that the gas-phase Fe abundances in H\,\textsc{ii} regions can be lower than those of the neighboring neutral ISM by 0.3 to 2 dex. This difference is likely an offset in the amount of Fe depleted into dust grains. As accretion of gas-phase Fe is likely not effective at the temperatures of the H\,\textsc{ii} regions, Fe depletion into solid form would have occurred in the dense atomic or molecular clouds prior to star formation. Stronger depletion in the H\,\textsc{ii} regions shows that Fe-bearing grains survive destruction in the first few million years following ionization. Our observations highlight that Fe depletion in H\,\textsc{ii} regions can be a useful tracer of Fe depletion in dense molecular clouds, which are challenging to observe directly via UV absorption.

astro-ph.GA↗

arXiv2026

The GlimmIr: Spectroscopic Variability in a z~7 LRD Indicates Rapid Changes in Both the Narrow and Broad Line Regions

The enigmatic population of ``Little Red Dots'' (LRDs) sit at the center of some of the largest debates in extragalactic astronomy today. The source(s) of ionizing emission and the physical scale over which it governs is still largely unknown. We show for the first time spectroscopic variability in a z ~ 7 LRD. Comparing a recently obtained 10.2 hr JWST/NIRSpec F290LP/G395M spectrum via the C3PO survey to an 8.4 hr F290LP/G395M spectrum taken 99 days earlier (~13 rest-days) via the THRILS survey, we find a ~30% $ difference in the continuum and broad-line flux, and a 42% difference between [OIII]5008 flux in the two epochs. Through rigorous testing, we confirm that such differences are not the result of differing MSA slit placements on source nor merely flux calibration offsets. These results are further corroborated by both a similar continuum and [OIII]5008 flux differences found in NIRSpec prism/clear observations of the source at an epoch taken approximately a year earlier than the THRILS observations via RUBIES and an additional observation fortuitously taken during the THRILS epoch (within a rest-day) via the CAPERS survey. Assuming LRDs are a type of accreting black hole system, this implies direct sight-lines must exist from the accretion disk to the surrounding nebular gas on scales beyond the broad-line region, and thus any high-density gas interpretations must allow for covering fractions < 100%. Furthermore, these results show the [OIII] line emission is likely not galaxy process-dominated, with a significant population of the narrow-line emitting gas closest to the broad-line region being directly ionized by the LRD. Finally, these results highlight the need for new approaches in inferring black hole properties of these systems, accounting for the lack of significant ionization via star formation, and/or exploring more exotic host-galaxy conditions at these early epochs.

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arXiv2026

The case for super-Eddington accretion in JWST broad-line AGN during the first billion years

A multitude of JWST studies reveal a surprising over-abundance of over-massive accreting super-massive black holes (SMBHs) -- leading to a deepening tension between theory and observation in the first billion years of cosmic time. Across X-ray to infrared wavelengths, models built off of pre-JWST predictions fail to easily reproduce observed AGN signatures (or lack thereof), driving uncertainty around the true nature of these sources. Using a sample of JWST AGN identified via their broadened H$α$ emission and covered by the deepest X-ray surveys, we find neither any measurable X-ray emission nor any detection of high-ionization emission lines frequently associated with accreting SMBHs. We propose that these sources are accreting at or beyond the Eddington limit, which reduces the need for efficient production of heavy SMBH seeds at cosmic dawn. Using a theoretical model of super-Eddington accretion, we can produce the observed relative dearth of both X-ray and ultraviolet emission, as well as the high Balmer decrements, without the need for significant dust attenuation. This work indicates that super-Eddington accretion is easily achieved through-out the early Universe, and further study is required to determine what environments are required to trigger this mode of black hole growth.

astro-ph.HE↗

arXiv2025

ALMA and JWST Identification of Faint Dusty Star-Forming Galaxies up to z~8

We exploit a new sample of around 400 bright dusty galaxies from the ALMA CHAMPS Large Program, together with the rich JWST multi-band data products in the COSMOS field, to explore and validate new selection methods for identifying dusty star-forming galaxies (DSFGs). Here, we present an effective empirical selection criterion based on a newly defined parameter: I_star = log(M_star) x log(SFR). Incorporating the F277W-F444W color as a second parameter further improves the purity of the selection. We then apply this method to the COSMOS2025 catalog to search for fainter dusty galaxy candidates below the ALMA CHAMPS detection limit and, through a stacking technique, identify a population of high-redshift (z=6-8) DSFGs with an average flux density of$S_1.2mm = 0.15uJy and a space density of ~6E-6 Mpc^-3. This faint population seems to have been missed by most of the previous submillimeter/millimeter surveys, and ground- and space-based UV-to-NIR surveys. Finally, we discuss the possibility of an evolutionary connection between the z > 10 UV-bright galaxies recently discovered by JWST, the faint dusty z=6-8 galaxies identified here, and the population of z=3-5 massive quiescent galaxies, potentially linked as progenitor-descendant populations based on their abundance, redshifts, and stellar masses.

astro-ph.GA↗

arXiv2025

Sparse by the River: Diverse Environments of z > 3 Massive Quiescent Galaxies

High-redshift ($z > 3$), massive quiescent galaxies (QGs) offer a significant window into early Universe galaxy formation. Previous works have predicted miscellaneous properties for these quiescents, from an overdensity of neighbors to elevated quenched fractions among such neighbors (i.e. galactic conformity). However, due to a scarcity in highly-resolved deep-field observations until recently, these properties have not been closely examined and pose unresolved questions for galaxy evolution. With new photometric-redshift catalogs from JWST data in the COSMOS-Web field, we present the S$\mathrm{\hat{O}}$NG sample, comprising 171 photometrically selected massive ($\geq10^{10}$ M$_\odot$) QGs with $3\leq$ $z\mathrm{_{phot}}$ $<$ 5. We look for low-mass neighbors around our sample and find substantial populations of star-forming galaxies (SFGs), contrasting the conformity effect at low-$z$. Our QGs also exhibit diverse clustering, from having no neighbors to potentially residing in environments no denser than star-forming equivalents, to being accompanied by SFGs with more stellar mass than the QG itself. Using a geometric method, we also report filamentary signals for 4\% of our sample, suggestive of some QGs' rejuvenation via cold gas accretion. We reapply the analysis on seven spectroscopically confirmed QGs in COSMOS-Web (M$_*$ $\sim$ $10^9-10^{11}$ M$_\odot$) and note similar patterns. Lastly, we report on Saigon, the most distant low-mass quiescent galaxy known to date ($z =$ 4.55, M$_*$ $= 1.33 \times10^9$ M$_\odot$); this spectroscopically confirmed QG resides in a protocluster candidate with 11 SFGs. These results pave new paths towards understanding QG environment, while also signaling an opportune era to examine their evolution with JWST.

astro-ph.GA↗

arXiv2025

Testing Photometric Techniques for Measuring the Rest-Frame UV Spectral Slope Against JWST PRISM Spectroscopy

We present a sample of 53 galaxy spectra at z_spec ~ 5-12 from the JWST CEERS and RUBIES surveys, combining NIRSpec PRISM spectroscopy with NIRCam photometry. We aim to use these data to establish best practices for measuring the UV spectral slope ($β$) in the era of JWST. We adopt power-law fits to the rest-frame UV continuum from the spectroscopic data as our fiducial, or `true', $β$ values, and compare them to photometric estimates derived through four methods: (1) photometric power-law fitting, (2) power-law fitting to an SED model fitted to the photometry, (3) single-color fitting near the Lyman break, and (4) single-color fitting at fixed rest-frame wavelengths. We find that photometric power-law fitting most closely recovers the spectroscopic slopes, with minimal bias and scatter. SED fitting performs moderately well, and can be preferable in cases of low signal-to-noise where photometric power-law fitting may become unreliable. Single-color estimates, while commonly used in past studies, show the most significant deviations and are not recommended when more than a single color is available. Our results highlight the limitations and strengths of each approach and provide practical guidance for measuring $β$ from photometry when spectra are unavailable or are of insufficient quality.

astro-ph.GA↗

arXiv2025

An almost NIRCam-dark dusty star-forming galaxy at z=6.63

We present AC-2168, an almost NIRCam-dark, millimetre-bright galaxy in the COSMOS field. The source was identified blindly in ALMA Band-4 continuum data and remains undetected in the COSMOS-Web DR1 NIRCam catalogue. We spectroscopically confirm a redshift of $z_{\rm spec}=6.631$ from [CII] 158 $μ$m and four tentatively detected CO lines in NOEMA and ALMA data. SED fitting to near-IR to millimetre photometry yields $\rm L_{IR}=1.6\times10^{12}\,L_\odot$, an SFR of $\rm 244\,M_\odot/yr$, heavy dust attenuation $\rm A_V=5.4$ mag, and a stellar mass $\rm M_\star=3.7\times10^{10}\,M_\odot$. From the millimetre continuum and [CII] emission, we infer a warm ISM with $\rm T_{\rm dust}=60K$, $\rm M_{dust}=3.0\times10^{8}\,M_\odot$ and $\rm M_{gas}=4.1\times10^{10}\,M_\odot$. AC-2168 has a gas fraction ($f_{\rm gas}=M_{\rm gas}/(M_\star+M_{\rm gas})$) of $\sim52\%$, a short depletion time of $\rm \sim170Myr$, a compact ($\rm \sim1kpc$) dust-continuum size, and an SFR consistent with the star-forming main sequence at its mass. These properties match expectations for progenitors of massive quiescent galaxies at the peak of their assembly, as implied by NIRSpec-based SFHs of $z\sim4-5$ systems. Using the blind detection, we estimate a space density of $\rm 7.8^{+18.0}_{-6.5}\times10^{-6}\,cMpc^{-3}$ for AC-2168-like NIRCam-dark galaxies at $z\sim6-7$, $\sim42\%$ of the abundance of massive quiescent galaxies at $z\sim4-5$. No overdensity of Ly$α$ emitters or Lyman-break galaxies is found nearby, suggesting AC-2168 does not lie in a prominent protocluster and highlighting the importance of unbiased blind surveys for this population.

astro-ph.GA↗

arXiv2025

Discovery of a Little Red Dot candidate at $z\gtrsim10$ in COSMOS-Web based on MIRI-NIRCam selection

JWST has revealed a new high-redshift population called little red dots (LRDs). Since LRDs may be in the early phase of black hole growth, identifying them in the early universe is crucial for understanding the formation of the first supermassive black holes. However, no robust LRD candidates have been identified at $z>10$, because commonly-used NIRCam photometry covers wavelengths up to $\sim5\,{\rm μm}$ and is insufficient to capture the characteristic V-shaped spectral energy distributions (SEDs) of LRDs. In this study, we present the first search for $z\gtrsim10$ LRD candidates using both NIRCam and MIRI imaging from COSMOS-Web, which provides the largest joint NIRCam-MIRI coverage to date ($0.20\,{\rm deg^2}$). Taking advantage of MIRI/F770W to remove contaminants, we identify one robust candidate, CW-LRD-z10 at $z_{\rm phot}=10.5^{+0.7}_{-0.6}$ with $M_{\rm UV}=-19.9^{+0.1}_{-0.2}\,{\rm mag}$. CW-LRD-z10 exhibits a compact morphology, a distinct V-shaped SED, and a non-detection in F115W, all consistent with being an LRD at $z\sim10$. Based on this discovery, we place the first constraint on the number density of LRDs at $z\sim10$ with $M_{\rm UV}\sim-20$ of $1.2^{+2.7}_{-1.0}\times10^{-6}\,{\rm Mpc^{-3}\,mag^{-1}}$, suggesting that the fraction of LRDs among the overall galaxy population increases with redshift, reaching $\sim3\%$ at $z\sim10$. Although deep spectroscopy is necessary to confirm the redshift and the nature of CW-LRD-z10, our results imply that LRDs may be a common population at $z>10$, playing a key role in the first supermassive black hole formation.

astro-ph.GA↗

arXiv2025

SCUBADive II: Searching for $z>4$ Dust-Obscured Galaxies via F150W-Dropouts in COSMOS-Web

The relative fraction of obscured galaxies at $z>4$ compared to lower redshifts remains highly uncertain as accurate bookkeeping of the dust-obscured component proves difficult. We address this shortcoming with SCUBADive, a compilation of the JWST counterparts of (sub-)millimeter galaxies in COSMOS-Web, in order to further analyze the distribution and properties of massive dust-obscured galaxies at early times. In this paper, we present a subset of SCUBADive, focusing on 60 ``dark'' galaxies that dropout at 1.5\micron. Motivated by JWST observations of AzTECC71, a far-infrared bright F150W-dropout with $z_{\rm phot}=5.7^{+0.8}_{-0.7}$, we complete a systematic search of F150W-dropouts with SCUBA-2 and ALMA detections to find more candidate high redshift dusty galaxies. Within our subsample, 16 are most similar to AzTECC71 due to fainter F444W magnitudes ($>24$\,mag) and lack of counterparts in COSMOS2020. Despite high star formation rates ($\langle$SFR$\rangle=450^{+920}_{-320}$\,\mdot\,yr$^{-1}$) and large stellar masses ($\langle$log$_{10}$(\mstar)$\rangle=11.2^{+0.5}_{-0.6}$\,\mdot) on average, these galaxies may not be particularly extreme for their presumed epochs according to offsets from the main sequence. We find that heavily obscured galaxies, which would be missed by pre-JWST optical imaging campaigns, comprise $\gtrsim20$\% of galaxies across mass bins and potentially contribute up to 60\% at the very high mass end (log$_{10}$(\mstar/\mdot)$>11.5$) of the $z>4$ stellar mass function.

astro-ph.GA↗

arXiv2025

JWST+ALMA reveal the ISM kinematics and stellar structure of MAMBO-9, a merging pair of DSFGs in an overdense environment at $z=5.85$

We present high-resolution ALMA [CII] 158 micron observations and JWST/NIRCam+MIRI imaging of MAMBO-9, a pair of optically-dark, dusty star-forming galaxies at $z=5.85$. MAMBO-9 is among the most massive, gas-rich, and actively star-forming galaxies at this epoch, when the Universe was less than 1 Gyr old. The new, 400 pc-resolution [CII] observations reveal velocity gradients in both objects; we estimate dynamical masses and find a relative mass ratio of 1:5. The kinematics of both objects suggest both rotation and strong tidal interaction, suggesting that the pair has already experienced a close encounter. Indeed, the new JWST imaging reveals a continuous bridge of moderately dust-obscured material between the two. We perform spatially-resolved SED fitting using the high-resolution ALMA+JWST imaging, finding that the majority of recent star-formation is concentrated in extremely obscured ($A_V > 10$) clouds, while the majority of rest-optical light (stellar continuum and H$α$ emission) is emergent from moderate-to-highly obscured ($A_V\sim 1$-$5$) regions on the outskirts. Combining our new stellar and dynamical mass measurements with previous CO observations, we find that the mass budget of MAMBO-9 requires a CO-to-H$_2$ conversion factor ($α_{\rm CO}$) of roughly unity, indicative of a highly metal-enriched ISM. Finally, we show that MAMBO-9 resides in a large overdensity spanning the PRIMER-COSMOS field, with 39 galaxies spectroscopically confirmed within $\sim 25$ cMpc. With a total baryonic mass $\sim 10^{11}\,M_\odot$, MAMBO-9 can be considered a prototype of massive galaxy formation and likely progenitor of the brightest cluster galaxies (BCGs) in the lower-redshift Universe.

astro-ph.GA↗

arXiv2025

COSMOS-Web: Comprehensive Data Reduction for Wide-Area JWST NIRCam Imaging

We present the data reduction methodology used for the COSMOS-Web survey JWST NIRCam data. Covering 0.54 deg^2 with four broadband filters (F115W, F150W, F277W, F444W) and a total exposure time of approximately 270 hours, COSMOS-Web represents the largest contiguous field surveyed during JWST Cycle 1, posing unique data reduction challenges due to its extensive scale. By combining the official JWST Calibration Pipeline with custom improvements for noise removal, background subtraction, and astrometric alignment, we achieve high fidelity science-ready mosaics. We detail the systematic approach employed in the three stages of the JWST Calibration Pipeline. The data, collected in three epochs from January 2023 to January 2024, encompass 152 visits and have been processed into 20 mosaic tiles to optimize computational efficiency and data processing. The final data products achieve 5 sigma depths of 26.7-28.3 AB mag in 0.15" apertures. The processed and calibrated datasets are made available to the public.

astro-ph.IM↗

arXiv2025

An upper limit of 10$^6$ M$_\odot$ in dust from ALMA observations in 60 Little Red Dots

By virtue of their red color, the dust in little red dots (LRDs) has been thought to be of appreciable influence, whether that dust is distributed in a torus around a compact active galactic nucleus (AGN) or diffuse in the interstellar medium (ISM) of nascent galaxies. In Casey et al. (2024) we predicted that, based on the compact sizes of LRDs (unresolved in JWST NIRCam imaging), detection of an appreciable dust mass would be unlikely. Here we present follow-up ALMA 1.3mm continuum observations of a sample of 60 LRDs drawn from Akins et al. (2024). None of the 60 LRDs are detected in imaging that reaches an average depth of $σ_{rms}=22\,μJy$. A stack of the 60 LRDs also results in a non-detection, with an inverse-variance weighted flux density measurement of $S_{1.3mm}=2.1\pm2.9\,μJy$. This observed limit translates to a 3$σ$ upper limit of 10$^6$ M$_\odot$ in LRDs' dust mass, and $\lesssim10^{11}$ L$_\odot$ in total dust luminosity; both are a factor of 10$\times$ deeper than previous submm stack limits for LRDs. These results are consistent with either the interpretation that LRDs are reddened due to compact but modest dust reservoirs (with $A_{V}\sim2-4$) or, alternatively, that instead of being reddened by dust, they have extreme Balmer breaks generated by dense gas ($>10^{9}\,cm^{-3}$) enshrouding a central black hole.

astro-ph.GA↗