arXiv subjects

Anita Zanella

Publications and source records attributed to Anita Zanella.

At least 19 recordsLinked to original sources

arXiv2026

SN Helios: A Multiply Imaged Type II Supernova Opening Time-Delay Cosmography beyond redshift of 3

We report the discovery and spectroscopic confirmation of SN Helios, a strongly lensed, multiply imaged Type II supernova (SN) at $z=3.34$ behind the galaxy cluster RXC J0018.5+1626. The transient was identified as a red F150W-dropout source (F277W = 26.1 mag) in JWST/NIRCam imaging taken on 2026 July 3 for the VENUS survey, with no counterpart in 2024 December imaging. Director's Discretionary Time observations on 2026 August 17 added second-epoch NIRCam imaging and a deep NIRSpec prism spectrum. The spectrum shows a strong, spectrally resolved H$α$ emission line (FWHM $\approx 7900$ km/s) with a P-Cygni absorption blueshifted by 8800 km/s, together with corresponding H$β$ and He I $λ$10830 features, establishing a hydrogen-rich photosphere at $z=3.34$. Template fits classify SN Helios as a Type II SN, and light-curve comparisons with well-studied local SNe II yield a phase of $27.5\pm2.8$ rest-frame days past B-band maximum. Independent cluster lens models magnify the observed image by $μ\approx 10$-20 and predict another image within $\simeq 2$-6 yr, offering a rare opportunity for a time-delay measurement of $H_{0}$ with a source at $z>3$. The host galaxy is undetected in deep rest-frame ultraviolet and optical imaging, in emission lines, and in ALMA 2 mm dust continuum, leaving a heavily obscured counterpart unlikely. Its de-lensed $M_{\rm UV}\gtrsim -13$ places it far below blank-field limits and adds support to an elevated core-collapse rate per unit star formation in ultra-faint galaxies. Without the multi-epoch data, SN Helios would masquerade as a convincing $z\approx 16$ Lyman-break galaxy, with galaxy-template fits strongly disfavoring any low-redshift solution even with multiple medium bands. Its spectrum thus provides an empirical SED template with which high-redshift searches can vet similar dropout candidates.

astro-ph.GA↗

arXiv2026

A sub-100 pc view at z~5 of a Multiply-Imaged Massive Quiescent Galaxy

Recent James Webb Space Telescope spectroscopic surveys reveal an abundance of rapidly assembled early galaxies in the first billion years. Cosmological simulations struggle to reproduce this population because models require a rapid, highly efficient conversion of baryons into stars followed by an abrupt cessation of star-formation, known as quenching, in the early Universe. Testing these theoretical models remains difficult because high-redshift field galaxies appear compact and unresolved. This lack of resolution makes it impossible to distinguish compaction-driven quenching from secular evolution. Here we present VENUS-CLJ0152-QG1, a massive post-starburst galaxy at $z=5.18\pm0.15$, with inferred low-levels of star-formation in the last 30 Myr. The foreground cluster CL J0152.7-1357 gravitationally lenses the system into three images with median magnifications of $μ\sim$ 7, 11, and 0.6. This geometry yields a physical resolution of $\sim 70$ pc, a factor of $\sim 7$ better than the JWST/NIRCam fundamental blurring limit (full-width half-maximum of $\sim$0.48 kpc at $z\sim5$). Our two-dimensional mapping reveals a half-light radius twice as large as expected, a central stellar mass surface density half a dex lower than field analogs, and off-center ionized gas emission. Together, these features show that VENUS-CLJ0152-QG1 abruptly ceased star formation in the absence of a compaction event or an obvious active galactic nucleus. Consequently, this galaxy shows that unresolved observations likely overestimate the global densities of early galaxies, and that theoretical models require alternative pathways to quench massive sources without relying on compaction or unobscured AGN.

astro-ph.GA↗

arXiv2026

The MICADO first light imager for the ELT: the PSF Reconstruction Software

The MICADO instrument for the ESO ELT will provide Adaptive Optics (AO) diffraction-limited NIR imaging and spectroscopy. A critical component of MICADO is the Point Spread Function Reconstruction (PSF-R) Software, which is designed to perform a blind reconstruction of the PSF for all observations using AO telemetry data only, both with the Single Conjugate (SCAO) and Multi-Conjugate Adaptive Optics (MCAO) modes of MICADO. The SCAO PSF-R Pipeline is currently being completed, and validation with ERIS at VLT and SOUL+LUCI at LBT has demonstrated high accuracy, of percent-level on-axis and 5-20% off-axis. Initial scientific simulations confirm that the PSF-R software meets the necessary precision for high-quality analysis, securing its role for MICADO's first light and future ELT operations.

astro-ph.IM↗

arXiv2026

The MICADO first light imager for the ELT: Simulated Observations of Star Forming Clumps at Cosmic Noon

Galaxies at redshift z$\sim$2-6 exhibit prominent star-forming regions (clumps). Characterisation of these galactic structures requires both high sensitivity and spatial resolution of $\sim$100 pc or less. Currently, these spatial scales are achievable only with the aid of strong gravitational lensing. However, lensing introduces model-dependent uncertainties and limits the sample to galaxies behind massive clusters. The upcoming 40m-class telescopes, such as the ESO Extremely Large Telescope (ELT), will enable routine studies of clumps in ubiquitous, non-lensed z$\ge$2 galaxies. We assess the capability of MICADO, the first-light imager and spectrograph of the ELT, to characterise clumps in non-lensed z=2 galaxies. Specifically, we focus on two representative observing scenarios, providing the basis for an early scientific exploitation of the instrument. We modelled clumps in an idealised z=2 star-forming galaxy and produced mock MICADO observations in both broad- and narrow-band imaging. The latter leverages the strong emission lines typical of clumps to improve detection and characterisation. Reaching UV rest-frame magnitudes of clumps as faint as M$_{UV} \sim -15$ in optimal observing conditions, MICADO will be able to efficiently characterise clumps in non-lensed z=2 galaxies down to R$_{e} \sim 20$ pc. Moreover, the full MICADO $\sim$1 arcmin$^2$ field of view enables efficient surveys of multiple targets. This study demonstrates MICADO's potential to fill a crucial observational gap, shedding new light on clump formation and evolution at cosmic noon. Accurate PSF reconstruction will be crucial to fully harness this capability as it significantly impacts clump detection and characterisation, especially at the smallest sizes ($<$40 pc).

astro-ph.GA↗

arXiv2026

Breathing Fire: Hot Dust in the Big Three Dragons at z = 7.15

We present new Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 ($λ_\mathrm{obs} = 0.45\,\mathrm{mm}$) and 4 ($λ_\mathrm{obs} = 2.2\,\mathrm{mm}$) observations towards the Big Three Dragons, a pair of merging Lyman-break galaxies (LBGs) at $z=7.15$. The system was previously detected in ALMA Bands 6, 7 and 8 ($λ_\mathrm{obs} = 0.73 - 1.32\,\mathrm{mm}$), which, combined with our new observations, allows us to robustly constrain its dust temperature and obscured SFR. The unresolved Band 4 observations yield a $3.5σ$ detection, and the $0.4''$ Band 9 observations detect the Eastern and Western LBGs at $3.7$ and $3.5σ$, respectively. Through modified blackbody fitting, we infer a global dust temperature of $T_d = 78_{-23}^{+35}\,\mathrm{K}$ for the system, which implies a high IR luminosity of $\log(L_\mathrm{IR}/L_\odot) = 12.32_{-0.41}^{+0.43}$. This makes the Big Three Dragons one of the most IR-luminous systems known at $z>7$, with a total $\mathrm{SFR}_\mathrm{UV+IR} = 267_{-153}^{+418}\,M_\odot\,\mathrm{yr}^{-1}$ that is almost completely obscured ($f_\mathrm{obs} = 0.94_{-0.09}^{+0.04}$). Using resolved ALMA observations in Bands 6, 8 and 9, we confirm both LBGs have hot dust temperatures ($T_d \approx 67 - 84\,\mathrm{K}$) and correspondingly high obscured fractions ($f_\mathrm{obs} \approx 0.88 -0.95$). We find the Western LBG to fall $\sim1\,\mathrm{dex}$ above the canonical IRX-$β_\mathrm{UV}$ relation, suggesting patchy dust obscuration. The compact Eastern component, on the other hand, is consistent with a Calzetti- or SMC-like dust screen within the uncertainties. Together with the similarly hot dust temperature recently reported for the $z=8.31$ galaxy MACS0416-Y1, our results suggest a non-negligible fraction of star formation at the bright end of the UV luminosity function is highly dust-obscured, even at $z\gtrsim7$.

astro-ph.GA↗

arXiv2026

The role of sound and auditory displays in telescope control rooms: a pilot study

Astronomers are often stereotyped as gazing at the stars. Today, they mainly visually inspect digital data they receive from telescopes in remote observatories. One of the most complex is the Very Large Telescope (VLT) in Chile. Multiple streams of information converge in the VLT control room, where telescopes and instruments are managed, with operators handling numerous tasks on dozens of screens in a challenging environment, marked by harsh geography, intense work rhythms, and isolation. Sound and sonification can represent an efficient novel means for data monitoring with efficiency and operator wellness in mind. We present insights from a qualitative pilot study aimed at assessing the current use of sound in the VLT control room to identify directions for the design of new auditory displays. Based on the analysis of questionnaires and interviews with VLT personnel, we identify the key characteristics of the current use of sound and describe opportunities for improved design strategies to integrate auditory displays in telescope control rooms to support decision-making, improve situational awareness, and reduce cognitive load.

astro-ph.IM↗

arXiv2026

No evolution in the number density of little red dots from cosmic dawn to cosmic noon

We present our search for little red dots (LRDs) in the "J1030 field", a region of the sky around the $z\sim 6.3$ quasar SDSS J1030+0524, observed by the JWST EIGER program. Over 154 point-like sources selected in a JWST-based photometric catalog, we find five broad line emitters (with $FWHM \gtrsim 1000\ \rm km s^{-1}$) that are red ($F200W - F356W > 0$) and are undetected in the X-rays. We use these sources to derive the bolometric luminosity function (LF) of LRDs at $z = 2.4$ and $z = 4.5$. At $z = 2.4$, the space density of LRDs is only a factor of $\sim 2$ lower than that of all pre-JWST active galactic nuclei (AGNs) with bolometric luminosity $L_{\rm bol} \gtrsim 3 \times 10^{44}\ \rm erg\ s^{-1}$. At $z = 4.5$, our estimate is consistent with those derived for LRDs based on larger areas of the sky. A similar behaviour is observed in the black hole mass function. More importantly, we study the number density of LRDs from cosmic dawn to cosmic noon. We find that there is no significant evolution in the abundance of LRDs with $L_{\rm bol} \gtrsim 3 \times 10^{44}\ \rm erg\ s^{-1}$ at $z > 2$. We speculate that the drop at $z < 4$ seen by other studies is due to their sampling of only the bright-end of the LRDs LF. At cosmic noon, the abundance of LRDs is $n = 3.4^{+5.6}_{-2.4} \times 10^{-5}\ \rm Mpc^{-3}$, which is a factor of $\sim 350$ larger than recent model predictions and is comparable with that of X-ray selected AGNs with similar bolometric luminosity. Our result may imply that, if LRDs are the early, rapid stages of supermassive black hole growth, as suggested by some models, then the formation of black hole seeds can be efficient down to epochs as recent as cosmic noon. Alternatively, LRDs may simply be a high-accretion phase in already mature black holes.

astro-ph.GA↗

arXiv2026

Ram-pressure stripping caught in action in a young cluster at $z = 2.51$

Galaxy clusters in the local Universe are dominated by massive quiescent galaxies with old ages, formed at high redshifts. Whether their quenching is driven by internal processes or environmental effects is a matter of debate that has been challenging to resolve due to the lack of observations during their peak formation epoch. Here we report clear evidence from the Atacama Large Millimeter/submillimeter Array of extended and elongated gas tails in five galaxies in a forming cluster at z = 2.51. The single-tailed gas distributions, which extend notably beyond the stellar emission probed by JWST in galaxies that are relatively isolated and lack signatures of mergers or interactions (features that are very uncommon in the field), provide evidence of ram-pressure stripping. These very distant confirmed cases of ram-pressure stripping highlight the critical role of environmental effects in gas removal at high redshifts, an often-overlooked quenching pathway.

astro-ph.GA↗

arXiv2026

AMACA: Astronomy education with a Multi-sensory, Accessible, and Circular Approach

The AMACA project (Astronomy education with a Multi-sensory, Accessible, and Circular Approach) develops multi-sensory activities for accessible education and engagement in astronomy. Despite promising innovations, existing resources are often poorly documented, designed for one-time events, expensive, and lack interdisciplinary collaboration, user testing, and broad dissemination. AMACA addresses these challenges by creating multi-sensory activities for education and outreach, with a particular focus on accessibility for people with sensory disabilities. A circular approach informs its educational structure: (1) a PhD course on multi-sensory astronomy outreach develops hands-on activities with the support of astronomers, psychologists, and organizations for the visually impaired and the deaf; (2) PhD candidates teach High School (HS) students how to deliver the activities; (3) HS students lead the activities at the Astronomy Festival "The Universe in All Senses"; (4) HS students train teachers to implement the activities in their classrooms. AMACA also develops tools to guide project development and track participants' learning. Key findings show improved communication and accessibility awareness among PhD candidates, increased emotional engagement with astronomy among HS students, enhanced public engagement with research and accessibility awareness, and high teacher satisfaction with the flipped-roles, hands-on approach. Overall, AMACA enhances accessibility and engagement in astronomy education across audiences.

physics.ed-ph↗

arXiv2025

Image simulations of highly magnified clumpy galaxies

We present ClumPyLen, a Python-based simulator designed to produce realistic mock observations of strongly lensed, high-redshift, clumpy star-forming galaxies. The tool models galaxy components such as disks, bulges, and spiral arms using Sérsic profiles, and it populates them with stellar clumps whose properties are sampled from physically motivated distributions. ClumPyLen includes the effects of gravitational lensing through user-provided deflection angle maps and simulates realistic observational conditions by accounting for instrumental effects, Point-Spread-Function convolution, sky background, and photon noise. The simulator can support a wide range of filters and instruments; here we focus on HST/ACS, HST/WFC3-IR, and JWST/NIRCam. We demonstrate the capabilities of the code through two examples, including a detailed simulation of the z = 6.145 source Cosmic Archipelago lensed by MACS J0416.1-2403. The simulated images closely match the morphology and limiting magnitudes of real observations. ClumPyLen is designed to explore the detectability of stellar clumps in terms of mass and size, especially in the low-mass regime, and it allows the study of clump blending effects. Thanks to its modular design, the code is highly adaptable to a wide range of scientific goals, including lensing studies, galaxy evolution, and the generation of synthetic datasets for machine learning or forward modeling applications.

astro-ph.GA↗

arXiv2025

The Parallel Ionizing Emissivity Survey (PIE). I. Survey design and selection of candidate Lyman Continuum leakers at 3.1<z<3.5

We present the survey design and initial results from the Parallel Ionizing Emissivity (PIE) survey. PIE is a large HST survey designed to detect Lyman continuum (LyC) emitting galaxies at 3.1$<$ z $<$3.5 and stack their images in order to measure average LyC escape fractions as a function of galaxy properties. PIE has imaged 37 independent fields in three filters (F336W, F625W and F814W), of which 18 are observed with a fourth band (F475W) from the accompanying PIE+ program. We use photometric colors to select candidate Lyman Break Galaxies (LBGs) at 3.1$<$ z $<$3.5, which can be followed up using ground-based spectrographs to confirm their redshifts. Unlike previous surveys, we use many independent fields to remove biases caused by correlated absorption in the IGM. In this paper, we describe the survey design, photometric measurements, and the use of mock galaxy samples to optimize our color selection. With 3 filters, we can select a galaxy sample of which $\approx$90\% are LBGs and over 30\% lie in the 3.1$<$ z $<$3.5 range for which we can detect uncontaminated LyC emission in F336W. We also use mock IGM sightlines to measure the expected transmission of the IGM, which will allow us to determine escape fractions from our stacked galaxies. We color-select $\approx$1400 galaxies, and predict that this includes $\approx$80 LyC-emitting galaxies and $\approx$500 that we can use in stacking. Finally, we present the Keck/LRIS spectrum of a galaxy at z $\approx$ 2.99, demonstrating that we can confirm the redshifts of z $\sim$ 3 galaxies from the ground.

astro-ph.GA↗

arXiv2025

GAMA 526784: the progenitor of a globular cluster-rich ultra-diffuse galaxy? II. Molecular gas, neutral gas and environment

Aims. We investigate the gas reservoirs, star formation (SF) properties, and environment of the ultra-diffuse galaxy GAMA526784 to understand its formation history, the efficiency of molecular gas conversion into stars, and the possible role of an interacting companion in shaping its morphology. Methods. We analyse low and high-resolution CO observations to constrain the molecular gas content, compare with HI data, and examine the SF efficiency of GAMA526784. The potential influence of a newly identified nearby dwarf galaxy is assessed using photometric and spatial information. Results. GAMA526784 exhibits a regular HI reservoir (M_HI/M* = 2.88) but only upper limits on its molecular gas mass (M_H2(5sigma)/M* < 0.23). The HI reservoir and CO non-detection can be explained by several mechanisms: (1) predominance of CO-dark H2, invisible to CO observations but contributing to SF; (2) a time delay in HI-to-H2 conversion following a recent interaction; or (3) elevated turbulence inhibiting gas collapse. An identified companion, found at a projected distance of 48 kpc, shows similar colours and lies in the direction of young star clusters in GAMA526784, indicating a possible association. We hypothesise this companion may have triggered the formation of star clusters in GAMA526784 through a high-velocity encounter. Conclusions. Our findings suggest GAMA526784 may have undergone an interaction that influenced its gas reservoirs and SF activity. The presence of a nearby companion agrees with predictions of a high-speed encounter, potentially offering a rare example of such an interaction in progress. We suggest this encounter may have shaped the system's recent evolution. Future observations, particularly targeting molecular gas tracers beyond CO and resolved HI maps, will be crucial in determining the extent of GAMA526784's cold gas reservoir and the nature of its recent star formation.

astro-ph.GA↗

arXiv2025

GAMA 526784: the progenitor of a globular cluster-rich ultra-diffuse galaxy? I. Star clusters, stellar body and ionised gas properties

Context. Ultra-diffuse galaxies (UDGs) are an intriguing population of galaxies. Despite their dwarf-like stellar masses and low surface brightness, they have large half-light radii and exhibit a diverse range of globular cluster (GC) populations. Some UDGs host many GCs while others have none, raising questions about the conditions under which star clusters (SC) form in dwarfs. GAMA526784, an isolated UDG with both an old stellar body and an extended star-forming (SF) front, including many young SCs, provides an exceptional case to explore the link between UDG evolution and star cluster formation. Aims. This study investigates the stellar populations, SCs, ionised gas, and kinematics of GAMA526784, focusing on its potential to form massive GCs and its connection to broader UDG formation scenarios. Methods. Imaging from HST and Subaru/HSC, alongside MUSE spectroscopy, were used to analyse the galaxy's morphology, chemical composition, and kinematics. A combination of SED fitting and full spectral fitting was applied. Results. GAMA526784's central stellar body exhibits a low-metallicity ([M/H] ~ -1.0 dex) and an old age (~9.9 Gyr). The outskirts are much younger (~0.9 Gyr), but slightly more metal-poor ([M/H] ~ -1.2 dex). The stellar kinematics show low velocity dispersions (~10 km/s) and a coherent rotational field, while the ionised gas exhibits higher dispersions (~50 km/s), a misaligned rotation axis (~20 deg) and localised SF, what could be suggestive of a recent interaction. The young SCs span ages of 8-11 Myr and masses of log(M*/Mo)~5.0, while the old GCs have ~9 Gyr and stellar masses of log(M*/Mo)~5.5. Conclusions. GAMA526784's properties point to interactions that triggered localised SF, leading to the formation of young SCs. Future observations of its molecular and neutral gas will help assess its environment, and the trigger of this SF episode.

astro-ph.GA↗

arXiv2025

Clump formation in ram-pressure stripped galaxies: evidence from mass function

The mass function (MF) of young ($\mathrm{age\lesssim 200}$ Myr) stellar clumps is an indicator of the mechanism driving the collapse of the interstellar medium (ISM) into giant molecular clouds. Typically, the clump MF in main-sequence galaxies is described by a power law ($dN/dM_*\propto M_*^{-α}$) with slope $α=2$, hinting a turbulence-driven collapse. To understand whether the local environment affects star formation, we have modelled the clump MF of six cluster galaxies from the GASP survey, undergoing strong ram-pressure stripping. This process, exerted by the hot and high-pressure intra-cluster medium (ICM), has produced long tails of stripped ISM where clumps form far away from the galactic disk and surrounded by the ICM itself. Clumps were selected from HST-UVIS/WFC3 images, covering from near-UV to red-optical bands and including H$α$-line maps. The catalogue comprises 398 H$α$ (188 in tails, 210 in disk outskirts, the so-called extraplanar region) and 1270 UV clumps (593 tail, 677 extraplanar). Using mock images, we quantified the mass completeness and bias of our sample. Accounting for these two effects, we adopted a Bayesian approach to fit the clump mass catalogue to a chosen function. The resulting MFs are steeper than expected. In the tails, the H$α$ clumps have slope $α=2.31\pm0.12$, while the UV slope is larger ($2.60\pm0.09$), in agreement with ageing effects. Similar results are found in the extraplanar region, with H$α$ slope $α=2.45^{+0.20}_{-0.16}$ and UV slope $α=2.63^{+0.20}_{-0.18}$, even if they are consistent within uncertainties. We suggest that the steepening results from the higher-than-usual turbulent environment, arising from the interaction between ISM and ICM. As shown by recent works, this process can favour the fragmentation of the largest ISM clouds, inhibiting the formation of very massive clumps.

astro-ph.GA↗

arXiv2024

herakoi: a sonification experiment for astronomical data

Recent research is revealing data-sonification as a promising complementary approach to vision, benefiting both data perception and interpretation. We present herakoi, a novel open-source software that uses machine learning to allow real-time image sonification, with a focus on astronomical data. By tracking hand movements via a webcam and mapping them to image coordinates, herakoi translates visual properties into sound, enabling users to "hear" images. Its swift responsiveness allows users to access information in astronomical images with short training, demonstrating high reliability and effectiveness. The software has shown promise in educational and outreach settings, making complex astronomical concepts more engaging and accessible to diverse audiences, including blind and visually impaired individuals. We also discuss future developments, such as the integration of large language and vision models to create a more interactive experience in interpreting astronomical data.

astro-ph.IM↗

arXiv2024

The MICADO first light imager for the ELT: off-axis performance of PSF reconstruction

The highest scientific return, for adaptive optics (AO) observations, is achieved with a reliable reconstruction of the PSF. This is especially true for MICADO@ELT. In this presentation, we will focus on extending the MICADO PSF reconstruction (PSF-R) method to the off-axis case. Specifically, a novel approach based on temporal-based tomography of AO telemetry data has been recently implemented. Results from the PSF-R of both simulated and real data show that, at half isoplanatic angle distances, a precision of about 10-15% is achievable in both Strehl ratio and full-width at half maximum, paving the way to extend the MICADO PSF-R tool also to the multi-conjugated AO case.

astro-ph.IM↗

arXiv2024

The MICADO first light imager for the ELT: the PSF Reconstruction Software

MICADO is the first-light camera of the ESO ELT, allowing NIR imaging and long-slit spectroscopy assisted by adaptive optics. MICADO is now entering its construction phase, and the software for data reduction is reaching an adequate maturity level. The PSF Reconstruction (PSF-R) of MICADO is a software tool for the blind derivation of the PSF, only using adaptive optics telemetry data. An update of the status of the PSF-R service is provided here. The PSF-R prototype has been tested on ERIS@VLT data in order to check the reconstruction of on- and off-axis PSFs. The on-axis PSF-R is accurate at a few percent level on Strehl, FWHM, Encircled Energy, and half light radius, while for the off-axis case the match is within 10-15 percent at a distance of half isoplanatic angle. The first version of the workflow for the PSF-R pipeline has been developed and verified using the latest release of the ESO data processing system. A set of simulations has been implemented on the morphological analysis of distant galaxies, showing that the accuracy of the PSF-R matches the goals needed to study their morphology. In summary, the PSF-R team is on the right track towards the ELT first light.

astro-ph.IM↗

arXiv2024

JWST's first glimpse of a z > 2 forming cluster reveals a top-heavy stellar mass function

Clusters and their progenitors (protoclusters) at z = 2-4, the peak epoch of star formation, are ideal laboratories to study the formation process of both the clusters themselves and their member galaxies. However, a complete census of their member galaxies has been challenging due to observational difficulties. Here we present new JWST/NIRCam observations targeting the distant cluster CLJ1001 at z = 2.51 from the COSMOS-Web program, which, in combination with previous narrowband imaging targeting H-alpha emitters and deep millimeter surveys of CO emitters, provide a complete view of massive galaxy assembly in CLJ1001. In particular, JWST reveals a population of massive, extremely red cluster members in the long-wavelength bands that were invisible in previous Hubble Space Telescope (HST)/F160W imaging (HST-dark members). Based on this highly complete spectroscopic sample of member galaxies, we show that the spatial distribution of galaxies in CLJ1001 exhibits a strong central concentration, with the central galaxy density already resembling that of low-z clusters. Moreover, we reveal a "top-heavy" stellar mass function for the star-forming galaxies (SFGs), with an overabundance of massive SFGs piled up in the cluster core. These features strongly suggest that CLJ1001 is caught in a rapid transition, with many of its massive SFGs likely soon becoming quiescent. In the context of cluster formation, these findings suggest that the earliest clusters form from the inside out and top to bottom, with the massive galaxies in the core assembling first, followed by the less massive ones in the outskirts.

astro-ph.GA↗