arXiv subjects

Raphael Gobat

Publications and source records attributed to Raphael Gobat.

At least 19 recordsLinked to original sources

arXiv2026

Quiescent fractions in high-redshift galaxy groups reflect their hot-or-cold state of gas accretion

Cold accretion and quenching are closely related aspects of galaxy evolution, as sustained gas supply is required to maintain star formation. High-redshift galaxy groups therefore provide a valuable laboratory for testing how the thermal state of accreting gas relates to the emergence of quiescence. We measure quiescent fractions in a sample of 16 spectroscopically confirmed galaxy groups at $1.6<z<3.6$, spanning halo masses from $10^{12.8},{\rm M_\odot}$ to $10^{13.9},{\rm M_\odot}$, by fitting the SEDs of candidate member galaxies selected from the COSMOS2020 catalog and using a membership-probability approach to estimate group quiescent fractions. We compare these quiescent fractions to the expected cold or hot accretion state of each halo and find evidence for a correlation: quiescent fractions reach about 50 percent in groups in the hot-accretion regime and are consistent with zero in groups in the cold-accretion regime. In mature hot-accreting groups, massive quiescent galaxies are preferentially found in the inner regions ($R<0.5R_{\rm vir}$), with a 4.4-sigma excess relative to the outskirts. Most groups lack a clearly established brightest group galaxy and instead show small stellar-mass gaps, typically $M_{*,1}/M_{*,2}<3$, indicating that they remain in an active assembly phase rather than being dynamically evolved systems. Consistently, the stellar-mass excess of the dominant galaxy, measured relative to the SHMR expectation, does not predict the group quiescent fraction. Taken together, our results support a picture in which the cold-to-hot transition in gas accretion contributes to the onset of quiescence, possibly through inside-out starvation associated with filament disruption in shock-heated intra-group gas, and suggest that environment plays a greater role than internal processes in shaping the quiescent galaxy population in these structures.

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arXiv2026

Prevailing thermally-pulsing-asymptotic-giant branch stars in the near-infrared rest-frame spectra of distant quiescent galaxies: towards robust galaxy ages and masses

We recently reported the discovery of prominent features from the thermally pulsing asymptotic-giant-branch (TP-AGB) phase in the rest-frame near-infrared of a massive quiescent galaxy (QG) at z~1 observed with the JWST, which provides strong constraints on population synthesis (SPS) models. Here we extend this analysis to 27 JWST/NIRSpec PRISM spectra of QGs at z>1 from GO-5019 and CEERS, with signal-to-noise ratios of ~100 (15/27) and ~50 (12/27), respectively. Each spectrum is modeled with three SPS models: the latest Maraston (M13) models with a sizable TP-AGB phase, and widely-used Bruzual & Charlot 2003 (BC03) and Conroy & Gunn 2009 (C09) models, both with weaker TP-AGB contributions. M13 generally provides the best overall fit and the most consistency between the optical and the NIR. Only M13 yields consistent ages from separate fits to the optical, the NIR and the full wavelength ranges. Compared to BC03 and C09, M13 yields systematically younger mass-weighted ages (by <500 Myr) hence lower stellar masses (by <0.2 dex). All models favor super-solar (Z/Z_sun>1.5) metallicities. Signal-to-noise-weighted stacked spectra reveal that TP-AGB-related features are strongest in galaxies with mass-weighted ages of t=0.4-1.8 Gyr, consistent with the predicted peak TP-AGB contribution in M13. Further sample subdivisions show that these features are most pronounced in high-mass, dusty, and metal-rich systems. These results support a significant contribution from TP-AGB stars to the NIR spectra of high-redshift, intermediate-age galaxies and pave the way towards improved spectral population synthesis modeling and robust stellar ages and masses.

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arXiv2025

Cosmic Vine: High abundance of massive galaxies and dark matter halos in a forming cluster at z=3.44

The Cosmic Vine is a massive protocluster at z=3.44 in the JWST CEERS field, offering an ideal laboratory for studying the early phases of cluster formation. Using the data from the DAWN JWST Archive, we conduct a comprehensive study on the large-scale structure, stellar mass function (SMF), quiescent members, and dark matter halos in the Cosmic Vine. First, we spectroscopically confirm 136 galaxies in the Vine at z=3.44, and an additional 47 galaxies belonging to a diffuse foreground structure at z=3.34 which we dub the Leaf. We identify four subgroups comprising the Cosmic Vine and two subgroups within the Leaf. Second, we identified 11 quiescent members with log(M*/Msun)=9.5-11.0, the largest sample of quiescent galaxies in overdense environments at z>3, which gives an enhanced quiescent galaxy number density 2x10^(-4)cMpc^(-3) that is three times above the field level at log(M*/Msun) > 10. Notably, these quiescent members form a tight red sequence on the color-magnitude diagram, making it one of the earliest red sequences known to date. Third, by constructing the SMFs for both star-forming and quiescent members, we find that both SMFs are top-heavy, with a significantly enhanced quiescent fraction at log(M*/Msun)>10.5 compared to field counterparts. The stellar mass-size analysis reveals that star-forming members are more compact at higher masses than their field counterparts. Finally, we estimate a halo mass of log(Mh/Msun)=13.2+-0.3 for the protocluster core, and log(Mh/Msun)=11.9-12.4 for satellite subgroups. The phase-space analysis indicates that three subgroups are likely infalling to the core. This work reveals a high abundance of massive galaxies and dark matter halos in a forming cluster, demonstrating the accelerated assembly of massive galaxies in massive halos when the Universe was less than 2 billion years old.

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arXiv2025

Ly-alpha emission reveals two satellite halos around massive groups at z ~ 3: the puzzling case of a quiescent central galaxy

We present the discovery and characterisation of two Ly$\alpha$ nebulae (LANs), RO-1001-Sat and RO-0959-Sat, as satellite structures of two giant LANs at $z=2.920$ and 3.092. They are found neighbouring two out of four known giant LANs at $z\sim3$ in our MUSE follow-up observations, reinforcing the idea that Ly$\alpha$ emission can be used to trace massive dark matter halos at high-$z$. This high occurrence of massive satellite halos agrees with simulations. With sizes of $\simeq80\times160$ and $80\times100~\mathrm{pkpc}^2$, the two nebulae are both $\sim$300pkpc from the main LANs. The Ly$\alpha$ emission is only shifted by $\simeq100-300$ km s$^{-1}$ between each of the two pairs, suggesting connections via large-scale structure. RO-1001-Sat and RO-0959-Sat are estimated to have log$(M_\mathrm{h}/M_\odot)\simeq13.2\pm0.3$ and $12.8\pm0.3$, putting them potentially close to the regime of cold-mode accretion. The central brightest galaxies in the two halos are morphologically distinct despite having similar stellar mass $\sim10^{11}M_\odot$, one being an elliptical quiescent galaxy in RO-1001-Sat and the other being a dusty star-forming spiral in RO-0959-Sat. Intriguingly, the quiescent galaxy aligns well with the peak of the LAN as well as the potential well of the host halo, making it the first clear-cut case where the cold gas ought to be accreting onto the galaxy but with no observable star formation, either due to morphological quenching or, more likely, radio-mode feedback from an active galactic nucleus. Finally, we show a tentative detection of a Ly$\alpha$ filament connecting RO-1001 and RO-1001-Sat. This work shows how panoramic MUSE (and in the future, BlueMUSE) observations of massive halo seeds can be used to efficiently search for additional halos, unveiling their large-scale structure and enabling the study of Ly$\alpha$-selected galaxy groups.

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arXiv2025

A large thermal energy reservoir in the nascent intracluster medium at a redshift of 4.3

Most baryons in present-day galaxy clusters exist as hot gas ($\boldsymbol{\gtrsim10^7\,\rm}\mathrm{K}$), forming the intracluster medium (ICM). Cosmological simulations predict that the mass and temperature of the ICM rapidly decrease with increasing cosmological redshift, as intracluster gas in younger clusters is still accumulating and being heated. The thermal Sunyaev-Zeldovich (tSZ) effect arises when cosmic microwave background (CMB) photons are scattered to higher energies through interactions with energetic electrons in hot ICM, leaving a localized decrement in the CMB at a long wavelength. The depth of this decrement is a measure of the thermal energy and pressure of the gas. To date, the effect has been detected in only three systems at or above $z\sim2$, when the Universe was 4 billion years old, making the time and mechanism of ICM assembly uncertain. Here, we report observations of this effect in the protocluster SPT2349$-$56 with Atacama Large Millimeter/submillimeter Array (ALMA). SPT2349$-$56 contains a large molecular gas reservoir, with at least 30 dusty star-forming galaxies (DSFGs) and three radio-loud active galactic nuclei (AGN) in a 100-kpc region at $z=4.3$, corresponding to 1.4 billion years after the Big Bang. The observed tSZ signal implies a thermal energy of $\mathbf{\sim 10^{61}\,\mathrm{erg}}$, exceeding the possible energy of a virialized ICM by an order of magnitude. Contrary to current theoretical expectations, the strong tSZ decrement in SPT2349$-$56 demonstrates that substantial heating can occur and deposit a large amount of thermal energy within growing galaxy clusters, overheating the nascent ICM in unrelaxed structures, two billion years before the first mature clusters emerged at $\mathbf{z \sim 2}$.

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arXiv2025

Noema formIng Cluster survEy (NICE): A Census of Star Formation and Cold Gas Properties in Massive protoclusters at 1.5<z<4

Massive protoclusters at z~1.5-4, the peak of the cosmic star formation history, are key to understanding the formation mechanisms of massive galaxies in today's clusters. However, studies of protoclusters at these high redshifts remain limited, primarily due to small sample sizes and heterogeneous selection criteria. In this work, we conduct a systematic investigation of the star formation and cold gas properties of member galaxies of eight massive protoclusters in the COSMOS field, using the statistical and homogeneously selected sample from the Noema formIng Cluster survEy (NICE). Our analysis reveals a steep increase in the star formation rates per halo mass ($\Sigma_{\rm SFR} /M_{\rm halo}$) with redshifts in these intensively star-forming protoclusters, reaching values one to two orders of magnitude higher than those observed in the field at z>2. We further show that, instead of an enhancement of starbursts, this increase is largely driven by the concentration of massive and gas-rich star-forming galaxies in the protocluster cores. The member galaxies still generally follow the same star formation main sequence as in the field, with a moderate enhancement at the low mass end. Notably, the most massive protocluster galaxies ($M_\star$>8$\times$10$^{10}$M$_\odot$) exhibit higher $f_{\rm gas}$ and $\tau_{\rm gas}$ than their field counterparts, while remaining on the star forming main sequence. These gas-rich, massive, and star-forming galaxies are predominantly concentrated in the protocluster cores and are likely progenitors of massive ellipticals in the center of today's clusters. These results suggest that the formation of massive galaxies in such environments is sustained by substantial gas reservoirs, which support persistent star formation and drive early mass assembly in forming cluster cores.

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arXiv2025

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.

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arXiv2025

Revealing the hidden cosmic feast: A z=4.3 galaxy group hosting two optically dark, efficiently star-forming galaxies

We present the confirmation of a compact galaxy group candidate, CGG-z4, at $z=4.3$ in the COSMOS field. This structure was identified by two spectroscopically confirmed $z=4.3$ $K_s$-dropout galaxies with ALMA $870\rm\, \mu m$ and 3 mm continuum detections, surrounded by an overdensity of NIR-detected galaxies with consistent photometric redshifts of $4.0 4$. Their high CO(5-4)/CO(4-3) ratios indicate that the inter-stellar mediums (ISMs) are close to thermalization, suggesting either high gas temperatures, densities, and/or pressure, while the low [CI](1-0)/CO(4-3) line ratios indicate high star formation efficiencies. With [CI]-derived gas masses we found the two galaxies have extremely short gas depletion times of $99$ Myr and $<63$ Myr respectively, suggesting the onset of quenching. With an estimated halo mass of $\rm log (M_{\rm halo}[M_{\odot}])\sim12.8$, we suggest that this structure is likely in the process of forming a massive galaxy cluster.

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arXiv2024

Behind the dust veil: A panchromatic view of an optically dark galaxy at z=4.82

Optically dark dusty star-forming galaxies (DSFGs) play an essential role in massive galaxy formation at early cosmic time, however their nature remains elusive. Here we present a detailed case study of all the baryonic components of a $z=4.821$ DSFG, XS55. Selected from the ultra-deep COSMOS-XS 3GHz map with a red SCUBA-2 450$\mu$m/850$\mu$m colour, XS55 was followed up with ALMA 3mm line scans and spectroscopically confirmed to be at $z=4.821$ via detections of the CO(5-4) and [CI](1-0) lines. JWST/NIRCam imaging reveals that XS55 is a F150W-dropout with red F277W/F444W colour, and a complex morphology: a compact central component embedded in an extended structure with a likely companion. XS55 is tentatively detected in X-rays with both Chandra and XMM-Newton, suggesting an active galactic nucleus (AGN) nature. By fitting a panchromatic SED spanning NIR to radio wavelengths, we revealed that XS55 is a massive main-sequence galaxy with a stellar mass of $M_\ast=(5\pm1)\times10^{10}\,{\rm M_\odot}$ and a star formation rate of ${\rm SFR}=540\pm177~{\rm M_\odot\,yr^{-1}}$. The dust of XS55 is optically thick in the far infrared (FIR) with a surprisingly cold dust temperature of $T_{\rm dust}=33\pm2\,{\rm K}$, making XS55 one of the coldest DSFGs at $z>4$ known to date. This work unveils the nature of a radio-selected F150W-dropout, suggesting the existence of a population of DSFGs hosting active black holes embedded in optically thick dust.

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arXiv2024

A photo-z cautionary tale: Redshift confirmation of COSBO-7 at z=2.625

Photometric redshifts are widely used in studies of dusty star-forming galaxies (DSFGs), but catastrophic photo-$z$ failure can undermine all redshift-dependent results. Here we report the spectroscopic redshift confirmation of COSBO-7, a strongly lensed DSFG in the COSMOS-PRIMER field. Recently, a photometric redshift solution of $z\gtrsim7.0$ was reported for COSBO-7 based on ten bands of {\it James Webb} Space Telescope (JWST) NIRCam and MIRI imaging data. This $z$ value was favored by four independent spectral energy distribution (SED) fitting codes, and the result provided an appealing candidate for the most distant massive DSFG known to date. This photo-$z$ solution was also supported by a single line detection in Atacama Large Millimeter Array (ALMA) Band 3 consistent with CO(7-6) at $z=7.46$. However, our new ALMA observations robustly detect two lines in Band 6 identified as CO(7-6) and [CI](2-1) at $z_{\rm spec}=2.625$, and thus the Band 3 line as CO(3-2). These three robust line detections decidedly place COSBO-7 at $z=2.625$, refuting the photo-$z$ solution. We derive physical parameters by fitting near-infrared(NIR)-to-millimeter(mm) photometry and lens modeling, revealing that COSBO-7 is a main sequence galaxy. We examine possible reasons for this photo-$z$ failure and attribute it to (1) the likely underestimation of photometric uncertainties at 0.9\,$\mu$m and 1.15 \,$\mu$m; and (2) the lack of photometry at wavelengths beyond 20\,$\mu$m. Notably, we recover a bona fide $z_{\rm phot}\sim 2.3$ by including the existing MIPS $24\,\mu$m photometry, demonstrating the critical importance of mid-infrared (MIR) data in bolstering photo-$z$ measurements. This work highlights a common challenge in modeling the SEDs of DSFGs, and provides a cautionary tale regarding the reliability of photometric redshifts as well as pseudo-spectroscopic redshifts based on single line detection.

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arXiv2024

NOEMA formIng Cluster survEy (NICE): Characterizing eight massive galaxy groups at $1.5 < z < 4$ in the COSMOS field

The NOEMA formIng Cluster survEy (NICE) is a large program targeting 69 massive galaxy group candidates at $z>2$ in six deep fields. We report spectroscopic confirmation of eight groups at $1.65\leq z\leq3.61$ in COSMOS. Homogeneously selected as significant overdensities of red IRAC sources with red Herschel colors, four groups are confirmed by CO and [CI] with NOEMA 3mm observations, three are confirmed with ALMA, and one is confirmed by H$\alpha$ from Subaru/FMOS. We constructed the integrated FIR SEDs for the eight groups, obtaining total IR SFR $=260-1300~{\rm M_\odot}$~yr$^{-1}$. We adopted six methods to estimate the dark matter masses, including stellar mass to halo mass relations, overdensity with galaxy bias, and NFW profile fitting to radial stellar mass density. We found the radial stellar mass density are consistent with a NFW profile, supporting that they are collapsed structures hosted by a single dark matter halo. The best halo mass estimates are $\log(M_{\rm h}/{\rm M_\odot})=12.8-13.7$ with uncertainty of 0.3 dex. From halo mass estimates, we derive baryonic accretion rate ${\rm BAR}=(1-8)\times10^{3}\,{\rm M_{\odot}/yr}$ for this sample. We find a quasi-linear correlation between the integrated SFR/BAR and the theoretical halo mass limit for cold streams, $M_{\rm stream}/M_{\rm h}$, with ${\rm SFR/BAR}=10^{-0.46\pm0.22}\left({M_{\rm stream}/M_{\rm h}}\right)^{0.71\pm0.16}$ with a scatter of $0.40\,{\rm dex}$. Further, we compare halo masses and stellar masses with simulations, and find all structures are consistent with being progenitors of $M_{\rm h}(z=0)>10^{14}\,{\rm M_{\odot}}$ galaxy clusters, and the most massive central galaxies have stellar masses consistent with brightest cluster galaxies (BCGs) progenitors in the TNG300 simulation. The results strongly suggest these structures are forming massive galaxy clusters via baryonic and dark matter accretion.

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arXiv2024

Strong spectral features from asymptotic giant branch stars in distant quiescent galaxies

Dating the ages and weighting the stellar populations in galaxies are essential steps when studying galaxy formation through cosmic times. Evolutionary population synthesis models with different input physics are used for this purpose. Moreover, the contribution from the thermally pulsing asymptotic giant branch (TP-AGB) stellar phase, which peaks for intermediate-age 0.6-2 Gyr, has been debated for decades. Here we report the detection of strong cool-star signatures in the rest-frame near-infrared spectra of three young (~1Gyr), massive (~10^10Msun) quiescent galaxies at large look-back time, z=1-2, using JWST/NIRSpec. The coexistence of oxygen- and carbon-type absorption features, spectral edges and features from rare species, such as vanadium and possibly zirconium, reveal a strong contribution from TP-AGB stars. Population synthesis models with a significant TP-AGB contribution reproduce the observations better than those with a weak TP-AGB, which are commonly used. These findings call for revisions of published stellar population fitting results, as they point to populations with lower masses and younger ages and have further implications for cosmic dust production and chemical enrichment. New generations of improved models are needed, informed by these and future observations.

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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.

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arXiv2024

Cosmic Vine: A z=3.44 large-scale structure hosting massive quiescent galaxies

We report the discovery of a large-scale structure at z=3.44 revealed by JWST data in the Extended Groth Strip (EGS) field. This structure, called the Cosmic Vine, consists of 20 galaxies with spectroscopic redshifts at 3.43 70\%$). Comparisons with simulations suggest that the Cosmic Vine would form a cluster with halo mass $M_{\rm halo}>10^{14}M_\odot$ at z=0, and the two massive galaxies are likely forming the brightest cluster galaxies (BCGs). The results unambiguously reveal that massive quiescent galaxies can form in growing large-scale structures at z>3, thus disfavoring the environmental quenching mechanisms that require a virialized cluster core. Instead, as suggested by the interacting and bulge-dominated morphologies, the two galaxies are likely quenched by merger-triggered starburst or active galactic nucleus (AGN) feedback before falling into a cluster core. Moreover, we found that the observed specific star formation rates of massive quiescent galaxies in z>3 dense environments are one to two orders of magnitude lower than that of the BCGs in the TNG300 simulation. This discrepancy potentially poses a challenge to the models of massive cluster galaxy formation. Future studies comparing a large sample with dedicated cluster simulations are required to solve the problem.

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arXiv2024

The powerful lens galaxy cluster PLCK G287.0+32.9 (${\theta}_E \sim 43''$)

We present a new high-precision strong lensing model of PLCK G287.0$+$32.9, a massive lens galaxy cluster at $z=0.383$, with the aim to get an accurate estimation of its effective Einstein radius and total mass distribution. We also present a spectroscopic catalog containing accurate redshift measurements for 490 objects, including multiply-lensed sources and cluster member galaxies. We exploit high-quality spectroscopic data from three pointings of the VLT Multi Unit Spectroscopic Explorer, covering a central $3~\rm{arcmin}^2$ region of the cluster. We complete the spectroscopic catalog by including redshift measurements from VLT-VIMOS and KECK-DEIMOS. We identify 129 spectroscopic cluster member galaxies, with redshift values $0.360 \leq z \leq 0.405$ and $m_{\rm{F160W}} \leq 21$, and 24 photometric ones identified with a Convolutional Neural Network from ancillary HST imaging. We also identify 114 multiple images from 28 background sources, of which 84 images from 16 sources are new and the remaining ones were identified in previous work. The best-fitting lens model shows a root mean square separation value between the predicted and observed positions of the multiple images of $0.75''$, corresponding to an improvement in reconstructing the observed positions of the multiple images of a factor of $2.5$ with respect to previous models. Using the predictive power of our new lens model we find 3 new multiple images and we confirm the configuration of three systems of multiple images that were not used for the optimization of the model. The derived total mass distribution confirms this cluster to be a very prominent gravitational lens with an effective Einstein $\theta_{E} = 43.4'' \pm 0.1''$, that is in agreement with previous estimates and corresponds to a total mass enclosed in the critical curve of $M_E = {3.33}_{-0.07}^{+0.02} \times{ 10^{14} M_\odot}$.

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arXiv2023

Massive galaxy formation caught in action at z~5 with JWST

We report the discovery of a compact group of galaxies, CGG-z5, at z~5.2 in the EGS field covered by the JWST/CEERS survey. CGG-z5 was selected as the highest overdensity of galaxies at z>2 in recent JWST public surveys and it consists of six candidate members lying within a projected area of $1.5"\times3"$ (10$\times$20~kpc$^2$). All group members are HST/F435W and HST/F606W dropouts while securely detected in the JWST/NIRCam bands, yielding a narrow range of robust photometric redshifts $5.0 11$) at z~1. This implies that CGG-z5 could be a "proto-massive galaxy" captured during a short-lived phase of massive galaxy formation.

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arXiv2023

Noema formIng Cluster survEy (NICE): Discovery of a starbursting galaxy group with a radio-luminous core at z=3.95

The study of distant galaxy groups and clusters at the peak epoch of star formation is limited by the lack of a statistically and homogeneously selected and spectroscopically confirmed sample. Recent discoveries of concentrated starburst activities in cluster cores have opened a new window to hunt for these structures based on their integrated IR luminosities. Hereby we carry out the large NOEMA (NOrthern Extended Millimeter Array) program targeting a statistical sample of infrared-luminous sources associated with overdensities of massive galaxies at z>2, the Noema formIng Cluster survEy (NICE). We present the first result from the ongoing NICE survey, a compact group at z=3.95 in the Lockman Hole field (LH-SBC3), confirmed via four massive (M_star>10^10.5M_sun) galaxies detected in CO(4-3) and [CI](1-0) lines. The four CO-detected members of LH-SBC3 are distributed over a 180 kpc physical scale, and the entire structure has an estimated halo mass of ~10^13Msun and total star formation rate (SFR) of ~4000Msun/yr. In addition, the most massive galaxy hosts a radio-loud AGN with L_1.4GHz, rest = 3.0*10^25W/Hz. The discovery of LH-SBC3 demonstrates the feasibility of our method to efficiently identify high-z compact groups or forming cluster cores. The existence of these starbursting cluster cores up to z~4 provides critical insights into the mass assembly history of the central massive galaxies in clusters.

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arXiv2023

Accelerated structural evolution of galaxies in a starbursting cluster at z=2.51

Structural properties of cluster galaxies during their peak formation epoch, $z \sim 2-4$ provide key information on whether and how environment affects galaxy formation and evolution. Based on deep HST/WFC3 imaging towards the z=2.51 cluster, J1001, we explore environmental effects on the structure, color gradients, and stellar populations of a statistical sample of cluster SFGs. We find that the cluster SFGs are on average smaller than their field counterparts. This difference is most pronounced at the high-mass end ($M_{\star} > 10^{10.5} M_{\odot}$) with nearly all of them lying below the mass-size relation of field galaxies. The high-mass cluster SFGs are also generally old with a steep negative color gradient, indicating an early formation time likely associated with strong dissipative collapse. For low-mass cluster SFGs, we unveil a population of compact galaxies with steep positive color gradients that are not seen in the field. This suggests that the low-mass compact cluster SFGs may have already experienced strong environmental effects, e.g., tidal/ram pressure stripping, in this young cluster. These results provide evidence on the environmental effects at work in the earliest formed clusters with different roles in the formation of low and high-mass galaxies.

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