Raphael Gobat

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 conducted 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 confirmed 136 galaxies in the Vine at z ≈ 3.44, along with an additional 47 galaxies belonging to a diffuse foreground structure at z ≈ 3.34, which we dubbed the Leaf. We identified four subgroups comprising the Cosmic Vine and two subgroups within the Leaf. Second, we identified 11 quiescent members with log( M _* /M _⊙ ) = 9.5 − 11.0, the largest sample of quiescent galaxies in overdense environments at z > 3, which gives an enhanced quiescent galaxy number density ∼1 − 2 × 10 ⁻⁴ cMpc ⁻³ that is two to three times above the field level at log( M _* /M _⊙ ) > 10. Notably, these quiescent members form a tight red sequence on the colour-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 _* /M _⊙ ) > 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 estimated a halo mass of log( M _h /M _⊙ ) = 13.2 ± 0.3 for the protocluster core and log( M _h /M _⊙ ) = 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 this forming cluster, demonstrating the accelerated assembly of massive galaxies in massive halos when the Universe was less than 2 billion years old.

We present the discovery and characterisation of two Ly α nebulae (LANs), RO-1001-Sat and RO-0959-Sat, as satellite structures of two giant LANs at z = 2.920 and 3.092. These two satellite LANs are found neighbouring two out of four known giant LANs at z ∼ 3 in our Multi-Unit Spectroscopic Explorer (MUSE) follow-up observations, reinforcing the idea that Ly α emission can be used as a tracer of massive dark matter halos at high z . This high occurrence of massive satellite halos is in agreement with simulations. With sizes of ≃80 × 160 and 80 × 100 pkpc ² , the two nebulae are both ∼300 pkpc from the main LANs. With Ly α emission only shifted by ≃100 − 300 km s ⁻¹ between each of the two pairs, these two satellite structures are likely connected to their main halos by the large-scale structure. RO-1001-Sat and RO-0959-Sat are estimated to have log( M _h / M _⊙ )≃13.2 ± 0.3 and 12.8 ± 0.3, putting them potentially close to the regime of cold-mode accretion according to several models, which suggests that cold streams should be able to penetrate the virial radii to reach the halo centres without being shock-heated. The central brightest galaxies in the two halos are morphologically distinct despite having a similar stellar mass of ∼10 ¹¹ M _⊙ , 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 in which 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, as supported by excess yet weak radio emission. Finally, we show a tentative detection of a Ly α 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 α -selected galaxy groups.

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 Hα 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.

Structural properties of cluster galaxies during their peak formation epoch, z ∼ 2–4 provide key information on whether and how the environment affects galaxy formation and evolution. Based on deep Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) imaging toward the z = 2.51 cluster, J1001, we explore environmental effects on the structure, color gradients, and stellar populations of a statistical sample of cluster star-forming galaxies (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 _⋆ > 10^10.5 M _⊙), 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.

How much gas and dust is contained in high-redshift quiescent galaxies (QGs) is currently an open question with relatively few and contradictory answers, as well as important implications for our understanding of the nature of star formation quenching processes at cosmic noon. Here we revisit far-infrared (FIR) observations of the REQUIEM-ALMA sample of six z = 1.6 - 3.2 QGs strongly lensed by intermediate-redshift galaxy clusters. We measured their continuum emission using priors obtained from high resolution near-infrared (NIR) imaging, as opposed to focusing on point-source extraction, converted it into dust masses using a FIR dust emission model derived from statistical samples of QGs, and compared the results to those of the reference work. We find that, while at least the most massive sample galaxy is indeed dust-poor, the picture is much more nuanced than previously reported. In particular, these more conservative constraints remain consistent with high dust fractions in early QGs. We find that these measurements are very sensitive to the adopted extraction method and conversion factors: the use of an extended light model to fit the FIR emission increases the flux of detections by up to 50% and the upper limit by up to a factor 6. Adding the FIR-to-dust conversion, this amounts to an order of magnitude difference in dust fraction, casting doubts on the power of these data to discriminate between star formation quenching scenarios. Unless these are identified by other means, mapping the dust and gas in high-redshift QGs will continue to require somewhat costly observations.

We study the process of panspermia in Milky Way–like galaxies by modeling the probability of successful travel of organic compounds between stars harboring potentially habitable planets. To this end, we apply the modified habitability recipe of Gobat & Hong to a model galaxy from the McMaster Unbiased Galaxy Simulations suite of zoom-in cosmological simulations. We find that, unlike habitability, which only occupies a narrow dynamic range over the entire galaxy, the panspermia probability can vary by orders of magnitude between the inner (R, b = 1–4 kpc) and outer disk. However, only a small fraction of star particles have very large values for the panspermia probability and, consequently, the fraction of star particles where the panspermia process is more effective than prebiotic evolution is much lower than from naïve expectations based on the ratio between the panspermia probability and natural habitability.

We have obtained spectroscopic confirmation with Hubble Space Telescope WFC3/G141 of a first sizeable sample of nine quiescent galaxies at 2.4 < z < 3.3. Their average near-UV/optical rest-frame spectrum is characterized by low attenuation (A_V ∼ 0.6 mag) and a strong Balmer break, larger than the 4000 Å break, corresponding to a fairly young age of ∼300 Myr. This formally classifies a substantial fraction of classically selected quiescent galaxies at z ∼ 3 as post-starbursts, marking their convergence to the quenching epoch. The rapid spectral evolution with respect to z ∼ 1.5 quiescent galaxies is not matched by an increase of residual star formation, as judged from the weak detection of [O ii]λ3727 emission, pointing to a flattening of the steep increase in gas fractions previously seen from z ∼ 0 to 1.8. However, radio 3 GHz stacked emission implies either much stronger dust-obscured star formation or substantial further evolution in radio-mode AGN activity with respect to z ∼ 1.5.

Atacama Large Millimeter/submillimeter Array (ALMA) measurements for 93 Herschel-selected galaxies at 1.1 ≤ z ≤ 1.7 in COSMOS reveal a sizable (>29%) population with compact star formation (SF) sizes, lying on average >×3.6 below the optical stellar mass (M _⋆)–size relation of disks. This sample widely spans the star-forming main sequence (MS), having 10⁸ ≤ M _⋆ ≤ 10^11.5 M _⊙ and 20 ≤ star formation rate (SFR) ≤ 680 M _⊙ yr⁻¹. The 32 size measurements and 61 upper limits are measured on ALMA images that combine observations of CO(5–4), CO(4–3), CO(2–1), and λ _obs ∼ 1.1–1.3 mm continuum, all tracing the star-forming molecular gas. These compact galaxies have instead normally extended K _band sizes, suggesting strong specific SFR gradients. Compact galaxies comprise the 50 ± 18% of MS galaxies at M _⋆ > 10¹¹ M _⊙. This is not expected in standard bimodal scenarios, where MS galaxies are mostly steadily growing extended disks. We suggest that compact MS objects are early post-starburst galaxies in which the merger-driven boost of SF has subsided. They retain their compact SF size until either further gas accretion restores premerger galaxy-wide SF, or until becoming quenched. The fraction of merger-affected SF inside the MS seems thus larger than anticipated and might reach ∼50% at the highest M _⋆. The presence of large galaxies above the MS demonstrates an overall poor correlation between galaxy SF size and specific SFR.

We present a census of the molecular gas properties of galaxies in the most distant known X-ray cluster, CLJ1001, at z = 2.51, using deep observations of CO(1-0) with the Karl G. Jansky Very Large Array. In total, 14 cluster members with M _* > 10^10.5 M _⊙ are detected, including all of the massive star-forming members within the virial radius, providing the largest galaxy sample in a single cluster at z > 2 with CO(1-0) measurements. We find a large variety in the gas content of these cluster galaxies, which is correlated with their relative positions (or accretion states), with those closer to the cluster core being increasingly gas-poor. Moreover, despite their low gas content, the galaxies in the cluster center exhibit an elevated star formation efficiency (SFE; SFE = SFR/M _gas) compared to field galaxies, suggesting that the suppression on the SFR is likely delayed compared to that on the gas content. Their gas depletion time is around t _dep ∼ 400 Myr, which is comparable to the cluster dynamical time. This implies that they will consume all of their gas within a single orbit around the cluster center, and form a passive cluster core by z ∼ 2. This result is one of the first direct pieces of evidence for the influence of environment on the gas reservoirs and SFE of z > 2 cluster galaxies, thereby providing new insights into the rapid formation and quenching of the most massive galaxies in the early universe.

We obtained optical/near-IR rest-frame Magellan FIRE spectra (including Paβ and Paγ) of 25 starburst galaxies at 0.5 < z < 0.9, with average star formation rates (SFRs) seven times above the main sequence (MS). We find that Paschen-to-Balmer line ratios saturate around a constant value corresponding to A _V ∼ 2–3 mag, while line-to-IR-luminosity ratios suggest a large range of more extreme obscurations and appear to be uncorrelated with the former. This behavior is not consistent with standard attenuation laws derived for local and distant galaxies, yet is remarkably consistent with observations of starburst cores in which young stars and dust are homogeneously mixed. This model implies A _V = 2–30 mag attenuation to the center of starburst cores, with a median of ∼9 mag (a factor of 4000). X-ray hardness ratios for six AGNs in our sample and column densities derived from observed dust masses and radio sizes independently confirm this level of attenuation. In these conditions observed optical/near-IR emission comes from surface regions, while inner starburst cores are invisible. We thus attribute the high [N ii]/Hα ratios to widespread shocks from accretion, turbulence, and dynamic disturbances rather than to AGNs. The large range of optical depths demonstrates that substantial diversity is present within the starburst population, possibly connected to different merger phases or progenitor properties. The majority of our targets are, in fact, morphologically classified as mergers. We argue that the extreme obscuration provides in itself smoking gun evidence of their merger origin, and a powerful tool for identifying mergers at even higher redshifts.