Kei Ito

We present spectroscopic follow-up observations with Subaru/MOIRCS of four protocluster candidate regions identified by the deep-learning model PCFNet. In three candidate regions, we confirm four protoclusters at z = 3.316, 3.403, 3.408 and 3.425, exhibiting galaxy density peaks over five times the uniform distribution. The detection rate of confirmed members aligns with PCFNet’s predictions, underscoring its effectiveness. Analysis of spectral energy distributions (SEDs), incorporating up to ten photometric bands (uu*grizyJHK), reveals that protocluster members lie on the star-formation main sequence. The masses of the most massive halos in the protoclusters are estimated from the most massive stellar mass of the member galaxies, yielding log Mhalo/M⊙ = 12.4–12.8, which are consistent with those at this redshift predicted by cosmological simulations and are comparable to those of the progenitors of typical local clusters ($M_\mathrm{halo}^{z=0}\sim 10^{14}M_\odot$). Using the Voronoi Tessellation Monte Carlo technique based on the probability of protocluster membership from PCFNet, we find that our protoclusters exhibit a significantly higher SFRD than the cosmic SFRD at z ~ 3.4. Based on the [O iii]/Hβ emission line ratio, two protoclusters show marginally (1.4–2σ) higher average metallicities than the field, while the other protoclusters are statistically consistent with the field. These diverse metallicities highlight the environmental impact within protoclusters detected by PCFNet, likely reflecting the complex opposing interplay between metal-rich gas recycling and metal-poor gas inflow, which may depend on the surrounding environment of each protocluster. This work demonstrates PCFNet’s capability to unveil diverse samples of high-redshift protoclusters, paving the way for a more comprehensive understanding of their role in galaxy evolution.

Massive quiescent galaxies (QGs) at high redshifts are likely progenitors of massive elliptical galaxies in the local Universe. Recent discoveries of high-redshift QGs in overdensities (galaxy groups and protoclusters) have highlighted the relationship between massive galaxy quenching and their surrounding environment. We spectroscopically confirm a galaxy group associated with a massive QG at z _spec = 4.53 from the Lyman break feature using Subaru/FOCAS. This group consists of at least three star-forming galaxies within 150 pkpc of the QG, suggesting a physical association necessary for galaxy quenching. To investigate the role of the surrounding environment, we also perform a statistical analysis to characterize the typical environment of QGs at high redshifts. By selecting QGs using a star formation rate-based selection in the COSMOS field, we find that the quiescent fraction is higher in overdensities than in the field at 1 < z < 5. In particular, the elevated quiescent fraction within small-scale overdensities (∼200 pkpc) demonstrates that environmental quenching, possibly driven by galaxy mergers and interactions, plays a major role in the evolution of massive QGs at high redshifts. By extending the low-redshift mass and environmental quenching model up to z ∼ 5, we confirm similar trends to those inferred from the Sloan Digital Sky Survey red galaxy fraction at z < 1. While environmental dependence weakens at z ≳ 3 for the overall population, a clear correlation persists specifically for more massive galaxies. This mass-dependent environmental effect suggests a downsizing trend, highlighting that the accelerated evolution of massive galaxies in high-density regions is already in place in the early Universe.

We report on the spectroscopic confirmation of overdense regions of massive quiescent galaxies (QGs) in the early Universe with JWST/NIRSpec. Based on data from the DeepDive NIRSpec program and archival data from the Dawn JWST Archive, we confirm three QGs in the vicinity of Jekyll and Hyde, a pair of massive QG and a dusty star-forming galaxy, at z = 3.71 and two QGs around SXDS-27434 at z = 4.01. According to the analysis of galaxy number density with photometric redshifts, Jekyll and Hyde (SXDS-27434) are in an overdense region, where the number density of galaxies is three (four) times higher than the average in the COSMOS (SXDS) field. SED fitting suggests that most of the QGs follow similar star formation histories and have consistent formation and quenching epochs. The same trend is observed in other protoclusters hosting QGs that were already identified by ground-based telescopes, indicating that the large-scale environment plays an important role in the formation of QGs. In addition, JWST spectra reveal a broad H α emission line from SXDS-27434 and faint emission lines from the other three QGs, which are identified as AGN-driven based on their emission line ratios. The overdensity is also reproduced by the Illustris TNG300 simulation at z = 3.71, in which the member QGs also have similar quenching epochs. These results are consistent with a scenario in which the large-scale structure enhances merger activity and/or gas accretion and triggers AGN feedback, thereby driving simultaneous formation and quenching of QGs in the overdensity.

Massive galaxies stopped forming stars surprisingly early in the history of the Universe. However, after quenching, the galaxies continued to grow in size across time, as established in the literature up to cosmic noon. We assemble one of the largest and most comprehensive multiwavelength photometric sample of massive quenched galaxies at z > 3 from publicly available JWST observations, counting 137 quiescent candidates within ∼825 arcmin ² and redshift 3 ≤ z < 7 across the well-studied extragalactic fields GOODS, PRIMER, and CEERS. We modeled their surface brightness distribution across five bands mapping their UV-to-near-IR rest-frame wavelength with Sérsic profiles, and derived their sizes, concentrations, and ellipticities. The size-mass relation is consistent with previous studies, showing a shallower slope at low redshift z ∈ [3, 3.5] and a steeper slope at high redshift z > 3.5. The size decreases with increasing redshift, in agreement with previous studies and we extend them up to z _spec = 4.9. Leveraging the large sample statistics, we robustly constrain the intrinsic scatter of the mass–size relation to ∼0.3 dex with no relevant dependence on the redshift and the filter used. At the population level, our multiwavelength modeling reveals that the size decreases with increasing observed wavelength, and the wavelength gradient decreases with increasing stellar mass. This result proves that in the near-IR bands, massive elliptical galaxies appear to be more compact. The Sérsic index shows no significant dependence on wavelength, regardless of the stellar mass. Following a forward–backward Bayesian fit analysis to assess the significance of several parameters in predicting the size of our sample, we identified no significant secondary dependence of the size on the axis ratio q , Sérsic index n , UVJ -colors, or environment. The combination of stellar mass and redshift is sufficient to predict the size of quiescent galaxies at z > 3, albeit with a large scatter. This suggests that the commonly used parameters of a Sérsic distribution cannot explain the large intrinsic scatter around the stellar mass–size relation, suggesting that other physical quantities need to be taken into account to break the degeneracy between evolution paths across the galaxy population.

Neutral gas outflows play a crucial role in the baryon cycle of galaxies, regulating their evolution by removing gas and redistributing energy and momentum into the surrounding medium. Their properties provide key insights into the transition from star formation to quiescence. In this work, we investigate the neutral gas outflow properties of 23 massive (M_⋆ = 10^ 10.1-11.6 ,̊m M_⊙) quiescent galaxies (QGs) at z=2.82--4.61, selected from the JWST NIRSpec (̊m R ). However, the outflows are still unlikely to escape their hosts; in fact, they are suggestive of fountain-like recycling on relatively short timescales (sim3--180 Myr), depending on the assumed potential and launching radius. All spectroscopic and NIRCam imaging program We traced neutral gas outflows using the doublet absorption lines and detected excess D absorption in 13/23 (57%) targets. Of these, 7/23 (30%) displayed blueshifted excess absorption with velocity offsets |̌off|≳ 150,, . The z łeq _ -1 targets exhibited velocity offsets similar to those of their local massive quiescent counterparts and they are also in agreement with comparisons in the SFR--̌off space. We derived mass outflow rates and identified, in particular, the most extreme neutral gas outflow rate ever reported beyond the Local Universe, coincident with an X-ray active galactic nucleus (AGN). For all D-detected systems, the inferred mass outflow rate could, in principle, suppress ongoing star formation (i.e., mathrm SFR M out D-detected targets occupy the LI(N)ER region of the BPT diagram and/or are X-ray-detected. Still, we found no strong correlation between ongoing AGN activity and the neutral outflow: 2/4 broad-line/X-ray AGNs were not D-detected. However, the outflows could be powered by fossil or episodic AGNs and one broad-line target shows a possible P-Cygni profile that would indicate strong outflows. Since, on average, neutral outflows alone are not able to permanently quench star formation by removing gas in our sample at z the presence of gas cycling in and out of massive passive systems could be interpreted as a signature of feedback-regulated quenching-maintenance processes.

We present the spectroscopic confirmation and in-depth analysis of AURORA-LQG1, a low-mass quiescent galaxy at z _spec = 2.0834 with log( M _★ / M _⊙ ) = 9.6, observed with medium-resolution JWST/NIRSpec spectroscopy. Its stellar mass places approximately ten times lower than the knee of the stellar mass function for quiescent galaxies at z ∼ 2. The deep medium-resolution spectrum enables the measurement of its stellar velocity dispersion ( σ _★ = 95 ₋₃₃ ⁺³⁸ km s ⁻¹ ), the smallest value recorded among spectroscopically confirmed quiescent galaxies at z ∼ 2. Coupled with a compact size (0.41 ± 0.03 kpc in the rest-frame optical), the stellar velocity dispersion yields a dynamical mass estimate of log( M _dyn M _⊙ = 9.75 _−0.38 ^+0.29 , consistent with the stellar mass, confirming the true low-mass nature of this galaxy and placing a first constraint on its initial mass function. Joint spectro-photometric spectral energy distribution fitting reveals a star formation history in which half the stellar mass was in place ∼1 Gyr before the observed epoch, with quenching occurring ∼0.2 Gyr prior to z = 2.08. These results confirm that AURORA-LQG1 is genuinely quenched, rather than in a temporary phase of suppressed star formation. AURORA-LQG1 is consistent with the mass fundamental plane at z ∼ 2, which was previously constrained only by massive quiescent systems with M _★ ≥ 10 ¹¹ M _⊙ at cosmic noon. Compared with more massive counterparts at the same epoch observed with similar NIRSpec grating spectroscopy, the time since quenching for AURORA-LQG1 is among the shortest observed. The star formation history of AURORA-LQG1 is broadly consistent with predictions for quiescent galaxies of similar mass and redshift in the IllustrisTNG and SHARK models. The galaxy resides in a possible dense group-scale (∼50 kpc) environment containing one companion with tentative spectroscopic redshift and five low-mass companion candidates with similar redshifts, and it is embedded in a large known protocluster on megaparsec scales. A potential environmental influence on its evolution could explain the outside-in quenching suggested by the positive gradient of size with wavelength. This study demonstrates that deep JWST/NIRSpec spectroscopy enables low-mass quiescent galaxies at cosmic noon to be characterized with a level of detail long reserved for massive systems, offering valuable new insights into how quenching operates in these underexplored, low-mass systems.

High-z quasars are believed to reside in massive dark matter haloes (DMHs), suggesting that they reside in galaxy overdense regions. However, previous observations have shown a range of environments around them. These fields have been limited to luminous quasars (M1450 ≲ −25), for which photoevaporation may hinder galaxy formation in their vicinity. Here, we present Subaru/Hyper-Suprime Cam observations of the environments of four low-luminosity quasars (−24 < M1450 < −22) at z ∼ 6.18, which are expected to have a smaller photoevaporation effect. We detect Lyman α emitters (LAEs) with narrowband NB872 imaging, and measure the local LAE overdensity. One quasar (J0844−0132) resides in an overdense region (δLAE = 1.97 ± 0.40), whereas the other three fields are consistent with no overdensity. These results hold over the proximity zone of each quasar, suggesting that the diverse environment around quasars is independent of photoevaporation. We find no significant correlation between the LAE overdensities and the characteristics of host galaxies and supermassive black holes. Our quasars have host stellar mass measurements from JWST, allowing us to compare them with the LAE overdensity around galaxies without quasar activity with comparable stellar masses. We find that the LAE overdensity in the J0844−0132 field is stronger than that of galaxies with similar stellar mass at z ∼ 6.

The mass assembly and chemical enrichment of the first galaxies provide key insights into their star formation histories and the earliest stellar populations at cosmic dawn. Here we compile and utilise new, high-quality spectroscopic JWST/NIRSpec Prism observations from the JWST archive. In particular, we extend the wavelength coverage beyond the standard pipeline cut-off (5.3 μm) up to 5.5 μm, which enables for the first time a detailed examination of the rest-frame optical emission-line properties for galaxies at z ≈ 10. Crucially, the improved calibration allows us to detect H β and the [O III ] λλ 4959, 5007 doublet and resolve the auroral [O III ] λ 4363 line for the 11 galaxies in our sample ( z = 9.3 − 10.0) to obtain direct T _e -based metallicity measurements. We find that the interstellar medium (ISM) of all galaxies shows high ionisation fields and electron temperatures, with derived metallicities in the range 12 + log(O/H) = 7.1 − 8.3 (3–50% solar), consistent with previous strong-line diagnostics based on JWST data at high redshifts. We derive an empirical relation for M _UV and 12 + log(O/H) at z ≈ 10, useful for future higher-redshift studies, and show that the sample galaxies are ‘typical’ star-forming galaxies though with relatively high specific star formation rates (median sSFR = SFR _{H β} / M _★ = 38 Gyr ⁻¹ ) and with evidence of bursty star formation on 10 Myr versus 100 Myr timescales (log ₁₀ (SFR ₁₀ /SFR ₁₀₀ )≈0.7). Combining the rest-frame optical line analysis and detailed UV to optical spectro-photometric modelling, we determine the mass-metallicity relation (MZR) and the fundamental metallicity relation (FMR) of the sample, pushing the previous redshift frontier of these measurements to z = 10. These results, together with literature measurements, point to a gradually decreasing MZR at higher redshifts, with a break in the FMR at z ≈ 3, decreasing to metallicities ≈3× lower at z = 10 than observed in galaxies during the majority of cosmic time at z = 0 − 3, likely caused by massive pristine gas inflows diluting the observed metal abundances during early galaxy assembly at cosmic dawn.