Junya Arita

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.

We present James Webb Space Telescope (JWST) NIRSpec integral field unit (IFU) observations of six galaxies at z ∼ 6, obtained as part of the Aether project (General Observers program 5645). The targets were originally identified by the Subaru High- z Exploration of Low-Luminosity Quasars (SHELLQs) survey as candidate obscured quasars with luminous (≳10 ⁴³ erg s ⁻¹ ) but narrow (≲500 km s ⁻¹ ) Ly α emission. Two objects exhibit a broad component in their Balmer lines (FWHM > 3000 km s ⁻¹ ), indicating the presence of active galactic nuclei (AGNs), while the remaining four show similar profiles in permitted and forbidden lines. Combining these data with similar SHELLQs objects reported previously, we find that the presence of broad lines is strongly correlated with Ly α luminosity ( L _{Ly α} ); the inferred AGN fraction is >77% and L _{Ly α} = 10 ⁴⁴ erg s ⁻¹ , respectively. Dust-extinction corrections inferred from the Balmer decrement would imply unrealistically high Ly α luminosities, suggesting that the line-emitting gas consists of multiple zones. The IFU data reveal diverse spatial structures. The AGN hosts are compact, whereas the other galaxies show extended ionized gas on scales up to 10 kpc and star formation rates of 60–600 M _⊙ yr ⁻¹ . One of the extended objects exhibits a signature of rotation, while most of the others show little ordered kinematics, with velocity widths (FWHM) up to 200–300 km s ⁻¹ . These objects populate the intermediate luminosity regime between classical luminous quasars and the low-luminosity AGNs discovered by JWST, including little red dots, potentially linking the two populations.

We present an analysis of the rest-frame optical JWST NIRSpec Fixed Slit spectra of extended host galaxy emission in 12 moderate luminosity quasars from the Subaru High- z Exploration of Low-Luminosity Quasars sample at redshifts 6.0 < z < 6.4. We fit and subtract a point-spread model based on observations of a calibration star and use it to remove the quasar from the 2D spectra, leaving only extended emission, which we analyze. Ten of 12 systems show spatially extended line emission, and six of 12 systems show an extended stellar continuum. From the extended [O III ] λ 5008 emission line, we measure a 132 ± 22 km s ⁻¹ ionized outflow in one system and 52 ± 12 km s ⁻¹ rotation, suggesting a coherent disk, in another. From the extended narrow H α emission, which we hypothesize is ionized by star-forming regions rather than the quasar, we measure star formation rates ranging from ∼7 to 111 M _⊙ yr ⁻¹ , the majority of which are consistent with the star-forming main sequence (SFMS) at z ≈ 6. The positions of our host galaxies on the log 10 [ O III ] λ 5008 / H β versus log 10 [ NII ] 6584 / H α (R3N2) Baldwin–Phillips–Terlevich diagram indicate ionization rates typical of active galactic nucleus (AGN) activity in the low-redshift Universe, but are consistent with the placement of other z ≈ 6 quasar host galaxies, suggesting that the R3N2 line ratios cannot distinguish AGN- and star-formation-powered line emission at high redshifts. We conclude from the consistency between our quasar host sample with z ∼ 6 galaxies that the presence of a low-luminosity AGN causes little significant change in the properties of galaxies at z ≈ 6 on 10 Myr timescales.

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 relation between the masses of supermassive black holes (SMBHs) and their host galaxies encodes information on their mode of growth, especially at the earliest epochs. The James Webb Space Telescope (JWST) has opened such investigations by detecting the host galaxies of active galactic nuclei (AGN) and more luminous quasars within the first billion years of the Universe ( z ≳ 6). Here, we evaluate the relation between the mass of SMBHs and the total stellar mass of their host galaxies using a sample of nine quasars at 6.18 ≤ z ≤ 6.4 from the Subaru High- z Exploration of Low-luminosity Quasars survey with NIRCam and NIRSpec observations. We find that the observed location of these quasars in the SMBH–galaxy mass plane ( log M BH / M ⊙ ∼ 8 –9; log M * / M ⊙ ∼ 9.5 –11) is consistent with a nonevolving intrinsic mass relation with dispersion ( 0.8 0 − 0.28 + 0.23 dex) higher than the local value (∼0.3–0.4 dex) of their more massive descendants. Our analysis is based on a forward model of systematics and includes a consideration of the impact of selection effects and measurement uncertainties with an assumption on the slope of the mass relation. While degeneracies between parameters persist, the best-fit solution has a reasonable AGN fraction (2.3%) of galaxies at z ∼ 6 with an actively growing UV-unobscured black hole. In particular, models with a substantially higher normalisation in M _BH would require an unrealistically low intrinsic dispersion (∼0.22 dex). Consequently, our results predict a large population of AGN at lower black hole masses, as are now just starting to be discovered in focused efforts with JWST.

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 ∼5 μ m 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 ≳ 10 LRD candidates using both NIRCam and MIRI imaging from COSMOS-Web, which provides the largest joint NIRCam-MIRI coverage to date (0.20 deg ² ). Taking advantage of MIRI/F770W to remove contaminants, we identify one robust candidate, CW-LRD-z10 at z phot = 10 . 5 − 0.6 + 0.7 with M UV = − 19 . 9 − 0.2 + 0.1 mag . CW-LRD-z10 exhibits a compact morphology, a distinct V-shaped SED, and a nondetection in F115W, all consistent with being an LRD at z ∼ 10. Based on this discovery, we place the first constraint on the number density of LRDs at z ∼ 10 with M _UV ∼ −20 of 1 . 2 − 1.0 + 2.7 × 1 0 − 6 Mpc − 3 mag − 1 , suggesting that the fraction of LRDs among the overall galaxy population increases with redshift, reaching ∼3% at z ∼ 10. 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.

The advent of the James Webb Space Telescope (JWST) has opened new horizons in the study of quasar host galaxies during the reionization epoch ( z > 6). Building upon our previous initial measurements of stellar light from two quasar host galaxies at these redshifts, we now report the detection of the stellar light from the full Cycle 1 sample of 12 distant moderate-luminosity quasar ( M ₁₄₅₀ > −24 mag) host galaxies at z > 6 from the Hyper Suprime-Cam Subaru Strategic Program. Using JWST/NIRCam observations at 1.5 and 3.6 μ m combined with 2D image decomposition analysis, we successfully detect the host galaxies in 11 of the 12 targets, underscoring the high detection rates achievable with moderate-luminosity quasars. Based on two-band photometry and spectral energy distribution fitting, we find that our host galaxies are massive, with log M _* / M _⊙ = 9.5–11.0. The effective radii range from 0.6 to 3.2 kpc, comparable to the sizes of inactive galaxies with similar masses at z ∼ 6 as measured with imaging from COSMOS-Web. Intriguingly, the two quasar hosts with post-starburst features, which reside at the high-mass end of our sample and exhibit relatively compact morphologies, have similar size and stellar mass surface densities to quiescent galaxies at z ∼ 4–5. These findings suggest that the so-called galaxy compaction scenario is already in place at the reionization epoch, in which gas inflows during starburst phases drive centrally concentrated star formation followed by rapid quenching, bridging the structural transition of massive galaxies from relatively extended star-forming disks to compact quiescent systems.