Masayuki Akiyama

The buildup of dense stellar cores is expected to mark an important transition in the star formation and black hole growth of massive galaxies. Using spatially resolved spectral energy distribution (SED) fitting of James Webb Space Telescope near-infrared imaging, combined with a stacking analysis of Chandra X-ray data, we trace stellar mass buildup and average black hole accretion in the most massive galaxies at z < 5, selecting the 50 most massive galaxies per redshift bin at constant number density of ∼4.4 × 10 ⁻⁵ cMpc ⁻³ . To robustly constrain central stellar populations, we separate active galactic nucleus (AGN) components affecting the photometry using multiband morphological decomposition and SED analysis. We find that the sample selected with constant number density exhibits an evolutionary trend of rapid central compaction at z ∼ 4, during which the median central 1 kpc stellar mass increases by ∼0.60 dex over ∼400 Myr. The majority of X-ray-detected AGN (63% ± 12%) are hosted by galaxies undergoing the compaction, while we find neither individually detected X-ray sources nor a significant stacked X-ray signal at z > 4, indicating that substantial average black hole growth emerges primarily during, rather than before, the compaction. Following the compaction, central specific star formation rates (sSFRs) decline by ∼1.24 dex over ∼700 Myr at z ∼ 3 while remaining elevated galaxy-wide, signaling the onset of inside-out quenching. Despite this central suppression, the specific black hole accretion rate remains coupled to the total sSFR. Our results suggest that dense core formation in the most massive galaxies marks the onset of inside-out quenching and a transition toward an enhanced black hole to stellar growth ratio.

One of the most remarkable discoveries of JWST is a population of compact, red sources at z > 4, commonly referred to as Little Red Dots (LRDs). Spectroscopic identifications reported that most LRDs are active galactic nuclei, which are preferentially found around z ∼ 6 and could imply a key phase in the formation and growth of black holes (BHs) in the early Universe. Photometric surveys at lower redshift have recently been carried out to trace their evolution across cosmic time, and a small number of LRD-like galaxies have been spectroscopically identified at both Cosmic Noon and in the local Universe. Here we report the discovery of one of the lowest- z LRD-like galaxies, J204837.26-002437.2 (hereafter J2048) at z = 0.4330, using new Gemini-N/GMOS integral field unit observations combined with archival multiband photometric spectral energy distribution data. The GMOS data reveal extended blue emission from starburst with a star formation rate of 400 M _⊙ yr ⁻¹ , together with an extended, highly fast ionized outflow. This is the first spectroscopic confirmation of extended host emission and outflow in an LRD-like galaxy, providing a unique laboratory for understanding the nature of their high-redshift counterparts. Moreover, J2048 would host an extremely overmassive BH with a BH-to-stellar mass ratio of ≃60%, with the BH mass and host stellar mass estimated to be 10 ^10.2 and 10 ^10.4 M _⊙ , respectively. We discuss the origin and evolutionary fate of J2048, and the implications that such low- z analogs have for interpreting the properties of high- z LRDs.

We investigate the stellar shape and size–mass relationship of X-ray-selected active galactic nucleus (AGN) host galaxies using the high angular resolution and deep sensitivity in the near-infrared of the COSMOS-Web JWST survey field. We present the rest-frame 1 μ m size, stellar mass, Sérsic index, axis ratio, and Gini– M ₂₀ parameters of 690 moderate-luminosity active galactic nuclei (AGNs) between redshift 0 and 3 and with stellar mass log M s / M ⊙ ∼ 10.75 . We find that AGN host galaxies have effective radii of 1–5 kpc, which is in between star-forming galaxies (SFGs) and quiescent galaxies (QGs) of the same stellar mass. AGN hosts have similar size–mass trends to SFGs and QGs, being smaller at higher redshift for the same stellar mass. The slope of the size–mass relationship of AGN host galaxies is steeper than that of SFGs. Their rest-frame 1 μ m stellar morphology indicates a significant spheroidal component. We observed a low merger fraction (6%) in our sample, as well as substructures similar to disks, bars, and spiral arms in the residual images, which are in tension with evolutionary pathways that require major mergers. However, they may also be due to the different timescales between mergers and AGN activity.

Massive quiescent galaxies at high redshift show significantly more compact morphology than their local counterparts. To examine their internal structure across a wide redshift range and investigate potential redshift dependence, we performed spatially resolved spectral energy distribution fitting using piXedfit software on massive ( log ( M * / M ⊙ ) ∼ 11 ) quiescent galaxies at 0 < z < 4 with public James Webb Space Telescope and Hubble Space Telescope imaging data from the Public Release Imaging for Extragalactic Research and the Cosmic Evolution Early Release Science Survey. We find that at z ∼ 3.5 the half-mass radius is about 5.4 times smaller than at z ∼ 0.5. This growth is driven by stellar mass buildup in the outskirts ( r > 4 kpc), while the central regions ( r ∼ 1 kpc) remain largely unchanged, with stellar mass surface density similar to local quiescent galaxies. The estimated star formation rates are too low to explain the stellar mass growth, indicating that an additional stellar mass accumulation process, such as mergers, is necessary. We parameterize the size–mass relation of the most massive galaxies in our sample as log ( R e , mass ) ∝ α log ( M * ) and find α = 2.6 7 − 1.17 + 1.14 for z ⪅ 2, consistent with growth dominated by minor mergers, and α = 0.9 1 − 0.16 + 0.20 for z ⪆ 2, consistent with growth dominated by major mergers. These results indicate that massive quiescent galaxies originate from compact quenched systems and grow through combinations of minor and major mergers.

We present spectroscopic identification of 43 quasars and 11 candidate obscured quasars in the epoch of reionization (EoR) at 5.71 ≤ z ≤ 7.02, along with 29 galaxies at similar redshifts. This is the 24th publication from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) imaging survey to search for EoR quasars. The HSC-SSP survey has finished, and this paper is likely the final installment of major (unobscured) quasar discoveries from the SHELLQs project. In addition to the EoR objects, we identified five strong [O iii] line emitters at z < 1, 30 Galactic brown dwarfs, and 14 passive galaxies at z ∼ 2, which contaminated our sample of photometric quasar candidates. The present paper focuses on describing the immediate outcome of the spectroscopic observations, while a statistical analysis of the full SHELLQs sample will be presented in our next publication.

Context. The tight correlation between supermassive black hole (SMBH) and host galaxy masses suggests their coevolution. Dust-obscured galaxies (DOGs) are thought to represent this coevolution phase, with active galactic nuclei (AGNs) buried in dust and gas. Despite hosting rapidly growing SMBHs, the X-ray statistical properties of DOGs remain poorly understood due to their rarity and the lack of wide, uniformly sensitive X-ray surveys. Aims. We construct a sample of X-ray-detected DOGs in the eROSITA Final Equatorial Depth Survey (eFEDS) field and examine their X-ray statistical properties. Methods. To construct the DOGs sample, we combined data from the Subaru/HSC SSP (optical), VIKING (near-infrared), and WISE (mid-infrared) all-sky surveys. We then cross-matched the sample with eROSITA-detected sources to select X-ray-detected DOGs. Results. We report the discovery of 5738 IR-bright DOGs within the 60 deg² area covered by both eFEDS and VIKING, including 65 X-ray-detected DOGs (eFEDS-DOGs). Among these, 41 eFEDS-DOGs exhibit a near- to mid-IR power-law slope, indicating dust-obscured AGNs. Hydrogen column densities (N_H) of eFEDS-DOGs span 10²⁰ < N_H/cm⁻² ≲ 10²³, including even unobscured AGNs. Most IR-bright DOGs remain undetected in X-rays, implying heavy obscuration (N_H/cm⁻² > 10²³). eFEDS-DOGs, identified via the wide-area eROSITA survey, represent a less obscured DOG phase, possibly tracing the decline of dust/gas obscuration due to AGN feedback such as gas stripping or outflows. Some eFEDS-DOGs deviate up to ∼1 dex below the L_6 μm–L_{0.5 − 2 keV}^{(abs, corr)} relation, potentially indicating high Eddington ratios near the Eddington limit. This suggests that eFEDS-DOGs are promising candidates for rapidly growing black holes in an early AGN feedback phase.

Extreme emission line galaxies (EELGs) at high redshifts are considered key contributors to cosmic reionization at z > 6 due to their higher ionization efficiencies. We have identified 119 Hβ + [O iii] emitters at z ∼ 7 selected by a flux excess in the medium-band filter F410M in the public James Webb Space Telescope Cycle-1 fields. Our emitters exhibit a wide range in rest-frame Hβ + [O iii] equivalent width (EWs), 420 < EW₀ / Å < 6850 (with the median value of ∼1700 Å). Among them, 19 are EW₀ > 3000 / Å, which represent extreme populations even in the context of recent findings with JWST. They are characterized by (i) low stellar mass (∼3 × 10⁷ M _⊙), (ii) blue colors (β _UV ∼ −2.2), and (iii) low dust attenuation (A _V ∼ 0.1 mag). We discuss the physical mechanisms responsible for the observed high rest-frame Hβ + [O iii] EWs, including (1) photoionization by active galactic nucleus (AGN), (2) stellar photoionization in the vicinity of H ii regions, and (3) radiative shocks powered by outflows either from AGN or massive stars. Notably, we find 13 emitters with spatially offset Hβ + [O iii] emission compared to the UV and stellar components. Given the absence of obvious signatures of actively accreting black holes, these emitters are likely under strong feedback-driven winds from massive stars. Lastly, we report a unique overdensity of EELGs in one of the observed fields. The discovery of such a “star-bursting” overdensity supports the idea that large ionizing bubbles formed around some EEGLs in the early Universe.