Ichi Tanaka

We present a new constraint on the primordial helium abundance, Y _P , based on Subaru observations. A major source of uncertainty in previous Y _P determinations is the lack of extremely metal-poor galaxies (EMPGs; 0.01–0.1 Z _⊙ ), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; 0.1–0.4 Z _⊙ ) predominantly used in earlier studies, requiring substantial extrapolation to zero metallicity. Here, we perform Subaru near-infrared spectroscopy of 29 galaxies, including 14 EMPGs. By incorporating existing optical spectra, we derive He/H for each galaxy using photoionization modeling of helium and hydrogen emission lines, including the He i 10830 Å line to break the density–temperature degeneracy. After carefully selecting galaxies with robust He/H determinations, and adding 58 galaxies from previous studies, we obtain Y P = 0.240 2 − 0.0040 + 0.0040 . This Y _P value is ∼1 σ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the cosmic microwave background constraint from the Atacama Cosmology Telescope experiment. Our result indicates N eff = 2.5 4 − 0.25 + 0.20 , showing a mild (∼2 σ ) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry ( ξ _e ≠ 0), which would alleviate the tension with ξ e = 0.0 5 − 0.03 + 0.02 . More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously.

We report the successful detection of the Arid meteor shower (IAU#1130 ARD), predicted to emerge for the first time in 2021, using a publicly accessible YouTube live camera developed by us. This live camera, installed on the Subaru Telescope dome in the summit region of Maunakea, Hawai’i, features a wide field of view ($70^{\circ } \times 40^{\circ }$) and high sensitivity, and is capable of observing stars fainter than the sixth magnitude. Meteor detection was performed in two ways: visual inspection by citizen viewers and subsequent validation through automated detection. As a result, we confirmed that the number of meteors appearing from near the predicted radiant increased by more than six times (${\sim }9\, \sigma$) compared to the preceding and following days. Our observation time was 4–5 h after the predicted peak (solar longitude = ${193^{\circ}_{.}9}$), providing clear data indicating that the activity had not yet declined. Optical observations at this time from the Northern hemisphere are extremely limited and unique, making our observation point valuable. The meteors are characterized by their slow and faint appearance, but several brighter meteors with wakes were also observed. Simulations tracing the dust trails from the parent body, Comet 15P/Finlay, suggest that our detection can be explained by either of the dust trails released in 2008 or 2014, both requiring high ejection velocities. However, during the comet’s 2008 return, its activity was exceptionally quiet, making a high-velocity dust ejection questionable. On the other hand, multiple large outbursts were observed during the 2014 return, at which time a certain amount of high-velocity dust release is expected. We conclude that the dust source of the meteor shower detected in Hawai’i this time is likely attributable to high-velocity ($\sim$67 m s$^{-1}$) dust ejected during the 2014 outburst.

We present results from a dual narrowband imaging survey targeting the CL1604 supercluster at z = 0.9 using the Subaru Telescope. By combining the NB921 filter on Hyper Suprime-Cam and the NB1244 filter on SWIMS, we can detect redshifted H α and H β emission lines from the supercluster. This unique technique allows us to measure both star formation rates and dust extinction for a sample of 94 emission-line galaxies across the supercluster. We find that dust extinction, estimated from the Balmer decrement (H α /H β ratio), increases with stellar mass in star-forming galaxies, whereas relatively quiescent systems exhibit comparatively low extinction. Among galaxies with intermediate masses (10 ^8.5 < M _* < 10 ^10.5 M _⊙ ), the dust-corrected H α -based star formation rates align with the main sequence at this epoch. More massive galaxies, however, deviate from this relation, exhibit redder colors, and reside predominantly in higher-density environments. Although stellar mass, star formation rate, and galaxy color are clearly influenced by environment, we detect no strong, systematic environmental dependence of dust extinction for the whole sample. However, a trend of increasing dust extinction toward denser regions becomes evident only after excluding quiescent galaxies, as the massive quiescent populations in cluster cores contain little dust and thereby mask this relationship.

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.

This study examines the impact of cluster environments on galaxy properties using data from the Hyper Suprime-Cam Subaru Strategic Program and an optically selected CAMIRA cluster sample. Specifically, the study analyzes the fractions of quiescent and green valley galaxies with stellar masses above 10^8.6 M _⊙ at z ∼ 0.2 and 10^9.8 M _⊙ at z ∼ 1.1, investigating their trends in radius and density. The results indicate that a slow quenching mechanism is at work, as evidenced by a radially independent specific star formation rate reduction of 0.1 dex for star-forming galaxies in a cluster environment. The study also finds that slow quenching dominates fast quenching only for low-mass galaxies (9.2 M _⊙) near the cluster edge, based on their contributions to the quiescent fraction. After controlling for M _*, z, and local overdensity, the study still finds a significant radial gradient in the quiescent fraction, indicating active ram pressure stripping in dense environments. That said, analyzing the density trend of the quiescent fraction with other fixed parameters suggests that radial and density-related quenching processes are equally crucial for low-mass cluster galaxies. The study concludes that ram pressure stripping is the primary environmental quenching mechanism for high stellar mass galaxies in clusters. By contrast, ram pressure stripping and density-related quenching processes act comparably for low-mass cluster galaxies around the center. Near the cluster boundary, starvation and harassment become the leading quenching processes for low stellar mass galaxies.

The LIGO/Virgo detected a gravitational wave (GW) event, named GW200224_222234 (also known as S200224ca) and classified as a binary-black hole coalescence, on 2020 February 24. Given its relatively small localization skymap (71 deg² for a 90% credible region; revised to 50 deg² in GWTC-3), we performed target-of-opportunity observations using the Subaru/Hyper Suprime-Cam (HSC) in the r2 and z bands. Observations were conducted on 2020 February 25 and 28 and March 23, with the first epoch beginning 12.3 hr after the GW detection. The survey covered the highest-probability sky area of 56.6 deg², corresponding to a 91% probability. This was the first deep follow-up (m _r ≳ 24, m _z ≳ 23) for a binary-black hole merger covering >90% of the localization. By performing image subtraction and candidate screening including light-curve fitting with transient templates and examples, we found 22 off-nucleus transients that were not ruled out as the counterparts of GW200224_222234 with our Subaru/HSC data alone. We also performed GTC/OSIRIS spectroscopy of the probable host galaxies for five candidates; two are likely to be located within the 3D skymap, whereas the others are not. In conclusion, 19 transients remain as possible optical counterparts of GW200224_222234; but we could not identify a unique promising counterpart. If there are no counterparts in the remaining candidates, the upper limits of the optical luminosity are ν L ν < 5.2 − 1.9 + 2.4 × 10 41 erg s⁻¹ and ν L ν < 1.8 − 0.6 + 0.8 × 10 42 erg s⁻¹ in the r2 and z bands, respectively, at ∼12 hr after GW detection. We also discuss improvements in the strategies of optical follow-ups for future GW events.

We report the detection of an ionized gas outflow from an X-ray active galactic nucleus hosted in a massive quiescent galaxy in a protocluster at z = 3.09 (J221737.29+001823.4). It is a type-2 QSO with broad (W ₈₀ > 1000 km s⁻¹) and strong ( log ( L [ OIII ] /erg s⁻¹) ≈ 43.4) [O iii]λ λ 4959,5007 emission lines detected by slit spectroscopy in three-position angles using Multi-Object Infra-Red Camera and Spectrograph (MOIRCS) on the Subaru telescope and the Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) on the Keck-I telescope. In the all slit directions, [O iii] emission is extended to ∼15 physical kpc and indicates a powerful outflow spreading over the host galaxy. The inferred ionized gas mass outflow rate is 22 ± 3 M _⊙ yr⁻¹. Although it is a radio source, according to the line diagnostics using Hβ, [O ii], and [O iii], photoionization by the central QSO is likely the dominant ionization mechanism rather than shocks caused by radio jets. On the other hand, the spectral energy distribution of the host galaxy is well characterized as a quiescent galaxy that has shut down star formation several hundred Myr ago. Our results suggest a scenario that QSOs are powered after the shutdown of the star formation and help complete the quenching of massive quiescent galaxies at high redshift.

We present new Atacama Large Millimeter/submillimeter Array (ALMA) results obtained from spatially resolved CO J = 2–1 line (0.″4 resolution) and 870 μm continuum (0.″2 resolution) observations of cluster galaxies in XMMXCS J2215.9-1738 at z = 1.46. Our sample comprises 17 galaxies within ∼0.5 Mpc (0.6R ₂₀₀) of the cluster center, all of which have previously been detected in the CO J = 2–1 line at a lower resolution. The effective radii of both the CO J = 2–1 line and 870 μm dust continuum emissions are robustly measured for nine galaxies by modeling the visibilities. We find that the CO J = 2–1 line emission in all of the nine galaxies is more extended than the dust continuum emission by a factor of 2.8 ± 1.4. We investigate the spatially resolved Kennicutt–Schmidt (KS) relation in two regions within the interstellar medium of the galaxies. The relation for our sample reveals that the central region (0 < r < R _e,870μm) of galaxies tends to have a shorter gas depletion timescale, i.e., a higher star formation efficiency, compared to the extended region (R _e,870μm < r < R _e,CO). Overall, our result suggests that star formation activities are concentrated inside the extended gas reservoir, possibly resulting in the formation of a bulge structure. We find consistency between the ALMA 870 μm radii of star-forming members and the Hubble Space Telescope/1.6 μm radii of passive members in a mass–size distribution, which suggests a transition from star-forming to passive members within ∼0.5 Gyr. In addition, no clear differences in the KS relation nor in the sizes are found between galaxies with and without a close companion.