Subaru Seminars

Abstract: Gaseous media in the universe are essential to both galaxy evolution and the growth of supermassive black holes. In this talk, I will introduce two distinct projects we have undertaken that each focus on gaseous environments through different observational approaches. The first part focuses on the gaseous environment in the vicinity of active galactic nuclei (AGNs). In this project, we utilized the infrared flux variability, which is characteristic of the AGN, to distinguish between dusty gas and dust-free gas components around the AGN for the first time, and discussed the radiation-pressure effect on these different gas structures. I will also present our recent results on variability analysis of the broad Hα line for local AGNs with unique characteristics uncovered by our independent analysis. The second part focuses on the gaseous environment in submillimeter galaxies (SMGs). In this project, we performed [CII]158µm observations using ALMA for flux-limited SMG sample at z=3-5, originally detected by the SCUBA-2 survey. Thanks to the bright nature of our sample, we successfully detected the [CII] line for 16 out of 18 targets with S/N>10. Most of the detected [CII] lines exhibit complex profiles that reflect the kinematics of [CII]-emitting gas. I will also present our preliminary results regarding correlations between [CII] luminosity and several physical properties, such as star formation rate and molecular gas mass.

We systematically search for galaxy overdensities using 20 independent fields with a minimum of six filters from BEACON, the JWST Cycle 2 NIRCam pure-parallel imaging survey. We apply an adaptive kernel-density estimation method that incorporates the full photometric redshift probability distribution function of each galaxy to map galaxy overdensities, and identify 207 significant overdensities at 1.5<z<5. We measure the quenched-galaxy fraction, the median specific star formation rate (sSFR), the total halo mass, and the local galaxy density of each system. By investigating the correlation among these observables, we find that galaxy quenching proceeds in two paths: (i) Overdensities within more massive halos exhibit higher quenched fractions and lower averaged sSFRs. This trend weakens at z>2, consistent with cold gas streams penetrating shock-heated massive halos and sustaining star formation activity at early times. (ii) We also find a dependence of the same parameters on local densities at z<2, where the quenched fraction increases and the sSFR decreases toward higher densities. The environmental trend in sSFR weakens at z~2-3 and shows tentative evidence for a reversal at z>3, potentially due to a larger cold gas supply in earlier times. Our study reveals a complex interplay between individual galaxies and large-scale environmental properties, marking the onset of environmental effects on galaxy quenching in massive halos at cosmic noon.

The cold gas accretion to proto-clusters and to galaxies therein is expected to be very efficient at z>2 by the cold stream mode. As the proto-clusters grow in mass, the gas is shock-heated to a high temperature, leading to the full ionization of the ICM eventually. This transition in the gas phase is probably responsible for the rise and fall of star forming activities in proto-clusters. In order to observationally test this working hypothesis, we need to map cold gas distribution along large scale structures, in which proto-clusters are developed at intersections of filaments. We employ a unique method of panoramic pair narrow-band (NB) imaging focusing on the difference in attenuation between Hα and Lyα lines due to resonant scattering and dust extinction. We conducted a NB imaging with HSC/Subaru across 1.5-degree scale structures at z=2.23 hosting a known rich proto-cluster in the COSMOS field. We have identified over 400 Ly-alpha emitters (LAEs) over a 140 cMpc x 140 cMpc region. By comparing with H-alpha emitters (HAEs) traced by HiZELS NB imaging across the same field, we mapped out the HI gas along the filaments and in the group/core based on differential spatial distributions of the two populations and their radial light profiles of the lines. As a result, we discovered a cold gas association neatly tracing the structures. We will discuss how the cold gas is accreted to proto-cluster and how it regulates star formation in galaxies.

Galaxy evolution is strongly linked to surrounding large-scale structures, yet the physical mechanisms driving environmental dependence—such as star-formation, AGN activities, and morphologies—are not fully understood. In this talk, I focus on two complementary topics. First, we investigate where luminous AGN reside within large-scale structure and how they influence surrounding galaxies, using Hα narrow-band imaging of a quasar overdensity at z~2.2. We identify extended large-scale structures traced by star-forming galaxies, providing new insight into the environments where quasars emerge. Second, we study environmental effects on galaxy morphology and quenching by combining Euclid near-infrared imaging with Subaru/MOIRCS narrow-band data in a z~1.5 cluster. The results suggest environmental dependence in the evolutionary pathways of quiescent galaxies. Together, these studies demonstrate that near-infrared narrow-band imaging is a powerful tool for probing the environmental dependence of galaxy evolution. I will also introduce our upcoming observations in Hawaii and discuss future follow-up observations and prospects for extending these analyses to larger samples.

We are entering an era of extremely large spectroscopic surveys, with Subaru/Prime Focus Spectrograph (PFS) poised to play a leading role. Dense spectroscopy provides a powerful tools of probing the physical properties of statistically large samples of galaxies and their environments. In this talk, I present two key applications of dense spectroscopy to the study of galaxy clusters and their member galaxies. The first application is the stellar velocity dispersion function (VDF) of cluster galaxies, which serves as a probe of the underlying dark matter subhalo mass distribution. Using dense spectroscopic data, we measure the VDF in galaxy clusters and derive corresponding theoretical predictions from cosmological simulations. I will discuss the comparison between the observational measurements and theoretical expectations, along with their implications for cluster substructure and galaxy-halo connections. The second application is weak-lensing spectroscopic tomography ("spectrotomography"), which combines weak lensing with dense spectroscopy. This technique reconstructs the cluster lensing signal using exclusively spectroscopically confirmed background galaxies, thereby minimizing uncertainties associated with photometric redshifts. I will present the methodology and early results from a sample of 10 galaxy clusters. Spectrotomography provides not only an independent measurement of cluster mass but also a promising cosmological probe in the era of large spectroscopic surveys.

The SCExAO team at the Subaru Telescope has launched an intensive survey, OASIS, for substellar companions around accelerating stars. Dynamical masses derived from orbital fitting allow us to compare evolutionary models of age-mass-luminosity relations with observational results. For example, hot (Baraffe et al. 2003) vs. cold start (Marley et al. 2007) are competing hypotheses of initial entropy. Differences between two models are most prominent at young ages (≲ 200 Myr), making high-contrast imaging particularly sand exhibit temperature variations across their surfaces, making apparent magnitudes inclination-dependuitable for characterizing such young and hot substellar companions. However, age estimates from the color-magnitude diagram (CMD) suffer from large uncertainties and may even be misleading when metallicity effects are underestimated. Our survey targets are mainly early-type stars (B, A, and early-F), which typically rotate rapidly. We developed software that applies Bayesian analysis to jointly sample six parameters: mass, age, metallicity, rotation, inclination, and parallax. Our method better accounts for parameters affecting age-dating, providing more reliable age estimates than simple CMD-based results. We validated our software with well-known clusters: Hyades (600-800 Myr), Praesepe (600-800 Myr), and Pleiades (~100 Myr), finding good agreement with previous studies (e.g., Brandt & Huang 2015a,b,c). We further conclude well-constrained extinction and metallicity are essential for reliable ages with small uncertainties. In addition, we will present a newly detected brown dwarf candidate from our survey with a mass of 51 (+12, -16) M_Jup, a semi-major axis of 54.4 (+11, -8.3) AU, and an eccentricity of 0.36 (+0.13, -0.17). Its spectral type is M4-M5, with an effective temperature of 3148 (+175, -142) K. We also derive a Bayesian age of 484 (+248, -235) Myr. We will also compare it with ages from evolutionary models of substellar companions.

Ground-Based direct imaging of exoplanets is limited by atmospheric turbulence, which introduces phase errors resulting in complex "speckle" patterns forming in the Point Spread Function (PSF). Understanding the relationship between the wavefront sensor (WFS) telemetry and the speckles is essential for high-contrast imaging; however traditional analytical reconstructions often struggle with the non-linearities in the WFS response. An alternative is a machine learning-based approach using a U-Net architecture. By treating PSF reconstruction as an image-to-image translation task, the model is trained to map telemetry from the wavefront sensor directly to the corresponding focal plane intensity distribution. This approach aims to leverage the U-Net’s ability to capture spatial features across multiple scales, potentially offering a more robust method for handling atmospheric speckles compared to standard linear models. This presentation outlines the current status of the model architecture, training data, and current model performance.