GPPU Seminar

Date

15:00—17:00, January 13, 2026

Place

(hybrid) Room 745, Science Complex B (H-03), Zoom registration map

Abstract

1) The GroundBIRD telescope is a ground-based Cosmic Microwave Background (CMB) experiment at the Teide Observatory in Tenerife, Spain. It is designed to observe the intensity and polarization anisotropy of the CMB at large angular scales (ℓ > 6, up to ℓ ≈ 300), with the primary scientific objective of precisely measuring the optical depth (τ) to the Universe’s reionization epoch. Combined with QUIJOTE maps, GroundBIRD aims to achieve a precision of σ(τ) ~ 0.01, improving upon constraints set by the Planck satellite and providing a crucial cross-check on previous measurements. To mitigate atmospheric 1/f fluctuations, the telescope rotates continuously at a speed of up to 20 rotations per minute, maintaining an elevation of 70 degrees. This unique scanning strategy allows it to observe approximately 40% of the sky from the Northern Hemisphere in a single day. GroundBIRD employs NbTiN/Al hybrid microwave kinetic inductance detectors (MKIDs) at two frequency bands centered at 145 GHz and 220 GHz. The 145 GHz channel is tuned to the peak of the CMB spectrum, while the 220 GHz channel helps remove contamination from Galactic dust emission. Through detailed analysis of Moon maps, we identified ghost residuals at approximately -25 dB relative to the Moon's peak intensity, potentially impacting the telescope's mapping sensitivity. Using a combination of geometric optics simulations and quasi-optical analysis, we identified these ghost images were caused by stray light. Based on GRASP simulations, we optimized the design of the baffle to mitigate the stray light while keeping the beam pattern of the main beam and thermal loading emitted by the black body material pasted on the inside of the baffle. The simulation suggested that reducing the effective aperture angle from 27 deg to 18 deg significantly suppresses the ghost signal while causing only minor degradation in detector performance. The trade-off between the improved stray light suppression and the small increase in thermal noise is considered acceptable. In this work, we show the baffle design to mitigate the ghost signals based on the optical simulations and results of on-sky measurements with the new baffle.

2) Understanding how large-scale environments influence galaxy evolution remains a central challenge. We combine Euclid Q1 data with Subaru/MOIRCS narrow-band (NB) Hα imaging to perform a panoramic study of galaxy morphology in a spectroscopically confirmed z = 1.5 cluster. The NB data provide a robust census of star-forming members, while Euclid enables uniform classification of quiescent galaxies (QGs) and Sérsic-based structural measurements out to >2 Mpc with wide-field near-IR imaging. We find: (1) The core hosts a high fraction of disky QGs, suggesting that many quenched systems in the cluster core are not yet fully morphologically transformed; and (2) core QG sizes are larger than those in the outskirts, pointing to core-specific accelerated size growth or distinct evolutionary pathways. These results demonstrate Euclid’s ability to probe environment-driven galaxy transformation, and highlight the potential of future Euclid releases to extend such analyses to large cluster samples.

3) Heavily obscured active galactic nuclei (AGN) are an important galaxy population believed to represent the early rapid growth phases of the central supermassive black holes (SMBHs) in the early Universe. To uncover such hidden systems, which are missed in previous surveys, we conduct a mid-infrared (MIR) search using Spitzer/MIPS 24 um imaging in the COSMOS field. We identify 253 MIR-luminous, rapidly accreting SMBH candidates in the early Universe. Their inferred contribution to the cosmic accretion history is very large, suggesting that dust-obscured growth is a key mode of SMBH assembly at early epochs. In this talk, I will introduce the basic concepts of our study, summarize the main results, and then outline ongoing related work that utilizes high-resolution follow-up observations to place these sources in the context of SMBH-host galaxy co-evolution.

4) The "Muon Puzzle"—a phenomenon where the observed number of muons in extensive air showers significantly exceeds the predictions of hadronic interaction models—remains a challenge in cosmic-ray physics. This discrepancy suggests a need for a more complete understanding of forward hadronic interactions at high energy scales. This study employs two large underground detectors, KamLAND and Super-Kamiokande, which are located 1,000 meters underground in Kamioka, Japan and just ~200 m apart. By performing a coincidence search between these two detectors, it becomes possible to selectively observe high-energy muon fluxes while utilizing the shielding effect of the surrounding rock. This setup also enables the study of hadronic interaction models in a unique phase space by detecting spatially separated muons that are invisible to single underground detectors.

Point

GSP 1