The team led by Prof. Nagamine, conducts theoretical and numerical investigations of cosmic large-scale structure formation and galaxy evolution. The CROCODILE simulation, based on the GADGET3/4-OSAKA code, is a high-resolution cosmological simulation framework capable of modeling a wide range of baryonic processes in the Universe, including star formation, AGN feedback, cosmic reionization, dust evolution, and the Lyman-α forest. Using this platform, we pursue an integrated, multiscale, and multiphysics approach to galaxy formation and cosmic structure.

The Lyman-α forest refers to a series of absorption lines seen in the spectra of distant quasars, caused by intervening neutral hydrogen. It serves as a powerful probe of the large-scale structure of the Universe, enabling high-precision mapping of the baryon distribution across redshifts. Furthermore, the advent of IGM tomography, which reconstructs the 3D structure of the intergalactic medium, has opened new pathways to explore the baryon distribution around galaxies, filaments, and voids. This allows us to spatially assess the impact of stellar and AGN feedback on the IGM, offering tight physical constraints on feedback mechanisms.

On the observational side, the Subaru Strategic Program (SSP) with the new Prime Focus Spectrograph (PFS) has officially started in spring 2025, enabling simultaneous large-scale galaxy redshift surveys and IGM structure mapping.

Using this simulation, we have analyzed the cross-correlation between the Lyman-α forest and dark matter halos, revealing how different feedback models affect clustering statistics (Nakashima et al. 2025). We also demonstrated, for the first time, that feedback effects can cause a systematic shift in the BAO peak location, providing key theoretical insight into interpreting upcoming observational results (Sinigaglia et al. 2024).

CROCODILE also predicts the spatial distribution of metals, neutral hydrogen, and dust in the circumgalactic and intergalactic medium (CGM/IGM), as well as the surface brightness of Hα and [C II] emission lines. Comparisons with observations from ALMA and JWST are used to test and constrain feedback models (Oku & Nagamine 2024).

In addition, we are developing theoretical predictions for 21cm intensity mapping, quantifying the impact of astrophysical processes on bias estimation and preparing for data interpretation from future SKA surveys (Murakami et al. 2024, PRD; MNRAS).

At the galaxy scale, we are modeling dust evolution including grain formation, growth, and destruction processes, in order to understand spectral energy distributions and match infrared/submillimeter observations (Matsumoto et al. 2024; van der Giessen et al. 2024).

We also participate in the international AGORA project, conducting detailed comparisons of different simulation codes and physical models to assess their influence on galaxy mass assembly and satellite populations (Roca-Fàbrega et al. 2024; Jung et al. 2024).

We pursue comprehensive research combining theory and observations toward a unified understanding of supermassive black hole physics. In particular, we focus on elucidating the magnetic fields deeply involved in the formation mechanisms of hot plasma (corona) above accretion disks and relativistic jets. Although magnetic fields are believed to play essential roles in coronal heating and jet driving, their direct measurement has long been challenging.

To address this challenge, we theoretically demonstrated a method for measuring magnetic fields through millimeter wave radiation from coronae (Inoue & Doi 2014), and successfully detected this radiation component for the first time through ALMA observations. This has revealed that the magnetic field strength near black holes is approximately 10 Gauss (e.g., Inoue & Doi 2018; Michiyama et al. 2023). This achievement prompts a reconsideration of conventional coronal heating mechanisms and provides important implications for jet formation theories.

We are also advancing theoretical investigations into the possibility of coronae becoming sources of MeV gamma rays and high-energy neutrinos (e.g., Inoue et al. 2019; Inoue, Khangulyan, & Doi 2020; Inoue, Takasao, & Khangulyan 2024). Furthermore, we are researching non-thermal particle acceleration and multi-wavelength/multi-particle radiation in disk winds driven from accretion disks of active galactic nuclei, demonstrating that AGN disk winds can be novel sources of multi-messenger radiation (Yamada et al. 2024; Sakai et al. 2025). Recently, we have also proposed a new radiation mechanism in which gamma rays and neutrinos are generated through the photodisintegration of helium nuclei in the jet of the active galaxy NGC 1068 (Yasuda et al. 2025). This series of studies represents an important step toward an integrated understanding of the complex interactions of phenomena surrounding black holes.

Modern astronomy has entered an era of multi-wavelength and multi-messenger observations with the realization of gravitational wave detection, but there remains an energy band where an observational window to the universe has yet to be opened. This is the MeV gamma-ray band. This energy range bridges the thermal and non-thermal universe and is essential for the fundamental understanding of various phenomena including black holes, supernova remnants, active galaxies, and interstellar matter.

We have been conducting theoretical investigations of various science cases toward the realization of future MeV gamma-ray observation missions (e.g., Inoue et al. 2008; 2013; 2015; 2019). Currently, we are participating in the GRAMS mission being jointly promoted by Japan and the United States, playing a central role in the science planning for MeV gamma-ray and cosmic-ray observations using liquid argon TPC detectors (Aramaki et al. 2019).

As part of this effort, we are conducting theoretical research on nuclear reactions resulting from the interaction between galactic cosmic rays raining down on the lunar surface and lunar surface materials. By applying Geant4 Monte Carlo simulations to the Moon, we have precisely reproduced the gamma-ray spectrum of the Moon observed by Fermi-LAT and demonstrated the possibility of detecting nuclear gamma-ray lines from the Moon with future MeV missions (Fujiwara et al. 2025). These lines are key to deciphering the cosmic ray irradiation history over the past several million years and are expected to play an important role both as new observational targets for MeV gamma-ray astronomy and in evaluating the radiation environment for future lunar exploration.

When stars form in galaxies, heavy elements synthesized inside stars are released into the interstellar medium through stellar winds and supernova explosions. As a result, galaxies gradually become enriched in heavy elements over time. Because different elements are produced through different physical processes and released on different timescales, elemental abundance ratios in the interstellar gas provide important clues about the star formation history experienced by a galaxy.

To study such chemical evolution processes in galaxies, we have developed a galaxy evolution model that phenomenologically incorporates processes such as cosmological mass assembly, star formation, nucleosynthesis, and stellar feedback. By directly comparing model predictions with observations, we aim to identify the key physical mechanisms required to reproduce real galaxies. In previous work, we successfully reproduced the metallicity distribution of stars in the Milky Way and showed that our Galaxy likely experienced large-scale gas circulation during its formation (Toyouchi & Chiba 2016, 2018). In addition, by studying the chemical abundance ratios observed in distant active galactic nuclei, we suggested that the stellar initial mass function can differ significantly between typical galactic environments and galactic nuclear regions (Toyouchi et al. 2022).

More recently, we have focused on galaxies at redshifts z > 5, where JWST and ALMA have made significant observational advances. These distant galaxies have been reported to exhibit properties that differ significantly from those of nearby galaxies. Our theoretical model has demonstrated that the stellar masses, star formation rates, sizes, and metallicities of these high-redshift galaxies can be consistently explained within a unified framework (Toyouchi et al. 2025). At the same time, many open questions remain, including the abundance ratios of specific elements and the origin of large dust masses observed in early galaxies. Our research team continues to investigate these issues, addressing the fundamental question of the origin of heavy elements and cosmic dust from the perspective of theoretical studies of galaxy evolution.

To understand the origin of supermassive black holes, we study gas accretion processes through accretion disks. A proper description of this phenomenon requires accurately treating the thermodynamic interactions between radiation and gas. Using the radiation hydrodynamics code PLUTO, which we have independently extended and improved, we perform multidimensional radiation hydrodynamics simulations that connect the outer regions of accretion disks to the vicinity of a black hole’s event horizon. These simulations allow us to study the global structure of gas accretion flows.

Our studies show that high-energy radiation produced during accretion can drive large-scale outflows from the outer disk, which in turn suppresses gas accretion onto the black hole. However, when the outer disk becomes sufficiently optically thick, this suppression effect weakens, allowing super-Eddington black hole growth (Toyouchi et al. 2021, 2024).

More recently, we have also begun using another radiation hydrodynamics code Ramses-RT to investigate the continuous evolution from the formation of massive stars to the subsequent growth of the remnant black holes. These studies represent an important step toward a comprehensive understanding of the co-evolution of galaxies and black holes.

We mainly work on numerical simulations of compact stars (neutron stars), isolated and in binary systems, also with magnetic fields.

Binary neutron-star systems are particularly interesting, because they are prime sources of detectable gravitational waves. Through gravitational waves, we can investigate the equation of state of ultra-high density matter, and also obtain indications on the internal engine of short gamma-ray bursts, whose engine is indeed hypothesized to be connected to binary neutron-star mergers in which a significant accretion disk remains around the merged compact object (black hole).

Recently, our research focuses on finding observational signatures (in gravitational waves) of the equation of state of matter at the highest densities, and in particular signatures related to how the transition from hadronic matter to free-quark matter occurs.

We are part of the KAGRA collaboration for the construction and operation of the Japanese underground cryogenic interferometric detector of gravitational waves, working in conjunction with the LIGO and Virgo Collaborations.