Research Topics

Structure Formation of the Universe: Formation of Galaxies and Black Holes

Galaxy Formation from Cosmological Hydrodynamic Simulations

  We use theoretical models and cosmological hydrodynamic simulations to study how dark matter and dark energy affect the structure formation in the Universe from early times until the present. You can see some of our findings below from papers that were published in 2019-2020 using GADGET3-Osaka SPH code (Shimizu et al. 2019).

Observations of cosmic background radiation and quasars indicate that widely-distributed, intergalactic gas transitioned from a neutral state to an ionized state approximately one billion years (redshift z~6) after the Big Bang. This event is known as the “Epoch of Reionization,” and was primarily caused by ionizing photons from the first galaxies. Thus, understanding the nature and evolution of these first galaxies in one of the most important issues within cosmology. Recent observations have detected these galaxies at multiple wavelengths in the infrared (IR), which contain important dust and metal emission lines ([O III] 88 µm and [C II] 158 µm).

  It has become clear that the radiative properties of the first galaxies are diverse. We have clarified the origin of the observational diversity of the first galaxies and how it relates to galaxy evolution (Arata et al. 2019, 2020). Specifically, we investigated cosmological hydrodynamic simulations and multi-wavelength radiation transport calculations to investigate galaxy evolution and radiation properties at 6 < z < 15. As a result, the escape fraction of UV photons fluctuates between 20-80% at z < 10 due to changes in the distribution of dust and the intermittent star formation from supernovae. Additionally, we developed our own code to perform non-equilibrium calculations for [O III] and [C II] emission lines in order to calculate the emission line flux from the first galaxies. By stacking the emission from many simulated galaxies, we obtained the average [C II] emission profile. We find that the simulated galaxies have a centrally-concentrated component within 5 physical kpc, as well as an extended component beyond few tens of kpc (Fujimoto et al. 2019). This result is at odds with recent ALMA observations that suggest a more gentle slope, suggesting that there might be room for improvement with the current simulation feedback model.

  In addition to our study of the first galaxies, we also study the distribution of galaxies and baryons between 1 < z < 3 in order to understand how galaxies gain mass and grow. In recent years, a growing focus of research has been on observing the circumgalactic medium (CGM), which is gas that lies outside of a galaxy but still within the dark matter halo. With the Subaru Prime Focus Spectrograph (PFS) in mind, we investigated the distribution of neutral hydrogen (H I), galaxies, and metals in the universe by analyzing their cross-correlation functions (Momose et al. 2020a,b; Nagamine et al. 2020). We found that neutral hydrogen gas was correlated with galaxy mass and star formation rate, which is consistent with the normal galaxy bias. On the other hand, an interesting implication of the Lyman-𝛼 emitters’ CCF is that they tend to avoid the highest density regions. We also find the luminosity distribution of H𝛼 from the CGM after considering the effect of dust; similarly to the [C II] distribution described above, the H𝛼 distribution has a more gentle slope. In the future, we will examine the differences between our cosmological simulations and actual observations by focusing on outflow rate and mass-loading factor in order to improve our feedback model.

Direct Collapse Model of Supermassive Black Holes

  The direct-collapse model is a promising scenario for the formation of supermassive black holes in the early universe. We used cosmological hydrodynamic simulations to refine the direct-collapse model. We used the Enzo adaptive mesh refinement (AMR) code to create cosmological zoom-in simulations while taking into account the radiation from each fluid element (Luo, Nagamine, Shlosman 2016; Shlosman et al. 2016; Ardaneh et al. 2018; Luo et al. 2018). Typically, ultraviolet background radiation (UVB) due to nearby star formation, black holes, etc. inhibits the formation of molecular hydrogen (H2), but the exact structure of the UVB is not fully characterized. Therefore, we used three-dimensional radiative hydrodynamic simulation to examine the effects. As a result, we succeeded in finding a boundary region in the parameter space of the photodissociation rate of H2 and H- that switches from hydrogen atom cooling to H2 cooling in a more general form than in previous studies (Luo et al. 2020). In addition to typical continuum radiation such as UV and visible light, we will examine the effect of radiation pressure by the Lyα photons.

High-Energy Astrophysics

Black Hole Accretion Disks and Relativistic Jets

  In order to better understand supermassive black holes, it is necessary to study a variety of physical processes around these complex objects. We focus on the study of their magnetic fields, which are deeply related to the formation of relativistic jets as well as coronal heating above their accretion disks. Unfortunately, the magnetic field in the vicinity of a supermassive black hole has never been measured, so the details regarding magnetic fields remain shrouded in mystery. However, we have shown that the radio synchrotron emission from black hole coronae can be used to constrain the magnetic properties of black holes (Inoue & Doi 2014). Our recent observations with ALMA have confirmed the presence of coronal radio emission as we predicted, and we made the first ever measurement of a black hole magnetic field as about 10 Gauss (Inoue & Doi 2018). This important result forces us to reconsider the coronal heating mechanism and is a major achievement towards understanding jet formation. Additionally, we have also predicted coronal emission of both gamma-rays and neutrinos (Inoue et al. 2019; Inoue, Khangulyan, & Doi 2020).

Breaking New Ground with MeV Gamma-Ray Astronomy

  Astronomy has entered a new era of multi-wavelength & multi-messenger astronomy due to the recent advent of gravitational wave observations. However, there remains one energy range over which the window to the universe remains closed: MeV gamma rays. The MeV gamma ray band connects our understanding of the thermal and the non-thermal regimes of the universe and is necessary to probe various astronomical phenomena. However, because there is a lack of MeV gamma-ray observations, many important astronomical mysteries remain unsolved. We have been conducting various theoretical investigations for future MeV gamma-ray missions (Inoue et al. 2008, 2013, 2015, 2019). We are leading the collaboration between Japan and the United States on the GRAMS mission, a MeV gamma-ray and cosmic-ray detector that uses liquid argon TPC detectors that have also been used in projects for the detection of dark matter and neutrinos (Aramaki et al. 2019).

Stellar Physics: Solar Physics and Star Formation

Explosive Phenomena in Solar Atmosphere

  The sun exhibits many different energetic phenomena on several spatial and temporal scales. These phenomena are driven by the physical mechanism known as magnetic reconnection. A solar flare, a typical example of explosions, is caused by the release of magnetic energy stored in the solar atmosphere above a sunspot due to magnetic reconnection. We study the way this magnetic energy is stored and released via numerical simulations. Our research includes the formation process of sunspots that gives rise to large flares (Takasao et al. 2015b, Toriumi & Takasao 2017), the behavior of shock waves created by high-speed plasma flow driven by magnetic reconnection, and the observational characteristics of solar flares (Takasao et al. 2015a, Takasao & Shibata 2016).

Magnetohydrodynamic Simulations of Protostars Growing via Disk Accretion

  The sun, like all stars, began its life by the gravitational collapse of a molecular cloud. Newly-born stars, known as protostars or pre-main sequence stars, evolve by accretion from gaseous disks. In order to understand the final state of stars, it is necessary to understand the interaction between stars and their accretion disk. This interaction has been the missing link in star formation theory for many years because of the complex magnetohydrodynamics involved in the interaction between the stellar surface and the accretion disk. We try to solve this problem by using large-scale three-dimensional magnetohydrodynamic simulations based on our understanding of solar physics.

  The outflowing gas from the accretion disk, known as the disk wind, is expected to be important for extracting the mass and angular momentum of the gas within the disk. However, the accretion disks are turbulent, and the exact behavior of the disk wind is not well known. We have proposed a new method of accretion in which part of the disk wind falls onto the star and transitions into an accretion flow (Takasao et al. 2018).

  Protostars are known to produce supermassive flares that release 100 000 times more energy than solar flares. This is important because such huge flares can have profound effects on the surrounding environment of low-temperature star-forming regions. Although the origin of these flares is not fully understood, we succeeded in conducting the world’s first three-dimensional numerical simulation to reproduce protostar flares.

General-relativistic simulations of neutron stars

  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.

List of our referreed publications since 2013 (NASA/ADS).