"How did our universe begin, and how did it develop into what we see today?" This has been a fundamental question since the dawn of human history. The field of cosmology and astrophysics studies these questions from a physics perspective. The theory proposed by George Gamow in 1948 became a paradigm for discussing the origins of the universe and has been supported by various observations from the 20th and 21st centuries. It suggests that our universe was born in an extreamly hot and dense state (the Big Bang) 13.8 billion years ago and has since expanded to its current state.

The Big Bang theory predicts that in the early universe there was only hydrogen, helium and a small amount of lithium, with no heavier elements. However, about 2% of our solar system consists of heavier elements, and it is well known that our bodies are made up of elements like carbon, calcium, iron, phosphorus, and chlorine. Recent astronomical data has confirmed that heavy elements already existed in the observed early universe. So, when, where, and how were these elements created, and how did they spread throughout the universe? Furthermore, when and where did life, like ours, emerge during the evolution of the universe?

After the Big Bang, dark matter, which constitutes the bulk of the Universe, is thought to have spread throughout the Universe. In astrophysics, dark matter is essential for explaining the formation of t-galaxies and the large-scale structure of the Universe, but it is an unknown material that has not yet been detected by particle experiments. In the early universe, the self-gravity of dark matter increased the contrast in its spatial distribution, and baryons also accumulated in high-density regions of dark matter, where stars were born and star clusters and galaxies were formed. As stars, supernovae, etc. synthesised new elements and ejected them into interstellar space, various elements began to be distributed in the Universe.

The galaxies formed in this way continued to evolve through mergers and interactions with other galaxies. For example, large galaxies like the Milky Way and Andromeda are thought to have formed through repeated collisions and mergers of smaller galaxies over billions of years. During this cosmic history, the solar system formed in the Milky Way about 4 billion years ago, and within it, humanity emerged. In several billion years, the Milky Way and Andromeda are expected to collide and merge into a massive elliptical galaxy. Galaxy collisions and mergers not only severely stir up the motion of stars and gas and cause star formation, but also affect the activity of supermassive black holes at the centre of galaxies.

Our goal in developing the Standard Model of Galaxy Formation and Evolution is to unravel not only the mysteries of galaxy formation and evolution but also the creation and distribution of elements in the universe, the activity of black holes, and the nature of dark matter. The fundamental aim of our research group is to scientifically detail how galaxies and the universe have evolved over time and to consider what the future holds. Below, you will find an overview of our research and the activities of our research group.

The Dark Matter Paradox

The current standard theory of structure formation, the Cold Dark Matter (CDM) model, successfully explains the statistical properties of the large-scale structure of the universe. However, it faces critical challenges on the galactic scale, such as the "Cusp-Core Problem" and the "Missing Satellite Problem." These issues are closely related to the formation and evolution of galaxies and are crucial for understanding the nature of dark matter.

Cusp-Core Problem:
So far, studies on the mass distribution of dark matter halos suggest that these halos universally exhibit a cusp-like structure, where the mass density sharply increases towards the centre. However, precise observations of the rotation curves of nearby dwarf galaxies often show that the central mass density distribution of dark matter halos is almost constant (core-like) or smoother than the theoretical predictions. This discrepancy between theory and observation is widely known as the "Cusp-Core Problem." Additionally, the absence of massive satellite galaxies with highly concentrated dark matter halos, known as the "Too-Big-to-Fail Problem," is also recognised as a related issue. Although galaxy formation simulations have phenomenologically shown that feedback from supernovae can alter the gravitational field and transition dark matter halos from a cusp to a core structure, the fundamental physical processes governing this transition remain poorly understood in a viewpoint of the fundermental physics.

Missing Satellite Problem:
According to cosmological N-body simulations based on hierarchical structure formation theory, galaxies with masses similar to the Milky Way or Andromeda should theoretically have hundreds to thousands of subhalos. However, only about 50 satellite galaxies have been observationally identified in these galaxies. This discrepancy of more than an order of magnitude in the number of satellite galaxies is known as the "Missing Satellite Problem" and is one of the major challenges of the CDM model. One proposed solution is the existence of dark satellites—dark matter halos that failed to form stars and thus do not appear as observable galaxies. However, research on whether these dark satellites exist and how to detect them is still in progress.

These issues with dark matter halos are crucial for understanding the nature of dark matter itself. They are interconnected through astrophysical phenomena such as star formation processes and supernova feedback within the gravitational fields of dark matter halos. Therefore, it is essential to study the co-evolution of galaxies and dark matter halos in detail. Using our comprehensive models of the optical, chemical, and dynamical evolution of galaxies, we are tackling the challenges posed by the CDM model, including the "Cusp-Core Problem" and the "Missing Satellite Problem."

Universal Scaling Laws of Dark Matter Halos

Dark matter halos surrounding galaxies exhibit strong correlations among various observational parameters, known as the "scaling relations of dark matter halos." However, the origins of these correlations remain unclear, requiring extensive exploration to fully understand them. We utilized the correlation between the concentration and mass of dark matter halos (c-M relation) derived from cosmological N-body simulations based on the cold dark matter model to theoretical scaling relations between other physical quantities such as surface mass density, maximum rotation velocity, and scale radius. By comparing these theoretical scaling relations with observed ones across various mass scales, we found that the scaling relations observed in less-massive galaxies and massive galaxies originate from the c-M relation of dark matter halos. Furthermore, our theoretical scaling relations predict their validity even in galaxy clusters.

Additionally, we propose a new theoretical scaling relation incorporating the "cusp-to-core transition" believed to occur in cold dark matter halos. A cusp refers to a state where the halo's central region is very dense and divergent, whereas a core refers to a more flattened and spread-out central region. This transition is thought to be triggered by feedback mechanisms such as supernova explosions, and other baryonic processes. These processes cause the dark matter distribution to transition from a cusp to a core in the central region. We examine the possibility of observationally verifying this cusp-to-core transition process in dark matter halos. Recent studies have devised new methods for more detailed observation of this transition and emphasized the importance of improving the accuracy of observational data and simulation techniques. This will allow for more precise comparisons between theoretical models and observational results, leading to new insights into the nature of dark matter and galaxy formation mechanisms.

Collision Frequency of Dark Matter Subhalos and Induced Formation of Dwarf Galaxies

According to the current standard galaxy formation model, hierarchical structure formation by cold dark matter, galaxies are known to contain more than double their stellar mass in dark matter. However, in this century, there have been a non-negligible number of reports of dark matter-deficient galaxies with significantly less halo mass than theoretically predicted. To address this severe issue in cold dark matter cosmology, we investigated the physical processes of galaxy formation induced by head-on collisions of dark matter subhalos containing primordial gas.

By analytically estimating the collision frequency of dark matter subhalos associated with massive galaxy host halos, we found that within 10% of the host halo's virial radius, collisions can occur frequently with a short collision timescale of about 10 million years. We also succeeded in demonstrating that relative velocity increases with radius. Using analytical models and numerical simulations to investigate these collisions, we showed that the formation paths of normal galaxies rich in dark matter and dark matter-deficient galaxies depend on the collision velocity. When the relative velocity is low, two dark matter subhalos merge, resulting in the formation of a dark matter-rich galaxy. In contrast, when the relative velocity is moderate, the subhalos pass through each other, but the gas components collide, causing a rapid increase in gas density at the collision surface, leading to explosive star formation. This results in the formation of a galaxy with little dark matter at the collision surface, which we believe is a crucial process in the formation of dark matter-deficient galaxies. If the relative velocity is sufficiently large, shock waves generated at the collision surface reach the gas surface, causing a shock breakout that disperses most of the gas without binding it to the system, preventing galaxy formation.

Birth of Galaxies and Formation of Heavy Elements

After the Big Bang, the first stars formed from primordial gas, consisting only of hydrogen and helium. Among these stars, the relatively massive ones would end their lives in supernova explosions. During these events, elements created within the stars and by the supernovae were ejected into interstellar space, mixing with the interstellar medium. Over time, new stars formed from this metal-enriched gas, now containing trace amounts of heavy elements. When these stars eventually underwent supernova explosions, they released newly created heavy elements back into the interstellar medium. This cosmic recycling process repeated several times, gradually increasing the abundance of elements in the galaxies.

Cosmoic recycling (Credit: Masao Mori).

The figure below shows the results of a galaxy formation simulation given by Mori & Umemura (2006). After about 100 million years (leftmost panel), stars began forming within the primordial galaxy. Massive stars ended their lives in supernova explosions, significantly stirring the gas within the dwarf galaxy and creating numerous bubble-like structures. The heavy elements released by the supernovae accumulated in the low-density interiors of these bubble structures, while the surrounding high-density gas shells contained fewer heavy elements. This is because these shells were formed by sweeping up primordial gas that originally lacked heavy elements. In the very early stages of galaxy evolution, supernovae had not yet occurred frequently enough to uniformly pollute the entire interstellar medium, leading to varying degrees of chemical evolution within the gas.

Galaxy Evolution through Galaxy Collisions

The current standard model of galaxy formation, hierarchical structure formation, suggests that small astonomical objects formed in the early universe, which then grew into larger structures through repeated mergers. Galaxy formation is particularly driven by the mergers and interactions of galaxies, with collisions between dwarf galaxies and larger galaxies playing a crucial role. Large galaxies like the Milky Way and its sister galaxy, Andromeda, are thought to have evolved by interacting with smaller galaxies over billions of years. Galaxy mergers not only disturb the motions of stars, triggering new star formation, but also promote the evolution of supermassive black holes at the centres of galaxies, leading to intense radiation and jet formations. These galactic collision phenomena are key to understanding galaxy evolution and the co-evolution of central black holes. Advances in observational technology have allowed us to observe distant galaxies and have significantly impacted our understanding of the Milky Way and nearby galaxies. Observations of extremely faint stars, previously impossible, have revealed the true nature of galaxies.

Large-scale structures around the Andromeda Galaxy revealed by the Pan-Andromeda Archaeological Survey. The elongated stellar structure in the centre, known as the Andromeda Giant Stream, is estimated to be about 450,000 light-years long and have a mass exceeding 10 million solar masses. (Modified from McConnachie et al., Nature, 461, 66 (2009))

The above image shows the results of observations by McConnachie et al., revealing faint objects around the Andromeda Galaxy. The complex structures and elongated streams surrounding Andromeda consist of stellar groups with very low density. These are significant discoveries in observational astronomy, revealing that previously believed galaxy structures are just the tip of the iceberg. The sheer scale of these structures is illustrated by the inclusion of the Moon for comparison. So, how did these large-scale structures form?

We have utilised large-scale simulations with supercomputers to tackle this problem, revealing for the first time that a small galaxy, with about 1/400 the mass of the Andromeda Galaxy, was captured by Andromeda's strong gravity and torn apart approximately 1 billion years ago. The remnants of this galaxy formed the Andromeda Giant Stream, stretching over 400,000 light-years, and created overlapping shell structures of stars. This shows the significant impact of galaxy collisions on galaxy evolution. (See the figure below.) This discovery clarifies that galactic collisions predicted by hierarchical structure formation still occur in massive, mature galaxies like Andromeda. Precision observations of Andromeda's structures are a central project for future observations using the world's most advanced instruments, with theoretical groups, including ours, contributing to these efforts.

(Top) Simulation of a collision between the Andromeda Galaxy and a dwarf galaxy. The elongated blue structure represents the Andromeda Galaxy. (Mori & Rich, ApJ, 674, L77, 2008) (Bottom) Simulation of a collision involving a dwarf disc galaxy. The white represents the Andromeda Galaxy, while the yellow-red dots show the dwarf galaxy, which disintegrates due to the collision. (Modified from Kirihara, Miki, Mori, Kawaguchi & Rich, Monthly Notices of the Royal Astronomical Society, 469, 3390 (2017))

Dormant Central Black Holes Induced by Galaxy Collisions

When sufficient gas falls into a supermassive black hole at a galaxy's centre, it releases potential energy, causing the black hole to shine brightly as an active galactic nucleus. Gas supply to these black holes is hindered by angular momentum, forming a torus that acts as a reservoir. While galaxy collisions are believed to trigger the activity of black holes by feeding them gas, there is no established theory as to how this activity is halted and it is still considered an open question. Supermassive black holes shine for only about 100 million years out of the age of the universe, which is 13.8 billion-year, suggesting that most central black holes, including those in the Milky Way and Andromeda, are in a dormant state due to a lack of gas. Recently, many galaxies have been found to show signs of rapid fading of black hole activity, highlighting the need to identify the mechanisms behind this.

We consider the Andromeda Galaxy an optimal testbed to investigate the relationship between galaxy collisions and central black hole activity. We hypothesised that removing the gas supply to the central black hole via a galaxy collision would eventually cause the black hole to run out of gas and cease activity. Using 3D numerical hydrodynamic simulations and 1D analytical models, we tested this hypothesis with supercomputers at the Information Technology Center at the Universiyt of Tokyo and the Center for Computational Sciences at the University of Tsukuba. The above figure shows that if the column density of the gas in the infalling satellite galaxy is higher than that of the torus gas around the black hole, the gas is stripped away, nearly entirely removing the gas. This result led us to propose, for the first time, the physical process whereby galaxy collisions can suppress central black hole activity.

We further investigated the possibility of extending this hydrodynamic process to the fading of black hole activity in other galaxies beyond Andromeda. We found that the column densities of torus gas around many central black holes are within the range that can be stripped by galaxy collisions. This indicates that galaxy collisions could cease the activity of many central black holes. To estimate the frequency of such collisions, we performed precise orbit calculations of satellite galaxies based on data from the Gaia mission. We found that significant collisions occur approximately once every 100 million years, matching the period during which supermassive black holes are known to shine brightly. This finding is a significant step towards understanding the relationship between galaxy collisions and black hole activity.

Below is a list of recent publications, selected publications, and details of research funding obtained.

Recent publications

• "The structure of the stellar halo of the Andromeda galaxy explored with the NB515 for Subaru/HSC. I.: New Insights on the stellar halo up to 120 kpc", Ogami, Itsuki, Tanaka, Mikito, Komiyama, Yutaka, Chiba, Masashi, Guhathakurta, Puragra, Kirby, Evan N., Wyse, Rosemary F. G., Filion, Carrie, Gilbert, Karoline M., Escala, Ivanna, Mori, Masao, et al., arXiv:2401.00668


• "Novel hydrodynamic schemes capturing shocks and contact discontinuities and comparison study with existing methods", Yuasa, Takuhiro, Mori, Masao, New Astronomy,109,id.102208 (2024)


• "Frequency of the dark matter subhalo collisions and bifurcation sequence arising formation of dwarf galaxies", Otaki, Koki, Mori, Masao, Monthly Notices of the Royal Astronomical Society,525,2535-2552 (2023)


• "Transonic galactic wind model including stellar feedbacks and application to outflows in high/low-z galaxies", Igarashi, Asuka, Mori, Masao, Nitta, Shin-ya, Publications of the Astronomical Society of Japan, 75, 1214-1245 (2023)


• "Study of Galaxy Collisions and Thermodynamic Evolution of Gas Using the Exact Integration Scheme", Otaki, Koki, Mori, Masao, Lecture Notes in Computer Science,13378,373-387 (2022)


• "Destruction of the central black hole gas reservoir through head-on galaxy collisions", Miki, Yohei, Mori, Masao, Kawaguchi, Toshihiro, Nature Astronomy, 5, 478-484 (2021)


• "Discrimination of heavy elements originating from Pop III stars in z = 3 intergalactic medium", Kirihara, Takanobu, Hasegawa, Kenji, Umemura, Masayuki, Mori, Masao, Ishiyama, Tomoaki, Monthly Notices of the Royal Astronomical Society, 491,4387-4395 (2020)


• "Stellar Stream and Halo Structure in the Andromeda Galaxy from a Subaru/Hyper Suprime-Cam Survey", Komiyama, Yutaka, Chiba, Masashi, Tanaka, Mikito, Tanaka, Masayuki, Kirihara, Takanobu, Miki, Yohei, Mori, Masao, et al., Astrophysical Journal, 853, 29 (2018)


• "Systematic Identification of LAEs for Visible Exploration and Reionization Research Using Subaru HSC (SILVERRUSH). I. Program strategy and clustering properties of ~2000 Lya emitters at z = 6-7 over the 0.3-0.5 Gpc2 survey area", Ouchi, Masami, Harikane, Yuichi, Shibuya, Takatoshi, Shimasaku, Kazuhiro, Taniguchi, Yoshiaki, Konno, Akira, Kobayashi, Masakazu, Kajisawa, Masaru, Nagao, Tohru, Ono, Yoshiaki, Inoue, Akio K., Umemura, Masayuki, Mori, Masao, et al., Publications of the Astronomical Society of Japan, 70, 13O (2018)


• "The nature of the progenitor of the M31 north-western stream: globular clusters as milestones of its orbit", Kirihara, Takanobu, Miki, Yohei, Mori, Masao, Monthly Notices of the Royal Astronomical Society, 469,3390 (2017)


• "Formation of the Andromeda giant stream: asymmetric structure and disc progenitor", Kirihara, Takanobu, Miki, Yohei, Mori, Masao, Kawaguchi, Toshihiro, Rich, R. Michael, Monthly Notices of the Royal Astronomical Society, 464, 35 (2017)


• "ALMA Reveals Strong [C II] Emission in a Galaxy Embedded in a Giant Lya Blob at z = 3.1", Umehata, Hideki, Matsuda, Yuichi, Tamura, Yoichi, Kohno, Kotaro, Smail, Ian, Ivison, R. J., Steidel, Charles C., Chapman, Scott C., Geach, James E., Hayes, Matthew, Nagao, Tohru, Ao, Yiping, Kawabe, Ryohei, Yun, Min S., Hatsukade, Bunyo, Kubo, Mariko, Kato, Yuta, Saito, Tomoki, Ikarashi, Soh, Nakanishi, Kouichiro, Lee, Minju, Izumi, Takuma, Mori, Masao, Ouchi, Masami, Astrophysical Journal, 834, L16 (2017)


• "Universal Dark Halo Scaling Relation for the Dwarf Spheroidal Satellites", Hayashi, Kohei, Ishiyama, Tomoaki, Ogiya, Go, Chiba, Masashi, Inoue, Shigeki, Mori, Masao, Astrophysical Journal, 843, 97 (2017)


• "Polytropic transonic galactic outflows in a dark matter halo with a central black hole", Igarashi, Asuka, Mori, Masao, Nitta, Shin-ya, Monthly Notices of the Royal Astronomical Society, 470, 2225 (2017)

Selected publications

• "The connection between the cusp-to-core transformation and observational universalities of DM haloes", Ogiya, Go, Mori, Masao, Ishiyama, Tomoaki, Burkert, Andreas., Astrophysical Journal, 440, L7 (2014)


• "The Core-Cusp Problem in Cold Dark Matter Halos and Supernova Feedback: Effects of Oscillation", Ogiya, Go, Mori, Masao, Astrophysical Journal, 793,46 (2014)


• "The Once and Future Andromeda Stream", Mori, Masao, Rich, R. Michael, Astrophysical Journal, 674, L77 (2008)


• "The evolution of galaxies from primeval irregulars to present-day ellipticals", Mori, Masao, Umemura, Masayuki, Nature, 440, 644 (2006)


• "Early Metal Enrichment by Pregalactic Outflows. II. Three-dimensional Simulations of Blow-Away", Mori, Masao, Ferrara, Andrea, Madau, Piero, Astrophysical Journal, 571, 40 (2002)


• "Gas Stripping of Dwarf Galaxies in Clusters of Galaxies", Mori, Masao, Burkert, Andreas, Astrophysical Journal, 538, 559 (2000)


• "Dissipative Process as a Mechanism of Differentiating Internal Structures between Dwarf and Normal Elliptical Galaxies in a Cold Dark Matter Universe", Mori, Masao, Yoshii, Yuzuru, Tsujimoto, Ken'ichi, Astrophysical Journal, 511, 585 (1999)


• "The Evolution of Dwarf Galaxies with Star Formation in an Outward-propagating Supershell", Mori, Masao, Yoshii, Yuzuru, Tsujimoto, Takuji, Nomoto, Ken'ichi, Astrophysical Journal, 478, L21 (1997)

Research Grants

• 2024-2027: "Hunting Dark Satellites Problem: Elucidating the Dark Matter Paradox" - Principal Investigator, JSPS Grant-in-Aid for Scientific Research (C)


• 2024-2026: "Understanding Galactic Dark Matter through Stellar System Dynamics Theory and Its Applications" - Co-Investigator, JSPS Grant-in-Aid for Scientific Research (B) (Principal Investigator: Masashi Chiba)


• 2020-2023: "Do Dark Satellites Exist? Resolving Issues in the Cold Dark Matter Model" - Principal Investigator, JSPS Grant-in-Aid for Scientific Research (C)


• 2015-2019: "Exploring Cosmological Scale Matter Recycling with Subaru HSC and SDSS" - Co-Investigator, JSPS Grant-in-Aid for Scientific Research (A) (Principal Investigator: Masami Ouchi)


• 2013-2018: "Constructing the Galactic Embryology through Radiation Hydrodynamics Simulations" - Principal Investigator, JSPS Grant-in-Aid for Scientific Research (C)


• 2011-2014: "Challenging Cosmic Reionisation with Next-Generation Large-Scale Surveys and Simulations" - Co-Investigator, JSPS Grant-in-Aid for Scientific Research (A) (Principal Investigator: Masami Ouchi)


• 2009-2012: "Exploring Origin of Galaxies through the Fusion of Theory and Observations" - Principal Investigator, JSPS Grant-in-Aid for Scientific Research (A)


• 2008-2012: "Understanding the Cosmic Dark Ages from First Generation Objects to Proto-Galaxies" - Co-Investigator, JSPS Grant-in-Aid for Scientific Research (S) (Principal Investigator: Masayuki Umemura)


• 2006-2008: "Exploring Galactic Genesis with High-Precision Hybrid Simulations" - Principal Investigator, JSPS Grant-in-Aid for Scientific Research (C)


• 2004-2007: "Understanding the Origins of the First Generation Objects in the Universe through Hybrid Parallel Computing" - Co-Investigator, Specially Promoted Research (Principal Investigator: Masayuki Umemura)


• 2002-2004: "Broadband Modelling of Galaxy Evolution through Hybrid Simulations" - Principal Investigator, JSPS Grant-in-Aid for Young Scientists (B)


• 2003-2004: "Exploring Cosmic Recycling Processes with Parallel Computing" - Principal Investigator, Academic Promotion Fund by Japan Private School Promotion and Mutual Aid Corporation


• 2003-2003: "Formation of the First Generation of Galaxies: Strategies for Observational Validation of Physical Scenarios" - Japan-Italy Seminar by the Japan Society for the Promotion of Science (Principal Investigator: Ryoichi Nishi)


• 2002-2002: "Formation and Cosmological Significance of the First Generation of Galaxies" - Japan-Italy Seminar by the Japan Society for the Promotion of Science (Principal Investigator: Masayuki Umemura)


• 2001-2001: Research Grant by the Astronomical Promotion Foundation - Principal Investigator


• 1997-1999: Research Fellowship for Young Scientists by the Japan Society for the Promotion of Science - Principal Investigator