"How did our universe begin, and how did it evolve into what we see today?" This has been one of humanity's most profound questions since ancient times. Cosmology and astrophysics explore these mysteries through the lens of physics. The theory proposed by George Gamow in 1948 laid the foundation for modern cosmology and has since been supported by a wealth of observational evidence from the 20th and 21st centuries. According to this theory, the universe began in an extremely hot and dense state—the Big Bang—approximately 13.8 billion years ago and has been expanding ever since.
The Big Bang theory predicts that the early universe consisted almost entirely of hydrogen, helium, and a trace of lithium, with no heavier elements. Yet, about 2% of the solar system's mass comprises heavier elements, and our bodies are made of elements like carbon, calcium, iron, phosphorus, and chlorine. Recent observations have revealed the presence of heavy elements even in the early universe. This raises key questions: When, where, and how were these elements formed and dispersed? Furthermore, how and when did life emerge in this evolving universe?
Following the Big Bang, dark matter—believed to constitute most of the universe's mass—permeated the cosmos. While essential for explaining the formation of galaxies and the universe's large-scale structure, dark matter remains undetected in particle physics experiments. In the early universe, gravitational instabilities amplified the density fluctuations in dark matter. Baryonic matter followed, accumulating in these dark matter potential wells, where stars and galaxies began to form. As stars evolved and exploded as supernovae, they synthesised new elements and distributed them into the interstellar medium, enriching the universe.
Galaxies continued to evolve through mergers and interactions. For instance, massive galaxies like the Milky Way and Andromeda formed through successive mergers of smaller galaxies over billions of years. The solar system emerged in the Milky Way approximately 4 billion years ago, ultimately giving rise to life on Earth. In a few billion years, the Milky Way and Andromeda are predicted to merge into a giant elliptical galaxy. Such collisions not only trigger star formation by violently stirring interstellar matter but also influence the activity of supermassive black holes at galactic centers.
Our aim in developing the Standard Model of Galaxy Formation and Evolution is to unravel the processes behind galaxy assembly, the creation and distribution of elements, black hole dynamics, and the nature of dark matter. Our research group is dedicated to scientifically tracing the evolution of galaxies and the universe while contemplating its future.
Below are highlights of our research themes and activities, including a study featured in Nature on the connection between galaxy evolution and Lyman-alpha emitters, another from Nature Astronomy examining black hole activity induced by galactic collisions, and continuing efforts to understand the Andromeda galaxy — a topic that has gradually become a central focus of our group.
Cusp-Core Problem:
Studies of dark matter halo mass distributions suggest a universal cusp-like structure, where
density rises steeply toward the centre. However, precise measurements of rotation curves in
nearby dwarf galaxies often reveal a flatter, nearly constant density distribution at the
centre—a core-like structure that contradicts theoretical predictions. This mismatch between
simulations and observations is known as the "Cusp-Core Problem." A related issue, the
"Too-Big-to-Fail Problem," refers to the puzzling absence of massive satellite galaxies with
dense dark matter cores, as predicted by simulations. While galaxy formation simulations have
phenomenologically demonstrated that feedback from supernovae can flatten central density
profiles by modifying the gravitational potential, the fundamental physical processes underlying
this cusp-to-core transition remain poorly understood.
Missing Satellite Problem:
Cosmological N-body simulations based on hierarchical structure formation theory predict that
galaxies like the Milky Way and Andromeda should host hundreds or even thousands of dark matter
subhalos. Yet only about 50 satellite galaxies have been observed in each system. This large
discrepancy, known as the "Missing Satellite Problem," poses a significant challenge to the cold
dark matter (CDM) model. One proposed resolution is that many of these subhalos are dark
satellites—halos that did not form stars and thus remain undetectable by conventional means.
However, the existence of such objects and effective methods to identify them remain open areas
of investigation.
These problems are essential to understanding the fundamental nature of dark matter. They are closely linked to astrophysical processes such as star formation and supernova feedback within dark matter-dominated environments. To address these challenges, our research focuses on the detailed co-evolution of galaxies and dark matter halos. Using comprehensive models that incorporate optical, chemical, and dynamical evolution, we aim to confront the limitations of the CDM model, particularly in relation to the Cusp-Core and Missing Satellite Problems.
Dark matter halos surrounding galaxies exhibit well-defined correlations among various observable quantities, commonly referred to as "scaling relations." Yet, the physical origins of these relations remain elusive and demand further theoretical and observational investigation. In our study, we used the correlation between halo concentration and mass (the c–M relation), derived from cosmological N-body simulations under the Cold Dark Matter (CDM) paradigm, to establish theoretical scaling relations involving surface mass density, maximum rotation velocity, and scale radius. By comparing these predictions with observational data across a wide mass range, we demonstrated that the scaling relations observed in both low-mass and high-mass galaxies can be traced back to the c–M relation. Our models further predict that these relations extend to the scale of galaxy clusters.
We also proposed a new scaling relation that incorporates the so-called "cusp-to-core transition" expected to occur in CDM halos. In this context, a cusp refers to a steep density profile near the halo center, while a core denotes a flattened distribution. This transformation is believed to be triggered by baryonic processes such as supernova feedback, which redistribute dark matter in the inner halo regions. We explore the feasibility of observationally validating this transition. Recent advances in observation and simulation techniques offer promising avenues for testing the cusp-core transformation and enhancing our understanding of dark matter and galaxy formation.
In this presentation at the KASHIWA DARK MATTER SYMPOSIUM 2021, held at the University of Tokyo from November 29 to December 2, 2021, first-year master's student Yuka Kaneda (as of December 2021) presented her work on the dynamical evolution of dark matter halos. Her research was recognized with the Best Poster Award, selected by participant vote.
The standard model of galaxy formation, based on hierarchical structure formation in a CDM universe, predicts that galaxies are embedded in dark matter halos with more than twice the mass of their stellar components. However, recent discoveries have reported the existence of dark matter-deficient galaxies whose halo masses are significantly lower than predicted. To address this serious challenge, we investigated a new galaxy formation scenario driven by head-on collisions between dark matter subhalos containing primordial gas.
Our analytic estimates revealed that within 10% of a host halo’s virial radius, such subhalo collisions occur frequently, with characteristic timescales on the order of 10 million years. We also found that the relative velocity of subhalos increases with distance from the halo center. Using a combination of analytical modeling and N-body/hydrodynamic simulations, we demonstrated that the outcome of a collision—whether it results in a dark matter-rich or -deficient galaxy—depends strongly on the relative velocity. At low velocities, merging subhalos form a galaxy rich in dark matter. At moderate velocities, the gas components collide even as the dark matter passes through, leading to local gas compression and intense star formation. This results in the formation of a dark matter-deficient galaxy at the collision interface. At high velocities, however, the shockwave breaks out from the gas surface and disperses the gas before it can collapse, preventing galaxy formation altogether.
Adapted from Otaki & Mori, Monthly Notices of the Royal Astronomical Society, 25, 2535 (2023)
Following the Big Bang, the first stars emerged from primordial gas composed solely of hydrogen and helium. Among them, relatively massive stars concluded their lifespans as supernovae. These explosive events expelled elements produced within the stars and during the explosions into interstellar space, mixing with the surrounding medium. Subsequent generations of stars formed from this enriched gas, which now contained trace amounts of heavy elements. When these stars, in turn, ended as supernovae, they released newly synthesized elements into the interstellar medium. Through this cosmic recycling process, the abundance of heavy elements in galaxies gradually increased over time.
The figure below shows the results of a galaxy formation simulation by Mori & Umemura (2006). After approximately 100 million years (leftmost panel), star formation began within the primordial galaxy. Massive stars underwent supernova explosions, stirring the surrounding gas and producing numerous bubble-like structures. Heavy elements from the explosions accumulated in the low-density interiors of these bubbles, while the high-density shells surrounding them remained relatively unenriched, as they were composed of swept-up primordial gas lacking metals. In these early stages of galactic evolution, supernovae were still too infrequent to enrich the entire interstellar medium uniformly, leading to chemically inhomogeneous conditions.
Simulation results: each row shows, from top to bottom, the distributions of stars, gas, and heavy elements (oxygen). The panels represent time evolution from left to right: 100 million, 300 million, 500 million, and 1 billion years. (Adapted from Mori & Umemura, Nature, 440, 644 (2006))
(Top) Simulation snapshots. The top, middle, and bottom rows represent the density distributions of stars, gas, and heavy elements (oxygen), respectively. Each column corresponds to a different evolutionary stage: 100 million, 300 million, 500 million, and 1 billion years. (Bottom) Evolution of gas density: repeated supernova explosions produce complex structures, including dense gas shells and filaments. (See Mori & Umemura, Nature, 440, 644 (2006))
Large-scale structures surrounding the Andromeda Galaxy revealed by the Pan-Andromeda Archaeological Survey. The central elongated stellar feature, known as the Andromeda Giant Stream, extends approximately 450,000 light-years and contains more than 10 million solar masses. (Adapted from McConnachie et al., Nature, 461, 66 (2009))
Observations have revealed an intricate network of faint stellar structures around Andromeda, including shells and streams composed of extremely low-density stars. The inclusion of the Moon in the figure offers a sense of scale, underscoring how the visible structure of galaxies represents only a small portion of their full extent.
Using high-resolution simulations on supercomputers, we demonstrated that these features were produced by the tidal disruption of a small galaxy—approximately 1/400 the mass of Andromeda—that was captured by Andromeda’s gravity about 1 billion years ago. The remnants of this galaxy now form the Andromeda Giant Stream and a series of overlapping stellar shells, providing compelling evidence for the transformative role of galactic collisions in galaxy evolution.
This finding confirms that galaxy mergers, a cornerstone of hierarchical structure formation theory, continue to shape even massive, mature galaxies like Andromeda. Ongoing and future high-precision observations, supported by state-of-the-art instruments, will further uncover these structures. Our theoretical work contributes to this global effort to understand galactic history.
Simulation of a collision involving a dwarf disc galaxy. The white structure represents Andromeda; the yellow-red points show the dwarf galaxy, which is disrupted by tidal forces during the encounter. (Adapted from Kirihara, Miki, Mori, Kawaguchi & Rich, MNRAS, 469, 3390 (2017))
Simulation of a collision between the Andromeda Galaxy and a dwarf satellite. The extended blue structure represents Andromeda. (Mori & Rich, ApJ, 674, L77, 2008)
When large amounts of gas accrete onto a supermassive black hole (SMBH) at the center of a galaxy, the infalling matter releases enormous gravitational energy, igniting the black hole as an active galactic nucleus (AGN). However, gas inflow is often hindered by angular momentum, leading to the formation of a torus-like structure that acts as a reservoir. Galaxy mergers are thought to drive gas toward the galactic center, triggering AGN activity. Yet, the mechanisms responsible for turning off such activity remain elusive.
Despite the universe’s 13.8-billion-year age, SMBHs shine as AGNs for only a brief period—typically around 100 million years. This suggests that most SMBHs, including those at the centers of the Milky Way and Andromeda, are currently dormant due to insufficient gas supply. Recent discoveries of galaxies exhibiting rapid AGN fading have intensified interest in identifying the physical processes responsible for this shutdown.
Miki, Mori & Kawaguchi, Nature Astronomy, 5, 478 (2021)
Building on this model, we extended our investigation to other galaxies and found that the column densities of torus gas surrounding many SMBHs fall within a range that could be stripped during galaxy collisions. This suggests that such collisions may play a key role in terminating AGN activity across the universe. To evaluate the frequency of these events, we conducted high-precision orbital analyses of satellite galaxies using data from the Gaia mission. Our results indicate that major collisions capable of disrupting the torus occur approximately once every 100 million years—coinciding with the typical lifetime of AGN activity. These findings mark a significant advance in understanding the link between galaxy interactions and the life cycle of central black holes.