The Dark Matter Paradox

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

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 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 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.