Solving 'cosmological puzzles' (Credit: Bingqing Sun)

Ultra-diffuse galaxies

Ultra-diffuse galaxies (UDGs) are a large sample of recently observed galaxies whose sizes are as large as our Milky Way Galaxy while whose luminosities are as faint as dwarf galaxies. The nature of these galaxies is still being debated.

I lead a project of using the Auriga simulations, which are a set of state-of-the-art zoom-in magetohydrodynamic simulations, to study the formation of such puzzling galaxies. In Auriga, field UDGs form in dark matter haloes with larger spins compared to normal dwarfs in the field. Satellite UDGs, on the other hand, have two different origins: about half of them formed as field UDGs before they were accreted; the remaining half were normal field dwarfs that subsequently turned into UDGs as a result of tidal interactions.

Dwarf galaxies forming in filaments

In the classical picture, to form galaxies, primordial gas was first trapped into the potential wells created by dark matter haloes. Comparing with haloes, massive filaments have the same function to provide potential wells to trap gas. Understanding these processes in detail will be helpful in understanding the observed filamentary dependences of galaxy properties.

We use zoom-in hydrodynamic simulations to show that cold and dense gas preprocessed by dark matter filaments can be further accreted into residing individual low-mass haloes in directions along the filaments, resulting a very anisotropic gas accretion for filament haloes. Filament haloes have higher baryon and stellar fractions when comparing with their field counterparts. The results suggest that filaments assist gas cooling and enhance star formation in their residing dark matter haloes.

Dark matter-baryon segregation and spin misalignment

The standard galaxy formation theory assumes that baryons and dark matter are initially well-mixed before becoming segregated by radiative cooling. This implies that before cooling, baryons and dark matter in a halo should have identical specific angular momentum as they have experienced identical tidal torquing.

With non-radiative hydrodynamical simulations, we show that on the contrary to this assumption, baryons and dark matter can also be segregated because of different physics obeyed by gas and dark matter during halo assembly. As a result, baryons and dark matter in many haloes do not originate from the same Lagrangian region, experience different tidal torques, and thus their angular momentum vectors are often misaligned. The result challenges the precision of some semi-analytical approaches which utilize dark matter halo merger trees to infer gas properties.

Environmental effects on halo growth

The environmental effects on dark matter halo assembly history and structure are a key ingredient in understanding the environmental dependence of galaxy properties which has been observed in many galaxy observations.

With high-resolution cosmological N-body simulations, we trace the accretion history of each particle in each z = 0 halo and record its accretion time and accretion modes, to quantify the environmental effects on halo growth. On average, haloes in high-density environments accrete their z = 0 particles earlier at all radii than their low-density counterparts. High-density haloes tend to accrete their particles slightly more from major mergers and less from smooth accretion comparing to low-density haloes, and this effect is more evident for inner particles. The environmental effects are usually more pronounced for haloes with lower masses.

Universal angular momentum profile

Halo angular momentum is a key ingredient in understanding the formation of disk galaxies. Once haloes reach virial equilibrium, their properties are found to follow some universal forms which are valid for haloes with different masses, reflecting the equilibirum properties of collisionless systems. One example is the famous NFW density profile. It will be interesting to see if there is any universal spatial distribution for halo spins.

From cosmological simulations, we find that the spatial distribution of specific angular momentum in dark matter haloes is universal, and it can be fitted with a simple functional form. The profile tells us that the outer parts close to the equatorial plane of a halo tend to have larger specific angular momentum, while the inner parts near the polar direction usually have smaller specific angular momentum.

Angular momentum - mass relation

There is a well-known simple power-law relation between an object's angular momentum, J, and its mass, M, i.e., J ~ M^{5/3}, the so-called J-M relation.

To understand the origin of this J-M relation, I and my collaborators use simulations to study its time-evolution, J(t) = β(t) M(t)^α(t), trying to find out when it established. In the ΛCDM model, α starts with a value of ~1.5 at high redshift z, increases monotonically, and finally reaches 5/3 near z = 0, whereas β evolves linearly with time in the beginning, reaches a maximum and decreases, and, finally, stabilizes. A three-regime scheme is proposed to understand this newly observed picture. The tidal torque theory accounts for this time-evolution behavior in the linear regime, whereas α = 5/3 comes from the virial equilibrium of halos. An updated and more complete understanding of the J-M relation is given.

(Image: M101. Credit: Subaru, HST, Robert Gendler)

Simulation initial conditions

Grid and glass configurations are two widely used pre-initial conditions when generating initial conditions for cosmological N-body simulations.

Recently, I introduce an alternative method to prepare pre-initial conditions, the Capacity Constrained Voronoi Tessellation (CCVT), which originates from computer graphics. As a geometrical equilibrium state, the CCVT configuration is uniform and isotropic, follows perfectly the minimal power spectrum, P(k) ~ k^4, and is quite stable under gravitational interactions. This new method is shown to play as good as grid and glass in cosmological simulations, and it will be helpful in studying the numerical convergence of pre-initial conditions in cosmological simulations.