Milky-Way-like Galaxies with SIDM component

Abstract

Cold Dark Matter (CDM) model works well in explaining the large-scale structure of the universe. However, far from being perfect, predictions made by CDM only simulations still face many small-scale issues, including the Core-cusp Problem, the Diversity Problem, the Too-big-to-fail Problem, and the Missing Satellite Problem. As an attempt to resolve the crisis on the small scale, Self-interacting Dark Matter (SIDM) model is proposed as an alternative, assuming that dark matter particles interact with each other. We divide our study about galaxies with SIDM component into two main parts. The first part includes solving a semianalytic model for galaxy with SIDM components and the second part includes analyzing results produced by a numerical simulation. In the first part of our research, we derive Jeans Equations from the collisionless Boltzmann Equation. We impose the isothermal and isotropic condition on it to derive a system of PDEs that solves the dark matter gravitational potential given the baryon density profile. We also detail the numerical methods to solve this model under spherical symmetry and cylindrical symmetry. Close analysis on the density profiles produced by the semi-analytic model leads to two conclusions: First, the presence of baryon component in the galaxy makes the SIDM density profile less cored. Second, galaxies with SIDM component do not suffer from the Core-cusp Problem. By qualitatively comparing the cylindrical symmetry case with the spherical one, we argue that the spherical symmetry case makes good approximations on the dark matter density profile. Then we support this argument with the observation that the cylindrical matter density profile becomes spherically symmetric far away from the baryon disk. In the second part, we analyze the redshift z = 0 snapshot of two Milky-Way-like galaxies from the Feedback in Realistic Environment (FIRE) project. The only difference between the two galaxies is the dark matter component: m12i has CDM component and m12i-s has SIDM component. We apply a modern clustering algorithm-Density-Base Algorithm for Discovering Clusters in Large Spatial Database with Noise (DBSCAN)-to discover the different substructures formed in the two galaxies. We closely study the physical properties (age, metallicity, and velocity) of a shell-like substructure found in both m12i and m12i-s. The similarities found between the two shells leads to our first hypothesis: the two shells originate from the same source in the simulation. However, at redshift z = 0 the shell in m12i yields a less distinct structure compared to the shell in m12i-s. We present one possible explanation for this phenomenon: the shell structure in m12i has disintegrated over time and the CDM component might be related to this disintegration process. Then, by performing DBSCAN on the dark matter particles, we find a larger abundance of small CDM subhalos ( 106 Gev/cm^3) in m12i. Using this result, we further hypothesize that m12i has more small subhalos scattering around its host halo; those subhalos pulls the substructures apart gravitationally, disrupting the substructures formed in m12i.