A New Method to Simulate Dark Matter-Baryon Interactions and Application to an Isolated Disk Galaxy

Abstract

We report on a new method for incorporating interactions between dark matter (DM) and baryons in cosmological simulations, capable of handling the challenging regime in which the dark matter particle mass is comparable to or lighter than the baryon mass. The method hybridizes two distinct approaches, as gas particles receive momentum and energy transfer according to a mean-field calculation while DM particles undergo Monte Carlo scatterings, which are derived from the Boltzmann equation and proved to be statistically equivalent. We present an open-source implementation of this method in the simulation code GIZMO. As a first application, we investigate the effects of DM--baryon interactions on an isolated Milky Way-like disk galaxy for dark matter having twice the proton mass, which roughly maximizes the average energy transfer per collision. For cross sections of order 1 barn ($10^{-24}\, \mathrm{cm}^2$), these interactions cause strong changes to the mass distribution in the center of the galaxy in less than 1 Gyr, even when bar formation is suppressed by hand. For cross sections typical of hadronic interactions, $\lesssim 30 \, {\rm mb}$, high-fidelity galaxy formation simulations will be needed to assess the effects on observable features of galaxies.

Figures (13)

• Figure 1: Top-down view of the gas in the center of the disk (left) and the entire stellar disk (right) at a variety of cross sections (increasing from top to bottom) and times (increasing from left to right). The largest two cross sections show the development of an extremely compact and dense object in the central gas, as well as the concentration of stars in the center. No significant differences are seen between CDM and the smaller cross sections.
• Figure 2: Dark matter density profile at the end of the simulations (at about 1 Gyr). Plotted for reference are an NFW profile (dashed grey line) and a profile proportional to $r^{-2}$ (dotted grey line). The largest two cross sections (300 and 1000 mb) are much denser in the center than all other simulations, with 1000 mb becoming steeper even than $r^{-2}$.
• Figure 3: Enclosed dark matter (dashed) and gas (solid) mass profiles at the end (about 1 Gyr) of the simulations. Gas appears to concentrate in the center, with density increasing as a function of cross section. At the largest cross sections (300, 1000 mb), dark matter concentrates in the center as well.
• Figure 4: The median gas temperature computed in 1 kpc bins of cylindrical radius. For a cross section of 1000 mb, there is a significant temperature increase at all radii, but the effect is largely gone for cross sections below 300 mb.
• Figure 5: Star formation rate over 1 Gyr for all simulated cross sections. For a cross section of 1000 mb, the star formation rate is increased at nearly all times. At other cross sections, there are few significant differences.