Gaseous Dynamical Friction in the Early Universe

Gaseous Dynamical Friction in the Early Universe

Figure 1: Variation in drag force with Mach number for two perturber masses, with the numerical results shown by the dots, and the analytic prediction shown by the line (Morton et al. in prep)

Dynamical friction is the process by which a massive perturber, moving through some background medium, gravitationally interacts with that medium, producing a net retarding force to its motion. When the background medium is gaseous, the pressure forces present in the gas must be included in modelling the response of the medium, and so impact the resultant force. To numerically model this process, we used the gravo-hydrodynamic code GIZMO, which uses state of the art Lagrangian hydro solvers, to test highly idealised wind tunnel-like setups against the analytic predictions for this net force. We found that the numerical results significantly under produce the force predicted by analytic treatments of the problem. These solvers did not reproduce the predicted structure of the wake, instead smoothing out predicted density profiles, and producing unexpected bow wave structures. These results were found in regimes where the analytic prediction should hold. As mentioned above, dynamical friction is critical in driving the mergers of dark matter sub-halos into their larger hosts, as it provides a crucial mechanism by which angular momentum can be transferred away from the orbiting substructure, allowing it to spiral into the central object. This is particularly important in the early Universe, when gas fractions were higher. The reduced retarding force (shown below in Figure 1 for different perturber velocity Mach numbers) suggests that modern cosmological simulations, that are run using these solvers, could be underestimating the merger rates of early dark matter structure, and so incorrectly recovering the evolution of early galaxies.