An Acceleration is Worth a Hundred Thousand Phase Space Measurements

Abstract

It is now possible to directly measure the accelerations that arise from the distribution of (dark) matter in the Milky Way. These acceleration-based measurements of the local dark matter density are now becoming competitive with estimates obtained through traditional kinematic techniques such as Jeans modeling. While classical methods can now draw on the positions and velocities of many millions of stars, recent acceleration-based studies have used fewer than 100 sources, yet achieve comparable precision. A key limitation of kinematic approaches is their reliance on assumptions of dynamical equilibrium and symmetry; direct acceleration measurements do not inherently suffer from this constraint. We find that, for the specific problem of estimating the local dark matter density, a single direct acceleration measurement can provide information comparable to $\sim$10$^5$ stars. We test the theoretical performance of direct acceleration techniques and Jeans modeling in estimating the local dark matter density using hydrodynamical N-body simulations of a MW-like galaxy, both in isolation and including a Sagittarius-like dwarf to generate disequilibrium. In the equilibrium scenario, Jeans modeling requires one thousand times more sources to achieve the same precision as the direct acceleration approach. This confirms that the per-source information advantage is intrinsic, and does not require disequilibrium to manifest. However, in the perturbed disk, the acceleration-based approach outperforms the Jeans analysis regardless of how many stars are available, due to significant bias in the result inferred from kinematics alone. Our results support earlier findings that non-equilibrium dynamics in the Galactic disk cause Jeans-based methods to systematically overestimate the local dark matter density; we show that this issue may also be present in other types of kinematic studies.