Disentangling gamma-beta: the 4th-order velocity moments based on spherical Jeans analysis

Distinguishing a core and a cusp within dark matter halos is complexified by the existence of mass-anisotropy degeneracy, where various combinations of velocity anisotropy (β) and inner density slope (γ) yield similar observational signatures. We construct a dynamical model that incorporates the 4th-order velocity moments to alleviate this challenge. The inclusion of the 4th-order velocity moments enables stars' line-of-sight velocity distribution (LOSVD) to be flexible. This flexible LOSVD can cover from a thin-tailed to a heavy-tailed distribution that is inaccessible if only the 2nd-order moments are considered. We test our dynamical model using mock galaxies and find that a ratio of the global line-of-sight velocity dispersion of the mock galaxy to the velocity error measurement σlos,global / Δ vₗₒₛ 4 is required to avoid obtaining systematically biased results. This bias arises from the strong dependency of the 4th-order moments on the LOSVD tails, and not even increasing the sample size to 10⁴ stars can mitigate this effect. In that velocity ratio, β is recovered within ~ 1 σ even when the sample size is only 500 stars, regardless of the recovery of the other parameters. However, the estimation of γ varies, depending on the degree to which LOSVDs deviate from Gaussianity. Because of the more significant change in its LOSVDs, a cored dark halo is more likely to be identified than a cusp.