Joint Gas and Stellar Dynamical Models of WLM: An isolated dwarf galaxy within a cored, prolate DM halo

Abstract

We present multi-tracer dynamical models of the low mass ($M_{*} \sim 10^{7}$), isolated dwarf irregular galaxy WLM in order to simultaneously constrain the inner slope of the dark matter (DM) halo density profile ($γ$) and flattening ($q_\mathrm{DM}$), and the stellar orbital anisotropy ($β_{z}, β_{r}$). For the first time, we show how jointly constraining the mass distribution from the HI gas rotation curve and solving the Jeans' equations with discrete stellar kinematics leads to a factor of $\sim2$ reduction in the uncertainties on $γ$. The mass-anisotropy degeneracy is also partially broken, leading to reductions on uncertainty by $\sim 30\%$ on $M_\mathrm{vir}$ (and $\sim 70\%$ at the half-light radius) and $\sim 25\%$ on anisotropy. Our inferred value of $γ= 0.3 \pm 0.1$ is robust to the halo geometry, and in excellent agreement with predictions of stellar feedback driven DM core creation. The derived prolate geometry of the DM halo with $q_\mathrm{DM} = 2 \pm 1$ is consistent with $Λ$CDM simulations of dwarf galaxy halos. While self-interacting DM (SIDM) models with $σ/m_{X} \sim 0.6$ can reproduce this cored DM profile, the interaction events may sphericalise the halo. The simultaneously cored and prolate DM halo may therefore present a challenge for SIDM. Finally we find that the radial profile of stellar anisotropy in WLM ($β_{r}$) follows a nearly identical trend of increasing tangential anisotropy to the classical dSphs, Fornax and Sculptor. Given WLM's orbital history, this result may call into question whether such anisotropy is a consequence of tidal stripping in only one pericentric passage or if it instead is a feature of the largely self-similar formation and evolutionary pathways for some dwarf galaxies.