Joint gas and stellar dynamical models of WLM: an isolated dwarf galaxy within a cored, prolate DM halo

We present multitracer dynamical models of the low-mass (M_* ∼ 10⁷), 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_DM), and the stellar orbital anisotropy (β_z, β_r). For the first time, we show how jointly constraining the mass distribution from the H I gas rotation curve and solving the Jeans equations with discrete stellar kinematics lead to a factor of ∼2 reduction in the uncertainties on γ. The mass-anisotropy degeneracy is also partially broken, leading to reductions on uncertainty by ${\sim} 30{{\ \rm per\ cent}}$ on M_vir (and ${\sim} 70{{\ \rm per\ cent}}$ at the half-light radius) and ${\sim} 25{{\ \rm per\ cent}}$ on anisotropy. Our inferred value of γ = 0.3 ± 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_DM = 2 ± 1 is consistent with Lambda cold dark matter simulations of dwarf galaxy haloes. While self-interacting DM (SIDM) models with σ/m_X ∼ 0.6 can reproduce this cored DM profile, the interaction events may sphericalize 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 dwarf spheroidals, 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.