Modeling Nearly Spherical Pure-bulge Galaxies with a Stellar Mass-to-light Ratio Gradient under the ΛCDM and MOND Paradigms. II. The Orbital Anisotropy of Slow Rotators within the Effective Radius

We investigate the anisotropy of the stellar velocity dispersions within the effective radius, R e , in 24 pure-bulge galaxies, 16 of which are kinematic slow rotators (SRs). We allow the spherical anisotropy parameter β to be radially varying and allow a radial gradient in the stellar mass-to-light ratio ( M ⋆ / L ) through the parameter K introduced earlier. The median anisotropy for SRs depends on K as follows: with a = 0.19 ± 0.05, b = −0.13 ± 0.07 (ΛCDM) or a = 0.21 ± 0.05, b = −0.26 ± 0.08 (MOND), where β m refers to the radially averaged quantity. Under the ΛCDM paradigm, this scaling is tied to a scaling of , where f DM refers to the dark matter (DM) fraction within a sphere of r = R e . For K = 0 (constant M ⋆ / L ), we obtain radially biased results with , consistent with previous results. However, marginalizing over 0 < K < 1.5 yields with ; isotropy is preferred. This isotropy hides the fact that β m is correlated with kinematic features such as counterrotating cores (CRCs), kinematically distinct cores (KDCs), and low-level velocities (LVs); SRs with LVs are likely to be radially biased, while SRs with CRCs are likely to be tangentially biased, and SRs with KDCs are intermediate. Existing cosmological simulations allow us to understand these results qualitatively in terms of their dynamical structures and formation histories, although there are quantitative tensions. More realistic cosmological simulations, particularly allowing for M ⋆ / L gradients, may be required to better understand SRs.