2019ApJ...874...41C

We investigate the anisotropy of the stellar velocity dispersions within the effective radius, R_e, in 24 ATLAS^3D 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: <β_m>=a+bK 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 <f_DM>=(0.16±0.03)+(0.31±0.06)K, 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 <β_m>~0.2, consistent with previous results. However, marginalizing over 0 < K < 1.5 yields <β_m>=0.06_–0.14^+0.11 with <f_DM>=0.35±0.08; 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.