We investigate the anisotropy of the stellar velocity dispersions within the effective radius , R _ { e } , in 24 ATLAS ^ { 3 D } pure-bulge galaxies , 16 of which are kinematic slow rotators ( SRs ) .
We allow the spherical anisotropy parameter \beta to be radially varying and allow a radial gradient in the stellar mass-to-light ratio ( M _ { \star } / L ) through the parameter K introduced earlier .
The median anisotropy for SRs depends on K as follows : \langle \beta _ { m } \rangle = a + bK with a = 0.19 \pm 0.05 , b = -0.13 \pm 0.07 ( \Lambda CDM ) or a = 0.21 \pm 0.05 , b = -0.26 \pm 0.08 ( MOND ) , where \beta _ { m } refers to the radially averaged quantity .
Under the \Lambda CDM paradigm this scaling is tied to a scaling of \langle f _ { DM } \rangle = ( 0.16 \pm 0.03 ) + ( 0.31 \pm 0.06 ) K , where f _ { DM } refers to the DM fraction within a sphere of r = R _ { e } .
For K = 0 ( constant M _ { \star } / L ) , we obtain radially biased results with \langle \beta _ { m } \rangle \approx 0.2 consistent with previous results .
However , marginalizing over 0 < K < 1.5 yields \langle \beta _ { m } \rangle = 0.06 ^ { +0.11 } _ { -0.14 } with \langle f _ { DM } \rangle = 0.35 \pm 0.08 : isotropy is preferred .
This isotropy hides the fact that \beta _ { m } is correlated with kinematic features such as counter rotating 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 exist quantitative tensions .
More realistic cosmological simulations , particularly allowing for M _ { \star } / L gradients , may be required to better understand SRs .