  Global polarization of $\Lambda$ hyperons has been measured to be of the
order of a few tenths of a percent in Au+Au collisions at $\sqrt{s_{_{NN}}}$ =
200 GeV, with no significant difference between $\Lambda$ and $\bar{\Lambda}$.
These new results reveal the collision energy dependence of the global
polarization together with the results previously observed at
$\sqrt{s_{_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the
medium created in non-central heavy-ion collisions at the highest RHIC
collision energy. The signal is in rough quantitative agreement with the
theoretical predictions from a hydrodynamic model and from the AMPT (A
Multi-Phase Transport) model. The polarization is larger in more peripheral
collisions, and depends weakly on the hyperon's transverse momentum and
pseudorapidity $\eta^H$ within $|\eta^H|<1$. An indication of the polarization
dependence on the event-by-event charge asymmetry is observed at the $2\sigma$
level, suggesting a possible contribution to the polarization from the axial
current induced by the initial magnetic field.
