The 3-D Kinematics of the Orion Nebula Cluster: NIRSPEC-AO Radial Velocities of the Core Population

The kinematics and dynamics of stellar and substellar populations within young, still-forming clusters provides valuable information for constraining theories of formation mechanisms. Using Keck II NIRSPEC+AO data, we have measured radial velocities for 56 low-mass sources within 4 of the core of the ONC. We also re-measure radial velocities for 172 sources observed with SDSS/APOGEE. These data are combined with proper motions measured using HST ACS/WFPC2/WFC3IR and Keck II NIRC2, creating a sample of 136 sources with all three velocity components. The velocities measured are consistent with a normal distribution in all three components. We measure intrinsic velocity dispersions of ($sigma_{v_alpha}$, $sigma_{v_delta}$, $sigma_{v_r}$) = ($1.76pm0.12$, $2.16^{+0.14}_{-0.15}$, $2.54^{+0.16}_{-0.17}$) km s$^{-1}$. Our computed intrinsic velocity dispersion profiles are consistent with the dynamical equilibrium models from Da Rio et al. (2014) in the tangential direction, but not in the line of sight direction, possibly indicating that the core of the ONC is not yet virialized, and may require a non-spherical potential to explain the observed velocity dispersion profiles. We also observe a slight elongation along the north-south direction following the filament, which has been well studied in previous literature, and an elongation in the line of sight to tangential velocity direction. These 3-D kinematics, coupled with estimates of source masses, will allow future studies to determine the dominant formation mechanism, differentiating between models such as competitive accretion and turbulent fragmentation.