Resolution-dependent subsonic non-thermal line dispersion revealed by ALMA

We report here Atacama Large Millimeter/submillimeter Array (ALMA) N₂H⁺ (1-0) images of the Orion Molecular Cloud 2 and 3 (OMC-2/3) with high angular resolution (3″ or 1200 au) and high spatial dynamic range. Combining a dataset from the ALMA main array, Atacama Compact Array (ACA), Nobeyama 45-m Telescope and Very Large Array (VLA) (providing temperature measurement on matching scales), we find that most of the dense gas in OMC-2/3 is subsonic (σ_NT / C_s = 0.62) with a mean line width (Δυ) of 0.39 km s^-1 full width at half maximum (FWHM). This is markedly different from the majority of previous observations of massive star-forming regions. In contrast, line widths from the Nobeyama Telescope are transonic at 0.69 km s^-1 (σ_NT / C_s = 1.08). We demonstrated that the larger line widths obtained by the single-dish telescope arose from unresolved sub-structures within their respective beams. The dispersions from larger scales σ_ls (as traced by the Nobeyama Telescope) can be decomposed into three components such that ${\sigma }_{{\rm{ls}}}^{2}={\sigma }_{{\rm{ss}}}^{2}+{\sigma }_{{\rm{bm}}}^{2}+{\sigma }_{{\rm{rd}}}^{2}$ , where small-scale σ_ss is the line dispersion of each ALMA beam, bulk motion σ_bm is dispersion between peak velocity of each ALMA beam and σ_rd is the residual dispersion. Such decomposition, though purely empirical, appears to be robust throughout our data cubes. Apparent supersonic line widths, commonly found in massive molecular clouds, are thus likely due to the effect of poor spatial resolution. The observed non-thermal line dispersion (sometimes referred to as 'turbulence') transits from supersonic to subsonic at ∼ 0.05 pc scales in the OMC-2/3 region. Such transition could be commonly found with sufficient spatial (not just angular) resolution, even in regions with massive young clusters, such as the Orion molecular clouds studied here.