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We report here Atacama Large Millimeter/submillimeter Array (ALMA) N$_2$H$^+$(1-0) images of the Orion Molecular Cloud 2 and 3 (OMC-2/3) with high angularresolution (3'' or 1200 au) and high spatial dynamic range. Combining datasetfrom the ALMA main array, ALMA Compact Array (ACA), the Nobeyama 45m Telescope,and the JVLA (providing temperature measurement on matching scales), we findthat most of the dense gas in OMC-2/3 is subsonic ($\rm \sigma_{NT}/c_{s}$ =0.62) with a mean line width ($\Delta\upsilon$) of 0.39 km s$^{-1}$ FWHM. Thisis markedly different from the majority of previous observations of massivestar-forming regions. In contrast, line widths from the Nobeyama Telescope aretransonic at 0.69 km s$^{-1}$ ($\rm \sigma_{NT}/c_{s}$ = 1.08). We demonstratedthat the larger line widths obtained by the single-dish telescope arose fromunresolved sub-structures within their respective beams. The dispersions fromlarger scales $\sigma_{ls}$ (as traced by the Nobeyama Telescope) can bedecomposed into three components $\rm \sigma_{ls}^2 = \sigma_{ss}^2+\sigma_{bm}^2+ \sigma_{rd}^2$, where small-scale $\sigma_{ss}$ is the linedispersion of each ALMA beam, bulk motion $\sigma_{bm}$ is dispersion betweenpeak velocity of each ALMA beam, and $\sigma_{rd}$ is the residual dispersion.Such decomposition, though purely empirical, appears to be robust throughoutour data cubes. Apparent supersonic line widths, commonly found in massivemolecular 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 $\sim 0.05$ pc scales inOMC-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 Orion molecular clouds studied here.

Resolution-dependent Subsonic Non-thermal Line Dispersion Revealed by ALMA