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 angular resolution ( 3 \arcsec or 1200 au ) and high spatial dynamic range .
Combining dataset from the ALMA main array , ALMA Compact Array ( ACA ) , the Nobeyama 45m Telescope , and the JVLA ( providing temperature measurement on matching scales ) , we find that most of the dense gas in OMC-2/3 is subsonic ( \sigma _ { NT } / c _ { s } = 0.62 ) with a mean line width ( \Delta \upsilon ) of 0.39 km s ^ { -1 } 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 } ( \sigma _ { 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 \sigma _ { ls } ( as traced by the Nobeyama Telescope ) can be decomposed into three components \sigma _ { ls } ^ { 2 } = \sigma _ { ss } ^ { 2 } + \sigma _ { bm } ^ { 2 } + \sigma _ { rd } ^ { 2 } , where small-scale \sigma _ { ss } is the line dispersion of each ALMA beam , bulk motion \sigma _ { bm } is dispersion between peak velocity of each ALMA beam , and \sigma _ { 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 \sim 0.05 pc scales in 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 Orion molecular clouds studied here .