Resolution-dependent Subsonic Non-thermal Line Dispersion Revealed by
  ALMA
  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'' 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 ($\rm \sigma_{NT}/c_{s}$ =
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$^{-1}$ ($\rm \sigma_{NT}/c_{s}$ = 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 $\rm \sigma_{ls}^2 = \sigma_{ss}^2+
\sigma_{bm}^2+ \sigma_{rd}^2$, 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.
