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 , 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.

我们在此报告Orac分子云2和3（OMC-2 / 3）的Atacama大毫米/亚毫米阵列（ALMA）N ₂ H ⁺（1–0）图像，具有高角分辨率（3“或1200 au）和高空间动态范围。结合来自ALMA主阵列，阿塔卡马紧凑阵列（ACA），Nobeyama 45米望远镜和甚大阵列（VLA）的数据集（以匹配规模提供温度测量），我们发现OMC-2 /图3是亚音速（σ _NT / ç _小号= 0.62），平均线宽度（Δ υ）为0.39公里S ^-1半高全宽（FWHM）。这与先前关于大质量恒星形成区域的大多数观测结果明显不同。与此相反，从野边山望远镜线宽0.69公里S跨音速^-1（σ _NT / ç _小号= 1.08）。我们证明了单碟望远镜获得的更大的线宽是由于它们各自光束中未解析的子结构引起的。从更大的尺度的分散体σ _LS （如追踪由野边山望远镜）可以分解为三个组成部分，使得，其中小规模σ _SS是每个ALMA束的线中的色散，整体运动σ _BM是每个ALMA光束的峰值速度之间分散σ _RD是残余分散体。这种分解，尽管纯粹是经验性的，但在我们的所有数据立方体中似乎都是可靠的。由于空间分辨率差的影响，通常可能在大量分子云中发现的表观超音速线宽。在OMC-2 / 3区域中，观测到的非热线弥散（有时称为“湍流”）以〜0.05 pc的比例从超音速过渡到亚音速。即使在具有大量年轻簇的区域，例如此处研究的Orion分子云，通常也可以以足够的空间（而不只是角度）分辨率找到这种过渡。