To investigate the fundamental principles of H2 formation in a giant molecular cloud, we derive the H I and H2 surface density (ΣH I and ΣH2) images of the Perseus molecular cloud on sub-pc scales (~0.4?pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the ΣH2 distribution across Perseus. We find a relatively uniform ΣH I ~ 6-8 M ☉?pc–2 for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between ΣH2/ΣH I and ΣH I + ΣH2. The transition between the H I- and H2-dominated regions occurs at N(H I) + 2N(H2) ~ (8-14)?× 1020?cm–2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of "CO-dark" gas, f DG ~ 0.3. While significant spatial variations of f DG are found, we do not find a clear correlation with the mean V-band extinction.
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机译：为了研究巨大分子云中H2形成的基本原理，我们得出亚英仙分子分子云的H I和H2表面密度（ΣHI和ΣH2）图像的单位为pc级（〜0.4µpc）。我们使用IRAS测量的改进后处理中的远红外数据和COMPLETE测量提供的V波段消光图像来估算英仙座的尘埃柱密度图像。结合银河Arecibo L波段馈源阵列H I测量的H I数据和局部尘土与瓦斯比的估计值，我们可以得出整个英仙座的ΣH2分布。我们发现在暗区和恒星形成区都存在相对均匀的ΣHI〜6-8 M ☉pc–2，这表明屏蔽H2免受光解离所需的最小H I表面密度。结果，在ΣH2/ΣHI和ΣHI +ΣH2之间发现了非常紧密和一致的关系。 H I和H2为主的区域之间的过渡发生在N（H I）+ 2N（H2）〜（8-14）？×1020？cm-2。我们的发现与平衡状态下H2形成的预测一致，表明湍流对于H2形成可能不是最重要的。但是，温暖的中性介质对H2的屏蔽，内部辐射场以及H2形成的时间尺度的重要性仍然是悬而未决的问题。我们还比较了H2和CO的分布，并估计了“ CO暗”气体的分数f DG〜0.3。尽管发现了f DG的显着空间变化，但我们没有发现与平均V波段消光有明显的相关性。
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