CLUMPING AND THE INTERPRETATION OF kpc-SCALE MAPS OF THE INTERSTELLAR MEDIUM: SMOOTH H i AND CLUMPY, VARIABLE H2 SURFACE DENSITY
CLUMPING AND THE INTERPRETATION OF kpc-SCALE MAPS OF THE INTERSTELLAR MEDIUM: SMOOTH H i AND CLUMPY, VARIABLE H2 SURFACE DENSITY
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星际介质的结块和 kpc 尺度图的解释：光滑的 H i 和块状、可变的 H2 表面密度
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发表时间：
2013
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通讯作者：
F. Walter
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作者：
A. Leroy;Cheoljong Lee;A. Schruba;A. Bolatto;A. Hughes;J. Pety;K. Sandstrom;E. Schinnerer;F. Walter
许多最近的模型考虑了单个星际介质（ISM）云的结构，作为解释大部分星系观测的一种方式。为了将这些模式与观测结果进行比较，我们必须了解如何在大尺度上平均观测到的表面密度（10 kpc）与模型处理的单个云尺度上的表面密度（10 kpc尺度）之间进行转换。我们定义了一个“聚集因子”，它将这种转换捕获为质量加权表面密度（通常是物理感兴趣的量）与观察到的面积加权表面密度的比率。我们使用高空间分辨率（次kpc）的地图CO和H i发射附近的星系来测量原子和分子气体的聚集因子。分子和原子ISM表现出明显不同的聚集程度。因此，在100 kpc分辨率下测得的H2/H i比值不能简单地解释为云尺度的表面密度比。H i发射看起来非常平滑，其中聚集因子仅为1.3。基于稀缺的和异质的高分辨率数据，CO排放是远远超过一个广泛变化的聚集因子，中位数为1.77为我们的异质数据的聚集。我们的测量并没有提供证据证明分子气体的普遍质量加权表面密度，但也不能最终排除这种情况。我们建议，一个更复杂的治疗分子ISM结构，一个通知高空间分辨率CO地图，需要将云尺度模型的星系kpc尺度的观测。
Many recent models consider the structure of individual interstellar medium (ISM) clouds as a way to explain observations of large parts of galaxies. To compare such models to observations, one must understand how to translate between surface densities observed averaging over large (∼kpc) scales and surface densities on the scale of individual clouds (∼pc scale), which are treated by models. We define a “clumping factor” that captures this translation as the ratio of the mass-weighted surface density, which is often the quantity of physical interest, to the area-weighted surface density, which is observed. We use high spatial resolution (sub-kpc) maps of CO and H i emission from nearby galaxies to measure the clumping factor of both atomic and molecular gas. The molecular and atomic ISM exhibit dramatically different degrees of clumping. As a result, the ratio H2/H i measured at ∼kpc resolution cannot be trivially interpreted as a cloud-scale ratio of surface densities. H i emission appears very smooth, with a clumping factor of only ∼1.3. Based on the scarce and heterogeneous high-resolution data available, CO emission is far more clumped with a widely variable clumping factor, median ∼7 for our heterogeneous data. Our measurements do not provide evidence for a universal mass-weighted surface density of molecular gas, but also cannot conclusively rule out such a scenario. We suggest that a more sophisticated treatment of molecular ISM structure, one informed by high spatial resolution CO maps, is needed to link cloud-scale models to kpc-scale observations of galaxies.