标题： 星系中的X因子：I. 环境和尺度的依赖性 显示英文标题

标题： The X-factor in Galaxies: I. Dependence on Environment and Scale

摘要： 表征一氧化碳辐射与分子氢柱密度之间的转换因子X_CO在研究星系的气态含量、其演化以及与恒星形成的关系中至关重要。在大多数情况下，转换因子被假定接近银河系中巨分子云（GMCs）的值，除非是合并星系和星暴星系。然而，有物理依据表明，它还应取决于气体金属丰度、表面密度和星际辐射场的强度。由于分辨率有限的影响，XCO因子可能还依赖于CO辐射平均的尺度。我们使用一个模型来研究X_CO与气体特性和平均尺度的关系，该模型基于亚秒差距尺度磁流体动力学模拟的结果以及自洽的星系形成宇宙学模拟中的气体分布。我们的模型预测的X_CO值与银河系的值一致，适用于银河系典型的星际介质条件。对于这些条件，预测的X_CO在气体表面密度范围为\sim 50 - 500 M_sun / pc^2时仅变化两倍左右。然而，模型还预测，在GMC尺度上，X_CO是金属丰度的强函数，并依赖于柱密度和星际紫外线通量。我们明确展示了在观测估计中忽略这些依赖关系会导致推断的分子云H2柱密度分布比真实分布更窄且偏移。我们发现，当在\sim kpc尺度上平均时，X因子仅弱依赖于辐射场和柱密度，但仍然是金属丰度的强函数。预测的金属丰度依赖性可以近似表示为X_CO\sim Z^{-{\gamma }}，其中{\gamma }~ 0.5 - 0.8。 显示英文摘要

摘要： Characterizing the conversion factor between CO emission and column density of molecular hydrogen, X_CO, is crucial in studying the gaseous content of galaxies, its evolution, and relation to star formation. In most cases the conversion factor is assumed to be close to that of giant molecular clouds (GMCs) in the Milky Way, except possibly for mergers and star-bursting galaxies. However, there are physical grounds to expect that it should also depend on the gas metallicity, surface density, and strength of the interstellar radiation field. The XCO factor may also depend on the scale on which CO emission is averaged due to effects of limited resolution. We study the dependence of X_CO on gas properties and averaging scale using a model that is based on a combination of results of sub-pc scale magneto-hydrodynamic simulations and on the gas distribution from self-consistent cosmological simulations of galaxy formation. Our model predicts a value of X_CO that is consistent with the Galactic value for interstellar medium conditions typical for the Milky Way. For such conditions the predicted X_CO varies by only a factor of two for gas surfaced densities in the range \sim 50 - 500 M_sun / pc^2. However, the model also predicts that more generally on the scale of GMCs, X_CO is a strong function of metallicity, and depends on the column density and the interstellar UV flux. We show explicitly that neglecting these dependencies in observational estimates can strongly bias the inferred distribution of H2 column densities of molecular clouds to have a narrower and offset range compared to the true distribution. We find that when averaged on \sim kpc scales the X-factor depends only weakly on radiation field and column density, but is still a strong function of metallicity. The predicted metallicity dependence can be approximated as X_CO \sim Z^{-{\gamma}} with {\gamma} ~ 0.5 - 0.8.