论文标题
$ x _ {\ rm co} $的依赖性在自我调节的星际介质中对金属，强度和空间比例的依赖性
Dependence of $X_{\rm CO}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium
论文作者
论文摘要
我们研究CO（1-0） - 至h $ _2 $转换因子（$ x _ {\ rm co} $）和CO（2-1）-CO（2-1）-CO（1-0）（$ r_ {21} $）的线比在多种金属范围内（$ 0.1 \ leq leq z/z___ \ odot sim frys \ odq 3 $）自调节的多相星际培养基。我们通过辐射转移构建合成的CO发射图，并系统地改变“观察”梁的大小以量化比例依赖性。我们发现，如果假设稳态化学反应或假设恒星形成的气体为H $ _2 $，则可以在低$ z $下过高估计的KPC尺度$ x _ {\ rm co} $。在Parsec量表上，$ x _ {\ rm co} $因各个地方的数量级而有所不同，主要是由从原子碳到Co的过渡。 cm^{ - 2}〜（k〜km〜s^{ - 1}）^{ - 1}} $一旦灰尘屏蔽变得有效，独立于$ z $。 CO线在较低的$ Z $下变得越来越光学上稀薄，导致较高的$ r_ {21} $。大多数云区域都用高$ x _ {\ rm co} $和低$ r_ {21} $填充，而大多数CO发射起源于低$ x _ {\ rm co} $和高$ r_ {21} $的密集气体。采用常数$ x _ {\ rm co} $强烈过度 - （低）估计h $ _2 $密度（扩散）气体。线强度（积极地）与$ x _ {\ rm co} $（$ r_ {21} $）相关，因为它是列密度（体积密度）的代理。在很大的尺度上，$ x _ {\ rm co} $和$ r_ {21} $由光束平均决定，并且它们自然偏向着密集的气体中的值。我们的预测$ x _ {\ rm co} $是$ z $，线强度和梁大小的多元功能，可用于更准确地推断h $ _2 $质量。
We study the CO(1-0)-to-H$_2$ conversion factor ($X_{\rm CO}$) and the line ratio of CO(2-1)-to-CO(1-0) ($R_{21}$) across a wide range of metallicity ($0.1 \leq Z/Z_\odot \leq 3$) in high-resolution (~0.2 pc) hydrodynamical simulations of a self-regulated multiphase interstellar medium. We construct synthetic CO emission maps via radiative transfer and systematically vary the "observational" beam size to quantify the scale dependence. We find that the kpc-scale $X_{\rm CO}$ can be over-estimated at low $Z$ if assuming steady-state chemistry or assuming that the star-forming gas is H$_2$-dominated. On parsec scales, $X_{\rm CO}$ varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO. The pc-scale $X_{\rm CO}$ drops to the Milky Way value of $2\times 10^{20}\ {\rm cm^{-2}~(K~km~s^{-1})^{-1}}$ once dust shielding becomes effective, independent of $Z$. The CO lines become increasingly optically thin at lower $Z$, leading to a higher $R_{21}$. Most cloud area is filled by diffuse gas with high $X_{\rm CO}$ and low $R_{21}$, while most CO emission originates from dense gas with low $X_{\rm CO}$ and high $R_{21}$. Adopting a constant $X_{\rm CO}$ strongly over- (under-)estimates H$_2$ in dense (diffuse) gas. The line intensity negatively (positively) correlates with $X_{\rm CO}$ ($R_{21}$) as it is a proxy of column density (volume density). On large scales, $X_{\rm CO}$ and $R_{21}$ are dictated by beam averaging, and they are naturally biased towards values in dense gas. Our predicted $X_{\rm CO}$ is a multivariate function of $Z$, line intensity, and beam size, which can be used to more accurately infer the H$_2$ mass.
