Dependence of $X_{\rm CO}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium
Chia-Yu Hu, Andreas Schruba, Amiel Sternberg, Ewine F. van Dishoeck
备注：
Published in ApJ (typos in the abstract fixed). Code for interpolating Lagrangian (particle) data onto an adaptive mesh (and auxiliary scripts for RADMC-3D) available at https://github.com/huchiayu/ParticleGridMapper.jl
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.
Linking the Metallicity Enrichment History to the Star Formation History: An SFH-regulated Chemical Evolution Model and Its Implications for the Gas Cycling Process
Self-regulated galaxy evolution within a self-consistently varying galaxy-wide IMF
Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn（波恩大学赫尔姆霍兹射线与核物理研究所）；Argelander-Institut für Astronomie, Universität Bonn（波恩大学阿尔格兰德天文学研究所）；Astronomical Institute, Faculty of Mathematics and Physics, Charles University（查理大学数学与物理学院天文研究所）；Department of Astrophysics, University of Vienna（维也纳大学天体物理学系）
The "bubbly" interstellar medium as origin for the inhomogeneous internal metallicity distributions in large disk galaxies
The University of Melbourne（墨尔本大学）；California Institute of Technology（加州理工学院）；The Johns Hopkins University（约翰斯·霍普金斯大学）；Space Telescope Science Institute（太空望远镜科学研究所）
SILCC -- IX. The multi-phase interstellar medium at low metallicity
Universität zu Köln（科隆大学）；Max Planck Institute for Astrophysics（马克斯·普朗克天体物理研究所）；Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik（海德堡大学天文学中心理论天体物理研究所）；Astronomical Institute of the Czech Academy of Sciences（捷克科学院天文研究所）
METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble program. III. Interstellar Depletions, Dust-to-Metal, and Dust-to-Gas Ratios Versus Metallicity
Inferring Interstellar Medium Density, Temperature, and Metallicity from Turbulent H II Regions
University of Chicago（芝加哥大学）；Sub-department of Astrophysics, University of Oxford（牛津大学天体物理亚系）；Kavli Institute for Cosmological Physics, University of Chicago（芝加哥大学卡弗里宇宙物理学研究所）；Cosmic Dawn Center (DAWN)（黎明宇宙中心）；Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔研究所）
Active Galactic Nuclei-driven Metallicity Enrichment in the Interstellar Medium of Mrk 573
Astronomical Observatory of the Jagiellonian University（雅盖隆大学天文台）；Canadian Institute for Theoretical Astrophysics, University of Toronto（多伦多大学加拿大理论天体物理研究所）；NASA-Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）
HD/H2 as a probe of the roles of gas, dust, light, metallicity and cosmic rays in promoting the growth of molecular hydrogen in the diffuse interstellar medium
Supernova-driven turbulent metal mixing in high redshift galactic disks: metallicity fluctuations in the interstellar medium and its imprints on metal poor stars in the Milky Way
Metallicity fluctuation statistics in the interstellar medium and young stars -- II. Elemental cross-correlations and the structure of chemical abundance space
Australian National University（澳大利亚国立大学）；The Ohio State University（俄亥俄州立大学）；Durham University（杜伦大学）；Columbia University（哥伦比亚大学）
New SiS destruction and formation routes via neutral-neutral reactions and their fundamental role in interstellar clouds at low and high metallicity values
A geostatistical analysis of multiscale metallicity variations in galaxies [I]: Introduction and comparison of high-resolution metallicity maps to an analytic metal transport model
High water D/H ratio of the interstellar object 3I/ATLAS is consistent with a low-metallicity origin
RIKEN Pioneering Research Institute（理化学研究所前沿研究部）；NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；Catholic University of America（美利坚天主教大学）；Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université（巴黎天文台，巴黎文理研究大学，法国国家科学研究中心，索邦大学，巴黎西岱大学，塞吉-蓬图瓦兹大学）；Auburn University（奥本大学）；University of Washington（华盛顿大学）；LSST Interdisciplinary Network for Collaboration and Computing（LSST跨学科协作与计算网络）；Northern Arizona University（北亚利桑那大学）；Universität Bern（伯尔尼大学）；American University（美利坚大学）
Understanding Interstellar Metals during Reionization with Radiative SPH Simulation: Metallicity and Emission Lines from the ISM at $10 \geq z \geq 5$
New Mexico State University（新墨西哥州立大学）；Cosmic Dawn Center (DAWN) Niels Bohr Institute, University of Copenhagen / DTU-Space Technical University of Denmark（黎明中心（DAWN）尼尔斯·玻尔研究所，哥本哈根大学/丹麦技术大学空间系）
Jet Feedback and the Self-Regulated Growth of Black Holes Embedded in AGN Disks
Columbia University（哥伦比亚大学）；Northwestern University（西北大学）；Harvard University（哈佛大学）；Institute of Science and Technology Austria (ISTA)（奥地利科学技术研究所）；Stony Brook University（石溪大学）
