邻近恒星形成盘星系中分子云的生命周期
The lifecycle of molecular clouds in nearby star-forming disc galaxies
摘要：
它仍然是一个重大的挑战，推导出云尺度（$\lesssim100$ pc）恒星形成和反馈的理论，描述星系如何将气体转化为恒星作为星系环境的函数。由于缺乏对巨分子云（GMC）生命周期的强有力的经验约束，进展受到了阻碍。我们解决这个问题，通过系统地应用一种新的统计方法来测量的GMC生命周期，恒星形成的演化时间轴，并反馈到附近的9个盘星系的样本，观察作为PHANGS-ALMA调查的一部分。我们测量的空间分辨（$\sim100$ PC）CO-H $α$通量比，并找到一个普遍的去相关的分子气体和年轻恒星的GMC尺度，使我们能够量化的基本进化时间轴。GMC的寿命很短，通常为10-30百万年，并且在星系之间和星系内表现出环境变化。在kpc尺度的分子气体表面密度$Σ_{\rm H_2}\geqslant8$ M $_{\odot}$ pc $^{-2}$，GMC寿命与星系动力学过程的时间尺度相关，而在$Σ_{\rm H_2}\leqslant8$ M $_{\odot}$ pc $^{-2}$ GMC从星系动力学解耦，并为内部动力学时间尺度而活。经过一个长时间的惰性阶段，没有大质量恒星形成的H $α$（云寿命的75-90%），一旦大质量恒星出现，GMC在1-5百万年内就会分散。这种扩散很可能是由于早期的恒星反馈，导致GMC达到4-10%的综合恒星形成效率。这些结果表明，星系恒星的形成是由云尺度的、依赖于环境的、驱动快速进化循环的动力学过程控制的。GMC和HII区是经历这些生命周期的基本单位，在恒星形成盘中的平均间隔为100-300 pc。未来的工作应该研究驱动这些生命周期的多尺度物理和质量流。
英文摘要：
It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially-resolved ($\sim100$ pc) CO-to-H$α$ flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10-30 Myr, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $Σ_{\rm H_2}\geqslant8$M$_{\odot}$pc$^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $Σ_{\rm H_2}\leqslant8$M$_{\odot}$pc$^{-2}$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H$α$ (75-90% of the cloud lifetime), GMCs disperse within just 1-5 Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4-10% These results show that galactic star formation is governed by cloud-scale, environmentally-dependent, dynamical processes driving rapid evolutionary cycling. GMCs and HII regions are the fundamental units undergoing these lifecycles, with mean separations of 100-300 pc in star-forming discs. Future work should characterise the multi-scale physics and mass flows driving these lifecycles.
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