Density structure of molecular clouds as a probe of the ISM physics

了解控制星际介质(ISM)生命周期的物理过程是当今星系天体物理学的关键主题之一。这种重要性源于ISM作为新星诞生地的角色，因此也是星系演化不可分割的组成部分。ISM向恒星转化的确切方式与ISM的冷分子相结构有着错综复杂的联系。事实上，我们知道恒星的真正形成地点是星际分子云中的强密度增强。无论是什么过程控制了这些密度增强的演化，都非常具体地控制了气体到恒星的转化。然而，我们目前对分子云结构形成的图景有一个基本的警告：它几乎完全基于对附近只形成低质量恒星的低质量分子云的观察。这是因为，只有在附近的云中，传统的观测技术才能达到足够高的空间分辨率和灵敏度。然而，大多数恒星是在高质量分子云中形成的，而对星系ISM影响最大的是高质量恒星和星团。基于第一个ISM-SPP资助期所做的工作，我提出了一个计划，它将填补我们对大质量分子云内部结构的理解的空白。我们开发了一种新的观测技术，提供了对大质量分子云结构的无与伦比的看法。我们还开发了一种技术，利用这种高保真数据可以研究分子云结构的最基本度量之一--其体积密度的概率密度函数。有了这个计划，这些工具的潜力将得到充分发挥。我们将获得大量大质量分子云的高保真列密度数据，并对它们的内部结构进行前所未有的普查。我们还将使用最近的分子测线数据来追踪云的运动学性质。作为该计划的一个组成部分，我们将把获得的观测图片与最先进的、星系尺度的数值模拟联系起来。这种方法使我们能够解决该领域的关键问题：是什么物理驱动了大质量分子云中结构的形成？高质量云是如何碎裂的？高质量星云的结构是如何形成恒星的呢？这些过程在低质量分子云和高质量分子云中有区别吗？最重要的是，该程序允许我们建立一个关于分子云结构的视图，以及控制这种结构的过程，涵盖ISM中分子云的整个质量范围。

Understanding the physical processes that control the life-cycle of the interstellar medium (ISM) is one of the key themes in the astrophysics of galaxies today. This importance derives from the role of the ISM as the birthplace of new stars, and consequently, as an indivisible constituent of galaxy evolution. Exactly how the conversion of the ISM to stars takes place is intricately linked to the structure of the cold, molecular phase of the ISM. Indeed, we know that the true formation sites of stars are the strong density enhancements in the interstellar molecular clouds. Whatever processes control the evolution of these density enhancements very concretely control the gas-to-stars conversion. However, our current picture of the molecular cloud structure formation has a fundamental caveat: it is based almost entirely on observations of nearby, low-mass molecular clouds that form only low-mass stars. This is because only in nearby clouds, traditional observation techniques can reach an adequately high spatial resolution and sensitivity. Yet, it is the high-mass molecular clouds in which most stars form, and it is the high-mass stars and star-clusters that impact the ISM of galaxies most. Based on the work done during the first ISM-SPP funding period, I present a program that will fill the gap in our understanding of the internal structure of massive molecular clouds. We have developed a new observational technique that provides an unparalleled view of the structure of massive molecular clouds. We also have developed a technique with which such high-fidelity data can be used to study one of the most fundamental measures of the molecular cloud structure, the probability density function of their volume densities. With this program, the full potential of these tools will be put into use. We will derive high-fidelity column density data for a large number of massive molecular clouds, and build an unprecedented census of their internal structure. We will also use the data of recent molecular line surveys to trace the kinematic properties of the clouds. As an integral part of the program, we will connect the obtained observational picture to state-of-the-art, galaxy-scale numerical simulations. This approach allows us to address keyquestions in the field: What physics drive the structure formation in massive molecular clouds? How do high-mass clouds fragment? How does the structure of high-mass clouds gives rise to the star formation within? Are these processes different in low-mass and high-mass molecular clouds? Most crucially, the program allows us to build a view of the molecular cloud structure, and of the processes that control this structure, over the entire mass range of molecular clouds in the ISM.