Star Formation and the ISM Evolution of Galaxies Across Cosmic Time

星系由恒星群、中央超大质量黑洞和所谓的星际介质(ISM)组成。后者由气体和尘埃组成，除了氦(~10%)和其他一些丰度较低的原子和分子物种外，气体几乎全部由原子和分子氢组成。恒星在ISM中密度过高的区域形成，这些区域已经变得引力不稳定，而超大质量黑洞则通过星系中心区域的气体吸积而增长。换句话说，ISM为恒星的形成和黑洞的生长提供了“燃料”，而尘埃--超新星和恒星风的凝结--则是恒星形成的结果。显然，ISM在星系的形成和演化中扮演着重要角色。不幸的是，由于氢分子的对称性，大部分分子气体不能直接观测到，这意味着我们必须依赖于对其他分子和原子的发射线的观测。CO分子是仅次于氢的第二丰度分子，具有容易激发的转动量子能级，是分子ISM的主要示踪物。通过研究CO谱线的亮度以及其他有用的示踪剂，如HCO+和HCN，人们能够限制气体的密度、动力学温度和化学成分。虽然这些类型的观测已经在我们自己的银河系进行过，但我们对其他星系的ISM的理解存在巨大的差距。其原因是，对于低红移星系，ISM的关键诊断发射线位于远红外和亚毫米波长，由于地球大气的原因，从地面很难或不可能观测到这些波长。这项研究计划的主要目标是获得其他星系中ISM的详细图像，从我们宇宙后院的局部星暴星系到可观测宇宙边缘的最遥远的星系。利用CO、HCN和HCO+的全旋转线光谱以及中性和单电离碳的诊断线，我们将表征星系中ISM的物理和化学性质。通过描绘气体的微观性质(即密度、温度、化学丰度)如何影响恒星形成等宏观过程，我们将获得至关重要的新见解，了解整个宇宙历史上星系的恒星形成和ISM演化。为此目的，我们将利用空间和地面上新的极其强大的望远镜，如赫歇尔空间天文台和阿塔卡马大毫米阵列，它们能够克服迄今地面观测所面临的许多困难。这些设备代表着我们研究遥远星系中的气体和尘埃的能力的巨大飞跃，并将为研究星系中的恒星形成和ISM演化打开一扇新的窗口。

Galaxies consist of stellar populations, a central supermassive black hole and a so-called interstellar medium (ISM). The latter is made up of gas and dust, with the gas consisting almost entirely of atomic and molecular hydrogen except for He (~10%) and a few other much less abundant atomic and molecular species. Stars form in overdense regions of the ISM that have become gravitationally unstable, while the supermassive black hole grows via the accretion of gas in the central regions of a galaxy. In other words, the ISM provides the 'fuel' for star formation and black hole growth, while the dust - condensing out of supernovae and stellar winds - is a consequence of star formation. Clearly the ISM plays a fundamental role in the formation and evolution of galaxies. Unfortunately, the bulk of the molecular gas is not directly observable due to the symmetry of the hydrogen molecule, which means that we have to rely on observations of emission lines from other molecules and atoms. The CO molecule, being the second-most abundant molecule after hydrogen and having easily excitable rotational quantum levels, is the principal tracer of the molecular ISM. By studying the brightness of the CO lines along with those of other useful, albeit observationally more challenging, tracers such as HCO+ and HCN, one is able to constrain the density, kinetic temperature and chemical composition of the gas. While these types of observations have been carried out in our own Galaxy, there are huge gaps in our understanding of the ISM in other galaxies. The reason for this is that, for low redshift galaxies, the key diagnostic emission lines of the ISM lie at far-infrared and submillimetre wavelengths, which are difficult or impossible to observe from the ground due to the Earth's atmosphere. The main objective of this research proposal is to obtain a detailed picture of the ISM in other galaxies, from local starburst galaxies in our Cosmic backyard to the most distant galaxies at the edge of the observable Universe. Using the full rotational line spectra of CO, HCN and HCO+ as well as diagnostic lines of neutral and single-ionised carbon we will characterize the physical and chemical properties of the ISM in galaxies. By delineating how the microscopic properties of the gas (i.e. density, temperature, chemical abundances) affect macroscopic processes such as star formation we will gain vital new insight into the star formation and ISM evolution of galaxies throughout the history of the Universe. To this end we will make use of new, extremely powerful telescopes in space and on the ground such as the Herschel Space Observatory and the Atacama Large Millimeter Array, which are able to overcome many of the difficulties that have faced ground-based observations to date. These facilities represent a quantum leap in our ability to study the gas and dust in distant galaxies, and will open up a new window in the study of star formation and ISM evolution in galaxies.