Star Formation and the ISM Evolution of Galaxies Across Cosmic Time

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.