Thesis Topic: The evolution of the cosmic molecular gas mass budget

Thesis Supervisor: Martin Zwaan


The cosmic star formation history of the Universe shows a peak at about two billion years after the Big Bang, followed by an order of magnitude decline down to the present day. Our understanding of this evolution and of galaxy evolution in general, can only be complete when the most fundamental galactic component - cold gas - is fully taken into account. Cold gas provides the fuel for the formation of stars and many of the properties of galaxies are determined by the amount of gas they contain, and specifically, how efficient they are in converting their innate gas content into stars. It is therefore essential to probe the evolution of cold gas over cosmic time. Neutral hydrogen provides the essential fuel, but this fuel has to cool and transform to the molecular phase in order to provide the necessary conditions for star formation. Therefore, observations of the evolution of the molecular gas provide a much more direct link to star formation over cosmic time. 

The molecular gas content of galaxies largely relies on observations of the second most abundant molecule, CO, whose bright transitions are observable to the distant Universe. ALMA has already made significant steps in establishing the evolution of the molecular gas, in particular through deep ‘blind’ observations, or observations of individual galaxies. The results of such surveys are however far from conclusive for two reasons: 1) given the small size of the survey areas the effects of cosmic variance are potentially very large, and 2) due to the limited telescope time available the galaxy sample sizes are small, leading to large Poisson errors. 

In this project, we address both these issues by using ALMACAL, a survey based on ALMA calibrator data. ALMACAL observations cover the whole southern sky and the total survey volume exceeds any previous survey by at least a factor of 10. The survey is managed and hosted at ESO.

The main aims for the student project are to quantify the molecular gas budget of the Universe and the impact of the molecular gas content of galaxies on the Universe's star formation history. The student will develop algorithms to automate the construction of ALMA image cubes, run source-finding algorithms etc. They will construct CO-luminosity functions as a function of redshift and evaluate the total cosmic molecular gas mass density. The results will be compared with models and auxiliary observations and eventually interpreted in the general context of galaxy evolution.