An unbiased and resolved view of cosmic star formation

Remarkable progress in our understanding of the formation and evolution of galaxies has been made over the last few decades. Detailed observations of large samples of galaxies across cosmic time, combined with cosmological simulations, have provided considerable insight into the physical processes that must be driving galaxy evolution. In a simple picture, galaxies accrete gas from the surrounding large-scale structure (known as the cosmic web) and use that gas to form stars, thereby increasing their stellar mass and their luminosity. Mergers with other galaxies can also cause a galaxy to grow. At some point in its evolution, the galaxy's supply of gas is somehow shut off, perhaps due to the effects of supernova explosions or the energy input from a growing massive black hole at the galaxy centre; this causes the galaxy to stop forming stars and to transition in colour from blue to red. Within this simple picture, however, many fundamental questions remain about the precise physical processes that regulate star formation across cosmic time.

One of the main challenges in studying star formation on cosmological scales is that of simply measuring how much star formation is going on in a galaxy, and relating that to the other properties of the galaxy. Star formation is easily traced by the bright blue (or ultraviolet) light emitted by a collection of newly-formed stars. However, star-forming galaxies are generally filled with cosmic dust, and the effect of that dust is to absorb a lot of the blue light. This not only affects our estimate of the amount of on-going star formation, but also our picture of how that star formation is distributed within the galaxy. This project aims to overcome these shortcomings by using two new transformational surveys, led by the applicant, that will enable us to map out the star-formation within galaxies right out to the furthest reaches of the Universe, using techniques which are not affected by these dust biases.

First, our JELS survey (JELS is the 'JWST Emission Line Survey') takes advantage of the new capabilities of the James Webb Space Telescope (JWST) to observe galaxies in a few well-defined slices of cosmic time using emission from Hydrogen atoms: Hydrogen is the most common element in the Universe and is easily ionised within star-forming regions, so the amount of Hydrogen emission traces the amount of star formation that is going on. With the JWST we are able to overcome previous limitations caused by the Earth's atmosphere and study the Hydrogen emission in many hundreds of galaxies to earlier cosmic times than ever before. With the precision images of JWST, we can also measure the internal structures of the galaxies.

Second, we will take advantage of the deepest wide-area radio survey ever undertaken, known as the LoTSS Deep Fields. This is a survey being carried out with the Low Frequency Array (LOFAR), by observing the best-studied regions in the Northern sky repeatedly for hundreds of hours. The radio images reach sufficient sensitivity to be able to detect and map out star formation in very distant galaxies through the emission of their supernovae. This survey is highly complementary to JELS, by covering enough sky area to be able to study the rarer, luminous galaxies.

In combination, these two surveys will provide a transformative, unbiased view of star formation across cosmic time. Our analysis of the star-forming galaxy populations, and their internal structures, will help us towards a comprehensive understanding of the physical processes that drive star-formation in galaxies across cosmic time, as well as those which ultimately lead to it being quenched.