Understanding obscured star formation through cosmic time

Early observations of nearby galaxies led astronomers to a startling revelation - most local galaxies are easy to classify into one of two groups. Either they are spiral shaped with ongoing star-formation and contain many young, blue stars (like our own Milky Way), or they have spheroidal (elliptical) shapes and are mostly composed of old, red stars. This relatively simple observation leads to the important questions of how the different galaxies formed and evolved, and why they now mostly occupy two distinct classes. One way to untangle the different effects is to study galaxies that are further away, which, due to the finite speed of light, also means that we see them when the Universe was younger and the galaxies were earlier in their own evolution.

However, a key difficulty in examining young galaxies in the distant Universe is that they can contain significant quantities of dust - small particles that act like smoke and block out much of galaxies' optical light. This dust makes the galaxies faint and difficult (sometimes impossible) to observe with optical and near-infrared telescopes, so without knowing how much light is hidden by dust we cannot accurately measure distant galaxies. Even worse, the dustiest (and optically hardest to detect) galaxies are the most active - they form new stars about 1000 times quicker than our Milky Way - and they push the limits of our understanding of galaxy evolution. In fact, since the discovery 20 years ago of a large population of distant dusty galaxies, observations of them have repeatedly disagreed with models and triggered new revelations in understanding how galaxies form and evolve.

To trace dust in galaxies and accurately measure how many stars are being formed at different times in the history of the Universe I use far-infrared light. Far-infrared data have shown that very dusty and active galaxies are rare in the local Universe, but there are many of them in the young, distant Universe, so they are thought to be a key stage in the evolution of galaxies. However, even the latest models and simulations of galaxy formation and evolution struggle to accurately reproduce the number and properties these distant active galaxies. The problem is particularly prevalent at the earliest times (within about a billion years of the Big Bang) when simulations rarely generate enough of these very dusty "starburst" galaxies. Different simulations also disagree about how to generate the most active dusty galaxies - some require collisions of two massive galaxies, but others can create starbursts from just a single galaxy.

During this fellowship I will use data from a new, exceptionally powerful, infrared observatory in Chile - the Atacama Large Millimeter/Submillimetre Array (ALMA), as well as other international telescopes to study the dust in distant galaxies and determine through observations which (if any) of the different galaxy evolution models is correct. Firstly, I will use ALMA to determine whether galaxy mergers are needed to trigger massive distant starbursts. I will then examine samples of the earliest dusty galaxies to measure whether they are really as bright and massive as the first observations suggested (a big problem for simulations), or whether they may be "gravitationally lensed" i.e. magnified and made to appear brighter by the mass of another galaxy in the foreground acting as a lens. If many of them are gravitationally lensed then our previous measurements of their sizes and activity levels would have been wrong, and that will again affect the galaxy evolution models. Finally, I will perform crucial preparation for the next generation of wide-field surveys, using statistics to establish new methods to accurately pinpoint far-infrared bright galaxies, which is a crucial step in being to study many individual galaxies in detail.