A spectroscopic study of the high-redshift Universe

Galaxies are systems in which stars are formed out of gas and dust. In each galaxy, millions to hundreds of billions of stars reside and evolve as time moves on. The first galaxies were probably formed as early as 200 million years after the Big Bang, in other words, 13.6 billion years ago. Young and low-mass galaxies are typically forming stars at a relatively high pace, while older and very massive galaxies are found to stop forming stars at some point. This shows that galaxies evolve strongly with time, and we need to study galaxies at high redshift, meaning relatively shortly after the Big Bang, to understand this evolution. In this thesis, I used spectroscopy to study the properties of galaxies in the period from one to two billion years after the Big Bang. Through a detailed study of their spectra, I found that gas flows out of these galaxies, which are thought to be responsible for stopping star-formation, are present in almost all galaxies, but are strongest in more massive star-forming systems. Although this research was conducted with the best telescopes in space and at Earth, I used gravitational lensing to study even fainter objects. In this way, I discovered some of the faintest galaxies ever observed at such redshifts, that were forming stars at a very high pace. Their properties are as expected if these galaxies played a major role in one of the outstanding questions of astronomy: what reionized the Universe during the Epoch of Reionization?