Understanding how the first galaxies formed is one of the most important unanswered questions in astronomy. One way of studying the first galaxies is to directly observe the light emitted by the stars and gas they contain. This can be difficult though, as many of the galaxies are very faint. However, galaxies do not grow in isolation, but rather they are surrounded by a cosmic web of gas. Another way we can learn about the first galaxies is by studying this connection between the galaxies and the intergalactic gas.
Galaxies can interact with the gas surrounding them in different ways. After the Big Bang, the gas in the cosmic web was cold and neutral until the first galaxies in the Universe formed. Light from the stars formed in these galaxies ionized and heated the cosmic web, in a period known as the epoch of reionization. As well as this, outflows driven by supernovae in the galaxies enriched the cosmic web with elements such as carbon and oxygen. Carefully characterising the properties of the cosmic web in the early Universe can therefore provide insights into the nature of the first galaxies.
The cosmic web can be observed as a pattern of absorption features in light coming from distant, luminous sources. By making measurements of the statistics of these absorption features and comparing with statistics derived from cosmological simulations, the ionization state, temperature and chemical composition of the intergalactic gas can be unveiled. A key objective of the GalaxyConnect project was to combine theoretical models and observational data to understand how the cosmic web evolves.
As well as this, the light coming from the faintest galaxies can be measured using a technique called intensity mapping. This involves making a map across the sky of gas emission coming from certain elements or molecules. This process relates to another objective of the GalaxyConnect project, which was to understand the emission signature of galaxies into the reionization epoch.
