The First Stars and Galaxies

Galaxies in the early Universe

After the Big Bang, the Universe expanded, cooled off and eventually allowed protons and electrons to form neutral hydrogen. However, at some point during the first billion years of cosmic history, the Universe was flooded with highly energetic photons which brought the hydrogen in the intergalactic medium back into a highly ionized state. The origin of these ionizing photons remains unclear, which makes cosmic reionization one of the great unsolved puzzles of the early Universe. While a host of astronomical objects could in principle have contributed to this process, star-forming galaxies are usually seen as the top candidates, since they are already known to exist in great numbers at the relevant epoch.

Since the light from some of the most distant galaxies we currently know of have taken more than ten billion years to reach us, we can learn more about reionization-epoch galaxies by studying these extremely faint and far-away objects. Our team is using galaxies and galaxy clusters as gravitational lenses to hunt such objects down, and is using numerical models to learn more about their properties.

Image showing the Hubble eXtreme Deep Field. In the image is thousands of galaxies.
The Hubble eXtreme Deep Field – one of the deepest astronomical images ever taken – featuring more than 10,000 galaxies in a region about a hundred times smaller than the size of the full moon in the sky. This image contains some of the faintest and most distant galaxies currently known. Image: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team

The first stars

The very first stars likely formed when the Universe was about 100 million years old, prior to the formation of the first galaxies. As the elements that make up most of planet Earth had not yet formed, these primordial objects – known as population III stars – were made almost entirely of hydrogen and helium. As they exploded as supernovae, they ejected the heavy elements produced in their interiors into the interstellar medium. This started the cosmic chemical enrichment that led to the formation of the stars that we see in the Milky Way today, to rocky planets and eventually humans. Simulations and theoretical arguments predict population III stars to be substantially more massive than the chemically enriched stars that formed at later epochs. However, none of this has been observationally confirmed, since no bona fide population III objects have so far been found. Members of our group are using a combination of observations and numerical models to search for signatures of these stars in the first generations of galaxies.

Artistic image of the first stars in the Universe.
Artist's impression of the first stars in the Universe. Image: NASA

Contacts: Erik Zackrisson, Martin Sahlén

Last modified: 2021-12-21