The First Stars and Galaxies
Galaxies in the early Universe
After the Big Bang, the Universe expanded and cooled, eventually allowing protons and electrons to combine and form neutral hydrogen. However, during the first billion years of cosmic history, the Universe was flooded with highly energetic photons that reionized the hydrogen in the intergalactic medium. The exact source of these ionizing photons remains a mystery, making cosmic reionization one of the great unsolved puzzles of the early Universe. While various astronomical objects could have contributed to this process, star-forming galaxies are generally considered the primary candidates, as they were already abundant during this epoch.
Because the light from some of the most distant galaxies currently known has taken more than ten billion years to reach us, studying these extremely faint and far-off objects gives us valuable insights into reionization-era galaxies. Our team is using gravitational lensing imposed by galaxies and galaxy clusters to track down these objects and employing numerical models to investigate their properties.
The James Webb Space Telescope (JWST) has also provided new insights into the formation and assembly of galaxies at high redshifts (z > 10). Observations of the Balmer break in galaxies allow astronomers to assess the age and star formation history of these distant galaxies, providing a way to compare real data with simulations. Recent studies have shown that the observed strengths of the Balmer break generally match those predicted by simulations, although there are some outliers with weaker breaks than expected. These findings help refine our models of star formation in the early Universe.
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 around 100 million years old, long before the first galaxies emerged. These stars are referred to as primordial stars, or also known as Population III stars. Such stars are composed almost entirely of hydrogen and helium, as heavier elements had not yet been created. When these stars exploded as supernovae, they ejected the heavy elements formed in their cores into the interstellar medium, starting the process of cosmic chemical enrichment that eventually led to the formation of stars like those in the Milky Way, rocky planets, and, ultimately, life. Simulations suggest that Population III stars were much more massive than stars that formed later, but this remains unconfirmed, as no genuine Population III objects have been observed.
One of the key spectral features used to study early star formation is the Balmer break, which has been observed in galaxies such as MACS1149-JD1 at a redshift of about 9.1. The presence of a significant Balmer break in this galaxy indicates large fluctuations in the star formation rate before z ∼ 9, with extended periods of low star formation activity. While some simulations predict such variations, galaxies like MACS1149-JD1 appear to be rare in current models. Future observations with the JWST may help determine whether these galaxies are indeed uncommon or represent a more typical phase of early galaxy evolution.
Our team is combining observations and numerical models to search for signs of these first stars in the first generations of galaxies, aiming to uncover the secrets of the Universe's first light.
Artist's impression of the first stars in the Universe. Image: NASA