Ancient Galaxies and Cosmic Evolution: A Deep Look Back in Time

The Early Universe Through the Oldest Galaxies

The oldest galaxies in the universe correspond to stellar systems formed in the first billion years following the Big Bang, about 13.8 billion years ago. Their physical study allows us to understand the fundamental mechanisms of formation and evolution of large cosmic structures. These so-called "primordial" or "ancient" galaxies are generally observed at very high redshift, implying that the light we receive has traveled for more than 12 billion years.

Physical Characteristics of the Oldest Galaxies

Physically, these galaxies often have low stellar mass compared to current galaxies, with very high star formation rates—a phenomenon known as "starburst." Their chemical composition is poor in heavy elements (metals) because the first stellar generations had not yet enriched the interstellar medium. This chemical composition, known as "low metallicity," is a key marker of their antiquity.

Dynamically, these galaxies frequently exhibit irregular morphologies due to intense gravitational interactions and frequent mergers in the young universe. Their gravitational potential is dominated by dark matter, whose distribution strongly influences their structural evolution.

Galaxy BX 442 (10.7 billion years)

Galaxy z8_GND_5296

Observation Methods and Physical Measurements

The observation of the oldest galaxies relies on detection in the far infrared and near infrared using space telescopes like Hubble or James Webb. The redshift $z$ is measured by spectroscopic analysis of emission and absorption lines, notably the Lyman-$\alpha$ line, which is a tracer of the presence of ionized hydrogen in these galaxies.

The distance $d$ to these galaxies is related to the redshift by the cosmological relation integrating the expansion of the universe, $$ d = c \int_0^z \frac{dz'}{H(z')} $$ where $c$ is the speed of light and $H(z)$ is the Hubble parameter at the corresponding epoch. These measurements allow us to estimate not only the distance but also the cosmic age of the observed galaxy.

Cosmological Importance of Primordial Galaxies

Primordial galaxies, which are among the first galaxies formed in the universe, play a crucial role in our understanding of cosmology.

• Formation and Evolution of Galaxies: Primordial galaxies offer clues about the formation and evolution of galaxies. Their study allows astronomers to understand how galaxies formed from the primitive matter of the universe and how they evolved over time.
• Reionization of the Universe: Primordial galaxies played a key role in the reionization process of the universe. After the Big Bang, the universe was filled with a fog of neutral hydrogen. The light from the first galaxies helped ionize this gas, making the universe transparent to ultraviolet and visible light.
• Dark Matter: The study of primordial galaxies also helps understand the distribution and role of dark matter in the universe. Dark matter is an invisible component that influences the formation and dynamics of galaxies.
• Testing Cosmological Models: Observations of primordial galaxies allow testing and refining cosmological models. They provide essential data for understanding the large-scale structure of the universe and the physical processes that govern its evolution.
• Formation of Elements: Primordial galaxies are also important for understanding the formation of chemical elements. The first stars in these galaxies produced the first heavy elements, which were then dispersed throughout the universe.
• Telescopes and Technologies: The study of primordial galaxies drives the development of new technologies and telescopes, such as the James Webb Space Telescope, designed to observe these distant and faint objects.

In summary, primordial galaxies are essential for understanding the early stages of the universe and the processes that led to its current structure. Their study continues to reveal valuable information about cosmic history and evolution.