The James Webb Space Telescope (JWST) has revolutionized our understanding of the early universe, pushing the boundaries of what we can observe and study. In just four years, it has enabled astronomers to peer back to a time when the universe's first stars and galaxies were forming, an era known as cosmic dawn. This remarkable achievement is thanks to a recent multi-spectral survey, which has revealed a steep drop in galaxy formation just 150 to 200 million years after the Big Bang. This is an incredibly short window of time, comparable to the area of a full moon in the sky.
The study of these early galaxies provides a unique glimpse into the initial conditions that set the stage for the universe's evolution. It allows us to explore the formation of chemical abundances, supermassive black holes, and large-scale structures that we observe today. Richard Ellis, a professor of astrophysics at University College London (UCL), has been at the forefront of this research, driven by his passion for understanding the distant universe.
Ellis's journey began as an undergraduate 58 years ago, when high redshift meant luminous quasars, now known as active galactic nuclei (AGN). Back then, telescopes were limited, and photographic plates were used. Despite these constraints, Ellis's curiosity and determination led him to explore the cosmos, and he played a pivotal role in advocating for the James Webb Space Telescope's development.
The JWST has enabled Ellis and his colleagues to study the universe when it was only 200 million years old, revealing insights into how the universe evolves. They are witnessing the emergence of the first galaxies from darkness, a moment known as cosmic dawn. These early galaxies are incredibly small, only 60 to 70 light-years across, but they produce stars 20 times faster than our Milky Way galaxy.
Understanding galaxy evolution is a complex quest. One of the primary goals is to find short-lived Population III stars, which are free of heavy elements and only consist of hydrogen and helium. These stars are massive and live for only 5 million years before exploding, polluting the gas with heavy elements. Identifying these stars is crucial for understanding the early universe and the formation of subsequent generations of stars and planets.
Ellis highlights the importance of studying these early times, emphasizing that we are made of the material synthesized in stars. The chemistry that led to our existence began at cosmic dawn. Without understanding the first galaxies and stars, we cannot fully grasp astrobiology. As these stars explode, they create clouds of gas and dust that nurture the next generation of stars, potentially giving rise to planets with conditions suitable for life.
The search for cosmic dawn involves various methods. One approach is to look for chemically pristine galaxies that have not been polluted by supernova explosions. This is challenging, as it requires unequivocally demonstrating the absence of oxygen emissions. Another method involves tracing the declining abundance of star-forming galaxies and chemical elements with increasing redshift. This approach requires a vast amount of spectra, which is currently beyond our capabilities.
Additionally, there is an alternative route to finding cosmic dawn that doesn't rely on conventional telescopes. It involves searching for the Lyman alpha signature of hydrogen gas at cosmological distances. The Square Kilometer Array (SKA) in Western Australia has the potential to make this detection in the radio spectrum. When galaxies first lit up the universe with starlight, gas clouds were heated, emitting the Lyman alpha line, which can be observed against the Cosmic Microwave Background (CMB).
Ellis and his colleagues expect to see the 21cm line of hydrogen redshifted in absorption as it is observed against the CMB. This correlation between the Lyman alpha line and the 21cm line provides a fascinating insight into the stability of hydrogen at its most fundamental energy level. Despite some arguing that studying these early times is irrelevant to our daily lives, Ellis emphasizes the profound connection between our existence and the cosmos, from the Big Bang to cosmic dawn.
