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Astronomers have used the Atacama Large Millimeter/submillimeter Array (ALMA) to peer into the early universe and uncover the building blocks of galaxies during their formative years. The CRISTAL survey—short for [CII] Resolved ISM in STar-forming galaxies with ALMA—reveals cold gas, dust, and clumpy star formation in galaxies observed as they appeared just 1 billion years after the Big Bang.

"Thanks to ALMA's unique sensitivity and resolution, we can resolve the internal structure of these early galaxies in ways never possible before," said Rodrigo Herrera-Camus, principal investigator of the CRISTAL survey, professor at Universidad de Concepción, and Director of the Millennium Nucleus for Galaxy Formation (MINGAL) in Chile. "CRISTAL is showing us how the first galactic disks formed, how stars emerged in giant clumps, and how gas shaped the galaxies we see today."

CRISTAL, an ALMA Large Program, observed 39 typical star-forming galaxies selected to represent the main population of galaxies in the early universe. Using [CII] line emission, a specific type of light emitted by ionized carbon atoms in cold interstellar gas, as a tracer of cold gas and dust, and combining it with near-infrared images from the James Webb and Hubble Space Telescopes, researchers created a detailed map of the interstellar medium in each system.

Among the key findings of the study in Astronomy & Astrophysics, is that most galaxies exhibited stellar birth in large clumps, each spanning several thousand light-years, revealing how star-forming regions assemble and evolve. A subset of galaxies showed signs of rotation, indicating the early formation of disk-like structures, which are precursors to modern spiral galaxies. The [CII] emission often extended far beyond the visible stars, indicating the presence of cold gas that may fuel future star formation or be expelled by stellar winds.

"What's exciting about CRISTAL is that we are seeing early galaxies not just as points of light, but as complex ecosystems," said Loreto Barcos-Muñoz, co-author of the study, astronomer at the U.S. National Radio Astronomy Observatory (NRAO), and ALMA point of contact for the survey. "This project shows how ALMA can resolve the internal structure of galaxies even in the distant universe—revealing how they evolve, interact, and form stars."

Two galaxies in the survey stood out. CRISTAL-13 features massive clouds of cosmic dust that block visible light from newborn stars. This light is reprocessed into millimeter wavelengths detectable by ALMA, revealing structures that are entirely hidden from telescopes observing in optical or infrared wavelengths.

CRISTAL-10 presents a puzzling case: its ionized carbon emission is unusually faint relative to its infrared brightness, a trait only seen in rare, heavily obscured galaxies like Arp 220 in the nearby universe. This suggests extreme physical conditions or an unusual power source in its interstellar medium.

"These observations highlight ALMA's potential as a time machine, allowing us to peer into the early ages of the universe," said Sergio Martín, Head of the Department of Science Operations at ALMA. "Programs like CRISTAL demonstrate the power of ALMA's Large Programs to drive high-impact science. They allow us to tackle the big questions of cosmic evolution with the unprecedented depth and resolution that only a world-class observatory like ALMA can provide."

By conducting the first systematic survey of the cold gas in early galaxies and comparing it with their stars and dust, CRISTAL offers a new window into cosmic history. The survey sets the stage for future observations that may uncover how galaxies transition from turbulent early phases to the well-structured systems we see in the local universe.

"CRISTAL provides the kind of multi-wavelength data that allows us to test and refine our theories of galaxy evolution," said Herrera-Camus. "This is a major step toward understanding how galaxies like our Milky Way came to be."