Astronomers using the FAST radio telescope and DESI have discovered that the universe’s atomic hydrogen gas supply has barely dropped over 4.5 billion years, despite star formation falling by more than half.
Star Formation Plummets While Fuel Renders Abundant
Across the cosmos, stars are being born at less than half the rate they managed 4.5 billion years ago. For decades, astronomers operated under a straightforward assumption to explain this quiet period: galaxies simply burned through their raw materials, exhausting the atomic hydrogen necessary to spawn new stars. However, new measurements tell a starkly different story.
Data compiled by an international research team show that the atomic hydrogen gas feeding star formation has barely budged over that same 4.5-billion-year stretch. The measurement comes from Hong Guo at the Shanghai Astronomical Observatory, part of the Chinese Academy of Sciences. Guo worked alongside Chuan-Peng Zhang, Yizhou Gu, and researchers from Shanghai Jiao Tong University and the Dark Energy Spectroscopic Instrument, known as DESI, to map atomic hydrogen across roughly a third of the sky, encompassing close to 2.5 million galaxies.
Sorting those galaxies by distance allowed the team to compare the current era with one about 4.5 billion years ago. While star formation was roughly two and a half times higher back then, the atomic hydrogen supply told an opposing tale. Measured the same way across both eras, the gas supply dropped by a factor of only about 1.35. Once the team corrected for potential observational errors, that drop shrank further to roughly 1.12. At every stellar mass the team measured, the ratio of gas to stars remained steady in small galaxies and giants alike.
Combining FAST and DESI to Catch a Faint Signal
Capturing atomic hydrogen across deep cosmic space presents a severe technical hurdle. Hydrogen gives off a radio signal at a wavelength of 21 centimeters, but that signal is extraordinarily faint. A single distant galaxy’s share normally dissolves completely into background noise, forcing astronomers to choose between scanning a small patch of sky deeply or scanning the whole sky and missing the data entirely.
The research team circumvented this trade-off by combining two massive instruments built for entirely separate missions. They utilized FAST, a radio dish 500 meters across that spans roughly the length of five football fields laid end to end, making it the largest single-dish radio telescope on Earth. They paired it with DESI, a robotic instrument that records the exact distance to millions of galaxies at once while mapping the universe’s expansion to track dark energy.
Using DESI’s spatial map as a targeting guide, the team pointed FAST to accumulate hydrogen signals galaxy by galaxy until a clean reading emerged from the background noise. The resulting data demonstrated that the atomic hydrogen reservoir stayed full while the stellar river slowed to a trickle.
Testing Galaxy Models Against Cosmic Realities
Molecular gas is the material that actually collapses to form stars. While atomic hydrogen sits upstream like a full reservoir, something prevents it from densifying into molecular clouds.
Uncovering the Milky Way’s Primordial Merger
While Guo’s team investigated universal star formation brakes, other astronomical teams pushed further back into galactic history to reconstruct how our own galaxy assembled its early mass. Researchers studying stellar clusters near the center of the Milky Way discovered evidence that our galaxy swallowed a smaller dwarf galaxy roughly 11.8 billion years ago, when the universe was less than 15% of its current age.
Published in Nature Astronomy and led by Davide Massari of the Astrophysics and Space Science Observatory of Bologna and the National Institute for Astrophysics in Italy, the study utilized precision ages of dense stellar clusters to date the ancient collision. The incoming dwarf galaxy contained stars with a mass equivalent to 500 million Suns, roughly a quarter the size of the Milky Way at the time.
Davide Massari of the National Institute for Astrophysics stated that the key finding of their research was the unambiguous discovery of the first merger event experienced by their galaxy in its infancy.
The researchers named this primordial building block Low-energy–Kraken–Heracles, or LKH, combining designations from three earlier studies. Massari noted that while the collision of stars was peaceful, the gas interaction proved violent. Massari explained that the collision between the gas from LKH and that from the Milky Way triggered the formation of many stars, noting that the Milky Way later absorbed Gaia–Sausage–Enceladus around 10 billion years ago.
Probing Cosmic Dawn Through Gravitational Waves
Further expanding our view of the early universe, another study published in Physical Review D explores whether a mysterious cosmic hum detected by Pulsar Timing Arrays holds clues to the emergence of early supermassive black holes. Colgate University researchers Sohan Ghodla and Cosmin Ilie examined whether black hole seeds left behind by supermassive Dark Stars could generate a dominant share of the gravitational wave background currently measured by international pulsar networks.
Cosmin Ilie, Colgate University
Ilie noted that their work demonstrates how the signal may also contain information about how the ancestors of those black holes formed at cosmic dawn. The researchers modeled primordial Dark Stars that gathered material powered by dark matter rather than nuclear fusion, growing to a million solar masses before collapsing into heavy seeds. Their findings indicate that current gravitational wave measurements can constrain how common those primordial seeds were. As Ghodla cautioned, producing too many of these massive seeds would result in over-producing the PTA-detected signal, whereas producing too few would require other sources to efficiently assemble these supermassive black holes later in the life of the universe to match PTA observations.
