Astronomers have used U.S. National Science Foundation National Radio Astronomy Observatory radio telescopes in Chile and New Mexico to peer through cosmic smoke and haze, building one of the clearest pictures yet of how giant clusters of young stars are born in the hearts of nearby galaxies. Focusing on two barred spirals, NGC 3351 and NGC 1097, the team used the Atacama Large Millimeter/submillimeter Array (ALMA) and the U.S. National Science Foundation Very Large Array (NSF VLA) to track baby star clusters from their earliest, dust-shrouded beginnings to the point when they start carving out glowing bubbles in their surroundings. The youngest, most massive clusters are usually hidden behind thick curtains of dust that block visible and even much of the infrared light they emit, but radio waves sail straight through, revealing what optical telescopes cannot see.
In many spiral galaxies, gas flowing inward along a central bar piles up in a dense, star-forming ring a few hundred to a thousand light-years from the core, acting like a cosmic factory for star clusters. In NGC 3351 and NGC 1097, these “circumnuclear” rings pack gas into compact, high-pressure clumps and ignite bursts of star formation at rates and densities similar to typical galaxies seen when the universe was only a few billion years old, making them nearby stand-ins for the early cosmos. By observing the ring of NGC 3351 with the NSF VLA at 33 gigahertz and with ALMA at several higher frequencies up to 350 gigahertz, and the larger, more active ring in NGC 1097 with ALMA alone, the team isolated dozens of compact “hot spots” where clusters are forming.
Different kinds of radio emission reveal different parts of a cluster’s story. Radio signals from ionized hydrogen gas trace the energetic glow around the hottest young stars, other radio signals come from high-energy particles launched by supernova explosions, and still others track the cold dust in the cluster’s birth cloud. By carefully measuring how each compact source brightens or fades across the radio spectrum, and using a method that looks for sources appearing in more than one radio band at the same spot on the sky, the researchers were able to detect fainter objects while still filtering out noise. Across both galaxies, the compact radio sources line up along the rings in bright knots, span sizes from a few light-years to a few dozen light-years, and show the energy output of anywhere from a handful to more than a thousand massive stars, placing many firmly in the “young massive cluster” candidate category.
Because the radio data allow astronomers to separate these different emission types, the team can effectively assign each source to a stage in a young cluster’s early life. Dust-bright, radio-faint clumps mark very early phases; clusters with strong ionized-gas and dust emission but little supernova-related emission appear to be extremely young and still buried; sources with strong ionized-gas emission but weak dust likely host clusters that have blown away most of their birth material; and sources dominated by supernova-related radio emission indicate clusters where some of the most massive stars have already exploded. By counting how many radio sources fall into each category and combining their sizes with the ionizing output inferred from the radio data, the researchers estimate how long clusters remain deeply embedded, how quickly they clear their cocoons, whether they are massive and tightly bound enough to survive, and how rapidly feedback from massive stars and supernovae reshapes their immediate environment.
The team also compared their radio-selected sources to high-resolution images from NASA’s James Webb Space Telescope to confirm that the compact radio sources truly correspond to star clusters and their surroundings, without going into the details of the infrared analysis. The conditions in these rings—thick gas, strong turbulence, and crowded neighborhoods—closely resemble those in typical massive galaxies at the peak of cosmic star formation, making nearby systems like NGC 3351 and NGC 1097 valuable laboratories for studying how the most massive clusters formed in the early universe. By using ALMA and the NSF VLA together as a kind of “radio time machine,” astronomers are now able to watch young massive cluster candidates in different stages of their early lives within the same galactic ring, providing crucial reality checks for theories of how quickly clusters assemble, how efficiently they turn gas into stars, and how stellar feedback shapes the densest star-forming environments in galaxies.
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