MeerKAT Detects How Cosmic Hydrogen Clumps on the Largest Scales, a 21-cm First
After a decade of work, the MeerKLASS team pulled a faint signal out of foregrounds 10,000 times stronger, showing hydrogen follows the cosmic web.
An international team has made the first measurement of how neutral hydrogen clumps across the largest scales of the universe, a step toward using radio telescopes to chart the cosmos in three dimensions. The MeerKLASS Collaboration reported the result in the Monthly Notices of the Royal Astronomical Society, after a decade of work.
The group used data from the MeerKAT radio telescope in South Africa, through the MeerKAT Large Area Synoptic Survey. The method is called 21-cm intensity mapping. Instead of picking out individual galaxies, it adds up the hydrogen radio emission from many galaxies within big patches of sky, capturing the collective pattern even when single galaxies cannot be resolved. The goal is a huge 3D map of the universe.
For this detection the team analyzed observations covering about 250 square degrees of sky, roughly the area of 1,250 full moons. The signal they measured left its source about 4.5 billion years ago, when the universe was a little older than 9 billion years. They showed that the hydrogen's distribution follows the structure of the cosmic web, which cosmologists have sought to map on the largest scales for decades.
The hard part was noise. Astrophysical foreground emission is about 10,000 times stronger than the hydrogen signal. The paper's team compared the task to trying to make out a whispered conversation from the front row of a roaring rock concert.
Their solution was a new observing mode for MeerKAT. Rather than combine its 64 dishes into one interferometer, the team ran each dish as an independent telescope, trading sharp angular resolution for very high survey speed. That produced many independent maps from different dishes and observing times. Real cosmic structure shows up across all of them, while instrumental effects that differ from map to map are suppressed. Cross-comparing the maps helped beat down systematic errors and confirm a signal standing more than six times above the statistical noise.
Dr. Steven Cunnington, a senior research fellow at the University of Portsmouth's Institute of Cosmology and Gravitation, developed the analysis pipeline that made the detection possible. He said the result is the culmination of a journey that began when he started his Ph.D. at Portsmouth more than a decade ago. "Many of the challenges addressed in this work have shaped my own research career," he said.
Professor Mário Santos of the University of the Western Cape, principal investigator of MeerKLASS, called the result "a milestone in establishing 21-cm intensity mapping as a novel tool for cosmology." He said the measurement took a decade of patient work by researchers in seven countries on three continents, many of them early-career scientists, and credited the South African Radio Astronomy Observatory, which built and operates MeerKAT. The first author of the paper is Matilde Barberi-Squarotti.
