ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, W. Zheng (JHU), M. Postman (STScI), the CLASH Team, Hashimoto et al.

Distant galaxy hints at universe’s earliest stars

The universe’s first stars may have formed a mere 250 million years after the big bang—hundreds of millions of years earlier than thought, according to a new study.

The find comes thanks to observations of an ancient galaxy known as MACS1149-JD1, first detected in 2012. In 2016 and 2017, a team of astronomers took another look using the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 64 radio telescopes in Chile. They measured the frequency of a peak in the galaxy’s spectrum that comes from ionized oxygen gas. When that light was produced in MACS1149-JD1 it was in the infrared, but during its billions of years journeying to Earth, the expansion of the universe stretched it out to the microwave frequencies that ALMA is sensitive to. Measuring that shift in frequencies reveals that the light set out 13.3 billion years ago, when the universe was just 550 million years old.

The fact that it was an oxygen line is significant: No oxygen was produced in the big bang; it was formed later when hydrogen gas coalesced into the first generation of stars and fusion reactions in their cores forged hydrogen into oxygen and other elements. At the end of their lives, the stars exploded, spreading those elements through space.

For MACS1149-JD1 to contain substantial amounts of oxygen, many stars must have already gone through that whole life cycle. The researchers report in Nature today that they then observed the galaxy in optical frequencies using the Hubble and Spitzer space telescopes to understand its star formation, concluding that it must have begun 300 million years earlier.

The findings pose problems for theorists because other evidence points to a later start for the first stars. Whether MACS1149-JD1 is just an outlier or the tip of an iceberg will have to wait for more observations.