Hubble data suggest first stars formed even earlier than previously thought

The galaxy cluster MACS J0416 is one of six studied by European researchers who took advantage of gravitational lensing to look for signs of first-generation population III stars in background galaxies dating back to within 500 million years of the Big Bang. None such evidence was found, indicating the first stars must have formed even earlier. Image: NASA, ESA, and M. Montes (University of New South Wales, Sydney, Australia)

One of the many profound questions about the birth of the universe is when the first stars and galaxies formed in the cooling aftermath of cosmic creation.

Those presumably massive first-generation population III stars must have been composed of the hydrogen, helium and trace amounts of lithium and beryllium, the only elements forged in the initial fireball. But no population III stars have ever been observed, possibly because they all exhausted their nuclear fuel long ago and only remnants remain today.

But that still leaves the question of when they first formed and what effect they had on the surrounding interstellar medium. A team of European astronomers used the Hubble Space Telescope, data from NASA’s now-retired Spitzer Space Telescope and the European Southern Observatory’s Very Large Telescope to study low-mass galaxies dating back to between 500 million and 1 billion years after the Big Bang.

To detect those faint galaxies, the team studied six remote galaxy clusters with Hubble’s Wide Field Planetary Camera 3 and the Advanced Camera for Surveys. The combined mass of the galaxies within each cluster acts as a gravitational lens, magnifying the light from even more remote background galaxies. The observations, carried out between 2012 and 2017, are among the deepest ever made, revealing galaxies 10 to 100 times fainter than any previously seen.

“We found no evidence of these first-generation population III stars in this cosmic time interval” said Rachana Bhatawdekar of the European Space Agency.

Such stars and the first galaxies must have formed even earlier than previously believed. The many observed low-mass galaxies suggests those galaxies may have produced the radiation needed to reionise the universe, breaking neutral hydrogen atoms apart and ending the so-called dark ages that followed the Big Bang.

“These results have profound astrophysical consequences as they show that galaxies must have formed much earlier than we thought,” Bhatawdekar said. “This also strongly supports the idea that low-mass/faint galaxies in the early Universe are responsible for reionisation.”