When did the first stars start shining? Researchers are now closer than ever to the answer. Work led by scientists in the UK places cosmic dawn between 250 and 350 million years after the Big Bang. The researchers also believe the first galaxies hosting these first stars might soon become observable to our instruments.
The research, published in the Monthly Notices of the Royal Astronomical Society, set out to expand our understanding of one of the most mysterious times of our universe: the cosmic dark ages. For hundreds of millions of years, no (visible) light shone in the cosmos. Slowly but surely, gas began to clump up in large clouds – and from these clouds, due to gravitational collapse, the first stars were born.
The team estimated this by looking at six of the furthest galaxies ever discovered. Looking far into the universe is like looking into the past, due to the finiteness of the speed of light. The light of these half-a-dozen objects comes to us from when the Universe was just 550 million years old. They then estimated the age of these galaxies, suggesting when the stars in them were born.
“Witnessing the moment when the universe was first bathed in starlight is a major quest in astronomy,” lead author Dr Nicolas Laporte, from the University of Cambridge, said in a statement.
“Our observations indicate that cosmic dawn occurred between 250 and 350 million years after the beginning of the universe, and, at the time of their formation, galaxies such as the ones we studied would have been sufficiently luminous to be seen with the James Webb Space Telescope.”
Thanks to observations from the Hubble and Spitzer space telescopes, the team was able to estimate the presence of atomic hydrogen. During the cosmic dark ages, all hydrogen was atomic hydrogen, but the light of stars ripped the electrons from those hydrogen atoms (a process called ionization). By the end of cosmic dawn, the vast majority of hydrogen in the universe was once again ionized.
This fact is important – by estimating how much hydrogen is left to be ionized in a galaxy, you can work out how long its stars have been active. It is a good way to date the formation of these objects and their stars.
“This age indicator is used to date stars in our own neighbourhood in the Milky Way but it can also be used to date extremely remote galaxies, seen at a very early period of the universe,” added co-author Dr Romain Meyer, from University College London and the Max Planck Institute for Astronomy in Heidelberg, Germany.
“Using this indicator we can infer that, even at these early times, our galaxies are between 200 and 300 million years old.”
The light of the first galaxies might soon be in our grasp (and telescopes).