A team of astronomers from the United Kingdom and the United States has examined six of the most distant galaxies currently known and found that the distance of these galaxies away from Earth corresponded to a look back time of more than 13 billion years ago, when the Universe was only 550 million years old. Using data from the NASA/ESA Hubble Space Telescope, NASA’s Spitzer Space Telescope, and several ground-based observatories, they have calculated the age of these galaxies as ranging from 200 to 300 million years, allowing an estimate of when their stars first formed.
This artist’s impression shows the evolution of the Universe beginning with the Big Bang on the left followed by the appearance of the Cosmic Microwave Background. The formation of the first stars ends the cosmic dark ages, followed by the formation of galaxies. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
“Theorists speculate that the Universe was a dark place for the first few hundred million years, before the first stars and galaxies formed,” said study’s lead author Dr. Nicolas Laporte, an astronomer in the Kavli Institute for Cosmology and the Cavendish Laboratory at the University of Cambridge.
“Witnessing the moment when the Universe was first bathed in starlight is a major quest in astronomy.”
“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.”
Dr. Laporte and his colleagues analyzed starlight from the galaxies as recorded by the Hubble and Spitzer space telescopes, examining a marker in their energy distribution indicative of the presence of atomic hydrogen in their stellar atmospheres. This provides an estimate of the age of the stars they contain.
This hydrogen signature increases in strength as the stellar population ages but diminishes when the galaxy is older than a billion years.
The age-dependence arises because the more massive stars that contribute to this signal burn their nuclear fuel more rapidly and therefore die first.
“This age indicator is used to date stars in our own neighborhood in the Milky Way but it can also be used to date extremely remote galaxies, seen at a very early period of the Universe,” said study’s co-author Dr. Romain Meyer, an astronomer in the Department of Physics and Astronomy at University College London and the Max Planck Institute for Astronomy.
“Using this indicator we can infer that, even at these early times, our galaxies are between 200 and 300 million years old.”
In analyzing the Hubble and Spitzer data, the astronomers needed to estimate the redshift of each galaxy which indicates their cosmological distance and hence the look-back time at which they are being observed.
To achieve this, they undertook spectroscopic measurements using several powerful ground-based telescopes: ALMA, ESO’s Very Large Telescope, the twin Keck telescopes, and Gemini-South telescope.
These measurements enabled the researchers to confirm that looking at these galaxies corresponded to looking back to a time when the Universe was 550 million years old.
“Over the last decade, astronomers have pushed back the frontiers of what we can observe to a time when the Universe was only 4% of its present age,” said study’s co-author Professor Richard Ellis, an astronomer in the Department of Physics and Astronomy at University College London.
“However, due to the limited transparency of Earth’s atmosphere and the capabilities of the Hubble and Spitzer space telescopes, we have reached our limit.”
“We now eagerly await the launch of the James Webb Space Telescope, which we believe has the capability to directly witness cosmic dawn.”
“The quest to see this important moment in the Universe’s history has been a holy grail in astronomy for decades.”
“Since we are made of material processed in stars, this is in some sense the search for our own origins.”
The findings were published in the Monthly Notices of the Royal Astronomical Society.
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N. Laporte et al. 2021. Probing cosmic dawn: Ages and star formation histories of candidate z ≥ 9 galaxies. MNRAS 505 (3): 3336-3346; doi: 10.1093/mnras/stab1239
