Characterising the earliest galaxies in the Universe

Formed around 200 million years after the Big Bang, the earliest galaxies in our Universe have been successfully characterised by a team of astrophysicists.

Led by Professor Rennan Barkana from the Sackler School of Physics and Astronomy at Tel Aviv University, the team discovered that early galaxies were relatively small and dim compared to their modern relatives. Being fainter, they likely only processed around 5% of their gas into stars.

Additionally, the earliest galaxies did not emit radio waves at an intensity that was much higher than that of modern galaxies.

The groundbreaking results were published in the journal Nature Astronomy.

Detecting signals from ancient hydrogen

“This is a very new field and a first-of-its-kind study,” explained Professor Barkana. “We are trying to understand the epoch of the earliest galaxies in the Universe, known as the ‘cosmic dawn,’ about 200 million years after the Big Bang.

“The James Webb Space Telescope, for example, can’t really see these stars. It might only detect a few particularly bright galaxies from a somewhat later period. Our goal is to probe the entire population of the first stars.”

According to the standard explanation, before stars began to fuse heavier elements inside their core, the Universe consisted only of hydrogen atoms from the Big Bang. Today, the Universe is also filled with hydrogen, but in modern times it is mostly ionised due to radiation from stars.

“Hydrogen atoms naturally emit light at a wavelength of 21 centimetres, which falls within the spectrum of radio waves,” said Professor Barkana. “Since stellar radiation affects the light emitted by hydrogen atoms, we use hydrogen as a detector in our search for the first stars: if we can detect the effect of stars on hydrogen, we will know when they were born, and in what types of early galaxies.

“I was among the first theorists to develop this concept 20 years ago and now observers are able to implement it in actual experiments. Teams of experimentalists all over the world are currently attempting to discover the 21cm signal from hydrogen in the early Universe.”

One of these teams is EDGES, which uses a fairly small radio antenna to measure the average intensity on the entire sky of radio waves arriving from different periods of the cosmic dawn. In 2018, the EDGES team announced that it had found the 21cm signal from ancient hydrogen.

Did the signal really come from hydrogen?

Professor Barkana stated: “There was a problem with the findings. We could not be sure that the measured signal did indeed come from hydrogen in the early Universe. It could have been a fake signal produced by the electrical conductivity of the ground below the antenna.

“Therefore, we waited for an independent measurement that would either confirm or refute these results. Last year, astronomers in India carried out an experiment called SARAS, in which the antenna was made to float on a lake, a uniform surface of water that could not mimic the desired signal. According to the results of the new experiment, there was a 95% probability that EDGES did not detect a real signal from the early Universe. SARAS found an upper limit for the genuine signal, implying that the signal from early hydrogen is likely significantly weaker than the one measured by EDGES.”

Barkana continued: “We modelled the SARAS result and worked out the implications for the earliest galaxies, such as what their properties were, given the upper limit determined by SARAS. Now, we can say for the first time that galaxies of certain types could not have existed at that early time.”

Summing up the team’s discoveries about the earliest galaxies, Professor Barkana concluded: “Modern galaxies, such as our own Milky Way, emit large amounts of radio waves. In our study, we placed an upper limit on the star formation rate in the earliest galaxies and on their overall radio emission.

“This is only the beginning. Every year, the experiments become more reliable and precise. Consequently, we expect to find stronger upper limits, giving us even better constraints on the cosmic dawn. We hope that in the near future we will have not only limits, but a precise, reliable measurement of the signal itself.”



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