Astronomers have discerned a key signal whose absence makes it possible to determine which were and were not the first galaxies to exist, in a study of the so-called ‘cosmic dawn’.

Using data from the SARAS3 radio telescope in India, an international team of researchers led by the University of Cambridge (UK) have been able to observe the early Universe. -barely 200 million years after the Big Bang- and put limits on the mass and energy of the first stars and galaxies. Counterintuitively, the researchers were able to establish these limits in the first galaxies when they did not find the signal they were looking for, known as the 21-centimeter line of hydrogen. They publish results in Nature Astronomy. This non-detection allowed the researchers to make other determinations about the cosmic dawn, putting limits on the first galaxies, which allowed them to rule out scenarios that included galaxies that were inefficient heaters of cosmic gas and efficient producers of radio emissions. Although these first galaxies cannot yet be directly observed, the results represent an important step in understanding how our Universe went from being mostly empty to being filled with stars.

Understanding the early Universe, when the first stars and galaxies formed, is one of the main objectives of the new observatories. The results obtained with the SARAS3 data are a proof of concept that paves the way to understand this period of the development of the Universe. The SKA project – which includes two next-generation telescopes scheduled for completion by the end of the decade – will probably be able to image the early Universe, but for current telescopes the challenge is to detect the cosmological signal from the first stars re-aired by cloud clouds. thick hydrogen. This signal is known as the 21-centimeter line, a radio signal produced by hydrogen atoms in the early Universe. Unlike the recently launched JWST, which will be able to directly image individual galaxies in the early Universe, surveys of the 21-centimeter line, conducted with radio telescopes such as the REACH (Radio Experiment for the Analysis of Cosmic Hydrogen), led by Cambridge, they can tell us about entire populations of even earlier galaxies. The first REACH results are expected in early 2023. To detect the 21-centimeter line, astronomers look for a radio signal produced by hydrogen atoms in the early Universe, affected by light from the first stars and radiation behind the hydrogen fog. Earlier this year, the same researchers developed a method they say will allow them to see through the mists of the early universe and detect light from the first stars. Some of these techniques have already been put into practice in the current study.

In 2018, another research group operating the EDGES experiment published a result that hinted at a possible earlier detection of this light. The reported signal was unusually strong compared to what is expected in the simplest astrophysical image of the early Universe. Recently, data from SARAS3 refuted this detection: the EDGES result is still awaiting confirmation from independent observations. In a new analysis of the SARAS3 data, the Cambridge-led team tested a number of astrophysical scenarios that could explain the EDGES result, but found no corresponding signal. Instead, the team was able to set some limits on the properties of the first stars and galaxies.

The results of the SARAS3 analysis are the first time that radio-averaged 21-centimeter line observations have been able to provide insight into the properties of early galaxies in the form of boundaries of their main physical properties. The Cambridge team, working with collaborators from India, Australia and Israel, used data from the SARAS3 experiment to search for signs of the cosmic dawn, when the first galaxies formed. Using statistical modeling techniques, the researchers were unable to find any signals in the SARAS3 data. “We were looking for a signal of a certain amplitude,” explains Harry Bevins, a PhD student at Cambridge’s Cavendish Laboratory and lead author of the paper. “But by not finding that signal, we can put a limit on its depth. That, in turn, begins to tell us about the brightness of the first galaxies”.

“Our analysis has shown that the hydrogen signal can tell us about the population of the first stars and galaxies,” said co-author Dr Anastasia Fialkov, from the Cambridge Institute of Astronomy. “Our analysis places limits on some of the key properties of the first light sources, including the masses of the first galaxies and the efficiency with which these galaxies can form stars. We also address the question of the efficiency with which these sources emit X-ray, radio and ultraviolet radiation.” “This is a first step for us in what we hope will be a decade of discoveries about how the Universe went from darkness and a vacuum to the complex realm of stars, galaxies and other celestial objects that we can see from Earth today. “, highlights Dr. Eloy de Lera Acedo, from the Cavendish Laboratory in Cambridge, who co-directed the research.

“Our data also reveals something that has been hinted at before, which is that the first stars and galaxies might have made a measurable contribution to the background radiation that appeared as a result of the Big Bang and has been traveling towards us ever since,” he explains. De Lera Acedo– We are also setting a limit to that contribution.” “It’s amazing to be able to look so far back in time – to just 200 million years after the Big Bang – and be able to learn about the early Universe,” concludes Bevins.