First galaxies determined by failure to detect key signal

First galaxies determined by failure to detect key signal

Researchers led by the University of Cambridge were able to make some observations of the earliest galaxies to exist using data from India’s SARAS3 radio telescope.

In one of the first astrophysical studies of the period of the Primitive universe, known as the Cosmic Dawn, researchers were able to observe the very first Universe and place limits on the mass and energy production of early stars and galaxies. Unexpectedly, this was done by not finding the signal they were looking for, known as the 21 centimeter hydrogen line.

Thanks to this non-detection, the researchers were able to impose constraints on the first galaxies, allowing them to rule out cosmic dawn scenarios, including galaxies that were inefficient heaters of cosmic gas and efficient producers of emissions. radio.

Although we cannot yet directly observe these first galaxies, the results, published in the study ‘Astrophysical constraints of SARAS 3’s non-detection of the 21 cm signal averaged by the cosmic dawn sky,’ represent an important step in understanding how our universe evolved from largely empty to a world full of stars.

Detection of the 21 centimeter line

Many new observatories aim to understand the beginning of the Universe, when the first stars and galaxies formed. The recent results are a proof-of-concept study and will pave the way to understanding this period in the development of the Universe.

Images of the early Universe should be made by the end of the decade, thanks to the two new generation telescopes of the SKA project. For current telescopes, however, the challenge is to detect the cosmological signal of the first stars re-emitted by thick clouds of hydrogen.

This signal is the 21 centimeter line, a radio signal produced by the hydrogen atoms in the primordial Universe. Studies of the 21-centimeter line, carried out with radio telescopes such as the Cambridge-led REACH (Radio Experiment for the Analysis of Cosmic Hydrogen) can shed light on entire populations of even older galaxies. The first results of REACH are expected in early 2023.

Astronomers detect the 21 centimeter line by searching for a radio signal produced by hydrogen atoms in the early Universe. This is affected by the light from early stars and the radiation behind the hydrogen fog.

Earlier this year, researchers developed a method that will allow them to see through the fog of the early Universe and detect light from the first stars. Some of these techniques have already been practiced in the present study.

Observation of the properties of the first galaxies

In 2018, another research group exploiting the EDGES experiment published a result suggesting a possible detection of this first light. Recent data from SARAS3 has challenged this detection. The EDGES result, which reported an unusually strong signal, now awaits confirmation from independent observations.

To potentially explain the EDGES result, the Cambridge-led team reanalyzed the SARAS3 data and tested a variety of astrophysical scenarios. However, a corresponding signal was not found; instead, the team was able to put some limits on the properties of early stars and galaxies.

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The results of this analysis allowed radio observations of the 21 centimeter mean line to reveal for the first time the limits of the main physical properties of the first galaxies.

Signal not detected

The researchers used statistical modeling techniques to look for signals from cosmic dawn, when the first galaxies formed, but couldn’t find any.

“We were looking for a signal with a certain amplitude,” said Harry Bevins, a PhD student at Cambridge’s Cavendish Laboratory and lead author of the paper. “But by not finding that signal, we can limit its depth. This, in turn, begins to tell us about the brightness of early galaxies. »

“Our analysis showed that the hydrogen signal can tell us about the population of early stars and galaxies,” said co-lead author Dr Anastasia Fialkov of the Cambridge Institute of Astronomy. “Our analysis places limits on some of the key properties of early light sources, including the masses of early galaxies and the efficiency with which these galaxies can form stars. We also address the issue of how effectively 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 evolved from darkness and emptiness to the complex realm of stars, galaxies and other celestial objects that we can see from Earth today,” Dr Eloy said. by Lera Acedo of the Cavendish Laboratory in Cambridge, who co-led the research.

The study, the first of its kind, rules out scenarios in which early galaxies were both more than a thousand times brighter than current galaxies in their radio emission and were poor heaters of hydrogen gas.

“Our data also reveals something that has been suggested before, which is that early stars and galaxies may have had a measurable contribution to the background radiation that appeared in the aftermath of the Big Bang and has been heading our way ever since,” said of Lera Acedo “We also set a limit to this contribution.”

“It’s amazing to be able to look so far back in time – just 200 million years after the Big Bang – and be able to learn more about the early Universe,” Bevins said.

The research was partially funded by the Science and Technology Facilities Council (STFC), part of UK Research & Innovation (UKRI) and the Royal Society.

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