By not discovering something, astronomers have learned more about the first galaxies

Using data from India’s SARAS3 radio telescope, a research team led by PhD student Harry Bivins of the University of Cambridge has learned more about the properties of the first stars and galaxies in the very early universe (natural astronomy28 November).

Ironically, the researchers derived their new insights from the fact that the signal they were looking for – the so-called line 21 cm of hydrogen – not found. The non-discovery allows them to place restrictions on the first galaxies that existed 200 million years after the Big Bang. While we can’t yet directly observe these early galaxies, the new findings help us understand how the universe changed from a great void to a space full of stars.

Understanding the early universe, when the first stars and galaxies formed, is one of the main goals of the new large observatories under construction. One of these, the Square Kilometer Array (SKA), which should be operational by the end of this decade, will likely capture images of the early universe, but the challenge for current telescopes is detecting the cosmic signal from the first stars, as a rebroadcast by dense clouds. of hydrogen gas.

This radio signal – the 21-centimetre line – is caused by hydrogen atoms in the early universe. Unlike the recently launched Webb Space Telescope, which can directly image individual galaxies in the early universe, examining the 21-centimetre line with radio telescopes can tell us about entire groups of former galaxies.

To detect this signal, astronomers look for radio emissions from hydrogen atoms in the early universe, influenced by the light of the first stars and the radiation behind the “hydrogen haze.” Earlier this year, the same researchers developed a method they say allows them to see through this haze and detect light from the first stars. Some of these techniques have already been put into practice in the current research.

In 2018, another research group conducting the so-called EDGES experiment published a result that hints at a possible detection of this early light. However, we have not yet been able to confirm this result, and recent SARAS3 data seem to undermine it.

In their new analysis of the SARAS3 data, Bevins and colleagues tested several astrophysical scenarios that could explain the EDGES result, but found no corresponding signal. However, the team was able to impose some limitations on the properties of the first stars and galaxies.

In particular, the study rules out those scenarios in which, as suggested by the EDGES experiment, the oldest radio-wavelength galaxies were a thousand times brighter than today’s galaxies, but only moderately heated hydrogen gas.

The data also reveals something that has been hinted at before, that the first stars and galaxies may have made a measurable contribution to the cosmic background radiationIt is a leftover from the Big Bang. (EE)