We are yet to observe the very first stars inhabiting the very first galaxies in the universe. Astronomers have been hunting for indirect signals related to them with not much luck, as one such detection is considered controversial at best. But the lack of signals turns out to be equally informative and can tell us a lot about what these first cosmic objects were like.
Researchers on a new paper set out to measure the so-called 21-centimeter line, also known as the hydrogen line, which is a particular wavelength of light produced by neutral hydrogen. Most of the hydrogen in the universe today is ionized, meaning that the atom has lost its electron. But during the first few hundred million years after the Big Bang, before the light of stars ionized it, most of the hydrogen was neutral.
So, by mapping the intensity of this line when the universe was just 200 million years old, researchers hope to get an understanding of how the universe came out from the cosmic dark ages and how the first galaxies came to be. But the signal is not easily found.
"We were looking for a signal with a certain amplitude,” Harry Bevins, a PhD student from the University of Cambridge’s Cavendish Laboratory and the paper’s lead author, said in a statement. “But by not finding that signal, we can put a limit on its depth. That, in turn, begins to inform us about how bright the first galaxies were.”
Back in 2018, the EDGES experiment reported findings consistent with the detection of this important line. But the signal appeared too strong compared to the expectations from models, and it is yet to be corroborated independently. SARAS3 Data used in this study disputes this detection.
The lack of signals is still informative and allowed astronomers to rule out certain scenarios. Including one that saw the first galaxies being very bright in radio waves while being poor heaters of cosmic gas around them.
“Our analysis showed that the hydrogen signal can inform us about the population of first stars and galaxies,” added co-lead author Dr Anastasia Fialkov from Cambridge’s Institute of Astronomy. “Our analysis places limits on some of the key properties of the first sources of light including the masses of the earliest galaxies and the efficiency with which these galaxies can form stars. We also address the question of how efficiently these sources emit X-ray, radio and ultraviolet radiation.”
Follow-up work is expected in the first few months of 2023, with results from the REACH telescope (Radio Experiment for the Analysis of Cosmic Hydrogen).
The findings presented today were published in the journal Nature Astronomy.