In recent years, astronomers have made significant progress in understanding the properties of the first galaxies that formed in the early universe. These galaxies are thought to have formed shortly after the Big Bang, around 400 million years after the universe was born.
In one of the earliest astrophysics investigations of the cosmic dawn, the time in the early Universe when the first stars and galaxies emerged, scientists have been able to establish some important conclusions regarding the first galaxies to exist.
Researchers led by the University of Cambridge were able to look at the very early Universe just 200 million years after the Big Bang and set limitations on the mass and energy production of the first stars and galaxies using data from India’s SARAS3 radio telescope.
Counterintuitively, the researchers were able to place these limits on the earliest galaxies by not finding the signal they had been looking for, known as the 21-centimetre hydrogen line.
The lack of detection allowed the researchers to draw other conclusions about the cosmic dawn, putting constraints on the early galaxies, and ruling out possibilities such as galaxies that were both effective radio emitters and inefficient cosmic gas heaters.
The findings, which were published in the journal Nature Astronomy, mark a significant step in our understanding of how our Universe changed from being largely empty to one filled with stars, even though we are unable to directly view these early galaxies at this time.
Our data also reveals something which has been hinted at before, which is that the first stars and galaxies could have had a measurable contribution to the background radiation that appeared as a result of the Big Bang and which has been travelling towards us ever since. We are also establishing a limit to that contribution.
Dr. Eloy de Lera Acedo
One of the main objectives of new observatories is to understand the early universe, when the first stars and galaxies formed. The findings from the SARAS3 data are a proof-of-concept research that opens the door to comprehending this phase of the universe’s evolution.
It is likely that the SKA project, which will involve two next-generation telescopes and be finished by the end of the decade, will be able to capture images of the early Universe. However, the challenge for existing telescopes is to find the cosmological signal of the first stars, which is re-radiated by dense hydrogen clouds.
This signal is known as the 21-centimetre 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, studies of the 21-centimetre line, made with radio telescopes such as the Cambridge-led REACH (Radio Experiment for the Analysis of Cosmic Hydrogen), can tell us about entire populations of even earlier galaxies. The first results are expected from REACH early in 2023.
Astronomers search for a radio signal produced by hydrogen atoms in the early Universe that is influenced by light from the first stars and radiation hidden by the hydrogen fog in order to find the 21-centimeter line. Earlier this year, the same researchers discovered a technology which they say will allow them to see through the fog of the early cosmos and detect light from the earliest stars. Some of these methods have already been used in the investigation at hand.
In 2018, another research group operating the EDGES experiment published a result that hinted at a possible detection of this earliest light. The reported signal was unusually strong compared to what is expected in the simplest astrophysical picture of the early Universe. Recently, the SARAS3 data disputed this detection: the EDGES result is still awaiting confirmation from independent observations.
In a re-analysis of the SARAS3 data, the Cambridge-led team tested a variety of astrophysical scenarios which could potentially explain the EDGES result, but they did not find a corresponding signal. Instead, the team was able to place some limits on properties of the first stars and galaxies.
The SARAS3 analysis’s findings mark the first time that radio measurements of the averaged 21-centimeter line have been able to shed light on the characteristics of the early galaxies in the form of the upper and lower bounds of their primary physical features.
Working with collaborators in India, Australia and Israel, the Cambridge team used data from the SARAS3 experiment to look for signals from cosmic dawn, when the first galaxies formed. Using statistical modelling techniques, the researchers were not able to find a signal in the SARAS3 data.
“We were looking for a signal with a certain amplitude,” said Harry Bevins, a PhD student from Cambridge’s Cavendish Laboratory and the paper’s lead author. “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.”
“Our analysis showed that the hydrogen signal can inform us about the population of first stars and galaxies,” said 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.”
“This is an early step for us in what we hope will be a decade of discoveries about how the Universe transitioned from darkness and emptiness to the complex realm of stars, galaxies and other celestial objects we can see from Earth today,” said Dr. Eloy de Lera Acedo from Cambridge’s Cavendish Laboratory, who co-led the research.
The observational investigation, the first of its sort in many ways, rules out scenarios in which the earliest galaxies were both poor warmers of hydrogen gas and more than a thousand times as brilliant in their radio-band emission than contemporary galaxies.
“Our data also reveals something which has been hinted at before, which is that the first stars and galaxies could have had a measurable contribution to the background radiation that appeared as a result of the Big Bang and which has been travelling towards us ever since,” said de Lera Acedo, “We are also establishing 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,” said Bevins.
The research was supported in part by the Science and Technology Facilities Council (STFC), part of UK Research & Innovation (UKRI), and the Royal Society. The Cambridge authors are all members of the Kavli Institute for Cosmology in Cambridge.
By studying these galaxies, astronomers have been able to learn more about the early universe and the processes that led to the formation of galaxies as we know them today.
