More clues to understand our early Universe

An array of 350 radio telescopes in the Karoo desert of South Africa is getting closer to detecting “the Epoch of Re-ionization” — the era just after the first stars ignited and galaxies began to bloom.

This is a great step forward for the understanding of our early universe. In effect, these observations represent the best evidence we have of heating of the intergalactic medium by early Galaxies.  Eloy de Lara Acedo

In a paper accepted for publication in The Astrophysical Journal, the Hydrogen Epoch of Reionization Array (HERA) team reports that it has doubled the sensitivity of the array, which was already the most sensitive radio telescope in the world dedicated to exploring this unique period in the history of the universe.

While they have yet to actually detect radio emissions from the end of the cosmic dark ages, their results do provide clues to the composition of stars and galaxies in the early universe. In particular, their data suggest that early galaxies contained very few elements besides hydrogen and helium, unlike our galaxies today.

When the radio dishes are fully online and calibrated, ideally this fall, the team hopes to construct a 3D map of the bubbles of ionized and neutral hydrogen as they evolved from about 200 million years ago to around 1 billion years after the Big Bang. The map could tell us how early stars and galaxies differed from those we see around us today, and how the universe as a whole looked in its adolescence.

“This is moving toward a potentially revolutionary technique in cosmology. Once you can get down to the sensitivity you need, there’s so much information in the data,” said Joshua Dillon, a research scientist in the University of California, Berkeley’s Department of Astronomy and lead author of the paper. “A 3D map of most of the luminous matter in the universe is the goal for the next 50 years or more.”

“This is a great step forward for the project and for our understanding of the early Universe,” said Eloy de Lera Acedo from Cambridge’s Cavendish Astrophysics, and involved since the early days of the project to design and develop the new wideband receiver system and companion electromagnetic models that allow HERA to explore the Universe at the times when the very first stars were born. “In effect, these observations represent the best evidence we have of heating of the intergalactic medium by early Galaxies.”

Other telescopes also are peering into the early universe. The new James Webb Space Telescope (JWST) has now imaged a galaxy that existed about 325 million years after the birth of the universe in the Big Bang. But the JWST can see only the brightest of the galaxies that formed during the Epoch of Reionization, not the smaller but far more numerous dwarf galaxies whose stars heated the intergalactic medium and ionized most of the hydrogen gas.

HERA seeks to detect radiation from the neutral hydrogen that filled the space between those early stars and galaxies and, in particular, determine when that hydrogen stopped emitting or absorbing radio waves because it became ionized.

The fact that the HERA team has not yet detected these bubbles of ionized hydrogen within the cold intergalactic medium rules out some theories of how stars evolved in the early universe.

Specifically, the data show that the earliest stars, which may have formed around 200 million years after the Big Bang, contained few other elements than hydrogen and helium. This is different from the composition of today’s stars, which have a variety of so-called metals, the astronomical term for chemical elements, ranging from lithium to uranium, that are heavier than helium. The finding is consistent with the current model for how stars and stellar explosions produced most of the other elements.

“Early galaxies have to have been significantly different than the galaxies that we observe today in order for us not to have seen a signal,” said Aaron Parsons, principal investigator for HERA and a UC Berkeley associate professor of astronomy.

The atomic composition of stars in the early universe determined how long it took to heat the intergalactic medium once stars began to form. Key to this is the high-energy radiation, primarily X-rays, produced by binary stars where one of them has collapsed to a black hole or neutron star and is gradually eating its companion. With few heavy elements, a lot of the companion’s mass is blown away instead of falling onto the black hole, meaning fewer X-rays and less heating of the surrounding region.

The new data fit the most popular theories of how stars and galaxies first formed after the Big Bang, but not others. Preliminary results from the first analysis of HERA data, reported a year ago, hinted that those alternatives — specifically, cold reionization — were unlikely.

“Our results require that even before reionization and by as late as 450 million years after the Big Bang, the gas between galaxies must have been heated by X-rays. These likely came from binary systems where one star is losing mass to a companion black hole,” Dillon said. “Our results show that if that’s the case, those stars must have been very low ‘metallicity,’ that is, very few elements other than hydrogen and helium in comparison to our sun, which makes sense because we’re talking about a period in time in the universe before most of the other elements were formed.”

"HERA provides the best existing information on the first billion years of cosmic history,” said Anastasia Fialkov from Cambridge’s Institute of Astronomy, whose group is focused on astrophysical interpretations for HERA. “We use the non-detection of the 21-cm hydrogen signal to constrain astrophysical scenarios and learn what the properties of the earliest stars and galaxies could and couldn't be."

“Observations by HERA and other similar radio telescopes are already paving the way towards studies of the critical period when the Universe transitioned from mostly simple and dark to a Universe full of lights and complex celestial objects,” de Lera Acedo said. “Soon we will be able to add to these studies with observations from the Cambridge led REACH telescope, designed to provide insights on the physics of the early Universe back to even earlier times, across larger scales in the sky and for a continuous cosmic timeline.”

The HERA collaboration is led by UC Berkeley and includes scientists from across North America, Europe and South Africa. The construction of the array is funded by the National Science Foundation and the Gordon and Betty Moore Foundation, with key support from the government of South Africa and the South African Radio Astronomy Observatory (SARAO). Five per cent of the funds for the telescope construction have come from the UK through the University of Cambridge.

Adapted from a press release by Robert Sanders first published on UC Berkeley’s website.

Image: HERA radio telescopes on site at the Karoo Radio Reseve in South Africa. Credit: Dara Stoner, HERA collaboration.