Researchers explain how were galaxies different in early universe

WASHINGTON: The most sensitive telescope now searching for radio signals from cosmic dawn, an era around 200 million years after the Big Bang when stars ignited, has doubled its sensitivity, a new paper reports.

While not yet detecting this radiation, the redshifted 21-centimeter line they have put new limits on the elemental composition of galaxies during the Epoch of Reionization.

Early galaxies seem to be low in metals, fitting the most popular theory of cosmic evolution. 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." 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 hydrogen of the cosmic dark age 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 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.

"In particular, their X-ray characteristics have to have changed. Otherwise, we would have detected the signal we're looking for."

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."