An array of 350 radio telescopes in South Africa’s Karoo Desert is moving closer to detecting the “cosmic dawn” – the era after the Big Bang when stars first ignited and galaxies began to blossom.
In a paper accepted for publication in The Astrophysical Journalthe Hydrogen Epoch of Reionization Array (HERA) team reports that it has doubled the sensitivity of the array, which was already the world’s most sensitive radio telescope dedicated to exploring this unique period in the universe’s history.
Although they have yet to detect radio emissions from the end of the cosmic dark ages, their results provide clues about the composition of stars and galaxies in the early universe. In particular, their data suggest that early galaxies contained very few elements other than hydrogen and helium, unlike our galaxies today.
When the radio antennas are fully online and calibrated, ideally this fall, the team hopes to build a 3D map of the ionized and neutral hydrogen bubbles as they evolved from about 200 million years ago to about 1 billion years after the Big Bang. The map could tell us how the early stars and galaxies were different from the ones we see around us today, and what the universe as a whole looked like in its teens.
“This leads to a potentially revolutionary technique in cosmology. Once you can get to the sensitivity you need, there’s so much information in the data,” said Joshua Dillon, a researcher at the University of California, Berkeley’s Department of Astronomy. and the main 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 are also peering into the early universe. The new James Webb Space Telescope (JWST) has now photographed a galaxy that existed about 325 million years after the birth of the universe in the Big Bang. But JWST can only see the brightest of the galaxies that formed during the Reionization Era, not the smaller but more numerous dwarf galaxies whose stars have heated the intergalactic medium and ionized most of the hydrogen gas.
HERA is trying to detect the 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 in the cold hydrogen of the cosmic dark age rules out some theories about how stars evolved in the early universe.
Specifically, the data show that the oldest stars, which may have formed about 200 million years after the Big Bang, contained few elements other 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 from lithium to uranium, that are heavier than helium. The discovery is consistent with the current model for how stars and starbursts produced most other elements.
“The early galaxies must have been significantly different from the galaxies we observe today for us not to have seen a signal,” said Aaron Parsons, principal investigator for HERA and UC Berkeley associate professor of astronomy. “In particular, their X-ray characteristics must have changed. Otherwise, we would have detected the signal we were 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. The key to this is the high-energy radiation, primarily X-rays, produced by binary stars where one of them has collapsed into a black hole or neutron star and is gradually eating away at its companion. With few heavy elements, much of the companion’s mass is thrown into the air instead of falling onto the black hole, meaning fewer X-rays and less heating of the surrounding region.
The new data fits the most popular theories about how stars and galaxies first formed after the Big Bang, but not others. Preliminary results from the first analysis of the HERA data, reported a year ago, suggested that those alternatives – in particular, cold reionization – were unlikely.
“Our results require that even before reionization and up to 450 million years after the Big Bang, the gas between galaxies must have been heated by X-rays. These probably originate from binary systems where a star loses mass to a companion black. hole,” Dillon said. “Our results show that, if that’s the case, those stars must have had very low ‘metallicity,’ meaning very few elements other than hydrogen and helium compared to our sun, which makes sense because we’re talking about a period in time in the universe before most other elements formed.”
The age of reionization
The origin of the universe in the Big Bang 13.8 billion years ago produced a hot cauldron of energy and elementary particles that cooled for hundreds of thousands of years before protons and electrons combined to form atoms – in mainly hydrogen and helium. Looking at the sky with sensitive telescopes, astronomers have mapped in detail the faint temperature variations from this moment – what is known as the cosmic microwave background – just 380,000 years after the Big Bang.
Apart from this relic heat radiation, however, the early universe was dark. As the universe expanded, the mass of matter seeded galaxies and stars, which in turn produced radiation—ultraviolet and X-rays—that heated the gas between the stars. At some point, the hydrogen began to ionize—it lost its electron—and formed bubbles inside the neutral hydrogen, marking the beginning of the Age of Reionization.
To map these bubbles, HERA and several other experiments focus on a wavelength of light that neutral hydrogen absorbs and emits, but ionized hydrogen does not. Called the 21-centimeter line (a frequency of 1,420 megahertz), it is produced by the hyperfine transition, during which the electron and proton spins switch from parallel to antiparallel. Ionized hydrogen, which has lost its only electron, neither absorbs nor emits this radio frequency.
Since the Age of Reionization, the 21-centimeter line has been redshifted by the expansion of the universe to a wavelength 10 times longer—about 2 meters, or 6 feet. The fairly simple HERA antennas, a construction of chicken wire, PVC pipe and telephone poles, are 14 meters in diameter to collect and focus this radiation onto the detectors.
“At two meters wavelength, a chicken wire mesh is a mirror,” Dillon said. “And all the sophisticated stuff, so to speak, is in the backend of the supercomputers and all the data analysis that follows.”
The new analysis is based on 94 observing nights in 2017 and 2018 with approximately 40 antennas – phase 1 of the array. Last year’s preliminary analysis was based on 18 nights of Phase 1 observations.
The main result of the new work is that the HERA team has improved the sensitivity of the array by a factor of 2.1 for light emitted about 650 million years after the Big Bang (a redshift or wavelength increase of 7.9) and 2.6 for radiation emitted about 450 million years after the Big Bang (a redshift of 10.4).
The HERA team continues to improve the telescope’s calibration and data analysis in hopes of seeing those bubbles in the early universe, which are about 1 millionth the intensity of radio noise in Earth’s vicinity. Filtering out local radio noise to see radiation from the early universe was not easy.
“If it’s Swiss cheese, the galaxies are making the holes, and we’re looking for cheese,” said David Deboer, a research astronomer at UC Berkeley’s Radio Astronomy Laboratory, so far without success.
Extending this analogy, however, Dillon noted, “What I’ve done is I’ve said that the cheese must be warmer than if nothing had happened. if the cheese were warm”.
This largely rules out the cold reionization theory, which postulated a colder starting point. HERA researchers suspect, instead, that X-rays from X-ray binaries heated the intergalactic medium first.
“The X-rays will actually heat the entire block of cheese before the holes form,” Dillon said. “And those holes are the ionized bits.”
“HERA continues to improve and set better and better limits,” Parsons said. “The fact that we’re able to keep going and have new techniques that continue to pay off for our telescope is great.”
The HERA collaboration is led by UC Berkeley and includes scientists from North America, Europe and South Africa.
HERA collaboration, improved constraints on the 21 cm EoR power spectrum and X-ray heating of the IGM with HERA Phase I observations, arXiv (2022). DOI: 10.48550/arxiv.2210.04912
Provided by University of California – Berkeley
Citation: Were galaxies very different in the early universe? (2023, January 24) retrieved January 24, 2023 from https://phys.org/news/2023-01-galaxies-early-universe.html
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