A group of 350 radio telescopes in South Africa’s Karoo Desert is getting 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 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 history of being.
While they didn’t actually detect radio emissions from the end of the cosmic dark ages, their results provide clues to the formation of stars and galaxies in early universe. In particular, their data indicates 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 create a 3D map of the ionizing bubbles and neutral hydrogen It evolved from about 200 million years ago to about a billion years after the Big Bang. The map can tell us how the early stars and galaxies differed from those we see around us today, and what the universe as a whole looked like in adolescence.
said Joshua Dillon, a research scientist in the University of California, Department of Astronomy and lead author of the paper. “Making a three-dimensional map of most of the luminous matter in the universe is the goal for the next 50 years or more.”
Other telescopes are also looking at the beginnings of the universe. The new James Webb Space Telescope (JWST) has imaged a galaxy that existed about 325 million years after the birth of the universe in the Big Bang. But the JWST can only see the brightest 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 seeks to detect the radiation from the neutral hydrogen that filled the space between those early stars and galaxies, and in particular, to determine when this hydrogen stopped emitting or absorbing radio waves because it had become ionised.
The fact that the HERA team has not yet detected such bubbles of ionized hydrogen within the cold hydrogen of the cosmic dark age rules out some theories about how stars evolved in the early universe.
Specifically, the data shows that the first stars, which may have formed around 200 million years after the Big Bang, contained few elements other than hydrogen and helium. This differs from the composition of today’s stars, which contain a variety of so-called metals, the astronomical term for elements, ranging from lithium to uranium, that are heavier than helium. This finding is consistent with the current model of how stars and starbursts produce most other elements.
“The early galaxies must have been significantly different from the galaxies we observe today for us not to see a signal,” said Aaron Parsons, HERA principal investigator and associate professor of astronomy at UC Berkeley. “In particular, their X-ray properties must have changed. Otherwise, we would have detected the signal we are looking for.”
The atomic composition of stars in the early universe determined how long it took for the intergalactic medium to heat up once stars began to form. The key to this is the high-energy radiation, mainly X-rays, that binary stars produce as one of them collapses into a black hole or neutron star and gradually eats its companion. With less heavy elements, much of the companion’s mass is blown away rather than falling onto the black hole, which means fewer X-rays and less heating of the surrounding area.
The new data fits with the most popular theories about how stars and galaxies first formed after the Big Bang, but not the others. Preliminary results From the first analysis of HERA data, reported a year ago, it was hinted that those alternatives – specifically, cold reionization– it’s improbably.
“Our results require that even before reionization and even 450 million years after the Big Bang, intergalactic gas must have been heated by X-rays. These likely result from binary systems where a single star loses mass to its friend’s black hole, Dillon said. “Our results show that if this is the case, then those stars must have been very low ‘metallic’, i.e. very few elements other than hydrogen and helium compared to our sun, which makes sense because we are talking about a period in time in the universe before most other elements were formed.” “.
The era 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 – mainly hydrogen and helium. Looking up at the sky with sensitive telescopes, astronomers have detailed a map of faint temperature changes since this moment – what’s known as the cosmic microwave background – just 380,000 years after the Big Bang.
Aside from this thermal radiation, the early universe was dark. As the universe expanded, the agglomeration of matter spread out galaxies and stars, which in turn produced radiation—ultraviolet and X-rays—that heated interstellar gas. At some point, the hydrogen began to ionize—losing its electron—and bubbles formed within the neutral hydrogen, marking the beginning of the era of reionization.
To map these bubbles, HERA and several other experiments focus on the wavelength of light that neutral hydrogen absorbs and emits, but that ionized hydrogen does not. The line is called 21 cm (with a frequency of 1420 MHz), and it is produced by hyperfine transition, in which the electron and proton cycles flip from parallel to parallel. Ionized hydrogen, which has lost its only electron, does not absorb or emit this radio frequency.
Since the epoch of reionization, the 21-centimetre line has been redshifted by the expansion of the universe to a wavelength 10 times its length — about 2 meters, or 6 feet. HERA’s rather simple antennas, a structure of chicken wire, PVC pipe and telephone poles, are 14 meters wide in order to collect this radiation and focus it on the detectors.
“With a two-meter wavelength, chicken wire mesh is a mirror,” Dillon said. “And all the complicated stuff, so to speak, is in the background of the supercomputer and all the data analysis that comes after.”
The new analysis is based on 94 nights of observation in 2017 and 2018 with about 40 antennas – the first phase of the array. Last year’s preliminary analysis was based on 18 nights of Phase 1 observations.
The main finding of the new paper is that the HERA team improved the sensitivity of the array by a factor of 2.1 for light emitted about 650 million years after the Big Bang (redshift, or wavelength increase, of 7.9), and 2.6 for radiation emitted about 450 million years after the explosion. The Great (redshift of 10.4).
The HERA team continues to improve telescope calibration and data analysis in hopes of seeing those bubbles in the early universe, which are about a millionth the magnitude of radio noise in Earth’s neighborhood. Filtering out local radio noise to see radiation from the early universe wasn’t easy.
If it’s Swiss cheese, then… galaxies So far, it hasn’t worked out for making holes, and we’re looking for cheese.
Extending this analogy, Dillon notes, “What we’ve done is we’ve said that the cheese must be warmer than if nothing had happened. If the cheese was really cold, it would be easier to notice this spot than if the cheese were warm.”
This mostly rules out the cold reionization theory, which assumed a much colder starting point. HERA researchers suspect, instead, that X-rays from X-ray binary stars heated the intergalactic medium first.
“The X-rays will effectively heat the entire block of cheese before holes can form,” Dillon said. “And these holes are the ionized bits.”
“HERA just keeps getting better and setting better and better boundaries,” Parsons said. “The fact that we’re able to keep moving forward, and have new technologies still coming to fruition for our telescope, is amazing.”
The HERA collaboration is led by UC Berkeley and includes scientists from across North America, Europe, and South Africa.
HERA Collaboration, Improving Constraints on the 21-cm EoR Energy Spectrum and X-ray Heating of IGM with HERA Phase I Observations, arXiv (2022). doi: 10.48550/arxiv.2210.04912
University of California, Berkeley
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