Were galaxies much different in the early universe?

An array of 350 radio telescopes within the Karoo desert of South Africa is getting nearer to detecting “cosmic daybreak”—the period after the Large Bang when stars first ignited and galaxies started to bloom.

In a paper accepted for publication in The Astrophysical Journal, the Hydrogen Epoch of Reionization Array (HERA) crew stories that it has doubled the sensitivity of the array, which was already probably the most delicate radio telescope on the planet devoted to exploring this distinctive interval within the historical past of the universe.

Whereas they’ve but to truly detect radio emissions from the tip of the cosmic darkish ages, their outcomes do present clues to the composition of stars and galaxies within the early universe. Particularly, their knowledge counsel that early galaxies contained only a few parts moreover hydrogen and helium, not like our galaxies at present.

When the radio dishes are totally on-line and calibrated, ideally this fall, the crew hopes to assemble a 3D map of the bubbles of ionized and neutral hydrogen as they developed from about 200 million years in the past to round 1 billion years after the Large Bang. The map might inform us how early stars and galaxies differed from these we see round us at present, and the way the universe as a complete seemed in its adolescence.

“That is transferring towards a probably revolutionary approach in cosmology. As soon as you may get right down to the sensitivity you want, there’s a lot data within the knowledge,” mentioned Joshua Dillon, a analysis scientist within the College of California, Berkeley’s Division of Astronomy and lead writer of the paper. “A 3D map of many of the luminous matter within the universe is the objective for the following 50 years or extra.”

Different telescopes are also peering into the early universe. The brand new James Webb Area Telescope (JWST) has now imaged a galaxy that existed about 325 million years after the start of the universe within the Large Bang. However the JWST can see solely the brightest of the galaxies that fashioned in the course of the Epoch of Reionization, not the smaller however way more quite a few dwarf galaxies whose stars heated the intergalactic medium and ionized many of the hydrogen fuel.

HERA seeks to detect radiation from the impartial hydrogen that stuffed the space between these early stars and galaxies, and specifically, decide when that hydrogen stopped emitting or absorbing radio waves as a result of it turned ionized.

The truth that the HERA crew has not but detected these bubbles of ionized hydrogen inside the chilly hydrogen of the cosmic darkish age guidelines out some theories of how stars developed within the early universe.

Particularly, the information present that the earliest stars, which can have fashioned round 200 million years after the Large Bang, contained few different parts than hydrogen and helium. That is completely different from the composition of at present’s stars, which have quite a lot of so-called metals, the astronomical time period for parts, starting from lithium to uranium, which are heavier than helium. The discovering is in step with the present mannequin for a way stars and stellar explosions produced many of the different parts.

“Early galaxies should have been considerably completely different than the galaxies that we observe at present to ensure that us to not have seen a sign,” mentioned Aaron Parsons, principal investigator for HERA and a UC Berkeley affiliate professor of astronomy. “Particularly, their X-ray traits should have modified. In any other case, we’d have detected the sign we’re searching for.”

The atomic composition of stars within the early universe decided how lengthy it took to warmth the intergalactic medium as soon as stars started to kind. Key to that is the high-energy radiation, primarily X-rays, produced by binary stars the place certainly one of them has collapsed to a black hole or neutron star and is regularly consuming its companion. With few heavy parts, quite a lot of the companion’s mass is blown away as a substitute of falling onto the black hole, that means fewer X-rays and fewer heating of the encircling area.

The brand new knowledge match the preferred theories of how stars and galaxies first fashioned after the Large Bang, however not others. Preliminary results from the primary evaluation of HERA knowledge, reported a yr in the past, hinted that these options—particularly, chilly reionization—have been unlikely.

“Our outcomes require that even earlier than reionization and by as late as 450 million years after the Large Bang, the fuel between galaxies should have been heated by X-rays. These seemingly got here from binary techniques the place one star is dropping mass to a companion black hole,” Dillon mentioned. “Our outcomes present that if that is the case, these stars should have been very low ‘metallicity,’ that’s, only a few parts aside from hydrogen and helium compared to our sun, which is sensible as a result of we’re speaking a few interval in time within the universe earlier than many of the different parts have been fashioned.”

The Epoch of Reionization

The origin of the universe within the Large Bang 13.8 billion years in the past produced a scorching cauldron of vitality and elementary particles that cooled for a whole bunch of 1000’s of years earlier than protons and electrons mixed to kind atoms—primarily hydrogen and helium. Wanting on the sky with delicate telescopes, astronomers have mapped intimately the faint variations in temperature from this second—what’s often called the cosmic microwave background—a mere 380,000 years after the Large Bang.

Apart from this relict warmth radiation, nonetheless, the early universe was darkish. Because the universe expanded, the clumpiness of matter seeded galaxies and stars, which in flip produced radiation—ultraviolet and X-rays—that heated the fuel between stars. In some unspecified time in the future, hydrogen started to ionize—it misplaced its electron—and fashioned bubbles inside the impartial hydrogen, marking the start of the Epoch of Reionization.

To map these bubbles, HERA and a number of other different experiments are centered on a wavelength of sunshine that impartial hydrogen absorbs and emits, however ionized hydrogen doesn’t. Referred to as the 21-centimeter line (a frequency of 1,420 megahertz), it’s produced by the hyperfine transition, throughout which the spins of the electron and proton flip from parallel to antiparallel. Ionized hydrogen, which has misplaced its solely electron, would not take in or emit this radio frequency.

For the reason that Epoch of Reionization, the 21 centimeter line has been red-shifted by the enlargement of the universe to a wavelength 10 occasions as lengthy—about 2 meters, or 6 ft. HERA’s fairly easy antennas, a assemble of hen wire, PVC pipe and phone poles, are 14 meters throughout as a way to acquire and focus this radiation onto detectors.

“At two meters wavelength, a hen wire mesh is a mirror,” Dillon mentioned. “And all the subtle stuff, so to talk, is within the supercomputer backend and the entire knowledge evaluation that comes after that.”

The brand new evaluation relies on 94 nights of observing in 2017 and 2018 with about 40 antennas—phase 1 of the array. Final yr’s preliminary evaluation was based mostly on 18 nights of phase 1 observations.

The brand new paper’s primary result’s that the HERA crew has improved the sensitivity of the array by an element of two.1 for gentle emitted about 650 million years after the Large Bang (a redshift, or a rise in wavelength, of seven.9), and a couple of.6 for radiation emitted about 450 million years after the Large Bang (a redshift of 10.4).

The HERA crew continues to enhance the telescope’s calibration and knowledge evaluation in hopes of seeing these bubbles within the early universe, that are about 1 millionth the depth of the radio noise within the neighborhood of Earth. Filtering out the native radio noise to see the radiation from the early universe has not been straightforward.

“If it is Swiss cheese, the galaxies make the holes, and we’re searching for the cheese,” to date, unsuccessfully, mentioned David Deboer, a analysis astronomer in UC Berkeley’s Radio Astronomy Laboratory.

Extending that analogy, nonetheless, Dillon famous, “What we have executed is we have mentioned the cheese should be hotter than if nothing had occurred. If the cheese have been actually chilly, it seems it might be simpler to watch that patchiness than if the cheese have been heat.”

That largely guidelines out chilly reionization idea, which posited a colder start line. The HERA researchers suspect, as a substitute, that the X-rays from X-ray binary stars heated up the intergalactic medium first.

“The X-rays will successfully warmth up the entire block of cheese earlier than the holes will kind,” Dillon mentioned. “And people holes are the ionized bits.”

“HERA is continuous to enhance and set higher and higher limits,” Parsons mentioned. “The truth that we’re capable of preserve pushing by, and we’ve got new methods which are persevering with to bear fruit for our telescope, is nice.”

The HERA collaboration is led by UC Berkeley and consists of scientists from throughout North America, Europe and South Africa.