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 Huge 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 experiences 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 really 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 recommend that early galaxies contained only a few components moreover hydrogen and helium, in contrast to our galaxies at this time.

When the radio dishes are absolutely 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 advanced from about 200 million years in the past to round 1 billion years after the Huge Bang. The map may inform us how early stars and galaxies differed from these we see round us at this time, and the way the universe as a complete seemed in its adolescence.

“That is shifting towards a probably revolutionary method in cosmology. As soon as you will get right down to the sensitivity you want, there’s a lot data within the knowledge,” stated Joshua Dillon, a analysis scientist within the College of California, Berkeley’s Division of Astronomy and lead creator of the paper. “A 3D map of a lot of the luminous matter within the universe is the purpose for the subsequent 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 Huge Bang. However the JWST can see solely the brightest of the galaxies that fashioned through the Epoch of Reionization, not the smaller however much more quite a few dwarf galaxies whose stars heated the intergalactic medium and ionized a lot of the hydrogen gasoline.

HERA seeks to detect radiation from the impartial hydrogen that crammed 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 grew to become ionized.

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

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

“Early galaxies should have been considerably completely different than the galaxies that we observe at this time to ensure that us to not have seen a sign,” stated 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 in search of.”

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 type. Key to that is the high-energy radiation, primarily X-rays, produced by binary stars the place one in all them has collapsed to a black hole or neutron star and is step by step consuming its companion. With few heavy components, quite a lot of the companion’s mass is blown away as an alternative of falling onto the black hole, that means fewer X-rays and fewer heating of the encompassing area.

The brand new knowledge match the most well-liked theories of how stars and galaxies first fashioned after the Huge Bang, however not others. Preliminary results from the primary evaluation of HERA knowledge, reported a yr in the past, hinted that these alternate 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 Huge Bang, the gasoline between galaxies should have been heated by X-rays. These seemingly got here from binary methods the place one star is shedding mass to a companion black hole,” Dillon stated. “Our outcomes present that if that is the case, these stars should have been very low ‘metallicity,’ that’s, only a few components 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 a lot of the different components have been fashioned.”

The Epoch of Reionization

The origin of the universe within the Huge Bang 13.8 billion years in the past produced a sizzling cauldron of vitality and elementary particles that cooled for tons of of 1000’s of years earlier than protons and electrons mixed to type atoms—primarily hydrogen and helium. Trying 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 Huge Bang.

Except for this relict warmth radiation, nevertheless, 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 gasoline between stars. Sooner or later, hydrogen started to ionize—it misplaced its electron—and fashioned bubbles throughout 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 soak up or emit this radio frequency.

Because the 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 toes. HERA’s fairly easy antennas, a assemble of hen wire, PVC pipe and phone poles, are 14 meters throughout so as to gather and focus this radiation onto detectors.

“At two meters wavelength, a hen wire mesh is a mirror,” Dillon stated. “And all the subtle stuff, so to talk, is within the supercomputer backend and all the 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 primarily based on 18 nights of phase 1 observations.

The brand new paper’s principal 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 Huge 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 Huge 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 simple.

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

Extending that analogy, nevertheless, Dillon famous, “What we have finished is we have stated the cheese should be hotter than if nothing had occurred. If the cheese have been actually chilly, it seems it could be simpler to look at that patchiness than if the cheese have been heat.”

That principally guidelines out chilly reionization idea, which posited a colder place to begin. The HERA researchers suspect, as an alternative, 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 type,” Dillon stated. “And people holes are the ionized bits.”

“HERA is constant to enhance and set higher and higher limits,” Parsons stated. “The truth that we’re capable of hold pushing by way of, and we’ve new methods which can be persevering with to bear fruit for our telescope, is nice.”

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