For decades, astronomers looking through telescopes like Hubble have been trying to catch a glimpse of the ancient epoch when the Universe’s first generation of stars ignited. But the small galaxies that were the building blocks of the cosmos we know today were too faint to spot, even by the most powerful instruments. Now it seems astronomers finally have two things on their side: the Webb Space Telescope and a bit of luck.

In a recent paper in Nature, a team of scientists led by Kimihiko Nakajima, an astronomer at the Kanazawa University, Japan, used the James Webb Space Telescope to observe an ultra-faint galaxy called LAP1-B as it existed roughly 800 million years after the Big Bang. It’s the most chemically primitive galaxy we’ve ever seen.

The magnifying glass

The LAP1-B is 13 billion light-years away from Earth. To observe an object that faint and distant, even the huge, gold-coated beryllium mirrors of JWST were not enough on their own. We spotted it due to a massive cluster of galaxies called the MACS J046, which warps the spacetime between us and the LAP1-B.

“The galaxy was strongly magnified through the gravitational lensing effect,” Nakajima said. Specifically, the spacetime warped by the MACS J046 clusters magnifies light traveling from LAP1-B toward Earth by roughly 100-fold.

But even with this 100-fold boost in brightness, LAP1-B is so dim that neither the JWST nor Hubble could detect its stellar continuum—the steady background light of its stars. For Nakajima and his colleagues, though, even that worked as a clue. Knowing the distance separating us from the LAP1-B and the sensitivity of telescopes, they calculated that the hard upper limit of the stellar mass of LAP1-B must be equal to 3,300 Suns. That’s a tiny number compared to the roughly 100 billion solar masses in the Milky Way.

Much of the light from LAB1-B hitting the JWST mirrors was not coming from stars but from glowing gas. Taking a closer look at this gas, Nakajima and his colleagues realized LAB1-B was the closest thing to the first, pristine galaxies we have observed so far.

Primordial composition

According to Nakajima’s team, we can see LAB1-B’s glow because high-energy radiation from massive stars within the galaxy hits the surrounding interstellar gas clouds, causing them to fluoresce. Using JWST’s Near-Infrared Spectrograph, the researchers analyzed this glowing gas by breaking its light into a spectrum and searching for the telltale emission lines indicating its chemical composition.

“We wanted to measure how much oxygen was present in this object,” Nakajima said.

This analysis revealed a profound shortage of elements heavier than hydrogen and helium. The gas-phase oxygen-to-hydrogen ratio stood at just 0.4 percent of what we find in our Sun.

Another detail in the spectrum indicated the type of radiation that made the gas glow. The team detected emission lines from triply ionized carbon—a state where a carbon atom has lost half of its six electrons. Stripping multiple electrons away from carbon atoms requires extreme-ultraviolet photons, with energies exceeding 47.9 electronvolts. Standard stars, even the massive ones we see in our galactic vicinity, are not hot enough to produce radiation this intense.

The stars that could get this hot, Nakajima’s team suggests, were the very first that ignited in the Universe. These were made exclusively of hydrogen and helium forged in the Big Bang and lacked heavy elements to help them cool as they formed. “Such stars should be formed from primordial gas,” Nakajima said.

The faint supernovae

The stars we see today, including our Sun, are Population I stars. The older generation, found in the halo of our galaxy, are Population II stars, which have far lower levels of elements heavier than helium. Population III stars were the first to appear in the cosmos, and they’re theorized to be violent monsters with masses hundreds of times higher than the Sun squeezed into surprisingly small volumes. They burned extremely hot and died young in supernova explosions. Nakajima’s team has likely found traces of these explosions in LAP1-B.

Despite being incredibly poor in heavy elements, LAP1-B has an unusually high amount of carbon; its carbon-to-oxygen ratio is higher than our Sun’s. The researchers think the answer might lie in how these massive first-generation stars died.

According to our models, when a massive, Population III star reaches the end of its life, its core collapses into a black hole, but the resulting supernova explosion isn’t energetic enough to blow the entire star apart. “Their bounding energy of gravity is stronger that in the usual massive stars,” Nakajima said.

Instead, the collapse results in a faint supernova with significant fallback, in which the heavier elements from the star’s core, such as oxygen, are sucked back past the event horizon and trapped in the black hole beneath. At the same time, the lighter outer layers, which are rich in carbon, escape and are expelled into the surrounding gas. LAP1-B’s chemical makeup, with low oxygen but elevated carbon, looks like a fingerprint of a gas cloud produced by Population III stars supernovae.

But there was one other clue hidden in the gas in LAP1-B, and it was all about its speed.

The dark matter

By looking at how the emission lines in the spectrum were broadened by the Doppler effect, Nakajima and his colleagues measured that the gas is swirling around inside the galaxy at roughly 58 kilometers per second, a rather typical value for dwarf galaxies.

Using the laws of gravity, the team calculated how much mass must be present to keep gas moving at that speed from flying off into intergalactic space. “We estimated the amount of material at 10 million solar masses,” Nakajima said.

Because the stars account for less than 3,300 solar masses and the gas adds just a tiny bit more, the team concluded that the rest of the galaxy must be made up of dark matter.

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