It’s a discovery so rich with mind-bending ideas that it seems straight from science fiction: Using humanity’s biggest off-world observatory to focus on a tiny, faraway arc of light magnified by a quirk of spacetime, astronomers have glimpsed a faint galaxy as it was 13 billion years ago, when it was brimming with dark matter—as well as what may be fresh ashes from the universe’s earliest, strangest stars.
The small, faraway galaxy is named LAP1-B, the observatory is NASA’s James Webb Space Telescope (JWST), and the strange stars would have been members of what astronomers call “Population III”—titanic suns that burned bright and died young close to the dawn of time.
Such stars are the quarry that JWST was designed for—stellar orbs composed of pristine, primordial hydrogen and helium gas that were summoned into being by the big bang. These stars are not quite the stuff that most cosmologists’ dreams are made of but rather the sources for the atoms that made cosmologists themselves. The oxygen in your lungs, the iron in your blood, the calcium in your bones, the carbon in your cells, and even the silicon in your smartphone can all be traced back to Population III stars, which blasted out heavy-metal cosmic fertilizer (astronomers call all heavier-than-helium elements “metals”) upon their explosive deaths. The debris from their demise coalesced to form subsequent stellar generations—Population II and Population I stars—plus planets and eventually people.
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That’s the creation story astronomers tell themselves, anyway. The trouble with proving all of its details has been that these first stars are so distant in space and time that even the mighty JWST has yet to directly, definitively see them. Instead telltale hints of their existence primarily show up in studies of galaxies that are big and bright enough for JWST to see clear across the universe. Rather than gathering Population III stars’ light, JWST so far has only inferred their presence in such places via incandescent fogs that are eerily lit from within by the first stars’ intense radiation.
LAP1-B is different. It’s a wisp of glowing gas nestled in a pool of invisible dark matter, a “cosmic fossil” seen a mere 800 million years after the big bang yet resembling the swarms of “ultrafaint dwarf galaxies” (UFDs) astronomers find near our Milky Way. Cosmologists suspect that, in the early universe, such objects were like puzzle pieces, assembling into bigger galaxies; the UFDs we see around us today are part of a larger population of leftover scraps scattered throughout the cosmos that never found a larger home. JWST’s ability to see LAP1-B at all is only because of the galaxy’s fortuitous placement behind a cosmic behemoth called MACS J0416, a giant galaxy cluster that is so immense that its mass warps spacetime to create a “gravitational lens” that boosts LAP1-B’s feeble light 100-fold.
Ironically, this boost is so great that JWST, custom-built for the task of finding things like LAP1-B, didn’t discover it. Instead the object was first announced in 2020 from data gathered with a ground-based facility, the European Southern Observatory’s Very Large Telescope in Paranal, Chile, which had been following up on earlier Hubble Space Telescope studies of MACS J0416. Subsequent studies with JWST have progressively revealed more about this mysterious object. The latest, published in Nature today, strengthens the case that LAP1-B is a newborn cosmic puzzle piece packed with material freshly manufactured by Population III stars.
A false-color image of a portion of the MACS J0416 galaxy cluster as seen at multiple infrared wavelengths by NASA’s James Webb Space Telescope (JWST). The “cosmic fossil” LAP1-B—a small, faint background galaxy magnified into view by the gravitational lensing of MACS J0416—appears as a faint arc of light in a zoomed-in inset image. LAP1-B is thought to contain relics of the universe’s first generation of stars. Orange bars around LAP1-B denote slits used for JWST’s spectroscopic measurements of the galaxy.
Nakajima et al., Nature
“LAP1-B shows us the ‘first generation’ of element production,” says the study’s lead author Kimihiko Nakajima, an astronomer at Kanazawa University in Japan. “We see a galaxy that has just inherited its first batch of heavy elements from the very first stars to ever shine. It tells us that these tiny, dark-matter-filled galaxies were the fundamental building blocks of the universe, and we have finally caught the moment they first blinked into existence.”
These key insights arise from JWST’s ability to perform spectroscopy on LAP1-B, spreading the tiny galaxy’s light into a rainbowlike spectrum of colors; the specific mix of colors can reveal an object’s chemical composition. Reading this chemical “barcode,” Nakajima and and his colleagues found LAP1-B’s gas is mostly pure hydrogen and helium from the big bang, with meager traces of oxygen presumably pumped out by the first generation of stars. The data also show a surprising surfeit of carbon—a sign, Nakajima says, of Population III stars ending their lives in “weak” supernovas, as predicted by some theoretical models. This would involve the stars ejecting their carbon-rich outer layers while oxygen-rich inner layers get swallowed by a newly formed black hole at their core.
Their data further reveal that the galaxy’s gas is glowing from high-energy radiation, consistent with predicted Population III emissions. Yet the actual stars remained undetected in JWST’s instruments, allowing the team to set an upper limit on their number: LAP1-B contains no more than about 3,300 solar masses of stars (the Milky Way, by comparison, contains about a hundred billion solar masses). If it had more than that, JWST should’ve seen their glow. Meanwhile, the tiny galaxy’s gas is swirling so fast it would fly apart unless held together in the gravitational grip of a sprawling cloud of dark matter.
All this, Nakajima says, makes LAP1-B “exactly what we expect for the ancestors of the ultra-faint dwarfs we see today. Until now, we only saw these fossils in their ‘final’ state—old and quiet. LAP1-B has turned a theoretical ‘missing link’ into a physical reality we can now measure and analyze.”
Independent experts view the result with cautious optimism, noting the uncertainties associated with studying spectra from such a strange object across such vast distances.
“I do think this is a compelling object,” says Roberto Maiolino of the University of Cambridge, an astronomer using JWST to study early galaxies. “LAP1-B may indeed be tracing the transition between the earliest pristine stellar populations and the regime of chemically enriched galaxies.”
Evan Kirby, an astronomer studying the chemistry of dwarf galaxies at the University of Notre Dame, agrees. “This is the galaxy that chemical evolution experts have wanted JWST to find,” he says. However, he adds, the team’s interpretations of LAP1-B “will need corroboration by future observations and other research groups.”
Eros Vanzella, an astronomer at the National Institute for Astrophysics in Italy who has previously studied LAP1-B with JWST and led the team that first discovered the galaxy, finds these latest results vindicating—and promising.
“I’m very happy to see that our first claim on [LAP1-B’s] very low metallicity is confirmed with deeper spectroscopic observations,” he says, adding that direct detection of the tiny galaxy’s starlight might yet be possible through even deeper observations with JWST. “The story of this remarkable source is far from over.”
