Bizarre Dark Object Could Be First-Known ‘Empty’ Galaxy from the Early Universe

A galaxy has been discovered that, unlike any other known in the modern universe, seems to be essentially devoid of stars. The galaxy—a floating hunk of dust and gas aimlessly adrift in space, found only thanks to its subtle, serendipitous signature in a radio telescope—is nearly as massive as the Milky Way yet invisible to the naked eye. The discovery could “upend what we think we know about galaxy formation,” says Pieter van Dokkum, a Yale University astrophysicist, who was not part of the finding.

Called J0613+52, the object was discovered and reported by a team led by Karen O’Neil, senior scientist at the Green Bank Observatory in West Virginia. O’Neil and her colleagues had been using radio observations from the Green Bank Telescope, the Nançay Radio Telescope in Paris and the now collapsed Arecibo telescope in Puerto Rico to look at a class of known objects called low-surface-brightness (LSB) galaxies. LSBs are dim, minuscule galaxies that are often torn-apart chunks of larger galaxies or small dwarf galaxies.

During one of the planned observations using the Green Bank Telescope—a vast 330-foot single-dish telescope—the researchers realized they were pointing at the wrong point on the night sky. “We had mistyped the location,” O’Neil says. But rather than finding nothing at that targeted spot, the telescope detected a galaxylike blip of hydrogen gas some 270 million light-years distant. Yet in visible observations, “there was no object in that area,” she says. The finding was presented last week at the 243rd meeting of the American Astronomical Society in New Orleans.

That curious combination—a hydrogen-rich radio signal but no apparent visible light—suggests this galaxy is made almost wholly of gas and dark matter, bearing few if any stars. The team’s measurements indicate that the object is at least 100,000 times fainter than our own Milky Way, which shines with the combined light of more than 100 billion stars. The researchers also managed to measure the rate at which the galaxy-sized blob of gas is spinning, allowing them to estimate its inventory of ordinary matter to be at least two billion solar masses—on the order of 10 times less than that held by the Milky Way but much larger than the amount contained by other LSBs, which can be 100 times smaller. It’s likely there is “a decent amount of dark matter” present as well, O’Neil says, but lingering uncertainties about the dark galaxy’s exact physical size make associated dark-matter estimates hazy at best.

Most interesting of all is that J0613+52 is completely isolated, with no neighboring galaxy closer than 330 million light-years or so; our own Milky Way, in fact, appears to be the object’s closest-known companion. “In these void areas of the universe, gas should be too diffuse to form any galaxylike object,” O’Neil says. “Clearly that’s not quite true.” J0613+52, she and her colleagues speculate, may have been alone in space since the early universe, existing as a solitary pocket of gas scarcely changed since its formation billions of years ago. “It’s our first real look into a primordial galaxy,” she says. “It contains gas from the early universe that has never been transferred into stars.”

Van Dokkum says there is more work to be done before such claims can be made. “It would be very exciting,” he says, but “we need to reobserve that field and make sure there is no glitch in the data.” A telescope van Dokkum helps operate in New Mexico called Dragonfly could be used, alongside other telescopes, to look for any evidence of faint starlight emanating from the object’s gloom. “We are certainly planning to look,” he says. O’Neil says her team is also working with groups to take some observations of its own “hopefully sometime this year.”

Robert Minchin of the National Radio Astronomy Observatory in New Mexico, who watched O’Neil’s presentation, believes the team’s deduction is correct. “I think it’s definitely a real detection,” he says. “It does look like a primordial object. It’s a bit like discovering a living dinosaur and having it there to study.”

If J0613+52 is confirmed to be an extremely faint and isolated LSB, which would make it unique among the hundreds of such objects already known, it could offer vital insights into the lives of galaxies. “So far we’ve thought that if you have the mass of the Milky Way, it would be very hard not to make any stars,” van Dokkum says. “There’s so much material there.” And stars tend to beget yet more stars as young and massive suns explode in supernovae that enrich and expel enormous volumes of star-forming gas and dust. Yet for reasons unknown, something seems to have suppressed such activity in J0613+52, although the galaxy may harbor a sparse, faint stellar population that subsequent observations will discover. “It’s a fantastic laboratory to really test our ideas on how stars and galaxies form to the extreme,” O’Neil says.

The galaxy’s profound isolation could help explain its dearth of stars, O’Neil says, which could be caused by a lack of gravitational nudges from nearby galaxies that would otherwise cause star-forming gas clouds to clump up and collapse. “If you’re in a higher-density area, it’s fine, but in this case, there’s no neighbors, and you end up staying diffuse,” she says. Our Milky Way, for comparison, is in a much higher-density area, the Virgo Supercluster, with the closest known dwarf galaxy just 25,000 light-years away. If the J0613+52 is rotating as measurements suggest, that could give it some shape or structure that would match fully-fledged galaxies such as our own. “If it’s indeed rotating, you could maybe see extremely faint spiral structure in [any] stars” eventually detected there, van Dokkum says. “That would be really terrific.”

Upcoming surveys by the Green Bank Telescope, the Square Kilometer Array in Australia and South Africa and the Vera Rubin Observatory in Chile could find more objects like J0613+52 if any exist. But for now, it stands alone as the only “dark” galaxy of its kind. “The fact we found it through a complete mishap on our part is incredibly surprising,” O’Neil says. “It’s a needle in a haystack. I really would be shocked if it was the only one out there.”

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