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…