Astronomers have found yet a ghostly galaxy that appears to be devoid of dark matter. AGC 114905 is similar in size to our own spiral galaxy yet has 1,000 times fewer stars.
Astronomers have found yet another ghostly galaxy that appears to be devoid of dark matter. Researchers have reported several such sightings over the past few years, each time flagging so-called ultradiffuse galaxies that can be as large as the Milky Way but relatively bereft of stars. This latest object, known as AGC 114905, is similar in size to our own spiral galaxy yet has 1,000 times fewer stars.
Such observations led to the Lambda-CDM model of cosmology, where Lambda refers to dark energy and CDM to cold dark matter, which is thought to make up about 27 percent of the universe. Simulations using cold dark matter have been extremely successful at replicating patterns seen in the large-scale clustering of galaxies, as well as in the cosmic microwave background, the leftover light from about 380,000 years after the big bang.
But in April 2021 van Dokkum’s team released the results of an in-depth HST study of both anomalous galaxies, showing that their greater initial distance estimates were correct. If anything, the galaxies are a wee bit farther away, making the case for little or no dark matter even stronger. “This convinced people and, frankly, ourselves,” van Dokkum says.
AGC 114905 was the one galaxy out of six that the team chose for deeper investigation. Mancera Piña and his colleagues observed the galaxy for 40 hours, using a high-resolution configuration of the VLA. Previously, they had studied the galaxy’s rotation by looking at the speeds of gas at two locations along its radius; this time they looked at five. The results did not change. “The observation suggests that there is no room for dark matter,” Mancera Piña says.
The team took AGC 114905 to be circular and estimated its inclination to be about 32 degrees, plus or minus three degrees. Yet, Mancera Piña says, “if you want both MOND and cold dark matter to work, that inclination will need to be around 10 degrees, so the galaxy will need to look rounder. We have measured this as carefully as possible. And we find that the associated uncertainties of our measurement are very far away from those 10 degrees.
Still, he concedes that if the results are verified, the implications would be enormous. “Assuming it holds, the authors are totally right to think it poses a problem to both LCDM and MOND,” Famaey says.
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