New research posted on the pre-print site arXiv on 16 October provides strong evidence that the universe contains galaxies – or at least one galaxy – devoid of dark matter, which could have major implications for the way astronomers think about galaxy formation.
The findings are based on deep images of the galaxy called NGC 1052-DF4 – or, more specifically, of its brightest red giant stars (called the Tip of the Red Giant Branch, or TRGB) – taken by the Hubble Space Telescope.
Since TRGB stars all shine with the same true brightness when viewed in infrared, their apparent brightness is determined by nothing more than distance, which allowed the authors of the paper to confirm that DF4 is located some 61 million light-years away, thereby disproving earlier research which suggested a much smaller number.
“I think this is definitive,” co-author Pieter van Dokkum from Yale University told astronomy.com. “The TRGB cannot be argued with: it is caused by well-understood stellar physics, and [is] as direct as distance indicators get.”
The DF4 was already thought to lack dark matter at the moment of its discovery by van Dokkum and colleagues back in 20 March 2019, yet the accompanying paper was challenged by the astronomers Matteo Monelli and Ignacio Trujillo who purported to show that it’s actually much closer than van Dokkum claimed.
Using the new Hubble data, however, the latest paper demonstrates that Monelli and Trujillo have simply misidentified the DF4’s TRGB due to the images available at the time of publishing their paper, which lacked some of the fainter stars located in the oddball galaxy.
“In the new data, there really is no ambiguity,” said co-author Shany Danieli of Yale University. ”We think the new data really rule out the [closer distance derived by Trujillo’s group]. The TRGB is generally seen as definitive, as its physics is well understood.”
If confirmed, the findings may cast doubt on whether current science even understands what a galaxy is: “[…] we have no idea how star formation would proceed in the absence of dark matter,” van Dokkum explained. “All we can say is that there must have been very dense gas early on in their history”.
Even though the researchers behind the paper agree that such extraordinary findings will require an extraordinary amount of scrutiny moving forward, they also believe that we should already begin considering the implications of dark matter-free galaxies, as the measurements are not likely to be incorrect.