After 40 hours of observations, astronomers have found no trace of the mysterious substance
Our ideas about dark matter could be about to turn on their head, as astronomers have confirmed galaxy AGC 114905 appears to be free of dark matter.
The galaxy is an ultra-diffuse dwarf galaxy, meaning that despite being the same size as the Milky Way, it contains 1,000 times fewer stars. It was one of six galaxies found in 2019, which appeared to be devoid of dark matter – something that our current theories of how galaxies and dark matter works says should be impossible. Even alternate theories such as modified Newtonian dynamics struggle to explain the galaxy’s existence.
But such an extraordinary claim requires extraordinary evidence and so a team of astronomers, led by Pavel Mancera Piña from the University of Groningen and ASTRON in the Netherlands, observed the galaxy for 40 hours with the Very Large Telescope. These observations monitored the motion of gas in the galaxy’s centre and found that the motions could be completely explained by normal matter in the galaxy, with no need for dark matter.
“This is, of course, what we thought and hoped for because it confirms our previous measurements,” says Mancera Piña. “But now the problem remains that the theory predicts that there must be dark matter in AGC 114905, but our observations say there isn’t. In fact, the difference between theory and observation is only getting bigger.”
One theory is that the galaxy originally had dark matter, but that it was stripped away during an interaction with a larger galaxy, only there are none nearby.
Alternately, the angle of the galaxy could be different to what they expect. “But that angle has to deviate very much from our estimate before there is room for dark matter again,” says Tom Oosterloo from ASTRON (The Netherlands Institute for Radio Astronomy), who also took part in the study.
Earlier this year, another team undertook further – though less in-depth – observations of NGC 1052-DF2, another ultra-diffuse dwarf galaxy, which reinforced evidence about its lack of dark matter. Meanwhile, Mancera Piña’s team is now studying another galaxy. If these other targets prove to be just as devoid of the mysterious substance, it will strengthen the case for dark matter-free galaxies and prompt some serious reconsideration of our theories about dark matter itself. www.astron.nl