On the surface, the galaxy AGC 114905 250 million light-years away from Earth appear to be like any other, but a closer examination shows an oddity: it seems to be devoid of dark matter.
AGC 114905 is an ultra-diffuse galaxy (UDG). These galaxies are faint; AGC 114905 is about the same size as the Milky Way but has 1,000 times fewer stars.
A doctoral candidate in astronomy at the University of Groningen in the Netherlands and an astronomer at ASTRON (the Netherlands Institute for Radio Astronomy), Pavel Mancera Piña referred to the fact that dark matter is thought to be the glue that holds a galaxy’s stars, gas, and dust together.
He also said that, in principle, galaxies like this shouldn’t exist; he could not effectively explain them with any existing theory.
A galaxy without dark matter would upend fundamental theories about galaxy formation.
The rotational speed tells how much matter it contains; the more massive a galaxy is, the stronger its gravity is, and the faster it spins.
Astronomers can calculate how much extra invisible stuff — dark matter — must be present to account for the galaxy’s speed by comparing the rate with how much stuff can be seen — the number of stars, gas, and dust.
In 2019, Mancera Piña and his colleagues first looked at AGC 114905; they suspected it might not have dark matter because of how fast it was rotating.
However, because the galaxy in question is so faint, they didn’t have enough data to fully resolve the rotation speed to determine if it was completely devoid of dark matter at first. So scientists went back for a second look, using the Very Large Array, a radio observatory in New Mexico with 40 hours of observations.
The astronomers calculated how rapidly the gas in the galaxy was traveling based on their observations, which mapped the gas in the galaxy. They were able to determine the galaxy’s rotation speed and consequently how much dark matter is contained in the galaxy due to this.
However, the researchers concluded that there doesn’t seem to be any room for dark matter.The new findings were published on Nov. 30.
Astronomers have previously discovered some UDGs rich in dark matter and others devoid of it, as reported by Live Science. Some of the latter are found near more massive galaxies, implying that they may have lost their dark matter due to interactions with more giant nearby galaxies, whose gravity may have sucked the dark matter from the smaller galaxies.
That explanation is doubtful, though, because AGC 114905 lacks any giant galaxies nearby, according to Mancera Piña.