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Astronomers solve the mystery of a galaxy containing 99.99% dark matter

The mystery of a galaxy that shouldn’t have existed may now have a solution. Dragonfly 44, a faint galaxy that was found in 2016 made up of 99.99% dark matter, has been carefully re-examined, revealing a lower and more normal proportion of dark matter.

That would mean we don’t have to review our galaxy formation patterns to try and figure out how they could have produced such an extreme outlier – everything is behaving completely normal, the researchers said.

“Dragonfly 44 (DF44) has been an anomaly all these years that could not be explained by existing galactic formation models,” said astronomer Teymoor Saifollahi of the Kapteyn Astronomical Institute in the Netherlands.

“We now know that the previous results were wrong and that DF44 is not extraordinary. Time to move on.”


Dark matter is a real cosmic pickle. We don’t know what it is. We cannot detect it directly, as it does not absorb, reflect or emit any electromagnetic radiation. But based on the way some things move due to gravity – the rotation of galaxies, the way the path of light bends – we can tell how much mass there is in galaxies. And there is far more mass than can be accounted for in a normal, detectable matter census.

We call it dark matter of missing mass. According to multiple measurements, about 85% of the matter in the Universe is dark, although the proportions in galaxies vary by type.

And it’s a key component of our best galactic formation models. It is believed that a web of dark matter may have channeled matter into the galaxies of the early Universe and that it is the cosmic glue that holds galaxies together.

DF44, located about 330 million light-years away in the Coma cluster of galaxies, is of a type known as an ultra-diffuse galaxy. It is much less bright than the Milky Way; its brightness suggests that DF44 has 1,000 times fewer stars than our natal galaxy.

This made that 2016 measurement of its mass surprising. Astronomers led by Yale University astronomer Pieter van Dokkum have counted the number of globular clusters around DF44, densely packed spherical groups of stars orbiting the galactic center.

Because the number of globular clusters is related to the mass of a galaxy (although we’re not sure why), this allowed astronomers to calculate DF44’s mass – and they found it was almost as massive as the Milky Way.

Given the low number of stars in DF44, this implied that the galaxy was almost entirely made up of dark matter, making it an extreme outlier. Maybe too extreme.

So, Saifollahi and his team embarked on a recount of the globular clusters, using the same observation data from the Hubble Space Telescope from the previous study. They applied rigorous parameters to determine which globular clusters are bound to DF44 and eventually came up with a much smaller number.

“The fact that we found only 20 globular clusters in our work, compared to 80 previously reported, drastically reduces the amount of dark matter believed to contain the galaxy,” said astrophysicist Ignacio Trujillo of the Instituto de Astrofísica de Canarias in Canary Islands.

“Furthermore, with the number of globular clusters we have found, the amount of dark matter in Dragonfly 44 is in agreement with what is expected for this type of galaxy. The ratio of visible matter to dark matter is no longer 1 in 10,000 but one in 300. “

This is not Trujillo’s first encounter with ultra-diffuse galaxies with anomalous dark matter content. Last year, he found that two of these previously found galaxies named DF2 and DF4 (also by van Dokkum and colleagues) had little or no dark matter and were likely normal. The low dark matter measurement was due to a distance calculation error. Review the distance and the two galaxies are no longer cosmic monsters.

It is, he told ScienceAlert at the time, far more likely that humans have made a mistake than the fact that we have found extreme outliers that defy cosmological models. And that’s what Saifollai and his team discovered this time too.

“Our work shows that this galaxy is neither so singular nor unexpected,” said astrophysicist Michael Beasley of the Istituto de Astrofísica de Canarias. “This way the galaxy formation models can explain it without modification.”

But the show isn’t necessarily over. Following the Trujillo distance recalculations, Van Dokkum and colleagues also recalculated the distances from DF2 and DF4. These new analyzes yielded results consistent with the team’s initial discovery, so we expect the debate around DF44 to be similarly underway.

Such debates, however, are good for science. Whatever the outcome, trying to understand it will help us perfect our techniques for studying the cosmos. We would call it a clear win.

The research was published in The monthly notices of the Royal Astronomical Society.

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