The mystery of a galaxy that should not exist can now be solved. Dragonfly 44, a weak galaxy that was found to consist of 99.99% dark matter in 2016, has been scrutinized, revealing a lower and more normal portion of dark matter.
That would mean we don’t have to rethink our galaxy formation patterns to try to figure out how they could have created such an extreme deviation – everything is perfectly normal, the researchers said.
“Dragonfly 44 (DF44) is an anomaly all these years that cannot be explained by existing models of galaxy formation,” said astronomer Teymour Saifolahi of the Kapteyn Astronomical Institute in the Netherlands.
“Now we know that previous results were wrong and that DF44 is not unusual. It̵
Dark matter is really crazy about space pickling. We don’t know what that is. We cannot detect it directly because it does not absorb, reflect or emit any electromagnetic radiation. But based on the way some things move by gravity – the rotation of galaxies, the way light bends – we can tell how much mass there is in galaxies. And there is much more mass than can be counted in counting normal, detectable matter.
We call this missing mass dark matter. According to numerous measurements, approximately 85 percent of the matter in the universe is dark, although the proportions in galaxies vary by species.
And this is a key component of our best galaxy formation models. It is believed that a network of dark matter could have directed matter into galaxies in the early universe, and that cosmic glue is what holds galaxies together.
DF44, about 330 million light-years from them in a coma of galaxies, is of a type known as an ultradiffuse galaxy. It is much less bright than the Milky Way; its luminosity suggests that DF44 has 1,000 times fewer stars than our home galaxy.
This made measuring his mass in 2016 surprising. Astronomers led by astronomer Peter van Docum of Yale University have counted the number of globular clusters around DF44, densely packed spherical clusters of stars orbiting the galactic center.
Because the number of globular clusters is related to the mass of the galaxy (although we’re not sure why), this allowed astronomers to calculate the mass of DF44 – and they found it to be almost as massive as the Milky Way.
Given the low number of stars in DF44, this means that the galaxy consists almost entirely of dark matter, which makes it an extreme deviation. Probably too extreme.
Thus, Saifolahi and his team undertook a census of globular clusters, using the same data to observe the Hubble Space Telescope as in the previous study. They apply strict parameters to determine which globular clusters are associated with DF44 and ultimately come out in much smaller numbers.
“The fact that we found only 20 globular clusters in our work, compared to the previous 80, drastically reduces the amount of dark matter thought to contain the galaxy,” said astrophysicist Ignacio Trujillo of the Canary Islands Institute of Astrophysics in the Canary Islands.
“Furthermore, with the number of globular clusters we found, the amount of dark matter in Dragonfly 44 is in line with what is expected for this type of galaxy. The ratio of visible to dark matter is no longer 1 in 10,000, but one in 300.”
This is not Trujillo’s first accumulation of ultradiffuse galaxies with anomalous content of dark matter. Last year, he discovered that two such galaxies, called DF2 and DF4, discovered earlier (also by van Dokum and colleagues) that had little or no dark matter were also likely to be normal. The measurement of low dark matter is due to an incorrect distance calculation. Revise the distance and both galaxies are no longer space freaks.
At the time, he told ScienceAlert that humans were much more likely to have made a mistake than that we had found extreme deviations that contradicted cosmological models. And this is what Saifolay and his team discovered this time as well.
“Our work shows that this galaxy is neither so single nor unexpected,” said astrophysicist Michael Beasley of the Canary Institute of Astrophysics. “That way, patterns of galaxy formation can explain it without the need for modification.”
But the show is not necessarily over. After recalculating Trujillo’s distance, Van Docum and colleagues also recalculated the distances to DF2 and DF4. These new analyzes yielded results consistent with the team’s initial finding, so we expect the debate over DF44 to continue in the same way.
However, such debates are useful for science. Whatever the results, trying to understand it will help us improve our space exploration techniques. We would call this pure profit.
The study was published in The monthly notices of the Royal Astronomical Society.