As early as 2016, researchers claimed to have discovered a galaxy made almost entirely dark matter and almost no stars. Now, on closer inspection, this claim has disintegrated.
The galaxy, Dragonfly 44 (DF44), belongs to a class of mysterious objects known as ultradiffuse galaxies or UDGs. Researchers have been debating since the 1980s whether these huge, dark objects have a low mass, like dwarf galaxies smeared in vast expanses of space, or rather like heavy Milky Way galaxies which seem obscure for two reasons: because they have almost no stars, and because much of their mass is dark matter, located at the outer edges of the galaxy, in so-called halos of dark matter that do not emit light. In an article published in 201
But a new analysis published on October 8 in the journal Monthly notices of the Royal Astronomical Society, suggests that the earlier study was wrong. Researchers in the 2016 study suggest that a pile of mass is absorbed in the halo of dark matter; but in fact the new study showed a much lower total mass, which shows that DF44 is one of those low-mass dwarf galaxies distributed in space with normal percentages of dark matter.
Connected: 8 ways you can see Einstein’s theory of relativity in real life
DF44 is about 360 million light years from The Earthso astronomers cannot directly measure its mass. Instead, they rely on proxies. Features such as the speed at which objects orbit a galaxy can show how massive it actually is, as well as more gravity would cause objects to rotate faster.
In 2016, researchers claimed that DF44 has a huge halo because of how fast its globular clusters (the few they call the home of the galaxy) appear to revolve around its center. (Spherical clusters are patches of stars that accumulate around galaxies.) But these velocity measurements turned out to be incorrect in 2019.
However, this was not the end of the controversy that DF44 has a huge amount of dark matter. This is because the galaxy appears to have hosted a relatively large number of globular clusters.
Over time, researchers have noticed a common link between the number of globular clusters in a galaxy and the mass of that galaxy, said lead author Teymour Saifolahi, a PhD candidate at the University of Groningen in the Netherlands.
And DF44 seems to have had more globular clusters than you would expect for a galaxy with so few stars. Early observations estimated about 100 of these clusters, which was later narrowed to 80 in a 2017 report in Astrophysical Journal Letters. This would put the mass of DF44 right on the territory of the Milky Way – unpleasant results with huge consequences for how cosmologists understand the history of galaxy formation in space time. In the new model, galaxies would be mostly objects of dark matter, capable of forming without many stars or other luminous matter. All these bright spots in space would be just additional accessories.
Saifolahi and his colleagues did their own count and landed at a much lower number: 20. This would mean that DF44 has a normal mass of a dwarf galaxy – a much less exciting result.
Not surprisingly, the two research teams turned out to be so different, he said.
Why such a clear difference in ratings?
“It’s not as easy as just looking and counting,” Saifolahi told Live Science. “Images have all sorts[s] of astronomical objects and not all are globular clusters. Some are just stars in the middle of the road from us to the galaxy, and some are very distant objects that look small. “
There is always some level of uncertainty to understand what these objects are, he said. This is especially true when you consider that researchers suggest that globular clusters are too small and obscured to be seen from our point of view always existing around any galaxy.
The key difference between the 2017 analysis and the 2020 analysis, Saifolahi said, has to do with where they assumed most of the globular clusters in DF44 were located. The 2017 team made a rough guess as to how far the clusters would orbit from the center of the galaxy, based on standard numbers associated with dwarf galaxies, and then looked for candidates in the field. For the 2020 paper, researchers actually measured how far clusters extend from the center of the galaxy and found that star globes cluster much closer to the center of DF44 than expected. Counting possible clusters only in this smaller area gives a smaller number
“It’s also an interesting find from the side,” he said.
In future UDG research, he said, scientists will need to be more careful not to rely on standard assumptions about globular clusters. He and his colleagues plan to take a closer look at other UDGs, which have a high projected number of globular clusters, and see if these estimates hold up.
Originally published in Live Science