Puzzling Galaxy Made Almost Exclusively Of Dark Matter Turns Out To Be Not So Puzzling

The ultra-diffuse galaxy Dragonfly 44 taken with the Hubble space telescope. Teymoor Saifollahi and NASA/HST

Galaxy Dragonfly 44 was a bit of a mystery when it was first discovered in 2016. Based on the original observations the galaxy was thought to be made overwhelmingly of dark matter, the mysterious substance that permeates the cosmos. This was inexplicable with our theories.

Now, new observations reveal that the quantity of dark matter was overestimated and that the peculiar galaxy is not an outlier at all. The findings are reported in the Monthly Notices of the Royal Astronomical Society.

“Dragonfly 44 has been an anomaly all these years that could not be explained with the existing galaxy formation models. Now we know that the previous results were wrong and that DF44 is not extraordinary. It is time to move on,” lead author Teymoor Saifollahi, from the Kapteyn Institute, said in a statement.

Dark matter does not interact with light, so it is invisible to our instruments. Instead, its existence is inferred through the motion of stars in galaxies. This is a problem for objects such as Dragonfly 44 whose stellar population is incredibly spread out. These are known as ultra-diffuse galaxies. But there is a trick to estimate the mass of these objects.

Thanks to a special correlation, astronomers just have to identify globular clusters, compact aggregations made of many stars orbiting the main bodies of galaxies. The number of globular clusters is related to the mass of the galaxy, although the reason behind this is unclear.

The original study identified 80 globular clusters that appeared to be bound to this galaxy, but the new work suggests that the number is much lower. The revision changes the amount of dark matter in the galaxy to a more expected value.

“The fact that in our work we found only 20 globular clusters, compared with the 80 previously claimed, reduces drastically the amount of dark matter which the galaxy is believed to contain,” said co-author Dr Ignacio Trujillo from the Instituto de Astrofisica de Canarias.

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

So, Dragonfly 44 is no longer considered an extreme galaxy but the mystery of dark matter itself continues.

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