In this week’s image from the Hubble Space Telescope, you can see an unusual type of galaxy called an ultra-diffuse galaxy. The galaxy GAMA 526784 is shown as a smear of light across the center of the image and is around four billion light-years away located in the constellation of Hydra.
“Ultra-diffuse galaxies such as GAMA 526784 have a number of peculiarities,” Hubble scientists write. “For example, they can have either very low or high amounts of dark matter, the invisible substance thought to make up the majority of matter in the universe. Observations of ultra-diffuse galaxies found some with an almost complete lack of dark matter, whereas others consist of almost nothing but dark matter. Another oddity of this class of galaxies is their unusual abundance of bright globular clusters, something not observed in other types of galaxies.”
This image was collected as part of a Hubble project to learn more about ultra-diffuse galaxies by imaging them at ultraviolet wavelengths. These galaxies can be as big as 60,000 light-years across, which is around the same size as our Milky Way galaxy, but contain just 1% of the number of stars as the Milky Way. This has led to them being called the “fluffiest” galaxies.
The low star densities of these galaxies mean it is hard to say how they have survived, as it would be expected that they would have been pulled apart. That’s where the dark matter comes in — researchers think that those galaxies with high levels of dark matter might be protected by these dark matter cushions.
But then, how to explain the very diffuse galaxies which contain almost no dark matter? Researchers still don’t have a good answer to this question. The only possibility so far is that two of these ultra-diffuse galaxies which are lacking in dark matter, NGC 1052-DF2 and NGC 1052-DF4, could have formed in the same group at the same time and that there might be something odd about the particular environment they formed in.
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