A Lonely Galaxy's Puzzling Lack of Neighbors Could Shed Light On Dark Matter

Why would anyone not want to hang with a galaxy as beautiful as this one? ESA/Hubble and NASA

It doesn't matter how big you are, sometimes you just want someone else around. If galaxies had feelings, M94 would probably be sad, because it seems no one wants to get close to it. Other galaxies of similar size have smaller companions, but M94 stands nearly alone. The discovery is likely to force a rethink of theories on galaxy formation and the nature of dark matter.

M94 seems an unlikely cosmic pariah. Known since 1781, it's popular with amateur astronomers as a particularly beautiful example of a large spiral galaxy that's face on to us and at a relatively close 15 million light-years away. Its name comes from being 94th in Charles Messier's catalog of 110 objects, which in 19th-century telescopes could be confused with comets.

However, when University of Michigan graduate student Adam Smercina studied M94 with Hawaii's Subaru telescope, he found something unexpected. Galaxies this size are expected to have many small companions, but Smercina reported in Astrophysical Letters that M94 has only two.

The two satellite galaxies of M94. Smercina et al. 2018

The Milky Way has the Large and Small Magellanic Clouds, visible to the naked eye, along with approximately eight others with masses greater than 400,000 times that of the Sun and a long list of even smaller objects. Not only that, we're still finding more hidden from our sight behind our own galaxy's dust clouds. Four galaxies of similar size have 6-12 satellite galaxies each. The slightly larger Andromeda Galaxy has at least 14. These smaller galaxies are not just casual add-ons. The creative destruction that ensues from their interaction with the large galaxy can have a major impact on its development.

We don't expect every galaxy to have the exact same number of companions, but existing models predict fewer than one in 500 galaxies this size have only two, making it unlikely we'd find one so close to home.

"More than just an observational oddity, we show that the current crop of galaxy formation models cannot produce such a satellite system," Smercina said in a statement. "Our results indicate that Milky Way-like galaxies most likely host a much wider diversity of satellite populations than is predicted by any current model."

Since these existing models all depend on theories about the workings of the dark matter that help keep galaxies together, unexpected findings may indicate a flaw in our ideas about dark matter.

In small galaxies, supernovas expel gas. If the galaxy's gravity is low enough, but production of supernovas high enough, they can push out so much gas the galaxy “dies”, becoming unable to form new stars. Smercina proposes M94's isolation may indicate this happens more easily than we thought.

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