Researchers Investigate Why Milky Way-Sized Galaxy Is Missing Neighbors


A galaxy about the size of the Milky Way casts doubts over scientists' current understanding of how galaxies are formed.

Messier 94's Missing Galaxies

Previous observations revealed that the Milky Way is surrounded by about 10 satellite galaxies, each with at least a million stars. Two of those satellite galaxies, the Magellanic Clouds, have up to more than a billion stars.

Based on this information, scientists made predictions stating that other galaxies that are similar in size with the Milky Way would have the same number of satellite galaxies around it. However, recent observations are disproving these predictions.

Researchers at the University of Michigan peered at the Messier 94 (M94), a spiral galaxy located about 16 million light-years away in the constellation Canes Venatici. Scientists initially thought that M94 is roughly 30,000 light-years in diameter, but recent observations spotted two faint spiral arms that tripled its known size, making it about as big as the Milky Way (100,000 light-years in diameter).

The team used the powerful Subaru Telescope installed in Hawaii to see exactly how many satellite galaxies are orbiting M94. The large telescope can see galaxies that are five to 10 times the distance from the Milky Way.

They found that, despite its size, the M94 does not have nearly as many satellite galaxies as the Milky Way. In fact, they only found two in its midst and each had significantly fewer stars each.

"More than just an observational oddity, we show that the current crop of galaxy formation models cannot produce such a satellite system," explained Adam Smercina who led the study. "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."

A New Cosmic Model

The researchers believe that the current cosmic model, including how galaxies form in the first place, might not be accurate for M94. Galaxies have a dark halo — a bubble of dark matter that permeates and surround them.

Smercina postulates that galaxy formation in intermediate-size dark halos does not behave the same way as initially thought.

"We think that that scatter — the range of galaxies we expect to see — may be a lot higher than what people currently think for dark matter halos of a certain mass," he stated. "Nobody's under any illusions as to there being this huge scatter at the very lowest halo masses, but it's at these intermediate dark matter halos that the discussion is happening."

The study was published in The Astrophysical Journal.

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