Dark Matter Pushes Super Spiral Galaxies To Incredible Speeds

This mosaic features Hubble images of three massive spiral galaxies (top) and Sloan Digital Sky Survey images of much heavier super spirals (bottom). Top row: NASA, ESA, P. Ogle and J. DePasquale (STScI). Bottom row: SDSS, P. Ogle and J. DePasquale (STScI)

If you are currently reading this from the comfort of your sofa, inactive and presumed stationary, you are in fact whizzing through space at great speed. The Earth rotates on itself, moves around the Sun, and the entire Solar System moves with everything else in the Milky Way. Our galaxy moves at an impressive 210 kilometers (150 miles) per second but it is definitely not a record-breaker among spiral galaxies.

Astronomers have discovered about 100 super spirals, disk galaxies that are larger, brighter, and more massive than the Milky Way. Now researchers know that they also spin much faster, with the heaviest one, weighting 20 times our galaxy, spinning at about 570 kilometers (350 miles) per second. The findings are reported in The Astrophysical Journal Letters.   

“Super spirals are extreme by many measures,” lead author Patrick Ogle of the Space Telescope Science Institute in Baltimore said in a statement. “They break the records for rotation speeds.”

These galaxies are not the biggest galaxies in the universe, but they are the biggest spirals, and they don’t seem to follow an important rule. Astronomers previously found a relationship between the mass of a spiral galaxy and the speed at which it rotates, but these super spirals don’t seem to follow it; they haven’t got enough stars for their speed.

According to the current model of the universe, five-sixths of the matter in the universe doesn’t interact with light, only with gravity, hence "dark matter". This dark matter keeps galaxies in their shape by simply spreading out further than the visible matter, in what is called the dark matter halo. In these super spirals, the halo is much heavier, comparable to what’s usually found around a group of galaxies, rather than a single one.

“This is the first time we’ve found spiral galaxies that are as big as they can ever get,” Ogle said. “It appears that the spin of a galaxy is set by the mass of its dark matter halo.”

The findings strengthen the idea that dark matter, which we are yet to detect directly, is the correct theory to explain the motion of galaxies. Alternative theories such as the Modified Newtonian Dynamics (MOND) require changes to how gravity works and they wouldn’t be able to explain how super spirals can spin so fast.

Researchers will be able to investigate these galaxies further using the James Webb Space Telescope after it is launched in 2021.


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