Is the Milky Way bigger than we thought?

Ripples in the galactic disc could increase Milky Way's size by up to 50 per cent

The Monoceros Ring, identified in 2002, may in fact be just one of several ripples in the Milky Way's disc
Credit: Rensselaer Polytechnic Institute/Dana Berry

By Russell Deeks

The Milky Way could be a full 50 per cent larger than current estimates suggest, according to new research funded jointly by the USA’s National Science Foundation and China’s National Science Foundation and National Basic Research Program.

An international team of researchers led by Professor Heidi Jo Newberg from the Rensselaer Polytechnic Institute in New York looked at data gathered by the Sloan Digital Sky Survey in 2002, which had previously identified a bulging ‘ring’ of stars, dubbed the ‘Monoceros Ring’, lying outside the Milky Way’s accepted plane some 60,000 lightyears from the galactic centre.

On revisiting the data, Prof Newberg and her team concluded that this anomalous ring is in fact a ripple in the galactic plane itself – and that it is just one of several such ripples. If this is correct, then our Galaxy is around 150,000 lightyears across, and not 100,000 as was previously believed.

“In essence, what we found is that the disk of the Milky Way isn’t just a disc of stars in a flat plane – it’s corrugated,” says Prof Newberg. “We see at least four ripples in the disc. While we can only look at part of the galaxy with this data, we assume that this pattern is going to be found throughout the disc.”

The team’s research was published earlier this month in the Astrophysical Journal, in a paper entitled 'Rings and Radial Waves in the Disk of the Milky Way'. The four anomalous ‘ripple’ regions found by the team lie north of the galactic plane at two kiloparsecs (6500 light years) from the Sun, south of it at 4-6kpc, north at 8-10kpc and south at 12-16kpc from the Sun. The ‘Monoceros Ring’ represents the third ripple.

These ripples, it is suggested, may be the result of a dwarf galaxy or large quantity of dark matter passing through the Milky Way at some point in the distant past. “It’s very similar to what would happen if you throw a pebble into still water – the waves will radiate out from the point of impact,” says Newberg. “If a dwarf galaxy goes through the disk, it would gravitationally pull the disk up as it comes in, and pull the disk down as it goes through, and this will set up a wave pattern that propagates outward.”


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