Milky Way double haloes spin both ways
Milky Way halo The halo of stars surrounding the Milky Way is made up of two different parts that spin in opposite directions, according to an international team of astronomers.
The findings could help settle a long-standing debate about how our galaxy and its halo formed.
The research could also help astronomers pin down much sought-after astronomical evidence of conditions just after the Big Bang.
The Milky Way halo is a sphere of ancient stars, invisible to the naked eye, that surrounds the familiar flattened spiral disc of the galaxy.
"We believe there are two main parts to the halo. It's not one simple halo," says Carollo's co-author and PhD supervisor Professor John Norris.
For decades astronomers have debated how the Milky Way formed after the Big Bang 10 billion years ago.
Some argue the halo was left behind as the centre of a cloud of gas collapsed to form the flat spiral disc that is so characteristic of our galaxy.
Others say the halo was formed from smaller galaxies outside the collapsing gas cloud that were pulled in by gravity.
The problem, says Norris, is that these theories were derived from a relatively small sample of stars.
Carollo, Norris and colleagues instead studied over 25,000 stars from a recent large survey of the Milky Way called SEGUE.
This allowed them to analyse how stars move in the Milky Way halo, and their chemical composition.
Inner and outer haloes
The researchers found inner and outer haloes, which rotate in opposite directions.
They found the inner halo is a flattened sphere rotating in the same direction as our sun but more slowly.
The Milky Way's spiral disc takes 200 million years to rotate, at around 800,000 kilometres per hour, while the inner halo rotates at around 80,000 kilometres per hour.
The inner halo also contains 40 times fewer heavy elements than the sun because it is much older and heavy metals like iron took billions of years to accumulate in the universe.
These facts are consistent with the halo forming as a result of a collapsing cloud of gas, says Norris.
The outer halo is a sphere that rotates at around 160,000 kilometres per hour in the opposite direction to the sun.
It has even fewer heavy metals than the inner halo and is similar to the composition of the smaller galaxies that surround the Milky Way.
Norris says these facts are consistent with the other theory that suggests the halo was creating from smaller galaxies that were attracted by the forming Milky Way.
Both theories true
Such evidence suggests the two competing theories about how the Milky Way was formed may not be competing after all.
"Each of them is probably true," says Norris.
He says while only time will tell whether this is the case, the "hard fact" is that there are two components of the halo.
"Even if what I've told you about how it might have come about is all nonsense, it's an important fact that people have to explain [in future theories of how the Milky Way formed]."
Norris says the discovery could help astronomers find stars that are low in heavy elements.
These chemically primitive stars give insight into the conditions present just after the Big Bang, he says. They are fossils of the early universe but are extremely rare.
Finding them remains a classic "needle in a haystack problem", says Norris. But discovering a chemically distinct outer halo, "gives us a much better way to search the haystack."