Miércoles 19 de Diciembre de 2007, Ip nº 221

Milky Way's two stellar halos have opposing spins
Por Zeeya Merali

We call it home, but the Milky Way can still surprise us. It does not have just one halo of stars, as we thought, but two. The finding calls into question our theories for how our galaxy formed.

Daniela Carollo at the Torino Observatory in Italy and her colleagues were measuring the metal content and motion of 20,000 stars in the Milky Way, observed by the Sloan Digital Sky Survey, when they made their discovery.

They found that the halo can be divided into two distinct regions, rotating in opposite directions, and containing stars of different chemical composition. "We really weren't expecting to see anything like this," says Carollo.

The team found that the inner halo is flattened and extends out to about 4.6 x 1017 kilometres from the galactic centre, rotating at 20 kilometres per second, in the same sense that the Sun travels round the galactic centre. The outer halo is spherical, stretching out to over 6.0 x 1017 kilometres and spinning in the opposite direction at about 70 kilometres per second.

It seems odd that no one noticed this in the past, but Carollo points out that while astronomers had found a few stars that appeared to be moving in the "wrong direction", they did not have enough data to conclude that the halo was split into two parts.

Complicated history

By examining the spectrum of light emitted by the stars, the team also calculated that the inner-halo stars contain three times more heavy atoms than the outer-halo stars, raising questions about when the two halos formed.

Astronomers know that lightweight atoms formed fairly soon after the big bang, while heavy atoms were forged later within massive stars. This confirms suspicions that the galaxy could not have been formed in one simple stage, as astronomers once believed.

"The two haloes appear to have been formed at different times by different mechanisms," says Carollo.

Other, more recent models for the formation of the galaxy may also be in trouble. Such models speculate that the halo formed in stages, as sets of mini-halos emerged, says Carollo. "But these models can't explain why one halo is flattened, and the other is spherical," she says. "This result throws out all our current models of galaxy formation."

  12/12/2007. New Scientist Magazine.