Twin history.

The Milky Way and its nearest large galactic neighbor, Andromeda, are more alike than earlier evidence had indicated. A new study shows that the two spiral galaxies evolved in a highly similar fashion over the first 3 billion to 4 billion years of their histories.

The study reveals that the composition of some 1,000 stars in Andromeda's halo--a vast cloud that includes the outer reaches of the galaxy--are deficient in all elements heavier than hydrogen, just as stars in the halo of the Milky Way are. A halo "is the true fossil relic of the earliest formation of a spiral galaxy," notes Scott Chapman of the California Institute of Technology in Pasadena. The finding therefore suggests that the Milky Way and Andromeda had similar early histories, he says.

Chapman and his colleagues used ground-based telescopes including Keck II on Hawaii's Manna Kea. They describe their findings in an upcoming Astrophysical Journal.

It's not surprising that the two galaxies, which are only about 2 million light-years apart, have similar histories, notes Chapman. But studies over the past decade had indicated that stars in Andromeda's halo were abundant in elements heavier than hydrogen. It now appears that astronomers making those observations had been mistakenly examining stars in Andromeda's disk.

Some puzzling differences between the Milky Way and Andromeda remain. Andromeda has a larger, lumpier, and faster-rotating disk. Galactic disks arise several billion years after halos do. "Clearly, after the first 3 to 4 billion years ... some very different events shaped the evolutionary histories of the two galaxies," Chapman says.--R.C.