If a castle is built with nearby rocks, you'd expect the stone walls to be made of the same stuff as any remaining boulders in the area. Likewise, if our Milky Way galaxy grew through the merging of smaller dwarf galaxies, you'd expect it to contain the same types of stars as the remaining dwarfs that have not been incorporated. But detailed observations of four of these dwarf galaxies show that this is not the case, indicating that theories about how galaxies form are incomplete.
According to current cosmological wisdom, small primordial clouds--mainly consisting of mysterious cold dark matter--first coalesced into dwarf galaxies. Subsequently, the dwarfs merged to build up large galaxies. When dwarf galaxies are swallowed by the Milky Way, their constituent stars end up in our galaxy's halo--a large, nearly spherical distribution of mostly old stars surrounding the Milky Way on all sides. As a result, the stellar content of the halo should reflect the properties of dwarf galaxies.
But that doesn't seem to be true, according to work by an international team of astronomers led by Amina Helmi of the University of Groningen in the Netherlands. Observations made by the team with the European Very Large Telescope in Chile indicate the stars in four neighboring dwarf galaxies, known as Sculptor, Sextans, Fornax and Carina, have a markedly different chemical makeup than the stars in the halo. In particular, stars with an extremely small amount of heavy elements (just a thousandth of the sun's) occur in the Milky Way's halo but are not found in the dwarf galaxies. "The progenitors of the Milky Way and the [dwarf galaxies] must have been different," write Helmi and her colleagues in a paper in the 10 November Astrophysical Journal Letters.
"It's a very interesting find," says astronomer Leo Blitz of the University of California, Berkeley. "The accretion hypothesis is now firmly entrenched in the minds of people studying the evolution of galaxies, and this result calls the simplest version of the model into question." But don't go changing the models just yet, says theoretical astrophysicist Andrey Kravtsov of the University of Chicago. Although he finds the results "somewhat surprising," he notes that the stars that make up the Milky Way's halo came from dwarf galaxies that were accreted early in the Milky Way's history. As a result, they may have had less time to evolve than current outlying dwarf galaxies, and this may explain their different composition.