According to the model of galaxy formation hierarchical said, the current large galaxies grew by swallowing smaller neighbors. The Milky Way would be formed in part of small satellite galaxies absorbed during its history. But the apparent absence in the current satellite galaxies, stars chemically similar to the oldest of the Milky Way had cast doubt on this scenario. Spectroscopic analysis of an old star in the Sculptor dwarf galaxy by the team of Anna Frebel, the Harvard-Smithsonian Center for Astrophysics, raises the question today: its chemical composition is similar to that of stars the galactic halo, in agreement with the scenario based on the hierarchical model.

The dwarf galaxies are small structures bringing together millions of stars at most. They are mostly in orbit around larger galaxies like the Milky Way, home to hundreds of billions of stars. According to the hierarchical model, dwarf galaxies are the “building blocks” of larger galaxies. Many of them were shredded and treated to form the stellar population of the halo of the Milky Way, a vast spherical region of low density which is immersed in the galactic disk. If this is the case, we should find in the current satellite galaxies and the galactic halo of old stars formed in the same structures that have in part given rise to current satellites and was partly submerged, and therefore should have the same chemical composition.

The old stars are to identify their metallicity, That is to say to their content of chemical elements heavier than hydrogen and helium, collectively known as metals. Indeed, the heavy elements in trace amounts in the early universe, are synthesized by stars before being released into the interstellar medium in the death of them. Therefore, the old stars, formed from an interstellar medium of virgin metals generally have low metallicity.

The stars of the halo of the Milky Way are extremely metal-poor with a metallicity as 100 000 times smaller than the Sun, which is nevertheless a moderately rich star field. However, records of recent years had shown no evidence of stars as metal-poor in dwarf galaxies surrounding the Milky Way. In other words, dwarf galaxies do not seem quite old compared to the Milky Way.

In 2009, a star of low metallicity, a red giant named S1020549, however, was discovered in the Sculptor dwarf galaxy located about 280,000 years to light: its iron content – a good indicator of the overall metallicity – is 4000 times smaller than the Sun.

More specific analysis of the chemical composition were nevertheless required to establish the link with the stars in the galactic halo. This was achieved A. Frebel and colleagues using the spectroscope MIKE mounted on the Magellan-Clay telescope at Las Campanas, Chile. These measurements confirm first the very low iron content of S1020549. It is even five times lower than the star of the poorer in heavy elements known so far in the dwarf galaxies. The iron content is similar to that of stars of low metallicity typical of the galactic halo. The researchers then measured the abundance of a dozen other chemical elements like magnesium, calcium and titanium, and found that the levels of these elements also coincide with those of stars in the halo. This similarity suggests that chemical S1020549 has the same origin as the old stars in the galactic halo, which gives weight to the hypothesis of absorption of its satellites by the Milky Way.

But a single star is not proof, and it will discover many other stars of this type to confirm the link between dwarf galaxies and the galactic halo. According to A. Frebel, however, detection of a star of such low metallicity in a modest statement portends the discovery of other metal-poor stars in dwarf galaxies.

Source:pourlascience