Astroboffins spot sneaky signs that the Milky Way devoured smaller galaxies

Astrophysicists have found evidence that the Milky Way has gobbled up neighbouring massive dwarf galaxies over its 13 billion year lifetime.

A team of researchers using a spectroscope analysed the starlight from 28 red giant stars hovering on the outer halo of the Milky Way. They then worked out their chemical composition from data collected by the Very Large Telescope and the Magellan Telescopes in Chile, as well as the Hobby Eberly Telescope in the United States.

After studying the concentration of elements contained in the stars on the edge, they found it did not match those of similar stars in the inner regions. The results were published in Astronomy and Astrophysics.

"The abundance of some chemical elements in the stars in the external regions of the Milky Way halo as found to be surprisingly different from the information we had concerning the inner regions of the halo," said Giuseppina Battaglia, first author of the paper and an astrophysicist at the Instituto de Astrofísica de Canarias (IAC).

The team looked at the concentration of elements other than hydrogen and helium contained in the old stars, a property known as metallicity. The red giants had similar metallicities to other stars in nearby galaxies such as the Sagittarius Dwarf Spheroidal Galaxy and Large Magellanic Cloud, both massive dwarf galaxies at over 65,000 lights years and 158,000 light years away.

A star’s chemical composition provides a hint of what kind of environment it formed in. The contrast of elements shows that the furthest regions in the Milky Way’s halo probably contains the stellar remains of at least one massive dwarf galaxy that was engulfed earlier in the Milky Way's history.

Theories explaining how galaxies were created predict that their halo areas are mostly made up of stellar components stolen from smaller galaxies.

“Qualitatively this is in agreement with the observational findings of this study, where we found remnants of cannibalized dwarf galaxies around the Milky Way," Battaglia concluded.

More spectroscopic goodness

Elsewhere, in related Milky Way news, another group of scientists has spotted one of the galaxy’s first stars in a similar research project.

A paper published in The Astrophysical Journal this month estimates that the ancient ball of plasma, codenamed J0815+4729, could have formed 300 million years after the Big Bang, 13.8 billion years ago.

But despite its old age, the star is a slow burner and is still in the main sequence stage like our Sun. Despite being smaller, at 0.7 times the mass of the Sun, it is hotter at 6,215 Kelvin (~5,942˚C).

The star was first identified from the Sloan Digital Sky Survey and was left as an entry in a database as part of the Baryon Oscillation Spectroscopic Survey (BOSS) project.

"This star was tucked away in the data base of the BOSS project, among a million stellar spectra which we have analysed to identify it, which required a considerable observational and computational effort," said Carlos Allende Prieto, co-author of the paper.

"It needs high resolution spectroscopy on large telescopes to try to detect the various chemical elements in the star which can help us to understand the first supernovae and their progenitors."

J0815+4729 is smaller and has only a millionth part of the calcium and iron that the Sun contains, but it has a relatively large amount of carbon, almost 15 per cent of what is contained in the Sun.

“The star could be the most iron-poor unevolved star known, and is also amongst the ones with the largest over abundances of carbon,” the paper’s abstract said.

"We know of only a few stars, which can be counted on the fingers of a hand, of this type in the halo - where the oldest and most metal-poor stars in our Galaxy are found,” said David Aguado, first author of the paper and a research student at the IAC and the University of La Laguna. ®