What makes up a galactic peanut?

The stars at the centre of the Milky Way are not uniformly old, an international team led by astronomers from the European Southern Observatory (ESO) in Chile has established. The finding challenges some previous research, which suggested the stars were all roughly the same age.

At the heart of the Milky Way is a bulge, a vast area thousands of light years across that contains about 25% of the stars in the entire galaxy. The bulge comprises two elements: a spherical collection of stars in which in metal is scarce, and a population of other, metal-rich stars that form a wasp-waisted bar-like structure that has been compared to the letter “x”, or a peanut.

Previous studies aimed at establishing the age of the stars have produced conflicting results. In the latest study, a team led by the ESO’s Marina Rejkuba used two massive datasets.

The first comprised observed and simulated information for millions of stars obtained by ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA) at the Paranal Observatory in Chile. In particular, the team used data gathered as part of a VISTA project called Variables in the Via Lactea (VVV), which is looking at the bulge and the galaxy’s southern disc, using near-infrared wavelengths.

The second tranche of information derived from an ESO spectrograph dubbed GIRAFFE, which operates from the ESO’s  Very Large Telescope, also at Paranal. The spectrograph has measured the metal content of a sample comprising 6000 stars in the bulge.

“We analysed the colour and brightness of stars to find those that have just reached the point of exhausting their hydrogen fuel-burning in the core, which is a sensitive age indicator,” explains Rejkuba.

If all, or even most, of the stars in the bulge were the same age, then this should have been reflected in the results, but it was not. Instead, the findings indicated that star formation went on in the bulge for around four billion years, starting around 11 billion years ago.

The results – which will be presented in early April at the European Week of Astronomy and Space Science (EWASS) held in the UK city of Liverpool – are not the first to suggest that star formation at the Milky Way’s heart was a staggered process. 

However, they indicate some significant differences with earlier multi-age findings.{%recommended 2064%}

“The youngest stars that we see are at least seven billion years old, which is older than some previous studies had suggested,” says Rejkuba.

A 2017 paper published in the journal Astronomy & Astrophysics, for instance, found that star formation in the bulge took place in distinct episodes, at three, six, eight and 11 billion years ago.

The ESO astronomers operate on the cautious assumption that the metal-rich stars in the galactic peanut are younger than the metal-depleted ones in the surrounding sphere.

However, Rejkuba and colleagues concede that their current findings cannot confirm that, because the data does not allow them to distinguish which stars in the data belong to which structure. It is sufficient, however, to permit the construction of the first large-scale “age map” of the Milky Way.

The work to date forms the basis of a much larger and more ambitious project, which will enable the matter to be definitively sorted.

“The final map will show us the star formation rate as a function of both age and metallicity for the stars across the bulge,” says ESO astronomer Elena Valenti. “This will be an important ingredient in telling the complete story of the formation of the Milky Way bulge.”