A group of astronomers led by Dr Joshua Simon from the Observatories of the Carnegie Institution of Washington has discovered two stars in the Sculptor dwarf galaxy that were born shortly after the galaxy formed, approximately 13 billion years ago. According to the astronomers, the unusual chemical content of the stars may have originated in a single supernova explosion from the first generation of Sculptor stars.
This image shows the Sculptor dwarf galaxy. Image credit: ESO / Digitized Sky Survey 2.
The Sculptor dwarf is a small spheroidal galaxy located in the constellation Sculptor, approximately 290,000 light-years away. It is one of our Milky Way’s neighboring dwarf galaxies.
Large galaxies like the Milky Way can contain several hundred billion stars, but Sculptor is home to just a few million. Because Sculptor’s stars are all located the same distance away from us, their ages can be determined by studying the pattern of their colors and brightnesses. This technique tells astronomers that Sculptor, like many dwarf galaxies, stopped evolving long ago.
While the Milky Way has been forming stars throughout the Universe’s 14 billion year existence, Sculptor’s youngest stars are 7 billion years old.
Dwarf galaxies thus provide scientists an opportunity to see what galaxies looked like in the early epochs of the Universe.
Dr Simon and his colleagues studied five stars in the Sculptor dwarf galaxy, measuring the abundance of 15 elements in each one.
“The two most-primitive stars have less than half as much magnesium and calcium as would be expected based on their iron content and just 10 percent as much silicon as similar stars in other galaxies,” the astronomers said.
“The only way to explain the shortage of magnesium, calcium, and silicon in these stars is if their heavy elements were made by fewer than four supernovae, and those supernovae need to have been a rare kind of explosion,” said Dr Simon, who is the first author of the paper accepted for publication in the Astrophysical Journal (arXiv.org preprint)
The team concluded that these two primitive stars were probably formed from a gas cloud that had been seeded with heavy elements made by just one ancient supernova.
This parent star is thought to be one of the very first stars ever formed in Sculptor.
“Most likely, we are seeing the leftover traces of just a single supernova,” said co-author Dr Heather Jacobson of MIT’s Kavli Institute for Astrophysics and Space Research and Department of Physics.
“These stars are giving us an unprecedented view of the earliest history of another galaxy,” added co-author Dr Anna Frebel, also of MIT.
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Joshua D. Simon et al. 2015. Chemical Signatures of the First Supernovae in the Sculptor Dwarf Spheroidal Galaxy. ApJ, accepted for publication; arXiv: 1412.5176
