The Sculptor dwarf galaxy composed from
data from the Digitized Sky Survey 2, courtesy of ESO/Digitized Sky
Survey 2.
Pasadena, CA— A Carnegie-based search of nearby
galaxies for their oldest stars has uncovered two stars in the Sculptor
dwarf galaxy that were born shortly after the galaxy formed,
approximately 13 billion years ago. The unusual chemical content of the
stars may have originated in a single supernova explosion from the first
generation of Sculptor stars. The team, which includes Carnegie’s Josh
Simon, Ian Thompson, and Stephen Shectman, will publish their work in The Astrophysical Journal on Thursday.
The Sculptor dwarf is a small galaxy that orbits around our own Milky
Way, just as the Moon orbits around the Earth. 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.
Stars in all galaxies are born out of collapsing clouds of dust and
gas. Only a few million years after they begin burning, the most-massive
of these stars explode in titanic blasts called supernovae. These
explosions seed the surrounding gas with the elements that were
manufactured by the stars during their lifetimes. Those elements are
then incorporated into the formation of the next generation of stars.
Generally this process is cyclical, with each generation of stars
contributing more elements to the raw material from which the next set
of stars will be formed.
Astronomers hoping to learn about the first stages of galaxy
formation after the Big Bang can use the chemical composition of stars
to help them unravel the histories of our own and nearby galaxies.
Elements heavier than hydrogen, helium, and lithium can only be produced
by stars. The more stars a galaxy forms, the more enriched in heavy
elements it becomes.
Thus, young stars contain larger amounts of heavy elements produced
by dozens or hundreds of supernovae, while the oldest stars have a very
simple chemical makeup with few of the heavy elements. Typically, stars
are characterized by how much iron they contain, because iron is a
relatively common element and is almost always the easiest for
astronomers to detect.
The team—also including Heather Jacobson and Anna Frebel of the
Massachusetts Institute of Technology, as well as former Carnegie
postdoc Josh Adams—studied five stars in Sculptor, 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 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,” explained Simon.
The astronomers concluded that these two primitive stars were
probably formed from a gas cloud that had been seeded with heavy
elements made by just one previously exploded star. 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,” added Jacobson.
“These stars are giving us an unprecedented view of the earliest history of another galaxy,” Frebel said.
This work was supported in part by the National Science Foundation.
It made use of NASA’s Astrophysics Data System Bibliographic services.
Data were gathered with the 6.5-meter Magellan telescopes at Carnegie’s
Las Campanas Observatory in the Atacama desert of Chile.
