Milky Way’s 11 most distant stars are located roughly 300,000 light-years away from us. According to a team of astronomers at Harvard University, five of those stars might have been ripped from another galaxy, the Sagittarius dwarf elliptical galaxy.
Stellar streams around the Milky Way Galaxy. Image credit: NASA / JPL-Caltech / R. Hurt, SSC & Caltech.
The Sagittarius dwarf elliptical galaxy (SagDEG), also known as the Sagittarius dwarf spheroidal galaxy, is one of the nearest dwarf galaxies.
It was discovered in 1994 by University of Cambridge astronomer Rodrigo Ibata and co-authors.
“SagDEG is in a near polar orbit around the Milky Way and has experienced multiple passages through the disk,” the Harvard astronomers said.
“The resulting stream of tidally stripped stars wraps a full 360 degrees around the celestial sphere.”
“Coincidentally, the Sun’s location is close enough to the SagDEG orbital plane to likely lie within the width of the debris trail.”
Harvard University Professor Avi Loeb and PhD student Marion Dierickx used computer models to simulate the movements of SagDEG over the past 8 billion years.
They varied its initial velocity and angle of approach to the Milky Way to determine what best matched current observations.
“The starting speed and approach angle have a big effect on the orbit, just like the speed and angle of a missile launch affects its trajectory,” Prof. Loeb said.
At the beginning of the simulation, SagDEG weighed about 10 billion times the mass of our Sun, or about 1% of the Milky Way’s mass.
The team’s calculations showed that over time, the dwarf galaxy lost about 1/3 of its stars and 9/10 of its dark matter.
This resulted in three distinct streams of stars that reach as far as one million light-years from the Milky Way’s center.
They stretch all the way out to the edge of the Milky Way halo and display one of the largest structures observable on the sky.
Moreover, five of the 11 most distant stars in our Galaxy have positions and velocities that match what you would expect of stars stripped from SagDEG.
The other six do not appear to be from SagDEG, but might have been removed from a different dwarf galaxy.
The research has been accepted for publication in the Astrophysical Journal. The article is also publicly available at arXiv.org.
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Marion Dierickx & Abraham Loeb. 2017. Predicted Extension of the Sagittarius Stream to the Milky Way Virial Radius. ApJ, accepted for publication; arXiv: 1611.00089
This article is based on a press-release from the Harvard-Smithsonian Center for Astrophysics.
