Astronomers recently revealed that our galaxy, the Milky Way, devoured a so-called 鈥渟ausage鈥� galaxy about 10 billion years ago. Now it seems this sausage might make finding dark matter ever harder.
We can鈥檛 track dark matter directly because it is invisible and can pass straight through normal matter. But the location of dark matter appears to coincide with regular matter, so we can track its movements by looking to the stars.
Lina Necib at the California Institute of Technology and her colleagues used the biggest-ever map of stars and their velocities to track down how this mysterious stuff is moving around us.
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Galaxy mapping
They combined data from the Gaia satellite, which measured the distances and velocities of two million stars, and the Sloan Digital Sky survey, which collected information on their compositions.
The velocity map showed a population of stars with weird, elongated orbits. The composition data revealed that these same stars have an intermediate iron content compared to the two main population of stars in the Milky Way, the disk stars and the halo stars.
A star鈥檚 metal content is dictated by its age, so these stars are younger than the halo stars, which come from mergers extremely early in the galaxy鈥檚 history, but older than the ones in the disk, where were born here later on.
That indicates that these stars 鈥� and the dark matter they brought with them 鈥� weren鈥檛 born in our galaxy but instead were slurped up in a galactic merger. They are the remains of the sausage galaxy, which gets its name from the stretched-out shape of its stars鈥� orbits.
鈥淭his one particular merger is dominating quite a bit of the structure and quite a bit of the mass,鈥� says Necib 鈥� 65 per cent of the stars in the immediate area around the sun that weren鈥檛 born here are from the sausage galaxy. Her team is currently working to confirm exactly what proportion of the dark matter came from the merger, but she says it鈥檚 likely also about two thirds.
On the whole, these stars are moving slower than everything else, which could make their dark matter harder to detect. 鈥淗igher-velocity dark matter tends to be easier to see,鈥� says Matthew Buckley at Rutgers University in New Jersey. 鈥淚t鈥檚 got more kinetic energy, so it makes a bigger bang when it enters your detector.鈥�
That could mean that some theoretical models of dark matter are back on the drawing board, having previously been ruled out by the fact that we haven鈥檛 seen anything, he says.
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