Eleven new streams of stars may have been spotted in the Milky Way in a survey of a quarter million stars. The streams may provide new forensic evidence of the Milky Way's violence, as they were likely ripped from dwarf galaxies that were gobbled up by our galaxy.
The eleven possible streams turned up during a study of the velocities of stars in the Milky Way's inner halo – a bubble-like region that surrounds the galactic centre. Only two streams were previously known to exist in the area.
The stars in the proposed streams move at roughly the same speed. That suggests they all originate from the same place – most likely small galaxies that were pulled in to create the Milky Way.
A number of other dramatic structures, such as a bright stellar stream known as Sagittarius, sit farther from the galactic centre, where they can be easily spotted because the stars appear closely clumped together.
"These things definitely won't make the same kind of pretty pictures on the sky that previous stream discoveries did," says Kevin Schlaufman, a graduate student at the University of California, Santa Cruz, who did the new analysis.
Streams closer to the galactic centre are likely to be older. That may explain why they are not photogenic – they have had more time to be ripped apart. "The notion is that you probably build a galaxy up from the inside out," says Connie Rockosi, Schlaufman's advisor.
The find was made using data from the SEGUE survey, which was part of a mammoth mapping project called the Sloan Digital Sky Survey.
SEGUE was designed in part to measure stellar velocities to look for substructures such as streams. Velocity is "actually a signature that's much longer-lived than looking at stars that are bunched together on the sky, which only works with very recent accretion events", Rockosi told New Scientist.
The 11 candidate streams were identified in a study of 100 small patches of the sky. Extrapolating that rate outwards suggests the entire inner 75,000 light years of the galaxy may contain close to 1000 separate streams of stars – ripped from as many galaxies.
One and the same
Such statistics could help fill out galaxy formation models, which make predictions about how much substructure should form as large galaxies absorb smaller ones.
But it may be too early to say for certain whether all of the patches of uniformly moving stars are distinct streams. Some may constitute the same stream, which is simply being seen in two different parts of the sky.
The clumps do seem to have distinct velocities – "probably more different than one would expect if you looked at the same feature from different places on the sky", Schlaufman told New Scientist. The patches also seem to be fairly far from each other. "But that isn't conclusive proof of uniqueness by any means," he cautions.
Checks of the stars' chemical composition could help determine whether some patches share a common origin.
Studying the chemical composition could also indicate what conditions were like in the galaxies where these stars got their start, before the Milky Way intervened.
The research was presented at a conference on the Sloan Digital Sky Survey in Chicago on Saturday.