| Milky Way head count|
| According to Daniel Zucker, astronomers now accept 14 mini-galaxies as Milky Way satellites. This tally includes the Large and Small Magellanic Clouds (classed as dwarf irregular galaxies), and 10 dwarf spheroidal galaxies: Draco, Ursa Minor, Sextans, Sculptor, Leo I , Leo II, Fornax, Ursa Major, Sagittarius, and Carina. The two new satellites, Canes Venatici and Boötes, are also classed as dwarf spheroidals. |
Not included in this count are two possible disrupting satellites, Canis Major and the Virgo Overdensity, whose identifications remain somewhat controversial. The Virgo Overdensity was also found using SDSS data. F. R.
James Bullock, a theorist at the University of California, Irvine, not affiliated with the study, agrees there are implications for dark matter. "The fact that we can see a 'Field of Streams' like this suggests that dark-matter particles are very cold, or slow moving. If the dark matter was made up of warm, fast-moving particles, we wouldn't expect these thin streams to hang around long enough for us to find them," he explains.
Another prominent trail is the Monoceros stream, discovered previously by team members Heidi Jo Newberg of Rensselaer Polytechnic Institute, New York, and Brian Yanny of the Fermi National Accelerator Laboratory in Illinois. The Monoceros stream is all that's left of a dwarf satellite the Milky Way long ago absorbed.
Analysis of the SDSS data also revealed a new stream that spans over 70° of sky. Its source galaxy remains unknown. "We're looking for it right now," says team member Wyn Evans of Cambridge University.
Astronomers have known about merging events for years, but the SDSS Field of Streams provides them with a dramatic example of ongoing activity in our own galaxy. "This is happening all over the universe, as big galaxies grow by tearing up smaller ones into streams," says Yanny.