Off the beaten path: Gaia reveals GD-1 stars outside of the main stream

Understand

Tidally-disrupted globular clusters are transformed into thin, dynamically-cold streams of stars that are extremely valuable tracers of the large- and small-scale distribution of mass in the Galaxy.

• Using data from the Gaia second data release combined with Pan-STARRS photometry, we present a sample of highly-probable members of the longest cold stream in the Milky Way, GD-1.
• The resulting map of GD-1: (1) extends the apparent length of the stream by $20^\circ$, (2) reveals plausible locations for the progenitor, (3) detects high-contrast gaps along the stream, and (4) indicates the existence of stream members perturbed off the main stream track.
• These discoveries are only possible because of the exquisite astrometry from Gaia, which permits a clean separation of the stream from Milky Way stars.