The 100 ^ { \circ } -long thin stellar stream in the Milky Way halo , GD-1 , has an ensemble of features that may be due to dynamical interactions .
Using high-resolution MMT/Hectochelle spectroscopy we show that a spur of GD-1-like stars outside of the main stream are kinematically and chemically consistent with the main stream .
In the spur , as in the main stream , GD-1 has a low intrinsic radial velocity dispersion , \sigma _ { V _ { r } } \lesssim 1 \textrm { km } \textrm { s } ^ { -1 } , is metal-poor , \textrm { [ Fe / H ] } \approx - 2.3 , with little [ Fe/H ] spread and some variation in [ \alpha /Fe ] abundances , which point to a common globular cluster progenitor .
At a fixed location along the stream , the median radial velocity offset between the spur and the main stream is smaller than 0.5 \textrm { km } \textrm { s } ^ { -1 } , comparable to the measurement uncertainty .
A flyby of a massive , compact object can change orbits of stars in a stellar stream and produce features like the spur observed in GD-1 .
In this scenario , the radial velocity of the GD-1 spur relative to the stream constrains the orbit of the perturber and its current on-sky position to \approx 5 , 000 \deg ^ { 2 } .
The family of acceptable perturber orbits overlaps the stellar and dark-matter debris of the Sagittarius dwarf galaxy in present-day position and velocity .
This suggests that GD-1 may have been perturbed by a globular cluster or an extremely compact dark-matter subhalo formerly associated with Sagittarius .