Hubble Space Telescope Proper Motions along the Sagittarius Stream: I. Observations and Results for Stars in Four Fields

Understand

We present a multi-epoch Hubble Space Telescope (HST) study of stellar proper motions (PMs) for four fields spanning 200 degrees along the Sagittarius (Sgr) stream: one trailing arm field, one field near the Sgr dwarf spheroidal tidal radius, and two leading arm fields.

• We determine absolute PMs of dozens of individual stars per field, using established techniques that use distant background galaxies as stationary reference frame.
• Stream stars are identified based on combined color-magnitude diagram and PM information.
• The results are broadly consistent with the few existing PM measurements for the Sgr galaxy and the trailing arm.