Proper motion of the Sextans dwarf galaxy from Subaru Suprime-Cam data

Abstract

We have measured the absolute proper motion of the Sextans dwarf spheroidal galaxy using Subaru Suprime-Cam images taken at three epochs, with a time baseline of ∼10 yr. We astrometrically calibrate each epoch by constructing distortion-correction 'maps' from the best available Subaru Suprime-Cam dithered data sets and from Gaia Data Release 1 positions. The magnitude limit of the proper motion study is V ∼ 24. The area covered is 26.7 × 23.3 arcmin², which is still within the core radius of Sextans. The derived proper motion is (μ_α, μ_δ) = (-0.409 ± 0.050, -0.047 ± 0.058) mas yr^-1. The direction of motion is perpendicular to the major axis of the galaxy. Our measurement, combined with radial velocity and distance from the literature, implies a low eccentricity orbit, with a moderate inclination to the Galactic plane, and a period of 3 Gyr. Sextans is now some 0.4 Gyr away from its pericentre (r_peri ∼ 75 kpc), moving towards its apocentre (r_apo ∼ 132 kpc). Its orbit is inconsistent with membership to the vast polar structure of Galactic satellites.