Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and Hubble Space Telescope WFPC2 Data

We have measured the absolute proper motion of the Fornax dwarf spheroidal galaxy from a combination of photographic plate material and Hubble Space Telescope Wide Field Planetary Camera 2 data that provide a time baseline of up to 50 yr. The extragalactic reference frame consists of eight QSO images and 48 galaxies. The absolute proper motion is μ_α cos δ = 0.59 ± 0.16 mas yr^-1 and μ_δ = -0.15 ± 0.16 mas yr^-1. The corresponding orbit of Fornax is polar, with an eccentricity of 0.27 and a radial period of 4.5 Gyr. Fornax's current location is near pericenter. The direction of the motion of Fornax supports the notion that Fornax belongs to the Fornax–Leo I–Leo II–Sculptor–Sextans stream as hypothesized by both Lynden-Bell and Majewski. According to our orbit determination, Fornax crossed the Magellanic plane ∼190 Myr ago, a time that coincides with the termination of the star formation process in Fornax. We propose that ram pressure stripping due to the passage of Fornax through a gaseous medium denser than the typical intragalactic medium left behind from the LMC may have caused the end of star formation in Fornax. The excess, anomalous clouds within the south Galactic pole region of the Magellanic Stream, whose origin has long been debated in the literature as constituents of either the Magellanic Stream or of the extragalactic Sculptor group, are found to lie along the orbit of Fornax. We speculate that these clouds are stripped material from Fornax as the dwarf crossed the orbit of the Magellanic Clouds.