Systemic proper motions of milky way satellites from stellar redshifts: The carina, fornax, sculptor, and sextans dwarf spheroidals

The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute line-of-sight components that increase with angular distance from the dSph centers, inducing detectable gradients in stellar redshift. In the absence of an intrinsic velocity gradient (e.g., due to rotation or streaming), an observed gradient in the heliocentric rest frame (HRF) relates simply to a dSph's systemic proper motion (PM). Kinematic samples for the Milky Way's brightest dSph satellites are now sufficiently large that we can use stellar redshifts to constrain systemic PMs independently of astrometric data. Data from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF velocity gradients in Carina, Fornax, and Sculptor, and no significant gradient in Sextans. Assuming there are no intrinsic gradients, the data provide a relatively tight constraint on the PM of Fornax, (μ_α ^HRF,μ_δ;^{H RF})=(+48 ± 15,-25 ± 14) mas century^-1, that agrees with published HSTastrometric measurements. Smaller data sets yield weaker constraints in the remaining galaxies, but our Carina measurement, (μ_α^HRF, μ_δ^HRF)=(+25 ± 36, +16 ± 43) mas century^-1, agrees with the published astrometric value. The disagreement of our Sculptor measurement, (μ_α ^HRF,μ_δ^HRF)=(-40 ± 29,-69 ± 47) mas century^-1, with astrometric measurements is expected if Sculptor has a rotational component as reported by Battaglia and coworkers. For Sextans, which at present lacks an astrometric measurement, we measure (μ_α^HRF, μ_δ ^HRF)=(-26 ± 41, +10 ± 44) mas century^-1.