We combine Gaia data release 1 astrometry with Sloan Digital Sky Survey ( SDSS ) images taken some \sim 10 - 15 years earlier , to measure proper motions of stars in the halo of our Galaxy .
The SDSS- Gaia proper motions have typical statistical errors of 2 mas/yr down to r \sim 20 mag , and are robust to variations with magnitude and colour .
Armed with this exquisite set of halo proper motions , we identify RR Lyrae , blue horizontal branch ( BHB ) , and K giant stars in the halo , and measure their net rotation with respect to the Galactic disc .
We find evidence for a gently rotating prograde signal ( \langle V _ { \phi } \rangle \sim 5 - 25 km s ^ { -1 } ) in the halo stars , which shows little variation with Galactocentric radius out to 50 kpc .
The average rotation signal for the three populations is \langle V _ { \phi } \rangle = 14 \pm 2 \pm 10 ( syst . )
km s ^ { -1 } .
There is also tentative evidence for a kinematic correlation with metallicity , whereby the metal richer BHB and K giant stars have slightly stronger prograde rotation than the metal poorer stars .
Using the Auriga simulation suite we find that the old ( T > 10 Gyr ) stars in the simulated halos exhibit mild prograde rotation , with little dependence on radius or metallicity , in general agreement with the observations .
The weak halo rotation suggests that the Milky Way has a minor in situ halo component , and has undergone a relatively quiet accretion history .