The fall of the Northern Unicorn: Tangential motions in the Galactic Anti-centre with SDSS and Gaia

Understand

We present the first detailed study of the behaviour of the stellar proper motion across the entire Galactic Anti-centre area visible in the Sloan Digital Sky Survey data.

• We use recalibrated SDSS astrometry in combination with positions from {\it Gaia} DR1 to provide tangential motion measurements with a systematic uncertainty $<$5 kms$^{-1}$ for the Main Sequence stars at the distance of the Monoceros Ring.
• We demonstrate that Monoceros members rotate around the Galaxy with azimuthal speeds of ~230 kms$^{-1}$, only slightly lower than that of the Sun.
• Additionally, both vertical and azimuthal components of their motion are shown to vary considerably but gradually as a function of Galactic longitude and latitude.