Detecting the Orbital Motion of Nearby Supermassive Black Hole Binaries with Gaia

We show that a 10 year Gaia mission could astrometrically detect the orbital motion of ~1 sub-parsec separation supermassive black hole binary in the heart of nearby, bright active galactic nuclei (AGN). Candidate AGN lie out to a redshift of z=0.02 and in the V-band magnitude range $10 \lesssim m_V \lesssim 13$\mathrm{.}Thedistributionofdetectablebinarymassespeaksatafewtimes$10^7$solarmassesandistruncatedaboveafewtimes$10^8$ solar masses.