Detecting the orbital motion of nearby supermassive black hole binaries with Gaia

Abstract

We show that a 10-yr Gaia mission could astrometrically detect the orbital motion of ∼1 subparsec 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 ≲m_V≲13 . The distribution of detectable binary masses peaks at a few times ∼10⁷ M_⊙ and is truncated above a few times ∼10⁸ M_⊙.