SDSS 0956+5128: A broad-line quasar with extreme velocity offsets

We report on the discovery of a Type 1 quasar, SDSS 0956+5128, with a surprising combination of extreme velocity offsets. SDSS 0956+5128 is a broad-lined quasar exhibiting emission lines at three substantially different redshifts: a systemic redshift of z ∼ 0.714 based on narrow emission lines, a broad Mg II emission line centered 1200kms^-1 bluer than the systemic velocity, at z ∼ 0.707, and broad Hα and Hβ emission lines centered at z ∼ 0.690. The Balmer line peaks are 4100kms^-1 bluer than the systemic redshift. There are no previously known objects with such an extreme difference between broad Mg II and broad Balmer emission. The two most promising explanations are either an extreme disk emitter or a high-velocity black hole recoil. However, neither explanation appears able to explain all of the observed features of SDSS 0956+5128, so the object may provide a challenge to our general understanding of quasar physics.