Keck/ESI Long-slit Spectroscopy of SBS 1421+511: A Recoiling Quasar
  Nucleus in An Active Galaxy Pair?
  We present Keck/ESI long-slit spectroscopy of SBS 1421+511, a system
consisting of a quasar at z = 0.276 and an extended source 3" northern to the
quasar. The quasar shows a blue-skewed profile of Balmer broad emission lines,
which can be well modeled as emissions from a circular disk with a blueshift
velocity of ~1400 km/s. The blueshift is better interpreted as resulting from a
recoiling active black hole than from a super-massive black hole binary, since
the line profile almost kept steady over one decade in the quasar rest-frame.
Alternative interpretations are possible as well, such as emissions from a
bipolar outflow or a circular disk with spiral emissivity perturbations. The
extended source shows Seyfert-like narrow line ratios and a [OIII] luminosity
of >1.4\times10^8L_\odot, with almost the same redshift as the quasar and a
projected distance of 12.5 kpc at the redshift. SBS 1421+511 is thus likely to
be an interacting galaxy pair with dual AGN. Alternatively, the quasar
companion only appears to be active but not necessarily so: the gas
before/in/behind the companion galaxy is illuminated by the quasar as an
extended emission line region is detected at a similar distance in the opposite
direction southern to the quasar, which may be generated either by tidal
interactions between the galaxy pair or large-scale outflows from the quasar.
