A Broad-line Quasar with Unexplained Extreme Velocity Offsets: Post-shock Outflow?

Abstract

The quasar SDSS 0956 + 5128 exhibits three distinct velocity components with large offsets in emission: the systemic velocity of [O ii], [O iii], and [Ne iii] narrow lines have redshift z = 0.7142; the broad Mg ii line is shifted by −1200 km s⁻¹ with respect to the narrow lines; the broad Hα and Hβ lines are at −4100 km s⁻¹. We present new Hubble Space Telescope spectra of Lyα and C iv emission lines and high-resolution images of the quasar. The offsets of these lines are consistent with the velocity component of the Balmer emission, and the photometry in optical and near-infrared wavelengths does not show any signs of recent mergers in the host galaxy or irregularities in the location of the quasar. The data do not confirm predictions of the previous most likely hypotheses involving a special orientation and morphology of the quasar disk, such as in the recoiling black hole scenario, neither it is consistent with accretion disk winds. Instead, based on the cumulative evidence, we propose a new scenario, in which the broad-line region is in the state of outflow caused by a strong shock wave, with a supernova as a possible event for producing the shock ejecta.

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