A broad-line quasar with unexplained extreme velocity offsets: post-shock outflow?

TL;DR

This paper investigates the unusual velocity offsets in the emission lines of quasar SDSS 0956+5128, ruling out common hypotheses and proposing a novel outflow scenario caused by a shock wave, possibly from a supernova.

Contribution

It introduces a new explanation for extreme velocity offsets in quasar emission lines involving shock-induced outflows, challenging previous models like recoiling black holes or disk winds.

Findings

Emission line offsets are consistent with outflowing gas caused by shocks.

Host galaxy shows no signs of recent mergers or irregularities.

The proposed shock outflow scenario explains the observed velocity components.

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 $z=0.7142$; broad Mg II line is shifted by $-1200$ km s$^{-1}$ with respect to the narrow lines; broad H$\alpha$, H$\beta$ lines are at $-4100$ km s$^{-1}$. We present new Hubble Space Telescope spectra of Ly$\alpha$ 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.