SDSS 0956+5128: A Broad-line Quasar with Extreme Velocity Offsets

TL;DR

This paper reports the discovery of a quasar with unprecedented velocity offsets in its emission lines, challenging existing models of quasar physics.

Contribution

It presents the first known case of a quasar with such extreme differences between broad MgII and Balmer emission lines, suggesting new physics or phenomena.

Findings

Emission lines at three different redshifts in the quasar

Broad MgII line offset by 1200 km/s from systemic

Balmer lines offset by 4100 km/s, unmatched in previous objects

Abstract

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 MgII emission line centered 1200 km/s bluer than the systemic velocity, at z ~ 0.707, and broad H\alpha and H\beta emission lines centered at z ~ 0.690. The Balmer line peaks are 4100 km/s bluer than the systemic redshift. There are no previously known objects with such an extreme difference between broad MgII 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.