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 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.

💡 Research Summary

The paper presents the discovery and detailed spectroscopic analysis of SDSS 0956+5128, a Type 1 quasar that exhibits an unprecedented combination of velocity offsets among its emission lines. Using optical spectra from the Sloan Digital Sky Survey and near‑infrared data obtained with Keck/NIRES and VLT/X‑shooter, the authors first establish a systemic redshift of z≈0.714 based on narrow forbidden lines such as