The peculiar optical spectrum of 4C+22.25: Imprint of a massive black hole binary?

TL;DR

This paper reports peculiar optical spectral features in the quasar 4C+22.25, suggesting the possible presence of a massive black hole binary, based on spectral analysis and ruling out alternative explanations.

Contribution

It presents the discovery of unusual broad emission line features in 4C+22.25 and discusses the potential evidence for a massive black hole binary scenario.

Findings

Broad emission lines with extreme velocity offsets

No significant spectral variation over ~3 years

Excludes superposition and recoiling black hole interpretations

Abstract

We report the discovery of peculiar features in the optical spectrum of 4C+22.25, a flat spectrum radio quasar at z=0.4183 observed in the SDSS and in a dedicated spectroscopic follow-up from the Nordic Optical Telescope. The Hbeta and Halpha lines show broad profiles (FWHM~12,000 km/s), faint fluxes and extreme offsets (Delta v=8,700+/-1,300 km/s) with respect to the narrow emission lines. These features show no significant variation in a time lag of ~3.1 yr (rest frame). We rule out possible interpretations based on the superposition of two sources or on recoiling black holes, and we discuss the virtues and limitations of a massive black hole binary scenario.