Spectropolarimetric evidence for a kicked supermassive black hole in the Quasar E1821+643

TL;DR

Spectropolarimetric observations of quasar E1821+643 suggest it hosts a supermassive black hole that has been kicked at high velocity, providing evidence for gravitational recoil after a black hole merger.

Contribution

This study presents spectropolarimetric evidence supporting the existence of a recoiling supermassive black hole in a quasar, a novel observation of gravitational wave recoil effects.

Findings

Broad emission lines show velocity shifts in total and polarized flux.

The data are consistent with a two-component broad-line region model.

The SMBH may be moving at ~2100 km/s relative to the host galaxy.

Abstract

We report spectropolarimetric observations of the quasar E1821+643 (z=0.297), which suggest that it may be an example of gravitational recoil due to anisotropic emission of gravitational waves following the merger of a supermassive black hole (SMBH) binary. In total flux, the broad Balmer lines are redshifted by ~1000 km/s relative to the narrow lines and have highly red asymmetric profiles, whereas in polarized flux the broad H_alpha line exhibits a blueshift of similar magnitude and a strong blue asymmetry. We show that these observations are consistent with a scattering model in which the broad-line region has two components, moving with different bulk velocities away from the observer and towards a scattering region at rest in the host galaxy. If the high velocity system is identified as gas bound to the SMBH, this implies that the SMBH is itself moving with a velocity ~2100 km/s relative to the host galaxy. We discuss some implications of the recoil hypothesis and also briefly consider whether our observations can be explained in terms of scattering of broad-line emission originating from the active component of an SMBH binary, or from an outflowing wind.