Very blue-shifted broad Hα in a low-redshift type-1.9 active galactic nucleus: a disc-emitter or a recoiling black hole scenario?

Abstract

ABSTRACT In this paper, very blue-shifted broad Hα with a shifted velocity of ∼2200 km s–1 is reported in the low-redshift type-1.9 active galactic nucleus (AGN) SDSS J1052+1036. Blue-shifted broad emission lines may arise owing to the presence of a rotating gas disc around a central black hole (BH), but may also be a signature of rare phenomena such as the gravitational wave recoil of a supermassive BH (rSMBH) or the presence of a binary BH (BBH) system. Here, owing to the higher shifted velocity of the stronger and wider blue-shifted broad Hα, the BBH system is disfavoured. Meanwhile, if this object contains a rSMBH, intrinsic obscuration with E(B –V) ≤ 0.6 should lead to a detectable broad Hβ, indicating that the rSMBH scenario is not preferred. We find that the blue-shifted broad Hα can be well explained by emission from an AGN disc, indicating that SDSS J1052+1036 is likely a disc-emitting AGN. In order to determine which scenario, a rSMBH or a disc-emitter, is preferred, a re-observed spectrum in 2025 could provide robust clues, with a disc-emitter probably leading to clear variations of peak positions, peak separations, and/or peak intensity ratios in broad Hα, but with a rSMBH scenario probably leading to no variations of peak separations in broad Hα.