BASS. LII. The Prevalence of Double-peaked Broad Lines at Low Accretion Rates among Hard X-Ray Selected Active Galactic Nuclei

A fraction of active galactic nuclei (AGN) have double-peaked Hα, Hβ, and Mg II broad lines attributed to emission from rotating gas in the accretion disk. Using optical spectroscopy of a flux-limited sample of AGN selected via ultrahard X-rays from the BAT AGN Spectroscopic Survey (BASS), we systematically identify 71 double-peaked emitters (DPEs) among 343 broad-line AGN with redshifts 0.004 < z < 0.297 and X-ray luminosities of 40 < log L 2 − 10 KeV (erg s⁻¹) < 45.7, and provide their best-fit accretion disk parameters. We find that ∼21% of X-ray-selected broad-line AGN are DPEs, consistent with rates previously reported for z < 0.2 broad-line AGN selected for strong optical variability in the Zwicky Transient Facility. 11 of 71 DPEs (15%) exhibited a single-peaked Gaussian component to the broad line profile in addition to the double-peaked disk profile. In this sample, DPEs have intrinsically higher masses by ∼0.4 dex and lower Eddington ratios by ∼0.3 dex than other broad-line AGN, and have a preference for elliptical host galaxies, higher X-ray luminosities, and higher [O i] λ6302 to narrow Hα flux ratios than other broad-line AGN. We find that DPEs are not segregated from non-DPE broad-line AGN in the L_bol versus M_BH relation or their X-ray to radio luminosity ratios, and do not show a preference for intermediate Seyfert types over Seyfert 1s. We do not find differences in a wide range of multiwavelength properties when comparing DPEs to non-DPE broad-line AGN, including optical and mid-IR variability levels, Wide-field Infrared Survey Explorer colors, α_ox, the column density of neutral obscuring material N_H, and the rate of changing-look events. We discuss the two populations in the context of multicomponent disk-wind models of the AGN broad-line region and consider how unrecognized contributions of disk emission to the broad lines may introduce biases in “virial” supermassive black hole mass estimates, with consequences for the inferred M_BH-M_* relation.