2023ApJ...959...47Z

We present detailed analysis of the optical and near-infrared spectrum of the reddened radio-loud quasar F2M J154043.74+492323.9 with strong optical Fe II emission of ${R}_{4570}\equiv \tfrac{\mathrm{Fe}\,{\rm\small{II}}\lambda \lambda \,4434,4684}{{\rm{H}}\beta }=1.60\pm 0.14$. It is found that the peaks of broad emission lines (BELs), including Hα, Hβ, and Hγ, and the peaks of individual Fe II BELs, are blueshifted as large as v ≈ 1500 km s^–1 with respect to the systemic redshift determined by the low-ionization [O II] narrow emission line. BEL components at the systemic redshift are also significantly detected, though much weaker than the corresponding blueshifted components. Interestingly, the Mg II BEL also shows a similar double-peaked profile, yet with a weaker blueshifted component relative to the unshifted one. We interpret these exceptional BELs as originating from massive outflows, possibly with both densities and column densities much higher than those of the normal BEL region (BELR). Alternative scenarios, however, cannot be completely ruled out based on the data available, including a gravitationally bound binary of active supermassive black holes (SMBHs), each with an unattached BELR, or a recoiling SMBH caused by anisotropic gravitational radiation during the coalescence of the progenitor binary. Follow-up observations will be able to disentangle these scenarios, and to also provide valuable clues to understand the growth of SMBHs and the assembly of their host galaxies.