Origin of the double-peaked narrow emission lines in the optical spectra of X-shaped radio galaxies

Abstract

We investigate X-shaped radio galaxies (XRGs) with optical double-peaked narrow emission lines (DPNELs) as potential hosts of dual or binary supermassive black holes (SMBHs). Using available optical spectra from the Sloan Digital Sky Survey and the Dark Energy Spectroscopy Instrument survey, we identify 55 DPNEL sources among a sample of 187 XRGs. We find that the occurrence of [O III] DPNELs in XRGs is ∼30%, which is substantially higher than the ∼1% observed in the general galaxy population. Using optical (involving [O III] λ5007, Hα, Hβ, [N II] λ6584, and [S II] λλ6716, 6731) and mid-infrared diagnostic diagrams, we found that both components are predominantly powered by active galactic nuclei (AGNs), implying a ∼95% probability of hosting dual-AGN systems. In comparison, a stellar-mass-, color-, and redshift-matched control sample of non-XRG DPNEL galaxies shows a lower dual-AGN fraction, which depends strongly on radio luminosity, rising from 25% in radio-undetected galaxies to 54% in radio-detected galaxies, and reaching 95% in radio-bright XRGs and FR II radio galaxies. Interestingly, more than 30% of DPNEL XRGs have companion galaxies, supporting a merger-driven origin for these systems. Finally, we analyze parsec-scale radio structure in seven XRGs using Very Long Baseline Array (VLBA) observations at 1.4, 4.3, and 7.6 GHz, resolving the core in only one source. Nevertheless, flat radio spectral indices, AGN-like emission-line properties, and radio─optical VLBA─Gaia positional offsets found in five cases together support the interpretation that XRGs are strong dual/binary-AGN candidates.