Origin of the double-peaked narrow emission-lines in the optical spectra of X-shaped Radio Galaxies

Abstract: We investigate the X-shaped radio galaxies (XRGs) with optical double-peaked narrow emission (DPNEL) as potential hosts of dual or binary supermassive black holes (SMBHs). Using a sample of 187 XRGs selected from SDSS and DESI optical spectroscopic surveys, we check the AGN nature of both emission components using the BPT diagnostics of multiple emission lines, namely {[O III]$\lambda\lambda$4959,5007}, H$\alpha$, {[N II]$\lambda\lambda$6548,6584}, {[S II]$\lambda\lambda$6716,6731}, and H$\beta$, and mid-infrared colors. We find that the detection rate of [O III] DPNEL features in XRGs is 30% compared to just 1% in the general galaxy population (mostly radio quiet). The dual AGN fraction in DPNEL galaxies is found to depend strongly on the radio luminosity, increasing from $\sim$25% for radio-undetected to $\sim$58% in the radio-detected sample of general DPNEL galaxies. In contrast, the DPNEL XRGs and FR-II radio galaxies having higher radio power show a $\sim$95% likelihood of hosting a dual AGN. Secondly, the detection of companion galaxies in more than 30% of DPNEL XRGs suggests a vital role of mergers in the XRG formation. We also investigate the parsec-scale radio structure of the nuclei of several XRGs using Very Long Baseline Array (VLBA) maps at 1.4 GHz, 4.3 GHz or 7.6 GHz and find a resolved core for only one of the XRGs. However, the flat spectral indices of the VLBA cores along with the DPNEL components exhibiting AGN characteristics, together with the detection of radio-optical offsets between the VLBA and Gaia position, are strongly indicative of XRGs being likely candidates for hosting dual/binary AGNs.