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Searching for X-shaped Radio Galaxies Hosting Binary Supermassive Blackholes

Presentation #138.13 in the session “AGN and Quasars 1”.

Published onJan 11, 2021
Searching for X-shaped Radio Galaxies Hosting Binary Supermassive Blackholes

We present a study of X-shaped radio galaxies (XRGs) through radio imaging and spectral analysis as a means of identifying galaxies hosting binary supermassive black holes (SMBHs). XRGs are radio galaxies that have the typical radio lobes as well as a second set of fainter wings, forming an X-shape. By the relic emission model for XRGs, binary SMBHs may provide an explanation for the second set of lobes (the wings), as their strong gravitational interactions could cause a change in jet direction, leaving the wings of the XRG as relic emissions from the previous jet direction. In this study, a pool of 131 possible XRGs reported in literature were narrowed down to 39 strong candidates according to their X-shaped radio morphology. The radio continuum emissions at 1.4GHz from the Faint Images Radio Sky at Twenty-centimeters survey (FIRST) and 3GHz from the Very Large Array Sky Survey (VLASS) as well as r-band optical images from the Dark Energy Camera Legacy Survey (DECaLS) were compiled for these 39 candidates. Furthermore, a study of their lobe to wing ratios was conducted resulting in a majority of the XRGs having longer lobes than wings, with 5 notable exceptions, and the most frequent lobe to wing ratio being between 1.3-1.4. Of these 39 XRGs, 8 were identified by spectral decomposition analysis of the available optical spectra as strong candidates for hosting binary SMBHs due to the presence of double peaks in their [OIII] λ4959 Å and [OIII]λ5007 Å emissions. The velocity difference between the dual AGN in each of these 8 XRGs were calculated by spectral decomposition of the [OIII] emissions, resulting in velocity differences ranging from 308 ± 47 km/s to 910 ± 97 km/s, with a mean of 546 km/s, median of 524 km/s, and standard deviation of 189 km/s. The spectral regions of Hα λ6563 Å and Hβ λ4861 Å were also decomposed into their broad and narrow line components, with the interesting result that some Hβ and Hα emissions, both broad and narrow, appeared to be redshifted or blushifted in comparison with their host galaxies, indicating possible movement of the AGN relative to its host galaxy. This study suggests a possible connection between the observed radio morphology and kinematics of binary SMBHs and provides a handful of interesting candidates for follow-up observation for confirmation of double AGNs.


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