| Abstract : | We employ a new catalog of X-shaped radio galaxies (XRGs) from the FIRST and TGSS surveys, which essentially doubles their known number, to shed light upon their origin, which is actively debated. Diversion of the back-flowing synchrotron plasma from the two primary lobes, spin-flip of the central supermassive black-hole, and other processes linked to galaxy merger, such as a jet-shell interaction, as well as a dual-AGN scenario, are among the proposed mechanisms. We examine a \sample of 106 XRGs, and investigate its host galaxies. For 41 of the XRGs it has been possible to determine the main axis of the parent optical galaxy, as well as the major and minor radio axes defined by two-lobe pairs, namely, the primary and the secondary lobes. Although we confirm that in a large majority of cases the primary radio axis is aligned close to the major axis of the optical host galaxy, we also find several counter-examples. These are highlighted here, as they challenge the basic back-flow diversion model for the origin of the radio-wings. Comparison of the XRGs with a well-defined sample of normal radio galaxies of FRII morphology reveals that: (i) XRGs tend to have slightly less massive central black holes than FRII RGs (average SMBH masses for the two types being logM_BH ~8.81 M_sun and 9.07 M_sun, respectively); (ii) 80% of XRGs exhibit red mid-IR colors, indicating a significant population of young stars and/or enhanced dust mass, probably on account of relatively recent galaxy merger(s). A comparison of the large-scale environment (i.e., within 1 Mpc) around the XRGs and FRII radio sources shows that both populations inhabit similarly poor clustering environments while FRI RGs are in richer environments. As a population, the observed properties of XRGs seem difficult to reconcile with a single dominant physical mechanism for their origin. |
