The Host Galaxies of Hybrid Morphology Radio Sources

Abstract

Based on their differing radio morphologies, powerful radio galaxies can be separated into the Fanaroff–Riley I (FR I) and II (FR II) classes. Hybrid morphology radio sources (HyMoRS) contain morphologies consistent with each type of jet on either side: a powerful, highly relativistic FR I–like jet terminating in a hotspot on one side and an FR I–like plume on the other. HyMoRS present a unique opportunity to study the conditions that give rise to the dichotomy. Using host galaxy properties, we conduct the first multiwavelength investigation into whether orientation can explain HyMoRS morphology. Through optical spectroscopy and mid-infrared photometry, we analyze the emission characteristics, and evaluate the broad characteristics of five HyMoRS host galaxies at intermediate redshifts (0.4 < z < 1.5). The HyMoRS host galaxies in our sample have properties consistent with typical host galaxies of FR II sources, suggesting that the observed hybrid morphologies may be caused by a dense, cluster-like environment bending FR II jets combined with a favorable orientation that can make one side appear similar to an FR I jet. Our results thus support the hypothesis that HyMoRS are mainly caused by environment and orientation.