Hybrid Morphology Radio Sources from FIRST survey

Abstract. The so-called HYbrid MOrphology Radio Sources (HYMORS) are a class of objects that appear to have a mixed Fanaroff-Riley (FR) morphology in a single object i.e. a HYMORS has an FR I-type lobe on one side of its nucleus and an FR II-type lobe on the other side. Because of this unique feature and given that the origin of the FR morphological dichotomy is still unclear, HYMORS may possibly play a crucial role in our understanding of the FR-dichotomy. The number of known HYMORS is quite small – Gopal-Krishna & Wiita (2000) established a sample of six HYMORS by means of a literature survey. With the aim to increase that number, a few areas of the sky covered by the VLA FIRST survey were inspected and based upon the morphology shown in the FIRST images a sample of 21 HYMORS candidates was selected. They were observed using the VLA in B-conf. at 4.9 GHz. Three objects from the initial sample turned out to be actual HYMORS and two others very likely to fulfill the criteria. These five were subsequently re-observed with the VLA in A-conf. at 1.4 GHz. Our results provide strong support to the findings of Gopal-Krishna & Wiita (2000), namely that there are two different kinds of jets in HYMORS and, consequently, the existence of FR-dichotomy as a whole is difficult to reconcile with the class of explanations that posit fundamental differences in the central engine.