A physically-motivated perspective of Fanaroff–Riley classification of radio galaxies

A small subset of extragalactic double radio sources, termed HYbrid MOrpholgy Radio Sources (HYMORS), is distinguished by a very unusual, hybrid morphology in terms Fanaroff–Riley (FR) classification. In HYMORS, one lobe appears edge-darkened (FR I), while the other shows well-defined emission peak near its outer edge (edge-brightened, FR II). Such sources are rare, but critical for constraining mechanism responsible dichotomy, widely debated issue astrophysics. Here, we highlight need caution assigning type, view some upcoming observational campaigns to confirm HYMORS among candidates. To illustrate this, cases three which have been perceived be including galaxy 0500 $$+$$ 630 (4C 63.07), has claimed good, original example with I western and II eastern marked prominent terminal hot spot. However, recent VLASS map at 3 GHz revealed that actually extends much farther out than reported terminates peak. This implies source regular not HYMORS. We also provide brief perspective phenomenon underscore classification ruling additional characteristics, such as an inward lobe-widening spectral steepening, well lack jet within lobe.