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

TL;DR

This paper discusses the rare hybrid morphology radio sources (HYMORS) that challenge the Fanaroff-Riley classification, emphasizing the importance of accurate classification and presenting case studies to clarify their nature.

Contribution

The paper highlights the need for careful classification of HYMORS and revises the understanding of a specific source, illustrating the importance of detailed observations for accurate FR type determination.

Findings

HYMORS are rare but crucial for understanding FR dichotomy.

Reclassification of 0500+630 from HYMORS to FR II based on new VLASS data.

Emphasizes criteria for confirming FR I vs. FR II classifications.

Abstract

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