Abstract
A small subset of extragalactic double radio sources, termed HYbrid MOrpholgy Radio Sources (HYMORS), is distinguished by a very unusual, hybrid morphology in terms of 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 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 three 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.
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Notes
Using \(H_0\) = 70 km s\(^{-1}\) Mpc\(^{-1}\); the original reference used a different Hubble constant.
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Acknowledgements
We thank the reviewers for their constructive comments. GK acknowledges award of a Senior Scientist fellowship of the Indian National Science Academy. DP acknowledges the post-doctoral fellowship of the S.N. Bose National Centre for Basic Sciences, Kolkata, India, funded by the Department of Science and Technology (DST), India. This research has made use of the CIRADA cutout service at URL cutouts.cirada.ca, operated by the Canadian Initiative for Radio Astronomy Data Analysis (CIRADA). CIRADA is funded by a grant from the Canada Foundation for Innovation 2017 Innovation Fund (Project 35999), as well as by the Provinces of Ontario, British Columbia, Alberta, Manitoba and Quebec, in collaboration with the National Research Council of Canada, the US National Radio Astronomy Observatory and Australia’s Commonwealth Scientific and Industrial Research Organization.
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Gopal-Krishna, Wiita, P.J., Joshi, R. et al. A physically-motivated perspective of Fanaroff–Riley classification of radio galaxies. J Astrophys Astron 44, 44 (2023). https://doi.org/10.1007/s12036-023-09937-9
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DOI: https://doi.org/10.1007/s12036-023-09937-9