Unveiling the cause of hybrid morphology radio sources (HyMoRS)

TL;DR

This study uses high-resolution VLA observations to analyze the morphology and spectra of hybrid morphology radio sources, revealing that their unique features are likely due to orientation effects on FR II sources with non-parallel lobes.

Contribution

It provides the first well-resolved spectral analysis of HyMoRS and proposes a new model attributing their morphology to orientation effects on FR II sources.

Findings

FR I lobe appears center-brightened due to a compact acceleration region.

Spectra of apparent FR I lobes differ from classical FR I sources, likely due to line-of-sight plasma mixing.

HyMoRS are likely caused by orientation effects on intrinsically FR II sources with non-parallel lobes.

Abstract

Hybrid morphology radio sources (HyMoRS) are a rare group of radio galaxies in which differing Fanaroff & Riley morphologies (FR I/II) are observed for each of the two lobes. While they potentially provide insights into the formation of lobe structure, particle acceleration, and the FR dichotomy, previous work on HyMoRS has mainly been limited to low-resolution studies, searches for new candidates, and milliarcsecond-scale VLBI observations of the core region. In this paper, we use new multi-array configuration Very Large Array (VLA) observations between 1 and 8 GHz to determine the morphology of HyMoRS on arcsecond scales and perform the first well-resolved spectral study of these unusual sources. We find that while the apparent FR I lobe is centre-brightened, this is the result of a compact acceleration region resembling a hotspot with a spectrum more consistent with an FR II ("strong-flavour") jet. We find that the spectra of the apparent FR I lobes are not similar to their classical counterparts and are likely the result of line-of-sight mixing of plasma across a range of spectral ages. We consider possible mechanisms that could lead to the formation of HyMoRS under such conditions, including environment asymmetry and restarted sources, concluding through the use of simple modelling that HyMoRS are the result of orientation effects on intrinsically FR II sources with lobes non-parallel to the inner jet.