Revisiting the Fanaroff–Riley dichotomy and radio-galaxy morphology with the LOFAR Two-Metre Sky Survey (LoTSS)

Abstract

<jats:title>Abstract</jats:title> <jats:p>The relative positions of the high and low surface brightness regions of radio-loud active galaxies in the 3CR sample were found by Fanaroff and Riley to be correlated with their luminosity. We revisit this canonical relationship with a sample of 5805 extended radio-loud active galactic nuclei (AGN) from the LOFAR Two-Metre Sky Survey (LoTSS), compiling the most complete data set of radio-galaxy morphological information obtained to date. We demonstrate that, for this sample, radio luminosity does not reliably predict whether a source is edge-brightened (FRII) or centre-brightened (FRI). We highlight a large population of low-luminosity FRIIs, extending three orders of magnitude below the traditional FR break, and demonstrate that their host galaxies are on average systematically fainter than those of high-luminosity FRIIs and of FRIs matched in luminosity. This result supports the jet power/environment paradigm for the FR break: low-power jets may remain undisrupted and form hotspots in lower mass hosts. We also find substantial populations that appear physically distinct from the traditional FR classes, including candidate restarting sources and ‘hybrids’. We identify 459 bent-tailed sources, which we find to have a significantly higher SDSS cluster association fraction (at z &amp;lt; 0.4) than the general radio-galaxy population, similar to the results of previous work. The complexity of the LoTSS faint, extended radio sources not only demonstrates the need for caution in the automated classification and interpretation of extended sources in modern radio surveys, but also reveals the wealth of morphological information such surveys will provide and its value for advancing our physical understanding of radio-loud AGN.</jats:p>

The morphological classification of distant radio galaxies explored with three-dimensional simulations

“…Garofalo & Singh (2019) propose a sequence involving the FR 0 as an intermediary stage. However, Mingo et al (2019) find that, within the large LoTSS survey, radio luminosity does not reliably predict the FR type with the host galaxy properties also important. Jets of low power may still form hotspots in lower mass hosts.…”

Radio Sources Associated with Optical Galaxies and Having Unresolved or Extended Morphologies (ROGUE). I. A Catalog of SDSS Galaxies with FIRST Core Identifications

“…In the ROGUE I catalog these sources are classified as elongated. We note that Mingo et al (2019) also find about a factor of three more FR I sources than FR II sources in the LoTSS data release due to significantly lower surface brightness limit of the LoTSS survey as compared to the FIRST survey.…”

“…This shows that the proper identification of all parts of radio sources will be a challenging problem in future automatic searches. It also shows that measuring sizes and classification similar to the one proposed by Lin et al can be inaccurate in the case of bent sources, or sources with more than one pair of lobes (see also comments in Mingo et al 2019).…”

“…Our classification of bent sources is based only on the morphological features. We note that from the data used in our search we are not able to verify if the source is a cluster member which is often used as the feature of WAT/NAT sources (e.g., Leahy 1993, but, see also, Missaglia et al 2019Mingo et al 2019). In some galaxies the location and morphology of the radio emission allowed us to classify it as coming from star-forming regions (SFR).…”

Unveiling the cause of hybrid morphology radio sources (HyMoRS)

“…FR Is are generally the lower powered of the two classes having 178 MHz luminosity densities below 2×10 25 W Hz −1 sr −1 (Fanaroff & Riley 1974) with FR IIs residing above this threshold. Although recent studies have called this luminosity divide into question (Mingo et al 2019) there is strong evidence of differing evolution between the two classes (e.g. Wall 1980), particularly with respect to the jet structure, particle acceleration, and particle content on large scales (e.g.…”