Understanding the Fanaroff–riley Radio Galaxy Classification

Abstract

A simple, yet profoundly far-reaching classification scheme based on extended radio morphologies of radio galaxies, the Fanaroff-Riley (FR) classification has been a cornerstone in our understanding of radio galaxies. Over the decades since the recognition that there are two basic types of radio galaxy morphologies, there have been several findings in different wavebands that have reported properties on different scales. Although it is realized that there may be intrinsic as well as external causes, an overarching view of how we may understand the two morphological types is missing. With the radio-power-absolute-magnitude relation (the Owen-Ledlow diagram) as a backdrop, we review and develop an understanding of the two radio galaxy types in light of what is known about them. We have for the first time included the dust properties of the two FR classes together with the relative orientations of dust, host major axis, and the radio axis to present a qualitative framework within which to understand the conditions under which they form. We discuss how the host elliptical and its history can explain the distribution of radio galaxies in the Owen-Ledlow diagram. The mass of the host elliptical galaxy is a crucial player in deciding what type of a radio galaxy it can host in what conditions. Benign conditions, characterized by natural evolutionary processes, most easily give rise to FR-I-type sources in ellipticals of all mass regimes, whereas with FR-IIs we reason that it is hard to form them without mergers. In undisturbed conditions elliptical galaxies appear to acquire stable states where the black hole axis settles along the host minor axis. With the steady conditions and the continuous supply of ambient gas, the FR-Is in principle may be powered for a long time. Aided by mergers and interactions, low-mass ellipticals more easily form FR-II-type sources than more massive hosts. LEG FR-IIs may include dying as well as restarted FR-II radio galaxies.

What Are “X-shaped” Radio Sources Telling Us? II. Properties of a Sample of 87

“…While 3CRR FR-IIs are known to have their radio axes over a wide range of angles with respect to the host major axis both FR-Is (mostly lower power FR-Is) and giant radio galaxies (GRGs) are found to have radio axes predominantly close to the host minor axis (Battye & Browne 2009;Saripalli & Subrahmanyan 2009). In addition, dust axisoptical axis correlations and dust axis-radio axis correlations for FR-Is, FR-IIs and GRGs have been understood within a framework where mergers needed to produce FR-IIs invariably contribute to disturbing the black hole axis where as FR-Is and GRGs are "aided" in their respective formation in more benign conditions not necessarily requiring mergers: Combination of lower power jets and larger host elliptical galaxies in the case of FR-Is removes the requirement for external fuel sources like mergers and as for GRGs, lower resistance to jet paths in the form of minor axis propagation and void regions on larger scales both provide conditions for unimpeded growth of jets to giant extents (Saripalli 2012;Malarecki et al 2015).…”

Remnant Radio Galaxy Candidates of Small Angular Sizes

“…Unlike D21 both [7,8] have introduced a high flux density cutoff limits i.e., S 150 MHz ≥ 40 mJy and S 150 MHz ≥ 80 mJy in their samples, respectively. In addition to relatively high flux density cutoff limit, all the previous studies have also introduced a cutoff limit of 30 -60 on LAS [6,9,10]. For instance, [6] identified RRGs using a sample of extended source with LAS ≥ 60 , while [7] limited their search to the radio sources with LAS ≥ 40 in the 150 MHz LOFAR images.…”

Classifying Radio Galaxies with the Convolutional Neural Network

“…The FR classification scheme, which starts on a morphological basis, also corresponds to a division in radio power (P 1.4GHz = 10 25 W/Hz) and possibly host galaxy optical luminosity (Ledlow & Owen 1996). For a detailed discussion, see Saripalli (2012).…”