Melanie Gendre
Researcher in Astrophysics (2006-2013)
Physics Teacher


The FR Dichotomy

Active galactic nuclei (AGN) comprise the majority of currently observed radio galaxies. Their structure includes a central supermassive black hole (SMBH) whose gravitational potential energy is the main source of luminosity. The central engine is surrounded by a dust torus and accretion disk created by matter being pulled toward the SMBH. Outflows of energetic particles occur along the rotation axis of the disk, forming collimator radio-emitting jets. For the most powerful sources, in the potential of the black hole, rapidly-moving clouds of gas produce strong optical and UV emission lines (`broad-line region'). Well outside the torus, slower moving gas produces narrower emission lines (`narrow-line region'). However, beyond structural information, details of AGN physics are literally hidden from view. To this day, statistical studies of the various classes of AGN are the main probe of the mechanisms powering them.

AGN model

The Fanaroff-Riley (FR) categorisation (Fanaroff & Riley, 1974, MNRAS, 167, 31) provides a classification of extended radio sources. The FRI objects have the highest surface brightness along the jets near the core, reside in moderately rich cluster environments and include sources with irregular structures. In contrast, FRII sources show the highest surface brightness at the lobe extremities, as well as more collimated jets, are found in more isolated environments and generally display stronger emission lines. Fanaroff & Riley found these two classes to be divided in radio power, with a break luminosity P178MHz ≈ 1025 W/Hz/sr, with FRII sources lying above this limit. It was subsequently shown that the break was a function of both radio and optical luminosity (Owen & Ledlow, 1994, ASPC, 54, 319). It is important to note that the cut between FRI and FRII is also somewhat ambiguous: hybrid sources showing jets FRI-like on one side and FRII-like on the other have been observed.

FRI radio galaxy.
The highest surface brightness is located along the jets near the core.
FRII radio galaxy.
The highest surface brightness is located at the lobe extremities.

Intrinsic models suggest that the dichotomy arises from differences in the properties of the central black hole. In these scenarios, jets produced by low accretion-flow rate sources are generally weak with the majority having an FRI-type structure, whereas higher accretion flow rates give rise to stronger, mainly FRII-type jets. Extrinsic models, on the other hand, are purely based on the source environment. The hypothesis is that inter-galactic medium (IGM) density is the differentiating factor, where jets of sources in higher/lower density mediums experience a higher/lower degree of resistance, yielding sources with FRI/FRII structures respectively. In spite of these existing hypotheses, the mechanisms differentiating the two populations are still unknown. Studies of the radio luminosity function (RLF - density of sources with a given luminosity per unit of co-moving volume) of each population could shed some light on this issue: if sources with different FR classes undergo different evolution, this might imply that their fundamental characteristics, such as the black hole spin or jet composition, are different too.