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


Unified Models of AGN

The unified model of AGN proposed by Jackson & Wall (1999, MNRAS, 304, 160) assumes that the cosmic evolution of radio-loud AGN is based on a division of the radio sources into a low-luminosity component corresponding to FRIs, and a high-luminosity component corresponding to FRIIs. In this scheme, the various forms of AGN observed (FRI and FRII extended double sources, flat- and steep-spectrum RQSOs and BL-Lac objects) result from the orientation of the extended parent objects with respect to the observer's line-of-sight. Indeed, because the double-sided ejection of synchrotron blobs in AGN is at relativistic speed, the orientation of the ejection axis to the line-of-sight becomes crucial: sources viewed side-on appear as double radio galaxies (FRI or FRII) and sources viewed along the jets appear as RQSOs (beamed counterparts of FRII sources) or BL-Lac objects (beamed counterparts of FRI sources). The relativistically-boosted jet emission in the beamed counterparts of the extended sources dominates the extended emission, making the overall radio emission appear compact down to VLBI scales.

Unified model of AGN: FRI Unified model of AGN: FRII

There are however observational signs that this unified model is not as straightforward as beamed counterpart of FRII being QSOs and beamed counterparts of FRI being BL Lac objects. First, there are too few FRIs for them to be the only class of host galaxy for BL Lacs, and there is strong evidence that beamed counterparts of low-excitation FRIIs could also be seen as BL Lacs. Secondly, a population of FRI QSOs exists and contradicts the idea that torus opening angles in FRIs are too small to observe a quasar nucleus, the nature of the torus being a fundamental characteristic of the AGN central engine. Finally, Gendre, Best & Wall (2010) showed that FRI and FRII sources very probably share a common mechanism governing the luminosity-dependent evolution, making it less likely candidates as parent populations.

Another possible choice for parent populations could be high and weak AGN jet strength. It scales broadly with accretion powers into the central SMBH, which are related to emission-line strengths. Hence, strong, collimated jets are associated with radiatively-efficient accretion of cold gas and high-excitation galaxies (HEG), while weaker jets are associated with hot gas accretion and low-excitation galaxies (LEG).