The Triggering of Jets in Active Galactic Nuclei

Large bipolar jets of relativistic plasma have popularly been
identified as one of the defining characteristics of active galactic
nuclei (AGN). The majority of AGN however, do not exhibit
kiloparsec-scale radio jets. There is ongoing debate in the
literature on whether a dichotomy exists between these so-called
"radio-quiet" AGN and the ones with jets spanning intergalactic
scales (a.k.a. "radio-loud" AGN), the radio-loudness parameter
being a relative measure of the radio power versus the optical
power of the AGN. We will look critically at the current status
of the debate, and discuss the probable causes of such a dichotomy,
if one exists. Our own multi-wavelength (i.e., radio, optical, X-ray)
work on bright "radio-quiet" and faint "radio-loud" AGN, which are
straddling the "radio-loud"-"radio-quiet" divide, will be discussed
in this context.