FR0 radio galaxies and their place in the radio morphology
  classification
  So-called FR0 radio galaxies have recently emerged as a family of active
galaxies with all the same properties as FRI radio galaxies except for their
ratio of core to total emission which is about 30 times as high compared to FRI
sources. We show how their properties fit within the gap paradigm as low
prograde spinning black holes whose progenitors are powerful FRII quasars that
transitioned rapidly from cold mode into advection dominated accretion in a few
million years. The prediction is that if sufficient fuel exists, FR0 radio
galaxies will evolve into full-fledged FRI radio galaxies and the observational
dearth of FRI radio galaxies compared to FR0s at low redshift tells us about
the supply of gas in the low redshift FR0s. Given the model prescription, this
5 to 1 FR0 to FRI ratio implies that at low redshift the FRII quasar class of
active galaxies struggles to fuel its black hole beyond 1.3 times its original
mass. In addition to this, we illustrate model prescriptions for the black hole
mass, black hole spin, redshift and environment distribution for FR0 radio
galaxies by fitting them within a paradigm that views them as a continuous
class of active galaxies that are sandwiched between FRII quasars and FRI radio
galaxies.
