The properties of FR0 radio galaxies as intermediate objects in the evolution of radio galaxies

It is becoming increasingly clear that counter-rotation between black holes and accretion disks is key to understanding radio galaxies. Such an accretion configuration was introduced over a decade ago to elucidate the nature of the radio loud/radio quiet dichotomy and the jet-disk connection, but has since been applied to a plethora of observations across space and time, from isolated to rich environments and from the formation of the first quasars to the nature of mature objects like M87. We briefly review the paradigm in which counter-rotation is key for the triggering of radio galaxies and its observational support, to then apply it to a series of seemingly unrelated observations concerning FR0 radio galaxies which we argue trace their explanation to one overarching theoretical idea. FR0 radio galaxies appear to be radio galaxies in transition, with low spinning black holes and thus weaker but tilted jets with respect to an earlier radio quasar phase. As a result of this jet reorientation, FR0 radio galaxies are prescribed to be in an earlier phase of star formation suppression in radio galaxies, compared to a later phase that is unlikely to be less than tens of millions of years in their future if they have enough accretion fuel to evolve into more powerful FRI radio galaxies. FR0 radio galaxies will have a greater or lesser star formation suppression feedback effect depending on how long they live. Tilted jets also enhance stellar velocities in the bulge. Because FR0 jet lengths are of the same order of magnitude as the radius of the stellar bulge, FR0 jets are prescribed to have begun, more or less recently depending on their age, to affect stellar velocity dispersions as well. As a result, they will be associated with dispersion values that tend to be larger than for characteristically non-jetted active galaxies, but smaller than for giant radio galaxies such as M87 that have experienced a long-term tilted and more powerful FRI jet. With these ideas it is possible to make a coarse-grained prediction for the slope on the M-σ plane for FR0 radio galaxies with values between 4 and 8.