The lives of FR I radio galaxies

After a brief introduction to the morphological properties of FR I radio sources, we discuss the possibility that FR I jets are relativistic at their bases and decelerate quickly to non-relativistic velocities. From two-frequency data we determine spectral index distributions and consequently the ages of FR I sources. We show that in the large majority of cases synchrotron theory provides unambiguous and plausible answers; in a few objects re-acceleration of electrons may be needed. The derived ages are of the order 10(7)-10(8) years, similar to 2-4 times larger than the ages inferred from dynamical arguments and a factor similar to 5-10 larger than the ages of FR II sources. The linear sizes of FR I and FR II sources make it unlikely that many FR II's evolve into FR I's. A brief discussion is given of the possibility that radio sources go through different cycles of activity. (C) 2002 Elsevier Science B.V. All rights reserved.