ar X iv : a st ro - p h / 99 10 25 4 v 1 1 4 O ct 1 99 9 The lives of FR I radio galaxies
نویسندگان
چکیده
After a brief introduction to the morphological properties of FRI radio sources, we discuss the possibility that FRI 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 FRI 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, ∼ 2 − 4 times larger than the ages inferred from dynamical arguments and a factor ∼ 5 − 10 larger than the ages of FRII sources. The linear sizes of FRI and FRII sources make it unlikely that many FRII's evolve into FRI's. A brief discussion is given of the possibility that radio sources go through different cycles of activity. 1 FR I radio sources: morphologies and jet velocities Low and high luminosity radio sources appear to be very different, in fact so much so, that the Fanaroff and Riley classification not only stresses the morphological differences but also suggests fundamental physical differences.
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