Astronomy and Astrophysics – Astrophysics
Scientific paper
1999-10-14
New Astron.Rev. 46 (2002) 313-325
Astronomy and Astrophysics
Astrophysics
19 pages, including 13 figures, to appear in `Life Cycles of Radio Galaxies', ed. J. Biretta et al., New Astronomy Reviews
Scientific paper
10.1016/S1387-6473(01)00201-9
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.
de Ruiter Hans R.
Fanti Roberto
Murgia Mariella
Parma Paola
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