The evolutionary stage of a powerful radio source originated by an AGN is
related to its linear size. In this context, compact symmetric objects (CSOs),
which are powerful and intrinsically small objects, should represent the young
stage in the individual radio source life. However, the fraction of young radio
sources in flux density-limited samples is much higher than what expected from
the number counts of large radio sources.This indicates that a significant
fraction of young radio sources does not develop to the classical
Fanaroff-Riley radio galaxies,suggesting an intermittent jet activity. As the
radio jets are expanding within the dense and inhomogeneous interstellar
medium,the ambient may play a role in the jet growth, for example slowing down
or even disrupting its expansion when a jet-cloud interaction takes place.
Moreover, this environment may provide the thermal seed photons that scattered
by the lobes' electrons may be responsible for high energy emission, detectable
by Fermi-LAT.Comment: 4 pages, 5 figures; 2011 Fermi Symposium proceedings - eConf C11050