Steep-spectrum sources and the duty cycle of the radio emission
  It is currently accepted that intrinsically compact and bright radio sources
characterized by a convex spectrum peaking at frequencies ranging from 100 MHz
to a few GHz are young objects. Following the evolutionary models, these
objects would evolve into the population of classical radio galaxies. However,
the fraction of young radio sources in flux density-limited samples is much
larger than what expected from the number counts of large radio sources. This
may suggest that for some reason a significant fraction of young objects would
never become large radio galaxies with sizes up to a few Mpc. The discovery of
the young radio source PKS 1518+047 characterized by an uncommonly steep
spectrum confirms that the radio emission may switch off shortly after its
onset. Then the source spectrum steepens and evolves due to energy losses. If
the interruption is not temporary, the fate of the fading sources is to
disappear at frequencies lower than those explored by current radio telescopes.
Fossils of past activities have been recently found at pc-scale distances from
newly born radio sources, suggesting the presence of short-lived objects with
an intermittent radio emission.
