Symmetry Parameters of CSS Sources: Evidence of Fuelling?

The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources are widely believed to be young radio sources, with ages <~106 yr. If the activity in the nucleus is fuelled by the supply of gas, one might find evidence of this gas by studying the structural and polarisation characteristics of CSS sources and their evolution through this gas. In this paper we discuss some of the possible `smoking-gun' evidence of this gas which may have triggered and fuelled the radio source.