jets, quasars: individual: 3C 48

INTRODUCTION {#section:intro}
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Compact Steep Spectrum (CSS) sources are a population of powerful radio sources with projected linear size less than 20 kpc and steep high radio frequency spectrum $\alpha<-0.5$ [^2] (Peacock & Wall 1982, Fanti et al. 1990, and review by O’Dea 1998 [**and Fanti 2009**]{}). Kinematical studies of the hot spots and analysis of the high-frequency turnover in the radio spectrum due to radiative cooling imply ages for CSS sources in the range 10$^2$–10$^5$ yr (e.g., Owsianik, Conway & Polatidis 1998; Murgia et al. 1999). The sub-galactic size of CSS sources has been used to argue that CSS sources are probably young radio sources, (the ‘youth’ model: Fanti et al. 1995; Readhead et al. 1996). However, another interpretation attributes the apparent compactness of the CSS sources to being strongly confined by the dense ISM in the host galaxy (the ‘frustration’ model: van Breugel, Miley & Heckman 1984). Spectroscopic observations of CSS sources provide evidence for abundant gas reservoirs in the host galaxies and strong interaction between the radio sources and the emission-line clouds [@ODe02]. Some CSS sources have been observed to have high-velocity clouds (as high as $\sim500$ kms$^{-1}$) in the Narrow-Line Region 