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This paper describes an investigation of the early evolution of extragalactic radio sources using samples of faint and bright Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio galaxies. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. In addition, the data seem to suggest that the sources are in equipartition while they evolve. If GPS sources evolve to large size radio sources, their redshift dependent birth-functions should be the same. Therefore, since the lifetimes of radio sources are thought to be short compared to the Hubble time, the observed difference in redshift distribution between GPS and large size sources must be due to a difference in slope of their luminosity functions. We argue that this slope is strongly affected by the luminosity evolution of the individual sources. A scenario for the luminosity evolution is proposed in which GPS sources increase in luminosity and large scale radio sources decrease in luminosity with time. This evolution scenario is expected for a ram-pressure confined radio source in a surrounding medium with a King profile density. In the inner parts of the King profile, the density of the medium is constant and the radio source builds up its luminosity, but after it grows large enough the density of th e surrounding medium declines and the luminosity of the radio source decreases. A comparison of the local luminosity function (LLF) of GPS galaxies with that of extended sources is a good test for this evolution scenario [abridged].Comment: LaTeX, 11 pages, 8 figures; Accepted by MNRAS. Related papers may be found at http://www.ast.cam.ac.uk/~snellen . Valuable comments of referee incorporated. More discussion on simulation

Abstract. We present an overview of the occurrence and properties of atomic gas associated with compact radio sources at redshifts up to z = 0.85. Searches for H  21 cm absorption were made with the Westerbork Synthesis Radio Telescope at UHFhigh frequencies (725-1200 MHz). Detections were obtained for 19 of the 57 sources with usable spectra (33%). We have found a large range in line depths, from τ = 0.16 to τ ≤ 0.001. There is a substantial variety of line profiles, including Gaussians of less than 10 km s −1 , to more typically 150 km s −1 , as well as irregular and multi-peaked absorption profiles, sometimes spanning several hundred km s −1 . Assuming uniform coverage of the entire radio source, we obtain column depths of atomic gas between 1 × 10 19 and 3.3 × 10 21 (T sp /100 K)(1/ f ) cm −2 . There is evidence for significant gas motions, but in contrast to earlier results at low redshift, there are many sources in which the H  velocity is substantially negative (up to v = −1420 km s −1 ) with respect to the optical redshift, suggesting that in these sources the atomic gas, rather than falling into the centre, may be be flowing out, interacting with the jets, or rotating around the nucleus.

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