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.

This paper presents a new sample of Compact Radio sources at Low Redshift (CORALZ), which is used to study the onset and evolution of young radio‐loud active galactic nuclei (AGN) and their environments. The CORALZ sample consists of 28 sources, of which 17 form a ∼95 per cent statistically complete sample with S1.4 GHz >100 mJy and θ < 2 arcsec, over the redshift range 0.005 < z < 0.16. The sample is selected from the Very Large Array (VLA) FIRST 1.4‐GHz radio survey over an area of sky of 2850 deg2, cross‐correlated with the APM Palomar Sky Survey (APM/POSS‐I) catalogue. In contrast to most previous studies of young radio‐loud AGN, no selection has been made on radio spectral characteristics. This is to enable investigation of the relation between radio spectra and morphologies, orientation and Doppler boosting in young radio sources, in an unbiased way. Remarkably, more than 90 per cent of the sources in the complete CORALZ sample are classified as Gigahertz Peaked Spectrum (six) or Compact Steep Spectrum (10) sources, and only one object shows indication of strong flux density variability. Several projects are under way to study this unique sample in detail, including investigation of the optical host galaxies and their emission line characteristics, radio morphologies and dynamical evolution, and the neutral hydrogen distribution and kinematics in absorption towards these strong compact radio sources.

GPS and CSS radio sources are the objects of choice to investigate the evolution of young radioloud AGN. Previous investigations, mainly based on number counts and source size distributions, indicate that GPS/CSS sources decrease significantly in radio power when evolving into old, extended objects. We suggest this is preceded by a period of increase in radio luminosity, which lasts as long as the radio source is confined within the core-radius of its host galaxy. We have selected a sample of nearby compact radio sources, unbiased by radio spectrum, to determine their luminosity function, size distribution, dynamical ages, and emission line properties in a complete and homogeneous way. First results indicate that the large majority of objects (>80%) exhibit classical GPS/CSS radio spectra, and show structures consistent with them being compact double or compact symmetric objects. This sample provides an ideal basis to further test and constrain possible evolution scenarios, and to investigate the relation between radio spectra and morphologies, orientation and Doppler boosting in samples of young radio-loud AGN, in an unbiased way.

Abstract. We present total flux densities of 1049 radio sources in the frequency range from 151 MHz to 10.6 GHz. These sources belong to the B3-VLA sample, which is complete down to 100 mJy at 408 MHz. The data constitute a homogeneous spectral database for a large sample of radio sources, 50 times fainter than the 3C catalogue, and will be used to perform a spectral ageing analysis, which is one of the critical points in understanding the physics and evolution of extragalactic radio sources.

Abstract. We carried out VLA observations in A configuration at 74 MHz of a new sample of faint Compact Steep-Spectrum (CSS) radio sources as part of a comprehensive project to study the nature and evolution of peaked-spectrum radio sources. In conjunction with higher frequency spectral information this has enabled us to determine the peak frequency and peak flux density of the spectral turnover for 35 faint CSS radio sources. Spectra are presented. An analysis is given of the quality of the images and their suitability for integrated intensity measurements of the CSS radio sources.