Aims. We present a study of five "dying" nearby (z ≤ 0.2) radio galaxies belonging to both the WENSS minisurvey and the B2 bright catalogs WNB1734+6407, WNB1829+6911, WNB1851+5707, B2 0120+33, and B2 1610+29. Methods. These sources have been selected on the basis of their extremely steep broad-band radio spectra, which strongly indicates that either these objects belong to the rare class of dying radio galaxies or we are observing "fossil" radio plasma remaining from a previous instance of nuclear activity. We derive the relative duration of the dying phase from the fit of a synchrotron radiative model to the radio spectra of the sources.Results. The modeling of the integrated spectra and the deep spectral index images obtained with the VLA confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime, although their extended lobes have not yet completely faded away. We found that WNB1851+5707 is in reality composed of two distinct dying galaxies, which appear blended together as a single source in the WENSS. In the cases of WNB1829+6911 and B2 0120+33, the fossil radio lobes are seen in conjunction with a currently active core. A very faint core is also detected in a MERLIN image of WNB1851+5707a, one of the two dying sources composing WNB1851+5707. We found that all sources in our sample are located (at least in projection) at the center of an X-ray emitting cluster. Conclusions. Our results suggest that the duration of the dying phase for a radio source in a cluster can be significantly higher than that of a radio galaxy in the field, although no firm conclusions can be drawn because of the small number statistics involved. The simplest interpretation of the tendency for dying galaxies to be found in clusters is that the low-frequency radio emission from the fading radio lobes lasts longer if their expansion is somewhat reduced or even stopped. Another possibility is that the occurrence of dying sources is higher in galaxy clusters. We argue that radio sources in dense environments, such as the center of cooling core clusters, may have a peculiar accretion mode which results in a bursting duty cycle sequence of active and quiescent periods. This result could have important implications for theories of the life cycles of radio sources and AGN feedback in clusters of galaxies but awaits confirmation from future observations of larger, statistically significant, samples of objects.

We use high‐quality Very Large Array (VLA) images of the Fanaroff–Riley Class I radio galaxy 3C 31 at six frequencies in the range 1365–8440 MHz to explore the spatial scale and origin of the rotation measure (RM) fluctuations on the line of sight to the radio source. We analyse the distribution of the degree of polarization to show that the large depolarization asymmetry between the north and south sides of the source seen in earlier work largely disappears as the resolution is increased. We show that the depolarization seen at low resolution results primarily from unresolved gradients in a Faraday screen in front of the synchrotron‐emitting plasma. We establish that the residual degree of polarization in the short‐wavelength limit should follow a Burn law and we fit such a law to our data to estimate the residual depolarization at high resolution. We discuss how to interpret the structure function of RM fluctuations in the presence of a finite observing beam and how to address the effects of incomplete sampling of RM distribution using a Monte Carlo approach. We infer that the observed RM variations over selected areas of 3C 31, and the small residual depolarization found at high resolution, are consistent with a power spectrum of magnetic fluctuations in front of 3C 31 whose power‐law slope changes significantly on the scales sampled by our data. The power spectrum P(f) can only have the form expected for Kolmogorov turbulence [P(f) ∝f−11/3] on scales ≲5 kpc. On larger scales, we find . We briefly discuss the physical interpretation of these results. We also compare the global variations of RM across 3C 31 with the results of three‐dimensional simulations of the magnetic‐field fluctuations in the surrounding magnetoionic medium. We infer that the RM variation across 3C 31 is qualitatively as expected from relativistic‐jet models of the brightness asymmetry wherein the apparently brighter jet is on the near side of the nucleus and is seen through less magnetoionic material than the fainter jet. We show that our data are inconsistent with observing 3C 31 through a spherically symmetric magnetoionic medium, but that they are consistent with a field distribution that favours the plane perpendicular to the jet axis – probably because the radio source has evacuated a large cavity in the surrounding medium. We also apply our analysis techniques to the case of Hydra A, where the shape and the size of the cavities produced by the source in the surrounding medium are known from X‐ray data. We emphasize that it is essential to account for the potential exclusion of magnetoionic material from a large volume containing the radio source when using the RM variations to derive statistical properties of the fluctuations in the foreground magnetic field.

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Abstract.We have conducted a deep survey (rms noise 17 µJy) with the Very Large Array (VLA) at 1.4 GHz, with a resolution of 6 arcsec, of a 1 deg 2 region included in the VIRMOS VLT Deep Survey. In the same field we already have multiband photometry down to I AB = 25, and spectroscopic observations will be obtained during the VIRMOS VLT survey. The homogeneous sensitivity over the whole field has allowed to derive a complete sample of 1054 radio sources (5σ limit). We give a detailed description of the data reduction and of the analysis of the radio observations, with particular care to the effects of clean bias and bandwidth smearing, and of the methods used to obtain the catalogue of radio sources. To estimate the effect of the resolution bias on our observations we have modelled the effective angular-size distribution of the sources in our sample and we have used this distribution to simulate a sample of radio sources. Finally we present the radio count distribution down to 0.08 mJy derived from the catalogue. Our counts are in good agreement with the best fit derived from earlier surveys, and are about 50% higher than the counts in the HDF. The radio count distribution clearly shows, with extremely good statistics, the change in the slope for the sub-mJy radio sources.

Aims. Up till now very few dying sources were known, presumably because the dying phase is short at centimeter wavelengths. We therefore have tried to improve the statistics on sources that have ceased to be active, or are intermittently active. The latter sources would partly consist of a fossil radio plasma left over from an earlier phase of activity, plus a recently restarted core and radio jets. Improving the statistics of dying sources will give us a better handle on the evolution of radio sources, in particular the frequency and time scales of radio activity. Methods. We have used the WENSS and NVSS surveys, in order to find sources with steep spectral indices, associated with nearby elliptical galaxies. In the cross correlation we presently used only unresolved sources, with flux densities at 1.4 GHz larger than 10 mJy. The eleven candidates thus obtained were observed with the VLA in various configurations, in order to confirm the steepness of the spectra, and to check whether active structures like flat-spectrum cores and jets are present, perhaps at low levels. We estimated the duration of the active and relic phases by modelling the integrated radio spectra using the standard models of spectral evolution. Results. We have found six dying sources and three restarted sources, while the remaining two candidates remain unresolved also with the new VLA data and may be Compact Steep Spectrum sources, with an unusually steep spectrum. The typical age of the active phase, as derived by spectral fits, is in the range 10 7 −10 8 years. For our sample of dying sources, the age of the relic phase is on average shorter by an order of magnitude than the active phase.

We describe a study of the side‐to‐side asymmetries on kpc scales in the jets of FR I radio galaxies selected from the B2 sample. The basic data are jet surface brightnesses and widths determined by fitting transverse profiles to Very Large Array (VLA) images at a range of distances from the core. Differences between the jets at a given distance from the nucleus are interpreted as effects of Doppler beaming on intrinsically symmetrical flows and are compared with the model derived for 3C31 by Laing & Bridle and with simpler variants. The jet/counterjet brightness ratios where the main jet first brightens are correlated with core prominence, as expected for a relativistic flow. From the distribution of brightness ratios, we infer that jets have a maximum velocity ≈0.9c where they first flare and brighten, but there is also evidence for additional slower material. Deceleration to subrelativistic speeds occurs on scales which increase with radio power. Jets in the majority of sources with luminosities <1024 WHz‐1 at 1.4 GHz become essentially symmetrical (and therefore subrelativistic) within 2 kpc of the core. In more powerful sources, jets that flare within the first 2 kpc become symmetrical by 10 kpc, but a subset of the most luminous objects has jets which remain asymmetrical to larger distances. The point at which the brighter jet flares appears to correspond to a sudden increase in rest‐frame emissivity, but the ratio of distances to the flaring point in main and counterjets is anticorrelated with brightness ratio, as expected for a decelerating relativistic flow. Brightness and full width at half‐maximum (FWHM) ratios are also anticorrelated, an effect which we interpret as a result of Doppler beaming for a flow in which the velocity decreases radially outwards from the jet axis. Jet deceleration by entrainment of external material provides a natural explanation for these velocity gradients. The jet energy flux is roughly consistent with energy supply to the lobes over a source lifetime estimated from spectral index measurements. Our results are qualitatively consistent with unified models of FR I radio galaxies and BL Lac objects, but require some modifications to the standard picture.

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This paper is part of a series reporting the results of the ATESP radio survey obtained at 1.4 GHz with the Australia Telescope Compact Array. The survey consists of 16 radio mosaics with ∼8 × 14 resolution and uniform sensitivity (1σ noise level ∼79 µJy) over the whole area of the ESO Slice Project redshift survey (∼26 sq. degr. at δ ∼ −40 •). The ATESP survey has produced a catalogue of 2960 radio sources down to a flux limit (6σ) of ∼0.5 mJy. In this Paper we present the 1.4 GHz log N −log S relation derived from the ATESP radio source catalogue. The possible causes of incompleteness at the faint end of the source counts are extensively discussed and their effects are quantified and corrected for. The ATESP counts are consistent with others reported in the literature, even though some significant discrepancies are present at low fluxes (below a few mJy). We investigate whether such discrepancies may be explained in terms of field-to-field anisotropies, considering the fact that all the existing sub-mJy surveys cover small areas of sky (from a fraction of square degree to a few square degrees). We stress that the ATESP survey, covering 26 sq. degr., provides the best determination of source counts available today in the flux range 0.7 < ∼ S1.4 GHz < ∼ 2 mJy.