A. P. Schoenmakers

Radio galaxies with a ‘double-double’ morphology—II. The evolution of double-double radio galaxies and implications for the alignment effect in FRII sources

A double-double radio galaxy (DDRG) is defined as consisting of a pair of double radio sources with a common centre. In this paper we present an analytical model in which the peculiar radio structure of DDRGs is caused by an interruption of the jet flow in the central AGN. The new jets emerging from the restarted AGN give rise to an inner source structure within the region of the old, outer cocoon. Standard models of the evolution of FRII sources predict gas densities within the region of the old cocoon that are insufficient to explain the observed properties of the inner source structure. Therefore additional material must have passed from the environment of the source through the bow shock surrounding the outer source structure into the cocoon. We propose that this material is warm clouds (~10 4 K) of gas embedded in the hot IGM which are eventually dispersed over the cocoon volume by surface instabilities induced by the passage of cocoon material. The derived lower limits for the volume filling factors of these clouds are in good agreement with results obtained from optical observations. The long time-scales for the dispersion of the clouds (~10 7 yr) are consistent with the apparently exclusive occurrence of the DDRG phenomenon in large (≥700 kpc) radio sources, and with the observed correlation of the strength of the optical/UV alignment effect in z ~1 FRII sources with their linear size.

A new sample of giant radio galaxies from the WENSS survey - II. A multi-frequency radio study of a complete sample: Properties of the radio lobes and their environment

We have formed a complete sample of 26 low redshift (z less than or similar to 0.3) giant radio galaxies (GRGs) from the WENSS survey, selected at flux densities above 1 Jy at 325 MHz. We present 10.5-GHz observations with the 100-m Effelsberg telescope of 18 sources in this sample. These observations, together with similar data of the remaining eight sources, are combined with data from the WENSS, NVSS and GB6 surveys to study the radio properties of the lobes of these sources at arcminute resolution. We investigate radio source asymmetries, equipartition energy densities in the lobes, the presence of lobe pressure evolution with redshift, the spectral age and the density of the environments of these sources. We iind that the arm length asymmetries of GRGs are slightly larger than those of smaller sized 3CR radio galaxies and that these are difficult to explain as arising from orientation effects only. We also find indications that the lobes of the GRGs, despite their large sizes, are still overpressured with respect to their environment. Further, we argue that any evolution of lobe pressure with redshift in these large sources (e.g. Cotter 1998) is due to selection effects. For sources which could be used in a spectral ageing analysis, we find spectral ages which are large, typically a few times 10(7) Sr. This is comparable to earlier studies of some giant sources and indicates that such large spectral ages are common for this class of radio source. The advance velocities of the radio lobes are typically a few percent of the speed of light; which is higher than those found for smaller, low power (<10(26.5) W Hz(-1) at 178 MHz) radio sources, and more comparable to higher power radio sources. This suggests that the GRGs in our sample are the oldest members of the group of relatively high power radio sources whose radio powers have evolved to their currently observed lower values (cf. Kaiser et al. 1997).

Radio galaxies with a ‘double‐double’ morphology – III. The case of B 1834+620

We present a study of the peculiar radio galaxy B 1834+620. It is characterised by the presence of a 420-kpc large edge-brightened radio source which is situated within, and well aligned with, a larger (1.66 Mpc) radio source. Both sources apparently originate in the same host galaxy, which has a R_s-magnitude of 19.7 and a redshift of 0.5194, as determined from the strong emission-lines in the spectrum. We have determined the rotation measures towards this source, as well as the radio spectral energy distribution of its components. The radio spectrum of the large outer source is steeper than that of the smaller inner source. The radio core has a spectrum that peaks at a frequency of a few GHz. The rotation measures towards the four main components are quite similar, within

Double-double radio galaxies

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Double-double radio galaxies: further insights into the formation of the radio structures: DDRG: formation of the radio structures

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Towards a Complete Sample of Giant Radio Galaxies at Z > 0.4

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Radio galaxies with a ‘double-double morphology’- I. Analysis of the radio properties and evidence for interrupted activity in active galactic nuclei

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A new sample of giant radio galaxies from the WENSS survey. I. Sample definition, selection effects and first results

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A new sample of giant radio galaxies from the WENSS survey

. They are probably largely galactic in origin. We have used the presence of a bright hotspot in the northern outer lobe to constrain the advance velocity of the inner radio lobes to the range between 0.19c and 0.29c, depending on the orientation of the source. This corresponds to an age of this structure in the range between 2.6 and 5.8 Myr. We estimate a density of the ambient medium of the inner lobes of

Giant Radio Galaxies and the Inter Galactic Medium

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