We present new radio observations at frequencies ranging from 240 to 4860 MHz of the well-known, double-double radio galaxy (DDRG), J1453+3308, using both the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). These observations enable us to determine the spectra of the inner and outer lobes over a large frequency range and demonstrate that while the spectrum of the outer lobes exhibits significant curvature, that of the inner lobes appears practically straight. The break frequency, and hence the inferred synchrotron age of the outer structure, determined from 16$-$arcsec strips transverse to the source axis, increases with distance from the heads of the lobes. The maximum spectral ages for the northern and southern lobes are $\sim$47 and 58 Myr respectively. Because of the difference in the lengths of the lobes these ages imply a mean separation velocity of the heads of the lobes from the emitting plasma of 0.036c for both the northern and southern lobes. The synchrotron age of the inner double is about 2 Myr which implies an advance velocity of $\sim$0.1c, but these values have large uncertainties because the spectrum is practically straight.Comment: 10 pages, 10 figures, 5 tables, accepted for publication in MNRA

Multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) are used to determine the spectral breaks in consecutive strips along the lobes of a sample of selected giant radio sources (GRSs) in order to estimate their spectral ages. The maximum spectral ages estimated for the detected radio emission in the lobes of our sources range from ∼6 to 46 Myr with a median value of ∼23 Myr using the classical equipartition fields. Using the magnetic field estimates from the Beck & Krause formalism the spectral ages range from ∼5 to 58 Myr with a median value of ∼24 Myr. These ages are significantly older than smaller sources. In all but one source (J1313+6937) the spectral age gradually increases with distance from the hotspot regions, confirming that acceleration of the particles mainly occurs in the hotspots. Most of the GRSs do not exhibit zero spectral ages in the hotspots, as is the case in earlier studies of smaller sources. This is likely to be largely due to contamination by more extended emission due to relatively modest resolutions. The injection spectral indices range from ∼0.55 to 0.88 with a median value of ∼0.6. We discuss these values in the light of theoretical expectations, and show that the injection spectral index appears to be correlated with luminosity and/or redshift as well as with linear size.

We report the discovery of the largest giant radio galaxy yet, J1420À0545: a FR type II radio source with an angular size of 17.4 0 , identified with an optical galaxy at z ¼ 0:3067. Thus, the projected linear size of the radio structure is 4.69 Mpc (if we assume that H 0 ¼ 71 km s À1 Mpc À1 , m ¼ 0:27, and à ¼ 0:73). This makes it larger than 3C 236, which is the largest double radio source known to date. New radio observations with the 100 m Effelsberg telescope and the Giant Metrewave Radio Telescope, as well as optical identification with a host galaxy and its optical spectroscopy with the William Herschel Telescope, are reported. The spectrum of J1420À0545 is typical of elliptical galaxies in which continuum emission with the characteristic 4000 8 discontinuity and the H and K absorption lines are dominated by evolved stars. The dynamical age of the source, its jets' power, the energy density, and the equipartition magnetic field are calculated and compared with the corresponding parameters of other giant and normal-sized radio galaxies from a comparison sample. The source is characterized by the exceptionally low density of the surrounding IGM and an unexpectedly high expansion speed of the source along the jet axis. All of these may suggest a large inhomogeneity of the IGM.

Context. Determining the ages of powerful radio sources is crucial for understanding galaxy evolution, the activity cycle of galactic nuclei, and their influence on the surrounding intergalactic medium. So far, several different methods for estimating the age of classical double radio galaxies have been proposed and widely used in the literature, although each of them faces difficulty due to observational limitations and/or freedom in choosing the underlying model assumptions. Aims. We propose a new approach to determining the ages of FR II type radio sources that, on one hand exploits a dynamical model developed for these objects by Kaiser et al. (1997, MNRAS, 292, 723) and, on the other hand, uses multifrequency radio observations not necessarily restricted to the high-resolution ones.Methods. In particular, we applied the assumed dynamical model to a number of FR II type radio galaxies observed at different radio frequencies and fit -for each frequency separately -the model's free parameters to the quantities of the observed sources. Such a procedure, which in fact enlarged a number of observables, enabled us to determine relatively precise ages and other crucial characteristics (like the jets' kinetic power) for the analyzed sources. Results. The resulting age estimates agree very well with those obtained with the "classical" spectral aging method for objects not older than 10 Myr, for which good-quality spectral data are available. However, this method is also applicable in the case of older sources than this and/or those for which the only available low-resolution radio data do not allow for detailed spectral aging studies. Interestingly, the estimated ages always correspond to the realistic values of the jets' advance velocity of ∼0.01-0.1 c. Conclusions. Our analysis indicates that the main factor precluding precise age determination for FR II type radio galaxies is related to the poorly known shape of the initial electron energy distribution injected by the jet's terminal shocks to the expanding lobes/cocoons. We briefly considered this issue and conclude that the broad-band single power-law form assumed here may be accurate enough for the age estimates, although most likely it does not strictly correspond to some well-defined realistic particle acceleration process. Instead, it should be considered as the simplest model approximation of the initial electron continuum, averaged over a very broad energy range and over the age of the source, with the effective spectral index that may be different for different sources, but within the relatively narrow range of p = 2.0-2.4 suggested by our modeling.

Abstract.A new sample of very large angular size radio sources has been selected from the 1.4 GHz VLA surveys: FIRST and NVSS. This sample will be very useful for an observational constraint on the time evolution of double radio sources, especially their size, predicted by numerous analytical models of such evolution (cf. Introduction). In this paper we present radio and optical data for a large fraction of the sample sources. They are: high-frequency VLA maps with very weak radio cores detected, deep optical images showing the identified faint host galaxies not visible on the DSS images, and optical spectra of the identified galaxies brighter than about R ≈ 18.5 mag taken with the McDonald Observatory 2.1 m telescope. For 15 galaxies (of which 4 do not belong to the complete sample) the redshift has been determined. In the result, 44 per cent of galaxies in the complete sample have redshift data (with one exception all redshifts are less than 0.33), of which 70 per cent have a linear size exceeding 1 Mpc. The photometric redshift estimates for other 11 galaxies with 19 mag < R < 21.7 mag (0.3 < z < 0.5) predict their sizes much over 1 Mpc.