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تسجيل مستخدم جديدPossible Relic Lobes in Giant Radio Sources
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S. Godambe
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We present low-frequency observations with the GMRT of three giant radio sources (J0139+3957, J0200+4049 and J0807+7400) with relaxed diffuse lobes which show no hotspots and no evidence of jets. The largest of these three, J0200+4049, exhibits a depression in the centre of the western lobe, while J0139+3957 and J0807+7400 have been suggested earlier by Klein et al. and Lara et al. respectively to be relic radio sources. We estimate the spectral ages of the lobes. All three sources have compact radio cores. Although the radio cores suggest that the sources are currently active, we suggest that the lobes in these sources could be due to an earlier cycle of activity.
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Dynamical ages of the opposite lobes determined {sl independently} of each other suggest that their ratios are between $sim$1.1 to $sim$1.4. Demanding similar values of the jet power and the radio core density for the same GRS, we look for a {sl self
-consistent} solution for the opposite lobes, which results in different density profiles along them found by the fit. A comparison of the dynamical and spectral ages shows that their ratio is between $sim$1 and $sim$5, i.e. is similar to that found for smaller radio galaxies. Two causes of this effect are pointed out.
The dynamical ages of the opposite lobes of selected giant radio sources are estimated using the DYNAGE algorithm of Machalski et al., and compared with their spectral ages estimated and studied by Jamrozy et al. in Paper II. As expected, the DYNAGE
fits give slightly different dynamical ages and other models parameters for the opposite lobes modelled independently each other, e.g. the age ratios are found between ~1.1 to ~1.4. Demanding similar values of the jet power and the radio core density for the same source, we look for a self-consistent solution for the opposite lobes, which results in different density profiles along them found by the fit. We also show that a departure from the equipartition conditions assumed in the model, justified by X-ray observations of the lobes of some nearby radio galaxies, and a relevant variation of the magnetic-field strengths may provide an equalisation of the lobes ages. A comparison of the dynamical and spectral ages shows that a ratio of the dynamical age to the spectral age of the lobes of investigated giant radio galaxies is between ~1 and ~5, i.e. is similar to that found for smaller radio galaxies (e.g. Parma et al. 1999). Supplementing possible causes for this effect already discussed in the literature, like uncertainty of assumed parameters of the model, an influence of a possible departure from the energy equipartition assumption, etc. Arguments are given to suggest that DYNAGE can better take account of radiative effects at lower frequencies than the spectral-ageing analysis.The DYNAGE algorithm is especially effective for sources at high redshifts, for which an intrinsic spectral curvature is shifted to low frequencies.
Multifrequency observations with the GMRT and the 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 ag
es estimated for the detected radio emission in the lobes of our sample of ten sources has a median value of $sim$20 Myr. The spectral ages of these GRSs 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. This is likely to be largely due to contamination by more extended emission due to relatively modest resolutions. The injection spectral indices range from $sim$0.55 to 0.88 with a median value of $sim$0.6. We show that the injection spectral index appears to be correlated with luminosity and/or redshift as well as with linear size.
Multifrequency observations with the GMRT and the 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 ag
es estimated for the detected radio emission in the lobes of our sources range from $sim$6 to 36 Myr with a median value of $sim$20 Myr using the classical equipartition fields. Using the magnetic field estimates from the Beck & Krause formalism the spectral ages range from $sim$5 to 38 Myr with a median value of $sim$22 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 $sim$0.55 to 0.88 with a median value of $sim$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 a relic Giant Radio Galaxy (GRG) J021659-044920 at redshift $z sim 1.3$ that exhibits large-scale extended, nearly co-spatial, radio and X-ray emission from radio lobes, but no detection of Active Galactic Nuclei core, jets
and hotspots. The total angular extent of the GRG at the observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is found to be ${sim}$ 2.4 arcmin, that corresponds to a total projected linear size of $sim$ 1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz shows high spectral curvature (${alpha}_{rm 0.610~GHz}^{rm 1.4~GHz} - {alpha}_{rm 0.240~GHz}^{rm 0.325~GHz}$ $>$ 1.19) with sharp steepening above 0.325 GHz, consistent with relic radio emission that is $sim$ 8 $times$ 10$^{6}$ yr old. The radio spectral index map between observed frame 0.325 and 1.4~GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from outer-end to inner-end, indicating backflow of plasma in the lobes. The extended X-ray emission characterized by an absorbed power-law with photon index $sim$ 1.86 favours inverse-Compton scattering of the Cosmic Microwave Background (ICCMB) photons as the plausible origin. Using both X-ray and radio fluxes under the assumption of ICCMB we estimate the magnetic field in the lobes to be 3.3 $mu$G. The magnetic field estimate based on energy equipartition is $sim$ 3.5 $mu$G. Our work presents a case study of a rare example of a GRG caught in dying phase in the distant Universe.
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