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Register a new userA low-frequency study of two asymmetric large radio galaxies
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A. Pirya
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We present the results of multifrequency observations of two asymmetric, Mpc-scale radio sources with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). The radio luminosity of these two sources, J1211+743 and J1918+742, are in the Fanaroff-Riley class II (FRII) range, but have diffuse radio components on one side of the galaxy while the opposite component appears edge-brightened with a prominent hot-spot. Although the absence of a hot-spot is reminiscent of FRI radio galaxies, suggesting a hybrid morphology, the radio jet facing the diffuse lobe in J1211+743 is similar to those in FRII radio sources, and it is important to consider these aspects as well while classifying these sources in the FR scheme. The observed asymmetries in these Mpc-scale sources are likely to be largely intrinsic rather than being due to the effects of orientation and relativistic motion. The formation of a diffuse lobe facing the radio jet in J1211+743 is possibly due to the jet being highly dissipative. The low-frequency spectral indices of the lobes are in the range of approximately -0.8 to -1, while at the outer edges these vary from approximately -0.65 to -1.05 suggesting steep injection spectral indices, which need to be examined further from observations at even lower frequencies by telescopes such as the LOw Frequency ARray (LOFAR).
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In order to understand the possible mechanisms of recurrent jet activity in radio galaxies and quasars, which are still unclear, we have identified such sources with a large range of linear sizes (220 $-$ 917 kpc), and hence time scales of episodic activity. Here we present high-sensitivity 607-MHz Giant Metrewave Radio Telescope (GMRT) images of 21 possible double-double radio galaxies (DDRGs) identified from the FIRST survey to confirm their episodic nature. These GMRT observations show that none of the inner compact components suspected to be hot-spots of the inner doubles are cores having a flat radio spectrum, confirming the episodic nature of these radio sources. We have indentified a new DDRG with a candidate quasar, and have estimated the upper spectral age limits for eight sources which showed marginal evidence of steepening at higher frequencies. The estimated age limits (11 $-$ 52 Myr) are smaller than those of the large-sized ($sim$ 1 Mpc) DDRGs.
Aims. We present low-frequency radio imaging and spectral properties of a well defined sample of Seyfert galaxies using GMRT 240/610 MHz dual frequency observations. Radio spectra of Seyfert galaxies over 240 MHz to 5.0 GHz are investigated using 240 MHz, 610 MHz flux densities derived from GMRT, and 1.4 GHz and 5.0 GHz flux densities mainly from published VLA data. We test the predictions of Seyfert unification scheme by comparing the radio properties of Seyfert type 1s and type 2s. Methods. We choose a sample such that the two Seyfert subtypes have matched distributions in parameters that are independent to the orientation of AGN, obscuring torus and the host galaxy. Our sample selection criteria allow us to assume that the two Seyfert subtypes are intrinsically similar within the framework of the unification scheme. Results. The new observations at 240/610 MHz, together with archival observations at 1.4 GHz, 5.0 GHz show that type 1s and type 2s have statistically similar radio luminosity distributions at 240 MHz, 610 MHz, 1.4 GHz and 5.0 GHz. The spectral indices at selected frequency intervals as well as index measured over 240 MHz to 5.0 GHz for the two Seyfert subtypes have similar distributions with median spectral index $/sim$ -0.7, consistent with the synchrotron emission from optically thin plasma. In our snap-shot 240/610 MHz GMRT observations, most of the Seyfert galaxies show primarily an unresolved central radio component, except a few sources in which faint kpc-scale extended emission is apparent at 610 MHz. Our results on the statistical comparison of the multifrequency radio properties of our sample Seyfert galaxies are in agreement with the predictions of the Seyfert unification scheme.
We report multi-frequency observations of large radio galaxies 3C 35 and 3C 284. The low-frequency observations were done with Giant Metrewave Radio Telescope starting from $sim$150 MHz, and the high-frequency observations were done with the Very Large Array. We have studied the radio morphology of these two sources at different frequencies. We present the spectral ageing map using two of the most widely used models, the Kardashev-Pacholczyk and Jaffe-Perola models. Another more realistic and complex Tribble model is also used. We also calculate the jet-power and the speed of the radio lobes of these galaxies. We check for whether any episodic jet activity is present or not in these galaxies and found no sign of such kind of activity.
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