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Register a new userLow-frequency radio observations of Seyfert galaxies: A test to the unification scheme
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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.
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Aims: The unification scheme of Seyfert galaxies predicts that the observed differences between type 1 and type 2 Seyfert galaxies are solely due to the differing orientations of the toroidal-shaped obscuring material around AGN. The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected to be affected by higher absorbing column density due to the edge-on view of the obscuring torus. We study the 0.5 - 10 keV X-ray spectral properties of Seyfert type 1s and type 2s with the aim to test the predictions of Seyfert unification scheme in the X-ray regime. Methods: We use an optically selected Seyfert sample in which type 1s and type 2s have matched distributions in the orientation independent parameters of AGN and host galaxy. Results: The 0.5 - 10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best fitted with a model consists of an absorbed power-law, a narrow Gaussian fitted to the Fe K{alpha} emission line and an often seen soft excess component characterized by either a thermal plasma model with temperature kT sim 0.1 - 1.0 keV and/or a steep power-law. The 2.0 - 10 keV hard X-ray continuum emission in several Seyfert type 2s is reflection dominated and suggests the Compton-thick obscuration. Results on the statistical comparison of the distributions of the observed X-ray luminosities in the soft (0.5 - 2.0 keV) and hard (2.0 - 10.0 keV) bands, the X-ray absorbing column densities, the equivalent widths of Fe K{alpha} line and the flux ratios of hard X-ray to [OIII] {lambda}5007{AA} for the two Seyfert subtypes are consistent with the obscuration and orientation based unification scheme.
Diffuse radio emission from galaxy clusters in the form of radio halos and relics are tracers of the shocks and turbulence in the intra-cluster medium. The imprints of the physical processes that govern their origin and evolution can be found in their radio morphologies and spectra. The role of mildly relativistic population of electrons may be crucial for the acceleration mechanisms to work efficiently. Low frequency observations with telescopes that allow imaging of extended sources over a broad range of low frequencies ($< 2$ GHz) offer the best tools to study these sources. I will review the Giant Metrewave Radio Telescope (GMRT) observations in the past few years that have led to: i) statistical studies of large samples of galaxy clusters, ii) opening of the discovery space in low mass clusters and iii) tracing the spectra of seed relativistic electrons using the Upgraded GMRT.
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).
We present radio observations of ultraluminous infrared galaxies (ULIRGs) using the Giant Metrewave Radio Telescope (GMRT) and combine them with archival multi-frequency observations to understand whether ULIRGs are the progenitors of the powerful radio loud galaxies in the local Universe. ULIRGs are characterized by large infrared luminosities ($L_{IR}>$10$^{12}$L$odot$), large dust masses ($sim10^{8}M_{odot}$) and vigorous star formation (star formation rates $sim$10-100 $M_{odot}~$yr$^{-1}$). Studies show that they represent the end stages of mergers of gas-rich spiral galaxies. Their luminosity can be due to both starburst activity and active galactic nuclei (AGN). We study a sample of 13 ULIRGs that have optically identified AGN characteristics with 1.28~GHz GMRT observations. Our aim is to resolve any core-jet structures or nuclear extensions and hence examine whether the ULIRGs are evolving into radio loud ellipticals. Our deep, low frequency observations show marginal extension for only one source. However, the integrated radio spectra of 9 ULIRGs show characteristics that are similar to that of GPS/CSS/CSO/young radio sources. The estimated spectral ages are 0.4 to 20 Myr and indicate that they are young radio sources and possible progenitors of radio galaxies. Hence, we conclude that although most ULIRGs do not show kpc scale extended radio emission associated with nuclear activity, their radio spectral energy distributions do show signatures of young radio galaxies.
We have observed seven nearby large angular sized galaxies at 0.33 GHz using GMRT with angular resolution of $sim10$ and sub-mJy sensitivity. Using archival higher frequency data at 1.4 or $sim$6 GHz, we have then determined their spatially resolved non-thermal spectrum. As a general trend, we find that the spectral indices are comparatively flat at the galaxy centres and gradually steepen with increasing galactocentric distances. Using archival far infrared (FIR) MIPS 70 ${mu} m$ data, we estimate the exponent of radio-FIR correlation. One of the galaxy (NGC 4826) was found to have an exponent of the correlation of $sim1.4$. Average exponent from 0.33 GHz data for the rest of the galaxies was 0.63$pm$0.06 and is significantly flatter than the exponent 0.78$pm$0.04 obtained using 1.4 GHz data. This indicates cosmic ray electron (CRe) propagation to have reduced the correlation between FIR and 0.33 GHz radio. Assuming a model of simple isotropic diffusion of CRe, we find that the scenario can explain the frequency dependent cosmic ray electron propagation length scales for only two galaxies. Invoking streaming instability could, however, explain the results for the majority of the remaining ones.
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