arXiv subjects

D. J. Saikia

Publications and source records attributed to D. J. Saikia.

144 records · Page 8Linked to original sources

arXiv2003

GMRT Observations of NGC 3079

We present new observations at three frequencies (326 MHz, 615 MHz, and 1281 MHz) of the radio lobe spiral galaxy, NGC 3079, using the Giant Metrewave Radio Telescope. These observations are consistent with previous data obtained at other telescopes and reveal the structure of the nuclear radio lobes in exquisite detail. In addition, new features are observed, some with HI counterparts, showing broad scale radio continuum emission and extensions. The galaxy is surrounded by a radio halo that is at least 4.8 kpc in height. Two giant radio extensions/loops are seen on either side of the galaxy out to $\sim$ 11 kpc from the major axis, only slightly offset from the direction of the smaller nuclear radio lobes. If these are associated with the nuclear outflow, then the galaxy has experienced episodic nuclear activity. Emission along the southern major axis suggests motion through a local IGM (not yet detected) and it may be that NGC 3079 is itself creating this local intergalactic gas via outflows. We also present maps of the minimum energy parameters for this galaxy, including cosmic ray energy density, electron diffusion length, magnetic field strength, particle lifetime, and power.

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arXiv2003

3C459: A highly asymmetric radio galaxy with a starburst

Multifrequency radio observations of the radio galaxy 3C459 using MERLIN, VLA and the EVN, and an optical HST image using the F702W filter are presented. The galaxy has a very asymmetric radio structure, a high infrared luminosity and a young stellar population. The eastern component of the double-lobed structure is brighter, much closer to the nucleus and is significantly less polarized than the western one. This is consistent with the jet on the eastern side interacting with dense gas, which could be due to a merged companion or dense cloud of gas. The HST image of the galaxy presented here exhibits filamentary structures, and is compared with the MERLIN 5-GHz radio map. EVN observations of the prominent central component, which has a steep radio spectrum, show a strongly curved structure suggesing a bent or helical radio jet. The radio structure of 3C459 is compared with other highly asymmetric, Fanaroff-Riley II radio sources, which are also good candidates for studying jet-cloud interactions. Such sources are usually of small linear size and it is possible that the jets are interacting with clouds of infalling gas that fuel the radio source.

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arXiv2003

Symmetry Parameters of CSS Sources: Evidence of Fuelling?

The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources are widely believed to be young radiosources, with ages <10^6 yr. If the activity in the nucleus is fuelled by the supply of gas, one might find evidence of this gas by studying the structural and polarisation characteristics of CSS sources and their evolution through this gas. In this paper we discuss some of the possible `smoking-gun' evidence of this gas which may have triggered and fuelled the radio source.

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arXiv2003

Polarization asymmetry in CSS sources: evidence of AGN fuel?

The compact steep spectrum and gigahertz peaked spectrum sources are widely believed to be young radio sources, with ages <10^6 yr. If the activity in the nucleus is fuelled by the supply of gas, one might find evidence of this gas by studying the structural and polarization characteristics of CSS sources as these evolve through this gas. We present polarization observations of a sample of CSS sources, and combine our results with those available in the literature, to show that CSS sources are more asymmetric in the polarization of the outer lobes compared with the more extended ones. We suggest that this could be possibly due to interaction of the jets with infalling material, which fuels the radio source. We also investigate possible dependence of the polarization asymmetry of the lobes on redshift, since this might be affected by more interactions and mergers in the past. No such dependence is found for the CSS sources, suggesting that the environments on the CSS scales are similar at different redshifts. However, the polarization asymmetry of the oppositely-directed lobes is larger at higher redshifts for the more extended sources, possibly reflecting the higher incidence of interactions in the past.

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arXiv2002

High Latitude Dust and the 617 MHz - 850 μm relation in NGC 5775

We present continuum observations of the edge-on spiral galaxy NGC 5775 at 617 MHz with the Giant Metrewave Radio Telescope (GMRT) and at 850 μm with the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). We report the detection of dust at high-latitudes, extending to about 5 kpc from the disk of the galaxy, and spurs of non-thermal radio emission. The 617 MHz and 850 μm distributions are compared with one another and we find a strong correlation between metre wavelength radio emission and sub-mm emission in the disk and at high-latitudes. This suggests that there may be a more fundamental relationship between cold dust and synchrotron radiation other than the link via star formation.

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arXiv2002

CSSs in a sample of B2 radio sources of intermediate strength

We present radio observations of 19 candidate compact steep-spectrum (CSS) objects selected from a well-defined, complete sample of 52 B2 radio sources of intermediate strength. These observations were made with the VLA A-array at 4.835 GHz. The radio structures of the entire sample are summarised and the brightness asymmetries within the compact sources are compared with those of the more extended ones, as well as with those in the 3CRR sample and the CSSs from the B3-VLA sample. About 25 per cent of the CSS sources exhibit large brightness asymmetries, with a flux density ratio for the opposing lobes of $>$5, possibly due to interaction of the jets with infalling material. The corresponding percentage for the larger-sized objects is only about 5 per cent. We also investigate possible dependence of the flux density asymmetry of the lobes on redshift, since this might be affected by more interactions and mergers in the past. No such dependence is found. A few individual objects of interest are discussed in the paper.

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arXiv2002

B1524-136: A CSS quasar with two-sided radio jets

We present MERLIN, global VLBI and VLBA observations of the high-luminosity, compact steep-spectrum quasar B1524$-$136 at cm wavelengths. These observations reveal well-defined radio jets on both sides of the active nucleus, a situation which is almost unique amongst high-luminosity radio quasars. However, the radio jets on opposite sides are very dissimilar, and the overall radio structure appears highly distorted. We discuss possible implications of these observations.

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arXiv2002

Milli-arcsecond scale Rotation Measure in the CSS Quasars 0548+165 and 1524-136

Two Compact Steep-spectrum Sources (CSSs), 0548+165 and 1524-136, chosen from a list of CSSs with polarization percentages that decrease with decreasing frequency and high rotation measure values (RM > 450 rad/m**2) on arcsecond scales, were observed with the VLBA at 4.9 and 8.4 GHz. RM values up to ~10**4 rad/m**2 were found in several regions along the jets in both sources. We suggest that a thin screen of magneto-ionic material with about 1 kpc thickness is responsible for these high RMs. The observed depolarization may be due to beam depolarization and/or inhomogeneities in the magnetic field.

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arXiv2002

Radio observations of the circumnuclear ring in NGC6951

We present sensitive, high-resolution radio observations of the circumnuclear region of the barred spiral galaxy NGC6951. These observations reveal a ring of radio emission with many discrete components and a marginally resolved nuclear component. We compare the radio ring with observations at other wavelengths, and discuss the nature of the compact radio components.

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arXiv2000

A radio and optical study of Molonglo radio sources

We present multi-wavelength radio observations with the Very Large Array and narrow- and broad-band optical observations with the 2.5m telescope at the Las Campanas Observatory, of a well-defined sample of high-luminosity Fanaroff-Riley class II radio galaxies and quasars, selected from the Molonglo Reference Catalogue 1-Jy sample. These observations were carried out as part of a programme to investigate the effects of orientation and environment on some of the observed properties of these sources. We examine the dependence of the Liu-Pooley relationship, which shows that radio lobes with flatter radio spectra are less depolarized, on size, identification and redshift, and show that it is significantly stronger for smaller sources, with the strength of the relationship being similar for both radio galaxies and quasars. In addition to Doppler effects, there appears to be intrinsic differences between the lobes on opposite sides. We discuss the asymmetry in brightness and location of the hotspots, and present estimates of the ages and velocities from matched resolution observations at L- and C-bands. The narrow- and broad-band optical images of some of these sources were made to study their environments and correlate with the symmetry parameters. Extended emission-line region is seen in a quasar and in four of the objects, possible companion galaxies are seen close to the radio axis.

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arXiv2000

Compact steep-spectrum sources from the S4 sample

We present the results of 5-GHz observations with the VLA A-array of a sample of candidate Compact Steep Spectrum sources (CSSs) selected from the S4 survey. We also estimate the symmetry parameters of high-luminosity CSSs selected from different samples of radio sources, and compare these with the larger sources of similar luminosity to understand their evolution and the consistency of the CSSs with the unified scheme for radio galaxies and quasars. The majority of CSSs are likely to be young sources advancing outwards through a dense asymmetric environment. The radio properties of CSSs are found to be consistent with the unified scheme, in which the axes of the quasars are observed close to the line of sight, while radio galaxies are observed close to the plane of the sky.

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arXiv2000

Small-scale structures in Compact Steep-Spectrum and GHz-Peaked-Spectrum radio sources

We determine the small-scale structure for a large sample of Compact Steep Spectrum (CSS) and Gigahertz Peaked Spectrum (GPS) sources and a sample of larger radio sources at 327 MHz using the technique of Inter-Planetary Scintillation (IPS) with the Ooty Radio Telescope. Our observations provide structural information on angular scales ranging from about 50 to a few hundred milliarcsec. We estimate the size and flux density of the scintillating components from the IPS observations. The fraction of flux density of the hotspots estimated from both the IPS observations as well as from interferometric observations for larger sources from the literature exhibits no significant dependence on either the radio luminosity or linear size for the high-luminosity sources. We examine the collimation of radio jets using the sizes of hotspots from both IPS and interferometric observations. The hotspot sizes do not exhibit a significant dependence on the overall linear size for the larger sources, although the CSS and GPS sources were earlier found to evolve in a self-similar way. The IPS observations show evidence of larger-scale structures compared to the known VLBI-scale structures in 8 CSS and GPS radio sources. We discuss the origin of these structures. We also examine the spectra of compact components in GPS sources using both IPS and interferometric measurements, and attempt to distinguish between different processes for explaining their low-frequency spectra.

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arXiv2000

Relativistic beaming effects in the spectra of cores and hotspots in radio galaxies and quasars

We present the results of multi-wavelength observations of cores and hotspots at L, C, X and U bands with the Very Large Array, of a matched sample of radio galaxies and quasars selected from the Molonglo Reference Catalogue. We use these observations to determine the spectra of cores and hotspots, and test the unified scheme for radio galaxies and quasars. Radio cores have been detected at all wavelengths in all the quasars in our sample, while only $\sim$ 50% of the galaxies have detected cores in at least one wavelength. The degree of core prominence in this sample is consistent with the unified scheme for radio galaxies and quasars. A comparison of the distributions of the two-point spectral index of the cores in our sample of lobe-dominated quasars (LDQs) with a matched sample of core-dominated quasars (CDQs) shows that the distributions for these two classes are significantly different, consistent with the expectations of the unified scheme. The difference in the spectral indices of the two hotspots on opposite sides are also significantly larger for quasars than radio galaxies, as expected in the unified scheme. We also investigate the relationship between the spectral index of the hotspots and redshift or luminosity for our sample of sources.

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arXiv1999

Collimation of extragalactic radio jets in compact steep spectrum and larger sources

We study the collimation of radio jets in the high-luminosity Fanaroff-Riley class II sources by examining the dependence of the sizes of hotspots and knots in the radio jets on the overall size of the objects for a sample of compact steep-spectrum or CSS and larger-sized objects. The objects span a wide range in overall size from about 50 pc to nearly 1 Mpc. The mean size of the hotspots increases with the source size during the CSS phase, which is typically taken to be about 20 kpc, and the relationship flattens for the larger sources. The sizes of the knots in the compact as well as the larger sources are consistent with this trend. We discuss possible implications of these trends. We find that the hotspot closer to the nucleus or core component tends to be more compact for the most asymmetric objects where the ratio of separations of the hotspots from the nucleus, r_d > 2. These highly asymmetric sources are invariably CSS objects, and their location in the hotspot size ratio - separation ratio diagram is possibly due to their evolution in an asymmetric environment. We also suggest that some soures, especially of lower luminosity, exhibit an asymmetry in the collimation of the oppositely-directed radio jets.

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arXiv1999

Evolution of Giant Radio Sources

We present radio images of two giant quasars from the Molonglo/1Jy sample, and make a comparative study of giant radio sources selected from the literature with 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme. The luminosity-size diagram shows that the giant sources are less luminous than smaller-sized sources, consistent with evolutionary scenarios where the giants have evolved from the smaller sources, losing energy as they expand. For the giant sources the equipartition magnetic fields are smaller, and inverse-Compton losses with the microwave background radiation dominates over synchrotron losses, while the reverse is true for the smaller sources. The giant radio sources have core strengths similar to those of smaller sources of similar total luminosity; hence their large sizes are unlikely to be due to stronger nuclear activity. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme.

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arXiv1999

Giant Radio Sources

We present multi-frequency VLA observations of two giant quasars, 0437-244 and 1025-229, from the Molonglo Complete Sample. These sources have well-defined FRII radio structure, possible one-sided jets, no significant depolarization between 1365 and 4935 MHz and low rotation measure ($\mid RM \mid < 20 rad/m^2$). The giant sources are defined to be those whose overall projected size is $\geq$ 1 Mpc. We have compiled a sample of about 50 known giant radio sources from the literature, and have compared some of their properties with a complete sample of 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme for radio galaxies and quasars. We find an inverse correlation between the degree of core prominence and total radio luminosity, and show that the giant radio sources have similar core strengths to the smaller sources of similar total luminosity. Hence their large sizes are unlikely to be due to stronger nuclear activity. The degree of collinearity of the giant sources is also similar to the sample of smaller sources. The luminosity-size diagram shows that the giant sources are less luminous than our sample of smaller-sized 3CR sources, consistent with evolutionary scenarios where the giants have evolved from the smaller sources losing energy as they expand to these large dimensions. For the smaller sources, radiative losses due to synchrotron radiation is more significant while for the giant sources the equipartition magnetic fields are smaller and inverse Compton losses with the microwave background radiation is the dominant process. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme.

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arXiv1998

A Polarization Study of Radio Galaxies and Quasars selected from the Molonglo Complete Sample

We present total-intensity and linear-polarization observations made with the Very Large Array at $λ$20 and 6 cm of a representative sample of 42 radio galaxies and quasars selected from the Molonglo Complete Sample. The sources have been chosen to be of large size to probe the depolarizing medium on these scales using our present data and later with observations at lower frequencies with the Giant Metrewave Radio Telescope. The $λ$20 and 6 cm data are of similar resolutions and show that depolarization between these two wavelengths is seen largely in only those lobes which are within about 300 kpc of the parent galaxy. Examination of the depolarization of the lobes with arm-length asymmetry shows that depolarization is observed predominantly for the lobe which is closer to the nucleus. There is also a trend for the lobe closer to the nucleus to be brighter, consistent with the scenario that the nearer lobe is interacting with a denser environment which is responsible for the higher depolarization and greater dissipation of energy. We have also examined the depolarization asymmetry of the lobes on opposite sides of the nucleus for galaxies and quasars. This shows that the depolarization asymmetry for quasars is marginally higher than that for galaxies. The depolarization properties of our sample are possibly determined by an asymmetric environment as well as the effects of orientation.

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arXiv1998

Polarization of radio cores as a test of the unified schemes

In the unified schemes for extragalactic radio sources, the quasars and BL Lac objects are intrinsically similar to the Fanaroff-Riley class II and I radio galaxies respectively, but appear to be different because they are viewed at different angles to the line-of-sight. The quasars and BL Lac objects are observed at small viewing angles while the radio galaxies lie close to the plane of the sky. In such schemes, the quasar or BL Lac nucleus is hidden from our view by a putative torus when the source is inclined at a large angle to the line-of-sight and appears as a radio galaxy. Such a scenario should also affect the observed polarization properties of the cores, with the cores in the radio galaxies and weak-cored quasars being less polarized than the core-dominated quasars and BL Lacs due to Faraday depolarization by the magnetoionic medium in the obscuring torus or disk. In this paper, we report that the core polarization of radio galaxies and weak-cored quasars is indeed much smaller than for the core-dominated quasars and BL Lacs. We suggest that this is due to the depolarization caused by the obscuring torus or disk, providing further evidence in favour of the unified schemes.

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