D J Saikia | Tata Institute of Fundamental Research (original) (raw)

Papers by D J Saikia

Monthly Notices of the Royal Astronomical Society, Dec 11, 2003

We present new observations at three frequencies (326, 615 and 1281 MHz) of the radio lobe spiral... more We present new observations at three frequencies (326, 615 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 H I 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 ∼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 intergalactic medium (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.

Monthly Notices of the Royal Astronomical Society, 1990

Summary To investigate the nature of extragalactic radio sources with extended emission on only o... more Summary To investigate the nature of extragalactic radio sources with extended emission on only one side of the active nucleus, we have observed a sample of suspected candidates with both the VLA and MERLIN. In this paper, we present observations of 13 sources which were either suspected of being ‘one-sided’ or were known to have lobes of very different surface brightnesses. Many of the candidates were found to have emission on both sides of the nucleus. Using our observations as well as those available in the literature, we have compiled a sample of 4C quasars having one-sided radio structure. We have then compared some of their properties with a sample of 4C quasars which are two-sided, in order to determine whether their apparently onesided structure is due to relativistic beaming in sources inclined at small angles to the line-of-sight. We find that the median values of fc, the fraction of emission from the core used as a statistical measure of source orientation, and l, their p...

The Astronomy and Astrophysics Review, 2021

Compact steep-spectrum (CSS) and peaked spectrum (PS) radio sources are compact, powerful radio s... more Compact steep-spectrum (CSS) and peaked spectrum (PS) radio sources are compact, powerful radio sources. The multi-frequency observational properties and current theories are reviewed with emphasis on developments since the earlier review of O'Dea (1998). There are three main hypotheses for the nature of PS and CSS sources. (1) The PS sources might be very young radio galaxies which will evolve into CSS sources on their way to becoming large radio galaxies. (2) The PS and CSS sources might be compact because they are confined (and enhanced in radio power) by interaction with dense gas in their environments. (3) Alternately, the PS sources might be transient or intermittent sources. Each of these hypotheses may apply to individual objects. The relative number in each population will have significant implications for the radio galaxy paradigm. Proper motion studies over long time baselines have helped determine hotspot speeds for over three dozen sources and establish that these are young objects. Multifrequency polarization observations have demonstrated that many CSS/PS sources are embedded in a dense interstellar medium and vigorously interacting with it. The detection of emission line gas aligned with the radio source, and blue-shifted HI absorption and [OIII] emission lines indicates that AGN feedback is present in these objects-possibly driven by the radio source. CSS/PS sources with evidence of episodic AGN over a large range of timescales have been discussed. The review closes with a discussion of open questions and prospects for the future.

One of the interesting issues in our understanding of active galactic nuclei is the duration of t... more One of the interesting issues in our understanding of active galactic nuclei is the duration of their active phase and whether such activity is episodic. In this paper we summarize our recent results on episodic activity in radio galaxies obtained with the GMRT and the VLA.

The Astrophysical Journal, 2014

We report a new double-double radio quasar (DDRQ) J0746+4526 which exhibits two cycles of episodi... more We report a new double-double radio quasar (DDRQ) J0746+4526 which exhibits two cycles of episodic activity. From radio continuum observations at 607 MHz using the Giant Metrewave Radio Telescope and 1400 MHz from the Faint Images of the Radio Sky at Twenty-cm survey we confirm its episodic nature. We examine the Sloan Digital Sky Survey (SDSS) optical spectrum and estimate the black hole mass to be (8.2 ± 0.3)×10 7 M from its observed Mg ii emission line, and the Eddington ratio to be 0.03. The black hole mass is significantly smaller than for the other reported DDRQ, J0935+0204, while the Eddington ratios are comparable. The SDSS spectrum is significantly red-continuum-dominated, suggesting that it is highly obscured with E(B − V) host = 0.70±0.16 mag. This high obscuration further indicates the existence of a large quantity of dust and gas along the line of sight, which may have a key role in triggering the recurrent jet activity in such objects.

21-cm absorption measurements towards radio sources can be used to study the properties of neutra... more 21-cm absorption measurements towards radio sources can be used to study the properties of neutral atomic gas associated with these sources. Here we present the results of our study to examine the distribution and kinematics of H I properties for compact (i.e. sub-galactic-sized) radio sources, and a new 21-cm absorption detected towards the core of the radio galaxy 3C 452.

The Astrophysical Journal, 2008

We report the discovery of the largest giant radio galaxy, J1420-0545: a FR type II radio source ... more We report the discovery of the largest giant radio galaxy, J1420-0545: a FR type II radio source with an angular size of 17.4' identified with an optical galaxy at z=0.3067. Thus, the projected linear size of the radio structure is 4.69 Mpc (if we assume that H 0 =71 km s −1 Mpc −1 , Ω m =0.27, and Ω Λ =0.73). This makes it larger than 3C236, 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 J1420−0545 is typical of elliptical galaxies in which continuum emission with the characteristic 4000Å 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.

Publications of the Astronomical Society of Australia, 2003

Radio observations of linear polarisation provide a useful probe of the physical conditions in th... more Radio observations of linear polarisation provide a useful probe of the physical conditions in the gaseous environments of compact steep spectrum (CSS) sources. Here, we present polarimetric VLBA observations at 8.4 GHz of the CSS quasar 3C 43. The jet in this source curves gently up to about 220 mas from the core, at which point it bends sharply, probably due to a jet–cloud encounter. Regions of polarised emission are detected along the jet, while the core component shows no significant polarisation.

Publications of the Astronomical Society of Australia, 2003

The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources are widely believed ... more The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources are widely believed to be young radio sources, with ages ≤106yr. 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.

Publications of the Astronomical Society of Australia, 2003

We present global VLBI and VLBA observations of the compact steep spectrum quasar B1524–136. Thes... more We present global VLBI and VLBA observations of the compact steep spectrum quasar B1524–136. These observations reveal well-defined radio jets on both sides of the active nucleus. Also, the overall radio structure appears highly distorted and asymmetric with the counter-jet exhibiting several oscillations. A possible scenario is one in which jet and counter-jet are inclined at about 25° and 75° to the line of sight respectively and an environment which is dense on the jet side. Possible implications of these results are discussed.

Publications of the Astronomical Society of Australia, 2011

EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array ... more EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms ∼ 10 μJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as +30° declination, with a resolution of 10 arcsec. EMU is expected to detect and catalogue about 70 million galaxies, including typical star-forming galaxies up to z ∼ 1, powerful starbursts to even greater redshifts, and active galactic nuclei to the edge of the visible Universe. It will undoubtedly discover new classes of object. This paper defines the science goals and parameters of the survey, and describes the development of techniques necessary to maximise the science return from EMU.

Monthly Notices of the Royal Astronomical Society: Letters, 2007

We report the detection of H I absorption towards the inner double of the double-double radio gal... more We report the detection of H I absorption towards the inner double of the double-double radio galaxy (DDRG) J1247+6723 with the Giant Metrewave Radio Telescope (GMRT). The inner double is a gigahertz peaked spectrum (GPS) source with a linear size of 14 pc while the overall size defined by the outer double is 1195 kpc, making it a giant radio source. The absorption profile is well resolved and consists of a number of components on either side of the optical systemic velocity. The neutral hydrogen column density is estimated to be N(H I) = 6.73 × 10 20 (T s /100)(1.0/ f c) cm −2 , where T s and f c are the spin temperature and partial coverage of the background source, respectively. We explore any correlation between the occurrence of H I absorption and rejuvenation of radio activity and suggest that there could be a strong relationship between them.

Monthly Notices of the Royal Astronomical Society: Letters, 2006

We report the detection of 21-cm Hi absorption towards the core of the Fanaroff-Riley II radio ga... more We report the detection of 21-cm Hi absorption towards the core of the Fanaroff-Riley II radio galaxy 3C 452 (J2245+3941). The absorption profile is well resolved into three components; the strongest and narrowest component being coincident with the velocity corresponding to [O iii] emission lines while the other two components are blue-shifted with respect to it by ∼30 amd ∼115 km s −1. If the systemic velocity of the host galaxy is determined from low-ionization lines, which are red-shifted with respect to the [O iii] doublet by about ∼200 km s −1 , then both the [O iii] emission and 21-cm absorption lines are associated with outflowing material. The neutral hydrogen column density is estimated to be N(HI)=6.39×10 20 (T s /100)(f c /1.0) −1 cm −2 , where T s and f c are the spin temperature and partial coverage of the background source respectively. If the 21-cm absorber is also responsible for the nuclear extinction at infrared wavelengths and x-ray absorption, then for a spin temperature of ∼8000 K, the absorber occults only ∼10 per cent of the radio core.

Monthly Notices of the Royal Astronomical Society, 2009

We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of three gi... more We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of three giant radio sources (GRSs, 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 ages of the lobes. We also present Very Large Array (VLA) observations of the core of J0807+7400, and determine the core radio spectra for all three sources. Although the radio cores suggest that the sources are currently active, we explore the possibility that the lobes in these sources are due to an earlier cycle of activity.

Monthly Notices of the Royal Astronomical Society, 2009

We present multifrequency radio observations with the Giant Metrewave Radio Telescope and Very La... more We present multifrequency radio observations with the Giant Metrewave Radio Telescope and Very Large Array, and X-ray observations with the X-ray Multi-Mirror Mission (XMM-Newton) telescope of the giant radio source (GRS) 3C 457. We have detected the core, lobes and the environment of the GRS in X-ray. We examine the relationships between the radio and X-ray emission, determine the radio spectrum over a large frequency range and attribute the X-ray emission from the lobes to the inverse Compton scattering of cosmic microwave background photons. The magnetic field strength of the lobes is very close to the equipartition value. Both the lobes are in pressure balance near the hotspots and apparently underpressured towards the core. The X-ray spectrum of the core of the GRS consists of an unabsorbed soft power-law component and a heavily absorbed hard power-law component. The soft unabsorbed component is likely to be related to the radio jets. There is no strong evidence of Fe Kα emission line in our data.

Monthly Notices of the Royal Astronomical Society, 2011

We present radio continuum observations at frequencies ranging from ∼150 to 5000 MHz of the misal... more We present radio continuum observations at frequencies ranging from ∼150 to 5000 MHz of the misaligned double-double radio galaxy (DDRG) 3C 293 (J1352+3126) using the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). The spectra of the outer lobes and the central source are consistent with being straight, indicating spectral ages of 17-23 Myr for the outer lobes and 0.1 Myr for the central source. The northwestern lobe has a prominent hotspot suggesting that the interruption of jet activity is 0.1 Myr, consistent with the age of the inner double. The timescale of interruption of jet activity appears significantly smaller than observed in most other DDRGs which are often associated with giant radio sources. These observations suggest that there is a wide range of timescales of interruption of jet activity in active galaxies.

Monthly Notices of the Royal Astronomical Society, 2007

We present multifrequency radio continuum and H I observations of NGC 4438, the highly disturbed,... more We present multifrequency radio continuum and H I observations of NGC 4438, the highly disturbed, active galaxy in the Virgo cluster, with the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT). High-resolution observations of the central 1 kpc with the VLA at 4860 and 8460 MHz show the presence of an inverted-spectrum radio nucleus located between the highly asymmetric lobes of radio emission. This demonstrates that these lobes which are seen in radio continuum, Hα and X-ray wavelengths and are located at ∼230 and 730 pc from the nucleus arise due to an active galactic nucleus rather than a compact nuclear starburst. The low-frequency radio continuum observations made with the GMRT detect the extended emission on the western side of the galaxy whose spectral index is flatter at higher frequencies and suggests that it is a mixture of thermal and non-thermal emission. The H I observations show an elongated structure which is displaced by ∼4.1 kpc on the western side of NGC 4438 and has a size of ∼9.8 kpc and a mass of 1.8 × 10 8 M. The velocity field suggests systematic rotation, consistent with earlier observations. These observations also detect H I emission from the disc of the galaxy with a mass of 1.2 × 10 8 M. We detect a faint H I tail towards the north of NGC 4438 close to a stellar tail seen earlier in deep optical observations. This H I tail has a total extent of ∼50 kpc and a mass of 1.4 × 10 8 M if it is at the distance of NGC 4438. The velocity of the H I tail is ∼ −10 km s −1 similar to that of H I emission from IC 3355, but the possibility that the tail could be foreground Galactic emission cannot be ruled out. We discuss the different structures in light of different models which have been suggested for this disturbed galaxy, namely ram pressure stripping, tidal and interstellar medium (ISM)-ISM interactions.

Monthly Notices of the Royal Astronomical Society, 2011

We present the results of H I absorption measurements towards a sample of nearby compact steep-sp... more We present the results of H I absorption measurements towards a sample of nearby compact steep-spectrum (CSS) and gigahertz-peaked spectrum (GPS) radio sources, the Compact Radio sources at Low Redshift (CORALZ) sample, using the Giant Metrewave Radio Telescope (GMRT). We observed a sample of 18 sources and find seven new detections. These sources are of lower luminosity than earlier studies of CSS and GPS objects, and we investigate any dependence of H I absorption features on radio luminosity. Within the uncertainties, the detection rates and column densities are similar to the more luminous objects, with the GPS objects exhibiting a higher detection rate than for the CSS objects. The relative velocity of the blueshifted absorption features, which may be due to jet-cloud interactions, are within ∼−250 km s −1 and do not appear to extend to values over 1000 km s −1 seen for the more luminous objects. This could be due to the weaker jets in these objects, but requires confirmation from observations of a larger sample of sources. There appears to be no evidence of any dependence of H I column density on either luminosity or redshift, but these new detections are consistent with the inverse relation between H I column density and projected linear size.

Monthly Notices of the Royal Astronomical Society, 2008

Multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Ar... more 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.

Monthly Notices of the Royal Astronomical Society, 2010

We present low-frequency observations starting from ∼150 MHz with the Giant Metrewave Radio Teles... more We present low-frequency observations starting from ∼150 MHz with the Giant Metrewave Radio Telescope (GMRT), and high-frequency observations with the Very Large Array (VLA) of two large radio galaxies 3C46 and 3C452. These observations were made with the objectives of estimating their spectral ages and examining any evidence of diffuse extended emission at low radio frequencies due to an earlier cycle of activity. While no evidence of extended emission due to an earlier cycle of activity has been found, the spectral ages have been estimated to be ∼15 and 27 Myr for the oldest relativistic plasma seen in the regions close to the cores for 3C46 and 3C452 respectively. The spectra in the vicinity of the hotspots are consistent with a straight spectrum with injection spectral indices of ∼1.0 and 0.78 respectively, somewhat steeper than theoretical expectations.

Monthly Notices of the Royal Astronomical Society, Dec 11, 2003

We present new observations at three frequencies (326, 615 and 1281 MHz) of the radio lobe spiral... more We present new observations at three frequencies (326, 615 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 H I 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 ∼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 intergalactic medium (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.

Monthly Notices of the Royal Astronomical Society, 1990

Summary To investigate the nature of extragalactic radio sources with extended emission on only o... more Summary To investigate the nature of extragalactic radio sources with extended emission on only one side of the active nucleus, we have observed a sample of suspected candidates with both the VLA and MERLIN. In this paper, we present observations of 13 sources which were either suspected of being ‘one-sided’ or were known to have lobes of very different surface brightnesses. Many of the candidates were found to have emission on both sides of the nucleus. Using our observations as well as those available in the literature, we have compiled a sample of 4C quasars having one-sided radio structure. We have then compared some of their properties with a sample of 4C quasars which are two-sided, in order to determine whether their apparently onesided structure is due to relativistic beaming in sources inclined at small angles to the line-of-sight. We find that the median values of fc, the fraction of emission from the core used as a statistical measure of source orientation, and l, their p...

The Astronomy and Astrophysics Review, 2021

Compact steep-spectrum (CSS) and peaked spectrum (PS) radio sources are compact, powerful radio s... more Compact steep-spectrum (CSS) and peaked spectrum (PS) radio sources are compact, powerful radio sources. The multi-frequency observational properties and current theories are reviewed with emphasis on developments since the earlier review of O'Dea (1998). There are three main hypotheses for the nature of PS and CSS sources. (1) The PS sources might be very young radio galaxies which will evolve into CSS sources on their way to becoming large radio galaxies. (2) The PS and CSS sources might be compact because they are confined (and enhanced in radio power) by interaction with dense gas in their environments. (3) Alternately, the PS sources might be transient or intermittent sources. Each of these hypotheses may apply to individual objects. The relative number in each population will have significant implications for the radio galaxy paradigm. Proper motion studies over long time baselines have helped determine hotspot speeds for over three dozen sources and establish that these are young objects. Multifrequency polarization observations have demonstrated that many CSS/PS sources are embedded in a dense interstellar medium and vigorously interacting with it. The detection of emission line gas aligned with the radio source, and blue-shifted HI absorption and [OIII] emission lines indicates that AGN feedback is present in these objects-possibly driven by the radio source. CSS/PS sources with evidence of episodic AGN over a large range of timescales have been discussed. The review closes with a discussion of open questions and prospects for the future.

One of the interesting issues in our understanding of active galactic nuclei is the duration of t... more One of the interesting issues in our understanding of active galactic nuclei is the duration of their active phase and whether such activity is episodic. In this paper we summarize our recent results on episodic activity in radio galaxies obtained with the GMRT and the VLA.

The Astrophysical Journal, 2014

We report a new double-double radio quasar (DDRQ) J0746+4526 which exhibits two cycles of episodi... more We report a new double-double radio quasar (DDRQ) J0746+4526 which exhibits two cycles of episodic activity. From radio continuum observations at 607 MHz using the Giant Metrewave Radio Telescope and 1400 MHz from the Faint Images of the Radio Sky at Twenty-cm survey we confirm its episodic nature. We examine the Sloan Digital Sky Survey (SDSS) optical spectrum and estimate the black hole mass to be (8.2 ± 0.3)×10 7 M from its observed Mg ii emission line, and the Eddington ratio to be 0.03. The black hole mass is significantly smaller than for the other reported DDRQ, J0935+0204, while the Eddington ratios are comparable. The SDSS spectrum is significantly red-continuum-dominated, suggesting that it is highly obscured with E(B − V) host = 0.70±0.16 mag. This high obscuration further indicates the existence of a large quantity of dust and gas along the line of sight, which may have a key role in triggering the recurrent jet activity in such objects.

21-cm absorption measurements towards radio sources can be used to study the properties of neutra... more 21-cm absorption measurements towards radio sources can be used to study the properties of neutral atomic gas associated with these sources. Here we present the results of our study to examine the distribution and kinematics of H I properties for compact (i.e. sub-galactic-sized) radio sources, and a new 21-cm absorption detected towards the core of the radio galaxy 3C 452.

The Astrophysical Journal, 2008

We report the discovery of the largest giant radio galaxy, J1420-0545: a FR type II radio source ... more We report the discovery of the largest giant radio galaxy, J1420-0545: a FR type II radio source with an angular size of 17.4' identified with an optical galaxy at z=0.3067. Thus, the projected linear size of the radio structure is 4.69 Mpc (if we assume that H 0 =71 km s −1 Mpc −1 , Ω m =0.27, and Ω Λ =0.73). This makes it larger than 3C236, 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 J1420−0545 is typical of elliptical galaxies in which continuum emission with the characteristic 4000Å 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.

Publications of the Astronomical Society of Australia, 2003

Radio observations of linear polarisation provide a useful probe of the physical conditions in th... more Radio observations of linear polarisation provide a useful probe of the physical conditions in the gaseous environments of compact steep spectrum (CSS) sources. Here, we present polarimetric VLBA observations at 8.4 GHz of the CSS quasar 3C 43. The jet in this source curves gently up to about 220 mas from the core, at which point it bends sharply, probably due to a jet–cloud encounter. Regions of polarised emission are detected along the jet, while the core component shows no significant polarisation.

Publications of the Astronomical Society of Australia, 2003

The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources are widely believed ... more The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources are widely believed to be young radio sources, with ages ≤106yr. 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.

Publications of the Astronomical Society of Australia, 2003

We present global VLBI and VLBA observations of the compact steep spectrum quasar B1524–136. Thes... more We present global VLBI and VLBA observations of the compact steep spectrum quasar B1524–136. These observations reveal well-defined radio jets on both sides of the active nucleus. Also, the overall radio structure appears highly distorted and asymmetric with the counter-jet exhibiting several oscillations. A possible scenario is one in which jet and counter-jet are inclined at about 25° and 75° to the line of sight respectively and an environment which is dense on the jet side. Possible implications of these results are discussed.

Publications of the Astronomical Society of Australia, 2011

EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array ... more EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms ∼ 10 μJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as +30° declination, with a resolution of 10 arcsec. EMU is expected to detect and catalogue about 70 million galaxies, including typical star-forming galaxies up to z ∼ 1, powerful starbursts to even greater redshifts, and active galactic nuclei to the edge of the visible Universe. It will undoubtedly discover new classes of object. This paper defines the science goals and parameters of the survey, and describes the development of techniques necessary to maximise the science return from EMU.

Monthly Notices of the Royal Astronomical Society: Letters, 2007

We report the detection of H I absorption towards the inner double of the double-double radio gal... more We report the detection of H I absorption towards the inner double of the double-double radio galaxy (DDRG) J1247+6723 with the Giant Metrewave Radio Telescope (GMRT). The inner double is a gigahertz peaked spectrum (GPS) source with a linear size of 14 pc while the overall size defined by the outer double is 1195 kpc, making it a giant radio source. The absorption profile is well resolved and consists of a number of components on either side of the optical systemic velocity. The neutral hydrogen column density is estimated to be N(H I) = 6.73 × 10 20 (T s /100)(1.0/ f c) cm −2 , where T s and f c are the spin temperature and partial coverage of the background source, respectively. We explore any correlation between the occurrence of H I absorption and rejuvenation of radio activity and suggest that there could be a strong relationship between them.

Monthly Notices of the Royal Astronomical Society: Letters, 2006

We report the detection of 21-cm Hi absorption towards the core of the Fanaroff-Riley II radio ga... more We report the detection of 21-cm Hi absorption towards the core of the Fanaroff-Riley II radio galaxy 3C 452 (J2245+3941). The absorption profile is well resolved into three components; the strongest and narrowest component being coincident with the velocity corresponding to [O iii] emission lines while the other two components are blue-shifted with respect to it by ∼30 amd ∼115 km s −1. If the systemic velocity of the host galaxy is determined from low-ionization lines, which are red-shifted with respect to the [O iii] doublet by about ∼200 km s −1 , then both the [O iii] emission and 21-cm absorption lines are associated with outflowing material. The neutral hydrogen column density is estimated to be N(HI)=6.39×10 20 (T s /100)(f c /1.0) −1 cm −2 , where T s and f c are the spin temperature and partial coverage of the background source respectively. If the 21-cm absorber is also responsible for the nuclear extinction at infrared wavelengths and x-ray absorption, then for a spin temperature of ∼8000 K, the absorber occults only ∼10 per cent of the radio core.

Monthly Notices of the Royal Astronomical Society, 2009

We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of three gi... more We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of three giant radio sources (GRSs, 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 ages of the lobes. We also present Very Large Array (VLA) observations of the core of J0807+7400, and determine the core radio spectra for all three sources. Although the radio cores suggest that the sources are currently active, we explore the possibility that the lobes in these sources are due to an earlier cycle of activity.

Monthly Notices of the Royal Astronomical Society, 2009

We present multifrequency radio observations with the Giant Metrewave Radio Telescope and Very La... more We present multifrequency radio observations with the Giant Metrewave Radio Telescope and Very Large Array, and X-ray observations with the X-ray Multi-Mirror Mission (XMM-Newton) telescope of the giant radio source (GRS) 3C 457. We have detected the core, lobes and the environment of the GRS in X-ray. We examine the relationships between the radio and X-ray emission, determine the radio spectrum over a large frequency range and attribute the X-ray emission from the lobes to the inverse Compton scattering of cosmic microwave background photons. The magnetic field strength of the lobes is very close to the equipartition value. Both the lobes are in pressure balance near the hotspots and apparently underpressured towards the core. The X-ray spectrum of the core of the GRS consists of an unabsorbed soft power-law component and a heavily absorbed hard power-law component. The soft unabsorbed component is likely to be related to the radio jets. There is no strong evidence of Fe Kα emission line in our data.

Monthly Notices of the Royal Astronomical Society, 2011

We present radio continuum observations at frequencies ranging from ∼150 to 5000 MHz of the misal... more We present radio continuum observations at frequencies ranging from ∼150 to 5000 MHz of the misaligned double-double radio galaxy (DDRG) 3C 293 (J1352+3126) using the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). The spectra of the outer lobes and the central source are consistent with being straight, indicating spectral ages of 17-23 Myr for the outer lobes and 0.1 Myr for the central source. The northwestern lobe has a prominent hotspot suggesting that the interruption of jet activity is 0.1 Myr, consistent with the age of the inner double. The timescale of interruption of jet activity appears significantly smaller than observed in most other DDRGs which are often associated with giant radio sources. These observations suggest that there is a wide range of timescales of interruption of jet activity in active galaxies.

Monthly Notices of the Royal Astronomical Society, 2007

We present multifrequency radio continuum and H I observations of NGC 4438, the highly disturbed,... more We present multifrequency radio continuum and H I observations of NGC 4438, the highly disturbed, active galaxy in the Virgo cluster, with the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT). High-resolution observations of the central 1 kpc with the VLA at 4860 and 8460 MHz show the presence of an inverted-spectrum radio nucleus located between the highly asymmetric lobes of radio emission. This demonstrates that these lobes which are seen in radio continuum, Hα and X-ray wavelengths and are located at ∼230 and 730 pc from the nucleus arise due to an active galactic nucleus rather than a compact nuclear starburst. The low-frequency radio continuum observations made with the GMRT detect the extended emission on the western side of the galaxy whose spectral index is flatter at higher frequencies and suggests that it is a mixture of thermal and non-thermal emission. The H I observations show an elongated structure which is displaced by ∼4.1 kpc on the western side of NGC 4438 and has a size of ∼9.8 kpc and a mass of 1.8 × 10 8 M. The velocity field suggests systematic rotation, consistent with earlier observations. These observations also detect H I emission from the disc of the galaxy with a mass of 1.2 × 10 8 M. We detect a faint H I tail towards the north of NGC 4438 close to a stellar tail seen earlier in deep optical observations. This H I tail has a total extent of ∼50 kpc and a mass of 1.4 × 10 8 M if it is at the distance of NGC 4438. The velocity of the H I tail is ∼ −10 km s −1 similar to that of H I emission from IC 3355, but the possibility that the tail could be foreground Galactic emission cannot be ruled out. We discuss the different structures in light of different models which have been suggested for this disturbed galaxy, namely ram pressure stripping, tidal and interstellar medium (ISM)-ISM interactions.

Monthly Notices of the Royal Astronomical Society, 2011

We present the results of H I absorption measurements towards a sample of nearby compact steep-sp... more We present the results of H I absorption measurements towards a sample of nearby compact steep-spectrum (CSS) and gigahertz-peaked spectrum (GPS) radio sources, the Compact Radio sources at Low Redshift (CORALZ) sample, using the Giant Metrewave Radio Telescope (GMRT). We observed a sample of 18 sources and find seven new detections. These sources are of lower luminosity than earlier studies of CSS and GPS objects, and we investigate any dependence of H I absorption features on radio luminosity. Within the uncertainties, the detection rates and column densities are similar to the more luminous objects, with the GPS objects exhibiting a higher detection rate than for the CSS objects. The relative velocity of the blueshifted absorption features, which may be due to jet-cloud interactions, are within ∼−250 km s −1 and do not appear to extend to values over 1000 km s −1 seen for the more luminous objects. This could be due to the weaker jets in these objects, but requires confirmation from observations of a larger sample of sources. There appears to be no evidence of any dependence of H I column density on either luminosity or redshift, but these new detections are consistent with the inverse relation between H I column density and projected linear size.

Monthly Notices of the Royal Astronomical Society, 2008

Multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Ar... more 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.

Monthly Notices of the Royal Astronomical Society, 2010

We present low-frequency observations starting from ∼150 MHz with the Giant Metrewave Radio Teles... more We present low-frequency observations starting from ∼150 MHz with the Giant Metrewave Radio Telescope (GMRT), and high-frequency observations with the Very Large Array (VLA) of two large radio galaxies 3C46 and 3C452. These observations were made with the objectives of estimating their spectral ages and examining any evidence of diffuse extended emission at low radio frequencies due to an earlier cycle of activity. While no evidence of extended emission due to an earlier cycle of activity has been found, the spectral ages have been estimated to be ∼15 and 27 Myr for the oldest relativistic plasma seen in the regions close to the cores for 3C46 and 3C452 respectively. The spectra in the vicinity of the hotspots are consistent with a straight spectrum with injection spectral indices of ∼1.0 and 0.78 respectively, somewhat steeper than theoretical expectations.