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0000-0002-4464-8023
Current Organisation
Inter-University Centre for Astronomy and Astrophysics
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Publisher: Oxford University Press (OUP)
Date: 08-1984
Publisher: Oxford University Press (OUP)
Date: 02-09-2011
Publisher: EDP Sciences
Date: 02-10-2007
Publisher: Springer Science and Business Media LLC
Date: 16-02-2023
Publisher: Zenodo
Date: 2016
DOI: 10.5281/ZENODO.60367
Publisher: Oxford University Press (OUP)
Date: 1991
Publisher: Oxford University Press (OUP)
Date: 29-09-2011
Publisher: Oxford University Press (OUP)
Date: 21-09-2000
Publisher: Oxford University Press (OUP)
Date: 09-08-2011
Publisher: Oxford University Press (OUP)
Date: 05-1984
Publisher: Oxford University Press (OUP)
Date: 10-2008
Publisher: Elsevier BV
Date: 05-2002
Publisher: EDP Sciences
Date: 10-2020
DOI: 10.1051/0004-6361/202038344
Abstract: We present the first results of a project called SAGAN, which is dedicated solely to the studies of relatively rare megaparsec-scale radio galaxies in the Universe, called giant radio galaxies (GRGs). We have identified 162 new GRGs primarily from the NRAO VLA Sky Survey with sizes ranging from ∼0.71 Mpc to ∼2.82 Mpc in the redshift range of ∼0.03−0.95, of which 23 are hosted by quasars (giant radio quasars). As part of the project SAGAN, we have created a database of all known GRGs, the GRG catalogue, from the literature (including our new s le) it includes 820 sources. For the first time, we present the multi-wavelength properties of the largest s le of GRGs. This provides new insights into their nature. Our results establish that the distributions of the radio spectral index and the black hole mass of GRGs do not differ from the corresponding distributions of normal-sized radio galaxies (RGs). However, GRGs have a lower Eddington ratio than RGs. Using the mid-infrared data, we classified GRGs in terms of their accretion mode: either a high-power radiatively efficient high-excitation state, or a radiatively inefficient low-excitation state. This enabled us to compare key physical properties of their active galactic nuclei, such as the black hole mass, spin, Eddington ratio, jet kinetic power, total radio power, magnetic field, and size. We find that GRGs in high-excitation state statistically have larger sizes, stronger radio power, jet kinetic power, and higher Eddington ratio than those in low-excitation state. Our analysis reveals a strong correlation between the black hole Eddington ratio and the scaled jet kinetic power, which suggests a disc-jet coupling. Our environmental study reveals that ∼10% of all GRGs may reside at the centres of galaxy clusters, in a denser galactic environment, while the majority appears to reside in a sparse environment. The probability of finding the brightest cluster galaxy as a GRG is quite low and even lower for high-mass clusters. We present new results for GRGs that range from black hole mass to large-scale environment properties. We discuss their formation and growth scenarios, highlighting the key physical factors that cause them to reach their gigantic size.
Publisher: Oxford University Press (OUP)
Date: 28-08-2007
Publisher: Oxford University Press (OUP)
Date: 10-1988
Publisher: Oxford University Press (OUP)
Date: 02-2010
Publisher: Oxford University Press (OUP)
Date: 15-01-2000
Publisher: Oxford University Press (OUP)
Date: 12-2003
Publisher: Springer Science and Business Media LLC
Date: 06-1989
DOI: 10.1007/BF02719317
Publisher: American Astronomical Society
Date: 23-05-2011
Publisher: Springer Berlin Heidelberg
Date: 1988
Publisher: Springer Science and Business Media LLC
Date: 08-12-2022
Publisher: Oxford University Press (OUP)
Date: 11-05-2001
Publisher: Oxford University Press (OUP)
Date: 11-03-2006
Publisher: American Astronomical Society
Date: 10-06-2014
Publisher: Oxford University Press (OUP)
Date: 11-1979
Publisher: Oxford University Press (OUP)
Date: 02-12-2016
Publisher: Oxford University Press (OUP)
Date: 2005
Publisher: Springer Science and Business Media LLC
Date: 1989
DOI: 10.1007/BF00637340
Publisher: American Astronomical Society
Date: 18-06-2019
Publisher: Cambridge University Press (CUP)
Date: 2003
DOI: 10.1071/AS02059
Abstract: 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.
Publisher: Oxford University Press (OUP)
Date: 11-1982
Publisher: Cambridge University Press (CUP)
Date: 2003
DOI: 10.1071/AS02058
Abstract: The compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) 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 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.
Publisher: Proceedings of the National Academy of Sciences
Date: 05-12-1995
Abstract: The compact steep-spectrum sources (CSSs) are an interesting class of objects which are of subgalactic dimensions they occur more frequently in high-frequency surveys because their spectra often turn over at lower frequencies. We have estimated the symmetry parameters of a well-defined s le of CSSs and compared these with the larger 3CR sources of similar luminosity to understand the evolution and the consistency of CSSs with the unified scheme. We suggest that the majority of CSSs are likely to be young sources advancing outward through an asymmetric, inhomogeneous environment to form the larger ones. The radio properties of the CSSs are consistent with the unified scheme, where the axes of the quasars are seen closer to the line of sight while the radio galaxies lie closer to the plane of the sky. We discuss how radio polarization observations may be used to probe whether the physical conditions in the central regions of the CSSs are different from the larger ones. We present a simple scenario where the depolarization and high rotation measures seen in many CSSs can be consistent with the low rotation measures of cores in the more extended quasars and suggest further observations to test this scenario.
Publisher: Oxford University Press (OUP)
Date: 02-07-2013
DOI: 10.1093/MNRAS/STT937
Publisher: Oxford University Press (OUP)
Date: 12-1987
Publisher: Oxford University Press (OUP)
Date: 21-03-2010
Publisher: Cambridge University Press (CUP)
Date: 2003
DOI: 10.1071/AS02054
Abstract: 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.
Publisher: Oxford University Press (OUP)
Date: 08-2004
Publisher: EDP Sciences
Date: 03-2005
Publisher: Springer Science and Business Media LLC
Date: 12-2011
Publisher: EDP Sciences
Date: 04-2022
DOI: 10.1051/0004-6361/202141928
Abstract: Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGN) that propel megaparsec-scale jets. In order to understand GRQs and their properties, we have compiled all known GRQs (‘the GRQ catalogue’) and a subset of small (size 700 kpc) radio quasars (SRQs) from the literature. In the process, we have found ten new Fanaroff-Riley type-II GRQs in the redshift range of 0.66 z 1.72, which we include in the GRQ catalogue. Using the above s les, we have carried out a systematic comparative study of GRQs and SRQs using optical and radio data. Our results show that the GRQs and SRQs statistically have similar spectral index and black hole mass distributions. However, SRQs have a higher radio core power, core dominance factor, total radio power, jet kinetic power, and Eddington ratio compared to GRQs. On the other hand, when compared to giant radio galaxies (GRGs), GRQs have a higher black hole mass and Eddington ratio. The high core dominance factor of SRQs is an indicator of them lying closer to the line of sight than GRQs. We also find a correlation between the accretion disc luminosity and the radio core and jet power of GRQs, which provides evidence for disc-jet coupling. Lastly, we find the distributions of Eddington ratios of GRGs and GRQs to be bi-modal, similar to that found in small radio galaxies (SRGs) and SRQs, which indicates that size is not strongly dependent on the accretion state. Using all of this, we provide a basic model for the growth of SRQs to GRQs.
Publisher: EDP Sciences
Date: 14-06-2002
Publisher: Oxford University Press (OUP)
Date: 09-1982
Publisher: Oxford University Press (OUP)
Date: 11-08-1998
Publisher: American Astronomical Society
Date: 28-03-2013
Publisher: Oxford University Press (OUP)
Date: 02-2007
Publisher: Oxford University Press (OUP)
Date: 02-1999
Publisher: Oxford University Press (OUP)
Date: 11-10-1999
Publisher: Oxford University Press (OUP)
Date: 08-2005
Publisher: American Astronomical Society
Date: 07-05-2010
Publisher: Springer Netherlands
Date: 1999
Publisher: Springer Science and Business Media LLC
Date: 07-05-2014
DOI: 10.1038/509164C
Publisher: Springer Science and Business Media LLC
Date: 12-1982
DOI: 10.1007/BF02714886
Publisher: Springer Science and Business Media LLC
Date: 06-1982
DOI: 10.1007/BF02714800
Publisher: Springer Science and Business Media LLC
Date: 12-2011
Publisher: Oxford University Press (OUP)
Date: 05-1983
Publisher: Oxford University Press (OUP)
Date: 15-03-1991
Publisher: Oxford University Press (OUP)
Date: 21-12-2012
DOI: 10.1093/MNRAS/STS499
Publisher: American Astronomical Society
Date: 06-07-2012
Publisher: Oxford University Press (OUP)
Date: 1987
Publisher: EDP Sciences
Date: 14-06-2002
Publisher: American Astronomical Society
Date: 06-07-2012
Publisher: Oxford University Press (OUP)
Date: 05-1986
Publisher: EDP Sciences
Date: 11-1997
DOI: 10.1051/AAS:1997244
Publisher: Oxford University Press (OUP)
Date: 21-11-2009
Publisher: Wiley
Date: 02-2009
Abstract: A s le of 26 Compact Steep Spectrum sources selected from the original list drawn up by Dallacasa & Stanghellini (1990) have been observed with the VLA A‐array at X, U, and K bands. Images have been produced in both total intensity and polarisation at four widely‐spaced frequencies with the aim of deriving linear polarisation parameters. We present preliminary polarisation results on source fractional polarisation and electric vector angle at X and U bands. These new information will be completed with available archive data in order to perform a polarimetric analysis of a complete CSS s le (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Publisher: Oxford University Press (OUP)
Date: 15-05-2019
Publisher: Oxford University Press (OUP)
Date: 05-1979
Publisher: Oxford University Press (OUP)
Date: 21-10-2006
Publisher: American Astronomical Society
Date: 26-09-2018
Publisher: Oxford University Press (OUP)
Date: 11-1984
Publisher: Oxford University Press (OUP)
Date: 11-10-2009
Publisher: Oxford University Press (OUP)
Date: 11-09-2006
Publisher: Oxford University Press (OUP)
Date: 21-06-2007
Publisher: EDP Sciences
Date: 12-2022
DOI: 10.1051/0004-6361/202243182
Abstract: We present for the first time a study of peculiar giant radio galaxy (GRG) J223301+131502 using deep multi-frequency radio observations from GMRT (323, 612, and 1300 MHz) and LOFAR (144 MHz) along with optical spectroscopic observations with the WHT 4.2m optical telescope. Our observations have firmly established its redshift of 0.09956 and unveiled its exceptional jet structure extending more than ∼200 kpc leading to a peculiar kink structure of ∼100 kpc. We measure the overall size of this GRG to be ∼1.83 Mpc it exhibits lobes without any prominent hotspots and closely resembles a barbell. Our deep low-frequency radio maps clearly reveal the steep-spectrum diffuse emission from the lobes of the GRG. The magnetic field strength of ∼5 μG and spectral ages between about 110 to 200 mega years for the radio lobes were estimated using radio data from LOFAR 144 MHz observations and GMRT 323 and 612 MHz observations. We discuss the possible causes leading to the formation of the observed kink feature for the GRG, which include precession of the jet axis, development of instabilities and magnetic reconnection. Despite its enormous size, the Barbell GRG is found to be residing in a low-mass ( M 200 ∼ 10 14 M ⊙ ) galaxy cluster. This GRG with two-sided large-scale jets with a kink and diffuse outer lobes residing in a cluster environment, provides an opportunity to explore the structure and growth of GRGs in different environments.
Publisher: Oxford University Press (OUP)
Date: 13-04-2011
Publisher: Springer Berlin Heidelberg
Date: 2007
Publisher: Oxford University Press (OUP)
Date: 04-07-2023
Abstract: The excellent sensitivity and optimum resolution of LoTSS DR2 at 144 MHz has enabled us to discover a giant radio galaxy (J1225+4011) with three distinct episodes of jet activity, making it a member of a class of objects called triple-double radio galaxies (TDRGs). The source extends overall up to 1.35 Mpc in projected size, with the second episode extending to 572 kpc, and the inner episode to 118 kpc. J1225+4011 is only the fourth radio source showing a triple-double radio structure. All four sources have overall sizes greater than 700 kpc, making them giants. We also present the LoTSS 144 MHz map of the TDRG J0929+4146 and report its updated size. Lastly, we have summarized and discussed the radio properties of all TDRGs for the first time to understand their growth and evolution. Our observations suggest that the power of their jets may decrease with time.
Publisher: Oxford University Press (OUP)
Date: 02-2004
Publisher: Oxford University Press (OUP)
Date: 18-10-2012
Publisher: Oxford University Press (OUP)
Date: 21-03-2003
Publisher: Springer Netherlands
Date: 1983
Publisher: Oxford University Press (OUP)
Date: 21-02-2018
DOI: 10.1093/MNRAS/STY451
Publisher: Oxford University Press (OUP)
Date: 2012
Publisher: Oxford University Press (OUP)
Date: 05-2009
Publisher: Oxford University Press (OUP)
Date: 04-2008
Publisher: American Astronomical Society
Date: 28-12-2006
DOI: 10.1086/511153
Publisher: Springer Science and Business Media LLC
Date: 06-1986
DOI: 10.1007/BF02715035
Publisher: EDP Sciences
Date: 05-2003
Publisher: EDP Sciences
Date: 18-06-2013
Publisher: Annual Reviews
Date: 09-1988
Publisher: Oxford University Press (OUP)
Date: 04-02-2013
Publisher: American Astronomical Society
Date: 04-2000
DOI: 10.1086/301287
Publisher: Oxford University Press (OUP)
Date: 02-1987
Publisher: Springer Science and Business Media LLC
Date: 30-03-2021
Publisher: American Astronomical Society
Date: 20-05-2008
DOI: 10.1086/586703
Publisher: Oxford University Press (OUP)
Date: 17-04-2017
DOI: 10.1093/MNRAS/STX885
Publisher: Oxford University Press (OUP)
Date: 18-08-2014
Publisher: Oxford University Press (OUP)
Date: 21-04-2020
Abstract: We present results of H i absorption experiment done using the Giant Metrewave Radio Telescope (GMRT) towards 27 low- and intermediate-luminosity ($P_{\\rm 1.4 GHz}\\, \\sim 10^{23}$-1026 W Hz−1) radio active galactic nuclei (AGNs), classified as either low excitation radio galaxies (LERGs) or high excitation radio galaxies (HERGs) and with WISE colour W2[4.6 μm]−W3[12 μm]& 2. We report H i absorption detection towards seven radio AGNs, six of which are new. Combined with other sources from literature classified as LERGs or HERGs, we confirm our earlier result that compact radio AGNs with WISE colour W2−W3 & 2 have higher detection rates compared to those with W2−W3 & 2. We find that H i absorption detection rate is higher for HERGs (37.0$^{+15.8}_{-11.5}$ per cent) compared to LERGs (22.0$^{+3.9}_{-3.4}$ per cent), mainly due to a larger fraction of HERGs being gas and dust rich with a younger stellar population compared to LERGs. However, for similar compact radio structures and host galaxies with WISE colours W2−W3 & 2, we don’t find any significant difference in detection rates of two types of AGNs implying detection of H i gas may not necessarily mean high excitation mode AGN. We further analysed the kinematics towards these sources. We find that while LERGs show a wide range in the shift of centroid velocities ($\\sim \\, -$479 to +356 km s−1) relative to the optical systemic velocity, most of the HERGs have centroid velocity shift less than 200 km s−1, possibly due to differences in jet-interstellar medium interaction.
Publisher: Oxford University Press (OUP)
Date: 11-10-1998
Publisher: Oxford University Press (OUP)
Date: 14-12-2007
Publisher: EDP Sciences
Date: 07-2010
Publisher: AIP
Date: 2005
DOI: 10.1063/1.2141904
Publisher: Oxford University Press (OUP)
Date: 21-06-2009
Publisher: Oxford University Press (OUP)
Date: 15-10-1994
Publisher: Springer Science and Business Media LLC
Date: 12-1984
DOI: 10.1007/BF02714471
Publisher: Oxford University Press (OUP)
Date: 11-1981
Publisher: Springer Science and Business Media LLC
Date: 05-1989
DOI: 10.1038/339286A0
Publisher: American Astronomical Society
Date: 06-2023
Abstract: We present a detailed multiwavelength study of star formation in the dwarf galaxy NGC 4395, which hosts an active galactic nucleus (AGN). From our observations with the Ultra-Violet Imaging Telescope, we have compiled a catalog of 284 star-forming (SF) regions, out of which we could detect 120 SF regions in H α observations. Across the entire galaxy, we found the extinction-corrected star formation rate (SFR) in the far ultraviolet (UV) to range from 2.0 × 10 −5 M ⊙ yr −1 to 1.5 × 10 −2 M ⊙ yr −1 with a median of 3.0 × 10 −4 M ⊙ yr −1 , and the age to lie in the range of ∼1 to 98 Myr with a median of 14 Myr. In H α we found the SFR to range from 7.2 × 10 −6 M ⊙ yr −1 to 2.7 × 10 −2 M ⊙ yr −1 with a median of 1.7 × 10 −4 M ⊙ yr −1 , and the age to lie between 3 to 6 Myr with a median of 5 Myr. The stellar ages derived from H α show a gradual decline with galactocentric distance. We found three SF regions close to the center of NGC 4395 with high SFRs both from H α and UV, which could be attributed to feedback effects from the AGN. We also found six other SF regions in one of the spiral arms having higher SFRs. These are very close to supernova remnants, which could have enhanced the SFR locally. We obtained a specific SFR (SFR per unit mass) for the whole galaxy of 4.64 × 10 −10 yr −1 .
Publisher: Oxford University Press (OUP)
Date: 11-05-2009
Publisher: EDP Sciences
Date: 19-06-2003
Publisher: Springer Science and Business Media LLC
Date: 12-1984
DOI: 10.1007/BF02714470
Publisher: Springer Science and Business Media LLC
Date: 12-2011
Publisher: Oxford University Press (OUP)
Date: 12-1981
Publisher: Oxford University Press (OUP)
Date: 05-2003
Publisher: Springer Science and Business Media LLC
Date: 12-2011
Publisher: Oxford University Press (OUP)
Date: 07-2006
Publisher: Oxford University Press (OUP)
Date: 10-1999
Publisher: Oxford University Press (OUP)
Date: 11-02-2001
Publisher: EDP Sciences
Date: 09-07-2009
Publisher: Oxford University Press (OUP)
Date: 15-06-2010
Publisher: Oxford University Press (OUP)
Date: 1987
Publisher: Oxford University Press (OUP)
Date: 04-1986
Publisher: Oxford University Press (OUP)
Date: 11-12-2004
Publisher: EDP Sciences
Date: 29-07-2002
Publisher: Oxford University Press (OUP)
Date: 21-12-2011
Publisher: Oxford University Press (OUP)
Date: 09-1985
Publisher: Oxford University Press (OUP)
Date: 08-2006
Publisher: American Astronomical Society
Date: 23-11-2011
Publisher: EDP Sciences
Date: 12-2015
Publisher: Oxford University Press (OUP)
Date: 16-08-2013
Publisher: Oxford University Press (OUP)
Date: 05-1994
Publisher: Oxford University Press (OUP)
Date: 22-12-2016
Publisher: Oxford University Press (OUP)
Date: 09-1987
Publisher: Springer Berlin Heidelberg
Date: 1988
Publisher: Oxford University Press (OUP)
Date: 04-1986
Publisher: Oxford University Press (OUP)
Date: 09-1994
Publisher: Oxford University Press (OUP)
Date: 15-10-1995
Publisher: Oxford University Press (OUP)
Date: 21-07-2009
Publisher: Springer Berlin Heidelberg
Date: 1988
Publisher: Oxford University Press (OUP)
Date: 11-11-2006
Publisher: Oxford University Press (OUP)
Date: 03-2013
DOI: 10.1093/MNRAS/STT147
Publisher: American Astronomical Society
Date: 05-1993
DOI: 10.1086/116544
Publisher: Oxford University Press (OUP)
Date: 11-08-1998
Publisher: Elsevier BV
Date: 11-1999
Publisher: Oxford University Press (OUP)
Date: 11-03-2009
Publisher: Oxford University Press (OUP)
Date: 13-11-2006
Publisher: Elsevier BV
Date: 05-2002
Publisher: EDP Sciences
Date: 02-2002
Publisher: Oxford University Press (OUP)
Date: 20-09-2012
Publisher: Oxford University Press (OUP)
Date: 11-2004
Publisher: Oxford University Press (OUP)
Date: 09-2009
Publisher: Cambridge University Press (CUP)
Date: 2003
DOI: 10.1071/AS02031
Abstract: The role of radio polarimetry in the understanding of GPS and CSS sources is explored. After an initial discussion of what can be learned from polarimetry, the expectations of a simple evolutionary sequence of GPS/CSO to CSS to FR I /FR II sources are explored. Observational results are then compared with the expectations. Conclusions include: the GPS category is likely not a single homogeneous class of objects Faraday depth effects are very strong inside the inner 3 kpc of CSS and CSO sources in at least 3C 138 the Faraday screen has very fine scale (subparsec) structure and there is evidence for increased ionisation near bends in some CSS jets probably due to jet–ISM interaction. New results on 3C 138, 3C 43, and 3C 454 are given.
Publisher: Springer Science and Business Media LLC
Date: 12-2011
Location: India
No related grants have been discovered for Dhruba Jyoti Saikia.