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

S. Jeyakumar  D. J. Saikia 《Monthly notices of the Royal Astronomical Society》2000,311(2):397-404

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 (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 the result of their evolution in an asymmetric environment. We also suggest that some sources, especially of lower luminosity, exhibit an asymmetry in the collimation of the oppositely directed radio jets.  相似文献   

Thermogravimetric and model-free kinetic studies on CO<Subscript>2</Subscript> gasification of low-quality,high-sulphur Indian coals     

Tonkeswar Das  Ananya Saikia  Banashree Mahanta  Rahul Choudhury  Binoy K Saikia 《Journal of Earth System Science》2016,125(7):1365-1377

Coal gasification with CO₂ has emerged as a cleaner and more efficient way for the production of energy, and it offers the advantages of CO₂ mitigation policies through simultaneous CO₂ sequestration. In the present investigation, a feasibility study on the gasification of three low-quality, high-sulphur coals from the north-eastern region (NER) of India in a CO₂ atmosphere using thermogravimetric analysis (TGA-DTA) has been made in order to have a better understanding of the physical and chemical characteristics in the process of gasification of coal. Model-free kinetics was applied to determine the activation energies (E) and pre-exponential factors (A) of the CO₂ gasification process of the coals. Multivariate non-linear regression analyses were performed to find out the formal mechanisms, kinetic model, and the corresponding kinetic triplets. The results revealed that coal gasification with CO₂ mainly occurs in the temperature range of 800^°–1400^°C and a maximum of at around 1100^°C. The reaction mechanisms responsible for CO₂ gasification of the coals were observed to be of the ‘nth order with autocatalysis (CnB)’ and ‘nth order (Fn) mechanism’. The activation energy of the CO₂ gasification was found to be in the range 129.07–146.81 kJ mol^?1.  相似文献   

Deep low-frequency observations with the Giant Metrewave Radio Telescope: a search for relic radio emission     

S. K. Sirothia  D. J. Saikia  C. H. Ishwara-Chandra  N. G. Kantharia 《Monthly notices of the Royal Astronomical Society》2009,392(4):1403-1412

We present deep multifrequency observations using the Giant Metrewave Radio Telescope (GMRT) at 153, 244, 610 and 1260 MHz of a field centred on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as haloes and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey (WENSS), NRAO (National Radio Astronomy Observatories) VLA (Very Large Array) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty-Centimeters Survey (FIRST). Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index,  α∼ 0.8 [S(ν) ∝ν^−α  ], which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with  α≥ 1.3  . We examine their optical fields and discuss the nature of some of these sources.  相似文献   

STAGES: the Space Telescope A901/2 Galaxy Evolution Survey     

Meghan E. Gray  Christian Wolf  Marco Barden  Chien Y. Peng  Boris Häußler  Eric F. Bell  Daniel H. McIntosh  Yicheng Guo  John A. R. Caldwell  David Bacon  Michael Balogh  Fabio D. Barazza  Asmus Böhm  Catherine Heymans  Knud Jahnke  Shardha Jogee  Eelco van Kampen  Kyle Lane  Klaus Meisenheimer  Sebastian F. Sánchez  y Taylor  Lutz Wisotzki  Xianzhong Zheng  David A. Green  R. J. Beswick  D. J. Saikia  Rachel Gilmour  Benjamin D. Johnson  Casey Papovich 《Monthly notices of the Royal Astronomical Society》2009,393(4):1275-1301

We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at   z ∼ 0.165  has been the subject of an 80-orbit F606W Hubble Space Telescope (HST) /Advanced Camera for Surveys (ACS) mosaic covering the full     span of the supercluster. Extensive multiwavelength observations with XMM–Newton , GALEX, Spitzer , 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sérsic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.  相似文献   

325-MHz observations of the ELAIS-N1 field using the Giant Metrewave Radio Telescope     

S. K. Sirothia  M. Dennefeld  D. J. Saikia  H. Dole  F. Ricquebourg  J. Roland 《Monthly notices of the Royal Astronomical Society》2009,395(1):269-281

We present observations of the European Large-Area ISO Survey-North 1 (ELAIS-N1) at 325 MHz using the Giant Metrewave Radio Telescope (GMRT), with the ultimate objective of identifying active galactic nuclei and starburst galaxies and examining their evolution with cosmic epoch. After combining the data from two different days we have achieved a median rms noise of  ≈40 μJy  beam⁻¹, which is the lowest that has been achieved at this frequency. We detect 1286 sources with a total flux density above  ≈270 μJy  . In this paper, we use our deep radio image to examine the spectral indices of these sources by comparing our flux density estimates with those of Garn et al. at 610 MHz with the GMRT, and surveys with the Very Large Array at 1400 MHz. We attempt to identify very steep spectrum sources which are likely to be either relic sources or high-redshift objects as well as inverted-spectra objects which could be Giga-Hertz Peaked Spectrum objects. We present the source counts, and report the possibility of a flattening in the normalized differential counts at low flux densities which has so far been reported at higher radio frequencies.  相似文献   

H i content and star formation in the interacting galaxy Arp86     

Chandreyee Sengupta  K. S. Dwarakanath  D. J. Saikia 《Monthly notices of the Royal Astronomical Society》2009,397(1):548-557

We present the results of Giant Metrewave Radio Telescope (GMRT) observations of the interacting system Arp86 in both neutral atomic hydrogen, H  i , and in radio continuum at 240 606 and 1394 MHz. In addition to H  i emission from the two dominant galaxies, NGC 7752 and NGC 7753, these observations show a complex distribution of H  i tails and bridges due to tidal interactions. The regions of highest column density appear related to the recent sites of intense star formation. H  i column densities  ∼1–1.5 × 10²¹ cm⁻²  have been detected in the tidal bridge which is bright in Spitzer image as well. We also detect H  i emission from the galaxy 2MASX J23470758+2926531, which is shown to be a part of this system. We discuss the possibility that this could be a tidal dwarf galaxy. The radio continuum observations show evidence of a non-thermal bridge between NGC 7752 and NGC 7753, and a radio source in the nuclear region of NGC 7753 consistent with it having a low-ionization nuclear emission region nucleus.  相似文献   

A complete sample of 21-cm absorbers at z∼ 1.3: Giant Metrewave Radio Telescope survey using Mg ii systems     

N. Gupta  R. Srianand  P. Petitjean  P. Noterdaeme  D. J. Saikia 《Monthly notices of the Royal Astronomical Society》2009,398(1):201-220

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

C. H. Ishwara-Chandra  D. J. Saikia 《Monthly notices of the Royal Astronomical Society》2000,317(3):658-666

We present the results of multiwavelength 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 of the quasars in our sample, whereas only ∼50 per cent of the galaxies have cores detected in at least one of the wavelengths . 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, with the distributions in a matched sample of core-dominated quasars, shows that the distributions for these two classes are significantly different, and this is consistent with the expectations of the unified scheme. The difference in the spectral indices of the two hotspots on opposite sides is also significantly larger for quasars than for radio galaxies, as is expected in the unified scheme. We also investigate the relationship between the spectral index of the hotspots and the redshift or luminosity for our sample of sources.  相似文献   

The M w 6.9 Sikkim–Nepal earthquake of September 2011: a perspective for wrench faulting in the Himalayan thrust zone     

N. Purnachandra Rao  V. M. Tiwari  M. Ravi Kumar  Pinki Hazarika  Dipankar Saikia  R. K. Chadha  Y. J. Bhaskar Rao 《Natural Hazards》2015,77(1):355-366

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Extragalactic sources with asymmetric radio structure II. further observations of the quasar B2 1320 + 299     

T. J. Cornwell  D. J. Saikia  P. Shastri  L. Feretti  G. Giovannini  P. Parma  C. J. Salter 《Journal of Astrophysics and Astronomy》1986,7(2):119-129

We present VLA A-array observations at λ20, 6 and 2 cm and B-array observations at λ20 and 6 cm of the quasar B2 1320 + 299, which has a very unusual radio structure. In addition to a component, A, coincident with the quasar, there are two lobes of radio emission, B and C, on the same side of A. These are located at distances of -25 and 50 arcsec respectively from A. The present observations show that A has a flat-spectrum component coincident with the quasar and a weak outer component at a distance of-4 arcsec along PA - 100°. The morphology of B resembles a head-tail type of structure with its tail towards the north-east. The magnetic field lines in component B appear to follow the bend in the tail. Component C exhibits some extension towards the north-west. We discuss the possible nature of B2 1320 + 299 and suggest that while A appears to be an independent source, the relation between B and C, if they are associated at all, is unclear. Deep optical observations are essential to help clarify the situation.  相似文献