Cotton College State University

Guwāhāti, India

Cotton College State University

Guwāhāti, India
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Chandola Y.,CAS National Astronomical Observatories | Saikia D.J.,Cotton College State University
Monthly Notices of the Royal Astronomical Society | Year: 2017

HI absorption studies of active galaxies enable us to probe their circumnuclear regions and the general interstellar medium and study the supply of gas that may trigger nuclear activity. In this article, we investigate the dependence of the detection rate of HI absorption on the nature of radio galaxies based on their emission-line spectra and on the nature of host galaxies based on WISE colours and their radio structure, which may help us understand the different accretion modes. We find significant differences in the distributions of W2-W3 colour for sources with HI absorption detections and non-detections. We report a high detection rate of HI absorption in those galaxies with WISE infrared colours W2-W3 > 2, typical of gasrich systems, along with a compact radio structure. The HI detection rate for low-excitation radio galaxies (LERGs) with W2-W3 > 2 and compact radio structure is high (70.6 ± 20.4 per cent). In high-excitation radio galaxies (HERGs), compact radio structure in the nuclear or circumnuclear region could give rise to absorption by gas in the dusty torus, in addition to gas in the interstellar medium. However, the higher specific star-formation rate (sSFR) for LERGs with W2-W3 > 2 suggests that HI absorption may be largely due to star-forming gas in their hosts. LERGs with extended radio structure tend to have significantly lower values of W2-W3 compared with those with compact structure. Extended radio sources and those with W2-W3 < 2 have low HI detection rates. © 2016 The Authors.


Nandi S.,The Oskar Klein Center | Jamrozy M.,Jagiellonian University | Roy R.,The Oskar Klein Center | Larsson J.,The Oskar Klein Center | And 4 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2017

We present a radio and optical study of the double-double radio galaxy J1328+2752 based on new low-frequency Giant Metrewave Radio Telescope observations and Sloan Digital Sky Survey (SDSS) data. The radio data were used to investigate the morphology and to perform a spectral index analysis. In this source, we find that the inner double is misaligned by~30 from the axis of the outer diffuse structure. The SDSS spectrum shows that the central component has double-peaked line profiles with different emission strengths. The average velocity offset of the two components is 235 ± 10.5 kms-1. The misaligned radio morphology along with the double-peaked emission lines indicate that this source is a potential candidate binary supermassive black hole. This study further supports mergers as a possible explanation for repeated jet activity in radio sources. © 2016 The Authors.


Irwin J.A.,Queen's University | Brar R.S.,University of Ontario Institute of Technology | Saikia D.J.,University of Pune | Saikia D.J.,Cotton College State University | Henriksen R.N.,Queen's University
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the first maps of NGC 3044 and NGC 4157 at λ 450 μm and λ 850 μm from the JamesClerk Maxwell Telescope as well as the first maps at 617 MHzfrom the Giant Metrewave Radio Telescope. High-latitude emission has been detected in both the radio continuum and sub-mm for NGC 3044 and in the radio continuum for NGC 4157, including several new features. For NGC 3044, in addition, we find 617MHz emission extending to the north of the major axis, beginning at the far ends of the major axis. One of these low-intensity features, more than 10 kpc from the major axis, has apparently associated emission at λ 20 cm and may be a result of in-disc activity related to star formation. The dust spectrum at longwavelengths required fittingwith a two-temperaturemodel for both galaxies, implying the presence of cold dust (Tc = 9.5K for NGC 3044 and Tc = 15.3K for NGC4157).Dustmasses areMd = 1.6 × 108Mȯ andMd = 2.1 × 107Mȯ forNGC3044 and NGC 4157, respectively, and are dominated by the cold component. There is a clear correlation between the 617MHz and λ 850 μmemission in the two galaxies. In the case of NGC 3044 for which the λ 850 μm data are strongly dominated by cold dust, this implies a relation between the non-thermal synchrotron emission and cold dust. The 617MHz component represents an integration of massive star formation over the past 107-8 yr and the λ 850 μm emission represents heating from the diffuse interstellar radiation field (ISRF). The 617 MHz-λ 850 μm correlation improves when a smoothing kernel is applied to the λ 850 μm data to account for differences between the cosmic ray (CR) electron diffusion scale and the mean free path of an ISRF photon to dust. The best-fitting relation is L617MHz ∞ L850μm 2.1±0.2 for NGC 3044. If variations in the cold dust emissivity are dominated by variations in dust density, and the synchrotron emission depends on magnetic field strength (a function of gas density) as well as CR electron generation (a function ofmassive star formation rate and therefore density via the Schmidt law) then the expected correlation for NGC 3044 is L617MHz ∞ L850μm 2.2, in agreement with the observed correlation. © 2013 The Authors.


Deka R.K.,Gauhati University | Paul A.,Cotton College State University | Kalit N.,Board of Secondary Education
Frontiers in Heat and Mass Transfer | Year: 2015

A theoretical study of thermal radiation effects on unsteady MHD natural convection flow of an electrically conducting fluid past a vertical plate with variable temperature is considered. It is supposed that the temperature of the plate decays exponentially with time. Exact solutions to the nondimensionalised coupled linear partial differential equations representing the flow problem are obtained using Laplace transform technique. Effects of different physical parameters involved in the temperature and velocity profiles are investigated, shown graphically and discussed. Skin friction and Nusselt number are also derived and their variations with respect to the parameters are investigated. © 2015, Global Digital Central. All rights reserved.


Sengupta C.,Institute Astrofsica Of Andaluca Iaa Csic | Sengupta C.,Calar Alto Observatory | Dwarakanath K.S.,Raman Research Institute | Saikia D.J.,Tata Institute of Fundamental Research | And 2 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

We present results from Giant Metrewave Radio Telescope HI 21 cm line observations of the interacting galaxy pair Arp 181 (NGC 3212 and NGC3215) at z = 0.032. We find almost all of the detected HI (90 per cent) is displaced well beyond the optical discs of the pair with the highest densityHI located~70 kpc west of the pair. AnHI bridge extending between the optical pair and the bulk of HI together with their HI deficiencies provide strong evidence that the interaction between the pair has removed most of their HI to the current projected position. HI to the west of the pair has two approximately equal intensity peaks. The HI intensity maximum furthest to the west coincides with a small spiral companion SDSS J102726.32+794911.9 which shows enhanced mid-infrared (Spitzer), ultraviolet (UV; GALEX) and Ha emission indicating intense star-forming activity. The HI intensity maximum close to the Arp 181 pair, coincides with a diffuse optical cloud detected in UV (GALEX) at the end of the stellar and HI tidal tails originating at NGC3212 and, previously proposed to be a tidal dwarf galaxy in formation. Future sensitive HI surveys by telescopes like Australian Square Kilometre Array Pathfinder should prove to be powerful tools for identifying tidal dwarfs at moderate to large redshifts to explore in detail the evolution of dwarf galaxies in the Universe. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Deka R.K.,Gauhati University | Paul A.,Cotton College State University | Chaliha A.,Biswanath College
Ain Shams Engineering Journal | Year: 2015

This paper describes a one dimensional unsteady natural convection flow past an infinite vertical cylinder with heat and mass transfer under the effect of constant heat flux at the surface of the cylinder. Closed form solutions of the dimensionless unsteady linear governing boundary layer equations are obtained in terms of Bessel functions and modified Bessel functions by Laplace transform method. The numerical values of velocity, temperature and concentration profiles are obtained for different values of the physical parameters namely, thermal Grashof number, mass Grashof number, Prandtl number, Schmidt number and time and presented in graphs. Also, skin friction and Sherwood number are shown graphically and discussed. It is observed that the velocity and temperature increase unboundedly with time, while the concentration approaches steady state at larger times. © 2015 Faculty of Engineering, Ain Shams University.


Dixit A.S.,North - Eastern Hill University | Singh N.S.,Cotton College State University
Journal of Photochemistry and Photobiology B: Biology | Year: 2016

Seasonality in daily locomotor activity pattern was investigated in the subtropical tree sparrow by exposing a group of birds to natural day lengths (NDL) for 30 days and another group to 12L/12D for 14 days followed by transfer to constant dim light (LLdim) for another 15 days in four different seasons of the year. Serum testosterone levels were also measured during different seasons. Sparrows, under NDL, exhibited distinct circadian rhythmicity in their locomotor activity with almost similar general pattern in different seasons that restricted mainly to the light hours. However, they showed season-dependent differences in the characteristics of circadian locomotor activity rhythm. Birds, when exposed to 12L/12D, showed entrainment of their locomotor activity rhythm with the activity confined mainly during the light phase. Though, tau (τ) under free run conditions did not show any significant difference, the activity period varied significantly in different seasons. The highest level of testosterone was recorded in the spring season that corresponded with the maximum locomotor activity in spring months. The seasonality in daily locomotor activity correlates with the seasonal changes in testosterone levels suggesting the influence of gonadal steroids on endogenous circadian system which is indicative of adaptation of tree sparrow to local photoperiodic conditions. © 2016 Elsevier B.V. All rights reserved.


PubMed | Cotton College State University and North - Eastern Hill University
Type: | Journal: Journal of photochemistry and photobiology. B, Biology | Year: 2016

Seasonality in daily locomotor activity pattern was investigated in the subtropical tree sparrow by exposing a group of birds to natural day lengths (NDL) for 30days and another group to 12L/12D for 14days followed by transfer to constant dim light (LLdim) for another 15days in four different seasons of the year. Serum testosterone levels were also measured during different seasons. Sparrows, under NDL, exhibited distinct circadian rhythmicity in their locomotor activity with almost similar general pattern in different seasons that restricted mainly to the light hours. However, they showed season-dependent differences in the characteristics of circadian locomotor activity rhythm. Birds, when exposed to 12L/12D, showed entrainment of their locomotor activity rhythm with the activity confined mainly during the light phase. Though, tau () under free run conditions did not show any significant difference, the activity period varied significantly in different seasons. The highest level of testosterone was recorded in the spring season that corresponded with the maximum locomotor activity in spring months. The seasonality in daily locomotor activity correlates with the seasonal changes in testosterone levels suggesting the influence of gonadal steroids on endogenous circadian system which is indicative of adaptation of tree sparrow to local photoperiodic conditions.


Nandi S.,Aryabhatta Research Institute of Observational science ARIES | Nandi S.,Kumaun University | Saikia D.J.,University of Pune | Saikia D.J.,Cotton College State University
Bulletin of the Astronomical Society of India | Year: 2012

The radio structures and optical identifications of a sample of 242 sources classified as double-double radio sources by Proctor (2011) from a morphological study of sources in the FIRST (Faint Images of the Radio Sky at Twenty centimeters) survey (2003 April release, 811,117 entries) have been examined. We have been able to confirm only 23 of these as likely to be double-double radio galaxies (DDRGs), whose structures could be attributed to episodic nuclear activity in their host galaxies. A further 63 require either higher-resolution radio observations or optical identifications to determine whether these are DDRGs. The remaining sources are unlikely to be DDRGs. We have examined the luminosities, sizes and symmetry parameters of the DDRGs and the constraints these place on our understanding of these sources.


Pirya A.,Aryabhatta Research Institute of Observational science ARIES | Pirya A.,Kumaun University | Saikia D.J.,University of Pune | Saikia D.J.,Cotton College State University | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

The distributions of galaxies in the environments of 16 large radio sources have been examined using the Sloan Digital Sky Survey (SDSS). In the giant radio galaxy J1552+2005 (3C 326) which has the highest arm-length ratio, the shorter arm is found to interact with a group of galaxies which forms part of a filamentary structure. Although most large sources occur in regions of low galaxy density, the shorter arm is brighter in most cases suggesting asymmetries in the intergalactic medium which may not be apparent in the distribution of galaxies. In two cases with strong and variable cores, J0313+4120 and J1147+3501, the large flux density asymmetries are possibly also caused by the effects of relativistic motion. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

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