Browsing by Author "Kembhavi, A.K."

Now showing 1 - 20 of 26

Bar fraction in lenticular galaxies: Dependence on luminosity and environment
(2010-10-15) Barway, Sudhanshu; Wadadekar, Yogesh; Kembhavi, A.K.
We present a study of bars in lenticular galaxies based on a sample of 371 galaxies from the SDSS-DR 7 and 2MASS in optical and near-infrared bands, respectively. We found a bar in 15% of the lenticular galaxies in our sample, which is consistent with recent stud- ies. The barred galaxy fraction shows a luminosity dependence, with faint lenticular galaxies (MK > −24.5, total absolute magnitude in K band) having a larger fraction of bars than bright lenticular galaxies (MK < −24.5). A similar trend is seen when Mr = −21.5, the total absolute magnitude in SDSS r band is used to divide the sample into faint and bright lenticular galaxies.We ﬁnd that faint galaxies in clusters show a higher bar fraction than their counterparts in the ﬁeld. This suggests that the formation of bars in lenticular galaxies not only depends on the total luminosity of galaxy but also on the environment of the host galaxy.
Correlations among global photometric properties of disk galaxies
(2015-03-01) Khosroshahi, Habib G.; Wadadekar, Yogesh; Kembhavi, A.K.
Dielectronic recombination and stability of warm gas in active galactic nuclei
(2008-09) Chakravorty, Susmita; Kembhavi, A.K.
Understanding the thermal equilibrium (stability) curve may offer insights into the nature of the warm absorbers often found in active galactic nuclei. Its shape is determined by factors such as the spectrum of the ionizing continuum and the chemical composition of the gas. We ﬁnd that the stability curves obtained under the same set of the above-mentioned physical factors, but using recently derived dielectronic recombination rates, give signiﬁcantly different results, especially in the regions corresponding towarmabsorbers, leading to different physical predictions. Using the current rates we ﬁnd a larger probability of having a thermally stable warmabsorber at 105 Kthan previous predictions and also a greater possibility for itsmultiphase nature. The results obtained with the current dielectronic recombination rate coefﬁcients are more reliable because the warm absorber models along the stability curve have computed coefﬁcient values, whereas previous calculations relied on guessed averages for these because of a lack of available data.
Difference boosting neural network for automated star-galaxy classiﬁcation
(2002-03-01) Philip, Ninan S.; Wadadekar, Yogesh; Kembhavi, A.K.; et al.
In this paper we describe the use of a new artiﬁcial neural network, called the diﬀerence boosting neural network (DBNN), for automated classiﬁcation problems in astronomical data analysis. We illustrate the capabilities of the network by applying it to star galaxy classiﬁcation using recently released, deep imaging data. We have compared our results with classiﬁcation made by the widely used Source Extractor (SExtractor) package. We show that while the performance of the DBNN in star-galaxy classiﬁcation is comparable to that of SExtractor, it has the advantage of signiﬁcantly higher speed and ﬂexibility during training as well as classiﬁcation.
A dust lane in the radio galaxy 3C270
(2015-02-18) Mahabal, A; Kembhavi, A.K.; Singh, K. P.; Bhatt, P.N.; Prabhu, T.P.
Effective radii and color gradients in radio galaxies
(2015-03-01) Mahabal, Ashish; Kembhavi, A.K.; Mccarthy, P.J
We present de Vaucouleurs' effective radii in B and R bands for a sample of Molonglo Reference Catalogue radio galaxies and a control sample of normal galaxies. We use the ratio of the scale lengths in the two bands as an indicator to show that the radio galaxies tend to have excess of blue color in their inner region much more frequently than the control galaxies. We show that the scale length ratio is a useful indicator of radial color variation even when the conventional color gradient is too noisy to serve the purpose.
Journey of IUCAA & Career in Astrophysics
(2013-07-24) Kembhavi, A.K.
Lenticular Galaxy Formation - Possible Luminosity Dependence
(2007-03-27) Barway, Sudhanshu; Kembhavi, A.K.; Wadadekar, Yogesh; et al.
We investigate the correlation between the bulge eﬀective radius (re) and disk scale length (rd), in the near-infrared K band for lenticular galaxies in the ﬁeld and in clusters. We ﬁnd markedly diﬀerent relations between the two parameters as a function of luminosity. Lenticulars with total absolute magnitude fainter than MT = −24.5 show a positive correlation, in line with predictions of secular formation processes for the pseudo bulges of late-type disk galaxies. But brighter lenticulars with MT < −24.5 show an anti-correlation, indicating that they formed through a diﬀerent mechanism. The available data are insuﬃcient to reliably determine the eﬀect of galaxy environment on this correlation.
Multicolor surface photometry of lenticulars galaxies I. The Data
(2011-07-06) Barway, Sudhanshu; Mayya, Y. D.; Kembhavi, A.K.; et al.
We present in this paper multicolor surface and aperture photometry in the B, V,R and K′ bands for a sample of 34 lenticular galaxies from the UGC catalogue. From surface photometric analysis, we obtain radial proﬁles of surface brightness, colors, ellipticity, position angle and the Fourier coeﬃcients which describe the departure of isophotal shapes from purely elliptical form and ﬁnd the presence of dust lanes, patches and ring like structure in several galaxies in the sample. We obtain total integrated magnitudes and colors and ﬁnd that these are in good agreement with the values from the RC3 catalogue. Isophotal colors are correlated with each other, following the sequence expected for early- type galaxies. The color gradients in lenticulars are more negative than the corresponding gradients in ellipticals. There is a good correlation between B − V and B − R color gradients, and the mean gradient in the B−V,B−R and V −K′ colors are −0.13±0.06, −0.18±0.06, −0.25±0.11 magnitude per dex in radius respectively.
Near infrared photometric plane for ellipticals and bulges of spirals
(2000-09-20) Khosroshahi, Habib G.; Wadadekar, Yogesh; Kembhavi, A.K.; et al.
We report the existence of a single plane in the space of global photometric pa- rameters describing elliptical galaxies and the bulges of early type spiral galaxies. The three parameters which deﬁne the plane are obtained by ﬁtting the Sersic form to the brightness distribution obtained from near-infrared K band images. We ﬁnd, from the range covered by their shape parameters, that the elliptical galaxies form a more ho- mogeneous population than the bulges. Known correlations like the Kormendy relation are projections of the photometric plane. The existence of the plane has interesting implications for bulge formation models.
Non-standard cosmologies
(Gordon and Breach, 1988-03-12) Narlikar, J. V.; Kembhavi, A.K.
Nonstandard cosmologies
(-, 1980-01-01) Narlikar, J. V.; Kembhavi, A.K.
On the origin of the featureless soft X-ray excess emission from the Seyfert 1 galaxy ESO 198–G24
(IUCAA, 2015-02) Laha, Sibasish; Dewangan, Gulab Chand; Kembhavi, A.K.
Photometric Scaling Relations for Bulges of Galaxies
(2005-09-01) Ravikumar, C. D.; Kembhavi, A.K.; Barway, Sudhanshu; et al.
We study the photometric parameters of the bulges of galaxies of diﬀerent Hubble types including ellipticals, lenticulars, early and late type spirals and early type dwarf galaxies. Analyzing the distributions of various photometric parameters, and two and three-dimensional correlations between them, we ﬁnd that there is a diﬀerence in the correlations exhibited by bright (MK < −22) and faint bulges, irrespective of their Hubble type. Importantly, the bright bulges, which include typically E/S0 galaxies and bulges of early type spirals, are tightly distributed around a common photometric plane (PP), while their fainter counter parts, mainly bulges of late type spirals and dwarf galaxies show significant deviation from the planar distribution. We show that the speciﬁc entropy, determined from the bulge structural parameters, systematically increases as we move from late to early Hubble types. We interpret this as evidence for hierarchical merging and passive evolution scenarios for bright and faint bulges respectively.
Properties of warm absorbers in active galaxies: a systematic stability curve analysis
(2008-10-14) Chakravorty, Susmita; Kembhavi, A.K.
Signatures of warm absorbers are seen in soft X-ray spectra of about half of all Seyfert1 galaxies observed and in some quasars and blazars. We use the thermal equilibrium curve to study the inﬂuence of the shape of the ionizing continuum, density and the chemical composition of the absorbing gas on the existence and nature of the warm absorbers. We describe circumstances in which a stable warm absorber can exist as a multiphase medium or one with continuous variation in pressure. In particular we ﬁnd the following results: i) the warm absorber exists only if the spectral index of the X-ray power-law ionizing continuum α > 0.2 and has a multiphase nature if α ∼ 0.8, which interestingly is the spectral index formost of the observed Seyfert 1 galaxies; ii) thermal and ionization states of highly dense warm absorbers are sensitive to their density if the ionizing continuum is sufﬁciently soft, i.e. dominated by the ultraviolet iii) absorbing gas with super Solar metallicity is more likely to have a multiphase nature; iv) the nature of the warm absorber is signiﬁcantly inﬂuenced by the absence of iron and associated elements which are produced in the later stages of star formation history in supernovae of type Ia.
PyMorph: Automated galaxy structural parameter rstimation using python
(2010-07-20) Vikram, Vinu; et al.; Wadadekar, Yogesh; Kembhavi, A.K.
We present a new software pipeline – PyMorph – for automated estimation of structural parameters of galaxies. Both parametric ﬁts through a two dimensional bulge disk decomposition as well as structural parameter measurements like concentration, asymmetry etc. are supported. The pipeline is designed to be easy to use yet ﬂexible; individual software modules can be replaced with ease. A ﬁnd-and-ﬁt mode is available so that all galaxies in a image can be measured with a simple command. A parallel version of the Pymorph pipeline runs on computer clusters and a Virtual Observatory compatible web enabled interface is under development.
Quantitative measure of evolution of bright cluster galaxies at moderate redshifts
(2010-01-01) Vikram, Vinu; et al.; Wadadekar, Yogesh; Kembhavi, A.K.
Using archival data from the Hubble Space Telescope, we study the quantitative morphological evolution of spectroscopically conﬁrmed bright galaxies in the core regions of nine clusters ranging in redshift from z = 0.31 to z = 0.84. We use morphological parameters derived from two dimensional bulge-disk decomposition to study the evolution. We ﬁnd an increase in the mean bulge-to-total luminosity ratio B/T as the Universe evolves. We also ﬁnd a corresponding increase in the fraction of early type galaxies and in the mean S´ersic index. We discuss these results and their implications to physical mechanisms for evolution of galaxy morphology.
The soft x-ray excess in quasars
(2014-11-22) Kembhavi, A.K.; Fabian, A.C.
A soft x-ray excess in the energy range of a few tenths of a ke V has been detected in some quasars , and there are indications in the x-ray data which point to the excess being a feature universal to all quasars. The poor energy resolution in the concerned energy band make it impossible at the present to observationally constrain the excess quantitatively. We show in this paper that the shape of the excess sensitively determines the surface density therefore allows an assessment of the nature of the excess
Spacetime singularities and conformal gravity
(Proccding of Cambridge Philosophical Society, 1977-01-30) Narlikar, J. V.; Kembhavi, A.K.
A Sptzer Study Of Pseudobulges In so Galaxies - Secular Evoution of disk
(IUCAA, 2015-02) Vaghmare, Kaustubh; Barway, Sudhanshu; Kembhavi, A.K.