IUCAA Preprints
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Item 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.Item 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.Item Unravelling the morphologies of Luminous Compact Galaxies using the HST/ACS GOODS survey(2007-04-17) Rawat, A.; Kembhavi, A.K.; Barway, Sudhanshu; et al.Context. Luminous Compact Galaxies (LCGs) (MB ≤ −20, R1/2 ≤ 4.5 kpc and EW0(OII) ≥15Å) constitute one of the most rapidly evolving galaxy populations over the last ∼8Gyr history of the universe. Due to their inherently compact sizes, any detailed quantitative analysis of their morphologies has proved to be difficult in the past. Hence, the morphologies and thereby the local counterparts of these enigmatic sources have been hotly debated. Aims. Our aim is to use the high angular resolution, deep, multiband HST/ACS imaging data, from the HST/ACS GOODS survey, to study the quantitative morphology of a complete sample of LCGs in the redshift range 0.5 ≤ z ≤ 1.2. Methods. We have derived structural parameters for a representative sample of 39 LCGs selected from the GOODS-S HST/ACS field, using full 2-dimensional surface brightness profile fitting of the galaxy images in each of the four filters available. B435W − z850LP color maps are constructed for the sample to aid in the morphological classification. We then use the rest frame B band bulge flux fraction (B/T) to determine the morphological class of galaxies which are well fit by a ulge+disk two dimensional structure. Mergers were essentially identified visually by the presence of multiple maxima of comparable intensity in the rest frame B band images, aided by the color maps to distinguish them from HII regions. We also make use of the Spitzer 24 m source catalog of sources in the CDFS to derive the dust enshrouded star formation rates (SFR) for some of the sample LCGs Results. We derive the following morphological mix for our sample of intermediate redshift LCGs: Mergers: ∼36%, Disk dominated: ∼22%, S0: ∼20%, Early types: ∼7%, Irr/tadpole: ∼15%.We establish that our sample LCGs are intermediate mass objects with stellar mass ranging from 9.44 ≤ Log10(M/M⊙) ≤ 10.96, with a median mass of Log10(M/M⊙) = 10.32. We also derive SFR values ranging from a few to ∼ 65 M⊙/year as expected for this class of objects. We find that LCGs account for ∼26% of the MB ≤ −20 galaxy population in the redshift range 0.5 ≤ z ≤ 1.2. We estimate a factor ∼11 fall in the comoving number density of blue LCGs from redshifts 0.5 ≤ z ≤ 1.2 to the current epoch, even though this number is subject to large uncertainities given the small sample size at zero redshift available from the literature. Conclusions. The strong redshift evolution exhibited by LCGs, and the fact that a significant fraction of LCGs are in merging systems, seem to indicate that LCGs might be an important phase in the hierarchical evolution of galaxies. We envisage that some of the LCGs that are classified as merging systems, might go on to rebuild their disks and evolve into disk galaxies in the local universe.Item 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.Item Towards a robust estimate of the merger rate evolution using near-IR photometry(2008-04-01) Rawat, A.; Kembhavi, A.K.We use a combination of deep, high angular resolution imaging data from the CDFS (HST/ACS GOODS survey) and ground based near-IR Ks images to derive the evolution of the galaxy major merger rate in the redshift range 0.2 ≤ z ≤ 1.2. We select galaxies on the sole basis of their J-band rest-frame, absolute magnitude, which is a good tracer of the stellar mass. We find steep evolution with redshift, with the merger rate ∝ (1 + z)3.43±0.49 for optically selected pairs, and ∝ (1 + z)2.18±0.18 for pairs selected in the near-IR. Our result is unlikely to be affected by luminosity evolution which is relatively modest when using restframe J band selection. The apparently more rapid evolution that we find in the visible is likely caused by biases relating to incompleteness and spatial resolution affecting the ground based near IR photometry, underestimating pair counts at higher redshifts in the near-IR. The major merger rate was ∼5.6 times higher at z ∼ 1.2 than at the current epoch. Overall 41%×(0.5Gyr/τ ) of all galaxies with MJ ≤ −19.5 have undergone a major merger in the last ∼ 8Gyr, where τ is the merger timescale. Interestingly, we find no effect on the derived major merger rate due to the presence of the large scale structure at z = 0.735 in the CDFS.Item 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.Item 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.Item 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.Item 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.