Astronomy Research Works
Permanent URI for this collectionhttp://hdl.handle.net/1903/1587
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Item Two Dimensional Velocity Fields of Low Surface Brightness Galaxies(EDP Sciences, 2005) Kuzio de Naray, R.; McGaugh, S. S.; de Blok, W. J. G.; Bosma, A.We present high resolution two dimensional velocity fields from integral field spectroscopy along with derived rotation curves for nine low surface brightness galaxies. This is a positive step forward in terms of both data quality and number of objects studied. We fit NFW and pseudo-isothermal halo models to the observations. We find that the pseudo-isothermal halo better represents the data in most cases than the NFW halo, as the resulting concentrations are lower than would be expected for CDM.Item Simulating Observations of Dark Matter Dominated Galaxies: Towards the Optimal Halo Profile(Blackwell, 2007) de Blok, W.J.G.; Bosma, Albert Bosma; McGaugh, StacyLow Surface Brightness (LSB) galaxies are dominated by dark matter, and their rotation curves thus reflect their dark matter distribution. Recent high-resolution rotation curves suggest that their dark matter mass-density distributions are dominated by a constant-density core. This seems inconsistent with the predictions of Cold Dark Matter (CDM) models which produce halos with compact density cusps and steep mass-density profiles. However, the observationally determined mass profiles may be affected by non-circular motions, asymmetries and offsets between optical and dynamical centres, all of which tend to lower the observed slopes. Here we determine the impact of each of these effects on a variety of halo models, and compare the results with observed mass-density profiles. Our simulations suggest that no single systematic effect can reconcile the data with the cuspy CDM halos. The data are best described by a model with a soft core with an inner power-law mass-density slope ⍺ = −0.2±0.2. However, no single universal halo profile provides a completely adequate description of the data.Item The AGN and Gas Disk in the Low Surface Brightness Galaxy PGC045080(Blackwell, 2007) Das, M.; Kantharia, N.; Ramya, S.; Prabhu, T. P.; McGaugh, S. S.; Vogel, S. N.We present radio observations and optical spectroscopy of the giant low surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a moderately distant galaxy having a highly inclined optical disk and massive HI gas content. Radio continuum observations of the galaxy were carried out at 320 MHz, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the galaxy. The emission appears extended over the inner disk at all three frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We also did optical spectroscopy of the galaxy nucleus; the spectrum did not show any strong emission lines associated with AGN activity but the presence of a weak AGN cannot be ruled out. Furthermore, comparison of the H flux and radio continuum at 1.4 GHz suggests that a significant fraction of the emission is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be present in PGC 045080. The extended radio emission represents lobes/jets from the AGN. These observations show that although LSB galaxies are metal poor and have very little star formation, their centers can host significant AGN activity. We also mapped the HI gas disk and velocity field in PGC 045080. The HI disk extends well beyond the optical disk and appears warped. In the HI intensity maps, the disk appears distinctly lopsided. The velocity field is disturbed on the lopsided side of the disk but is fairly uniform in the other half. We derived the HI rotation curve for the galaxy from the velocity field. The rotation curve has a flat rotation speed of 190km s−1.Item The Balance of Dark and Luminous Mass in Rotating Galaxies(Copyright 2005 The American Physical Society, 2005) McGaugh, Stacy S.baryons to the total rotation velocity increases, the contribution of the dark matter decreases by a compensating amount. This poses a fine-tuning problem for CDM galaxy formation models, and may point to new physics for dark matter particles or even a modification of gravity.Item The pursuit of the whole NChilada: Virtual petaflops using multi-adaptive algorithms for gravitational systems(IBM, 2004) Lake, G.; Quinn, T.; Richardson, D. C.; Stadel, J.We describe the keys to meeting the challenges of N-body simulation: adaptive potential solvers, adaptive integration, and volume renormalization. With these techniques and a dedicated teraflop facility, simulation can keep pace with observations of the universe. We also describe some problems in simulating the formation and stability of planetary systems.Item Planetesimal clusters in a Keplerian disk I. gravitational evolution(EDP Sciences, 2002-08-30) Tanga, P.; Michel, P.; Richardson, D. C.It was recently demonstrated by numerical simulations that a turbulent flow in a rotating system is capable of efficiently concentrating passively advected particles having a density larger than the fluid – inside anti-cyclonic vortices. This process has important consequences on the distribution of solid particles in protoplanetary disks, since dust surface densities 1–2 orders of magnitude higher than the background are rapidly reached in vortex cores. However, until now, the role of self-gravitation of captured solids has been neglected. In this work we study the action of mutual gravitational interactions - after the gas has dissipated - over the dynamics of planetesimals inside clusters similar to those created in vortex cores. A comparison is made between the behavior of idealized clusters of planetesimals characterized by ad-hoc velocity profiles, and more complex initial conditions such as those obtained in previous hydrodynamical simulations. We show here that, within the explored interval of parameters, mutual scattering of particles can quickly disperse the cluster. Our results are demonstrated to be not dependent on the resolution employed. It can be concluded that if large planetesimals were formed inside vortex cores, they would be ejected by mutual perturbations.Item Gravitational instability and clustering in a disk of planetesimals(EDP Sciences, 2004-08-05) Tanga, P.; Weidenschilling, S. J.; Michel, P.; Richardson, D. C.For a long time, gravitational instability in the disk of planetesimals has been suspected to be the main engine responsible for the beginning of dust growth, its advantage being that it provides for rapid growth. Its real importance in planetary formation is still debated, mainly because the potential presence of turbulence can prevent the settling of particles into a gravitationally unstable layer. However, several mechanisms could yield strongly inhomogeneous distributions of solids in the disk: radial drift, trapping in vortices, perturbations by other massive bodies, etc. In this paper we present a numerical study of a gravitationally unstable layer. This allows us to go beyond the classical analytical study of linear perturbations, exploring a highly non-linear regime. A hierarchical growth of structure in the presence of dissipation (gas drag) can yield large, virialized clusters of planetesimals, the first time such clusters have been observed in the context of planetesimal disks.Item Nonequilibrium Phenomena in the Magnetosphere: Phase Transition, Self-organized Criticality and Turbulence(Springer, 2005) Sharma, A. Surjalal; Baker, Daniel N.; Borovsky, Joseph E.The magnetosphere is a large scale natural system powered by the solar wind that exhibits many nonequilibrium phenomena. A wide range of these phenomena are driven directly by the solar wind or arise from the storage-release processes internal to the magnetosphere. Under the influnce by the turbulent solar wind, the magnetosphere during geomagnetically active periods is far from equilibrium and storms and substorms are essentially non-equilibrium phenomena. In spite of the distributed nature of the physical processes and the apparent irregular behavior, there is a remarkable coherence in the magnetospheric response during substorms and the entire magnetosphere behaves as a global dynamical system. Alongwith the global features, the magnetosphere exhibits many multi-scale and intermittent characteristics. These features of the magnetosphere have been studied in terms of phase transitions, self-organized criticality and turbulence. In the phase transition scenario the global features are modeled as first-order transitions and the multi-scale behavior is interpreted as a manifestation of the scale-free nature of criticality in second order phase transitions. In the self-organized criticality framework substorms are considered as avalanches in the system when criticality is reached. Many features of the magnetosphere, in particular the power law dependence of scale sizes, can be viewed as a feature of a turbulent system.The common theme underlying these approaches is the recognition that the nonequilibrium phenomena in the magnetosphere could be understood in terms of processes generic to such systems. In many cases the power-law behavior of the magnetosphere seen in many observations is the starting point for these studies. This chapter is an overview of the recent understanding achieved using these different approaches, and identifies the common issues and differences.Item Magnetospheric Multiscale Mission:Cross-scale Exploration of Complexity in the Magnetosphere(Springer, 2005-01) Sharma, A. Surjalal; Curtis, Steven A.The physical processes in the magnetosphere span a wide range of space and time scales and due to the strong cross-scale coupling among them the fundamental processes at the smallest scales are critical to the large scale processes. For example, many key features of magnetic reconnection and particle acceleration are initiated at the smallest scales, typically the ion gyro-radii, and then couples to meso-scale and macro-scale processes, such as plasmoid formation. The Magnetospheric Muliscale (MMS) mission is a multi spacecraft mission dedicated to the study of plasma physics at the smallest scales and their cross-scale coupling to global processes. Driven by the turbulent solar wind, the magnetosphere is far from equilibrium and exhibits complex behavior over many scales. The processes underlying the multi-scale and intermittent features in the magnetosphere are fundamental to sun-earth connection. Recent results from the four spacecraft Cluster and earlier missions have provided new insights into magnetospheric physics and will form the basis for comprehensive studies of the multi-dimensional properties of the plasma processes and their inter-relationships. MMS mission will focus on the boundary layers connecting the magnetospheric regions and provide detailed spatio-temporal data of processes such as magnetic reconnection, thin current sheets, turbulence and particle acceleration. The cross-scale exploration by MMS mission will target the microphysics that will enable the discovery of the chain of processes underlying sun-earth connection.