Supernova Feedback Keeps Galaxies Simple

Galaxies evolve continuously under the influence of self-gravity, rotation, accretion, mergers and feedback. The currently favored cold dark matter cosmological framework, suggests a hierarchical process of galaxy formation, wherein the present properties of galaxies are decided by their individual histories of being assembled from smaller pieces. However, recent studies have uncovered surprising correlations among the properties of galaxies, to the extent of forming a one-parameter set lying on a single fundamental line. It has been argued in the literature that such simplicity is hard to explain within the paradigm of hierarchical galaxy mergers. One of the puzzling results, is the simple linear correlation between the neutral hydrogen mass and the surface area, implying that widely different galaxies share very similar neutral hydrogen surface densities. In this work we show that self-regulated star formation, driven by the competition between gravitational instabilities and mechanical feedback from supernovae, can explain the nearly constant neutral hydrogen surface density across galaxies. We therefore recover the simple scaling relation observed between the neutral hydrogen mass and surface area. This result furthers our understanding of the surprising simplicity in the observed properties of diverse galaxies.Comment: 5 pages, 1 figure, accepted for publication in Ap

Verification of the Rayleigh Scattering Cross Section

A simple experiment is described for the direct determination of the wavelength dependence of the Rayleigh scattering cross section using the classical example of the blue sky.Comment: 3 pages, 2 figures, submitted to AJ

Proving Abstractions of Dynamical Systems through Numerical Simulations

A key question that arises in rigorous analysis of cyberphysical systems under attack involves establishing whether or not the attacked system deviates significantly from the ideal allowed behavior. This is the problem of deciding whether or not the ideal system is an abstraction of the attacked system. A quantitative variation of this question can capture how much the attacked system deviates from the ideal. Thus, algorithms for deciding abstraction relations can help measure the effect of attacks on cyberphysical systems and to develop attack detection strategies. In this paper, we present a decision procedure for proving that one nonlinear dynamical system is a quantitative abstraction of another. Directly computing the reach sets of these nonlinear systems are undecidable in general and reach set over-approximations do not give a direct way for proving abstraction. Our procedure uses (possibly inaccurate) numerical simulations and a model annotation to compute tight approximations of the observable behaviors of the system and then uses these approximations to decide on abstraction. We show that the procedure is sound and that it is guaranteed to terminate under reasonable robustness assumptions

Image Registration Techniques: A Survey

Image Registration is the process of aligning two or more images of the same scene with reference to a particular image. The images are captured from various sensors at different times and at multiple view-points. Thus to get a better picture of any change of a scene or object over a considerable period of time image registration is important. Image registration finds application in medical sciences, remote sensing and in computer vision. This paper presents a detailed review of several approaches which are classified accordingly along with their contributions and drawbacks. The main steps of an image registration procedure are also discussed. Different performance measures are presented that determine the registration quality and accuracy. The scope for the future research are presented as well

Dynamic Heckscher-Ohlin Results from a 2x2x2x2 Overlapping Generations Model with Unequal Population Growth Rates

This paper considers a two-country world where the population in one country grows faster than the other, and investigates the implications of the addition of non-stationary population dynamics to a simple 2- commodity, 2-factor model of international trade within an overlapping- generations framework. The two countries in the world considered are assumed to be identical in every respect except, for their population growth rates initially. The effects of differential speed of population growth on relative factor endowments and patterns of international trade are explored by comparing simulation results obtained from the overlapping-generations general equilibrium model under autarky and trade scenarios. Unequal population growth rates are shown to give rise to differentials in wage rates and rentals for capital under autarky conditions. This, in turn, causes costs of production and relative prices to differ, creating the grounds for trade in the sense of Heckscher-Ohlin (HO). Yet, the results from simulation exercises indicate that static welfare results from the standard 2x2x2 HO model can not be generalized to hold in a dynamic setting with overlapping generations of individuals.Unequal population growth rates, Heckscher-Ohlin model, international trade, overlapping-generations