Neutral Gas Distributions and Kinematics of Five Blue Compact Dwarf Galaxies

We present the results of high spatial resolution HI observations of five intrinsically compact dwarf galaxies which are currently experiencing a strong burst of star formation. The HI maps indicate that these systems have a complex and clumpy interstellar medium. Unlike typical dwarf irregular galaxies, these Blue Compact Dwarf (BCD) galaxies exhibit strong central concentrations in their neutral gas distributions which may provide a clue to the origin of their strong star-burst activity. Furthermore, while all of the systems do appear to be rotating, based on observed velocity gradients, the kinematics are complex. All systems have non-ordered kinematic structure at some level; some of the extended gas is not necessarily kinematically connected to the main system. The observed gas distributions and kinematics place constraints on evolutionary scenarios for BCDs. Evolutionary links between BCDs, dwarf irregulars, and dwarf ellipticals have been postulated to explain their high star formation rates and low luminosity, low metallicity nature. The BCDs appear to have higher central mass concentrations in both gas and stellar content than the dwarf irregulars, indicating that evolutionary scenarios connecting these two classes will require mass redistribution. In addition, the fact that BCDs are rotationally supported systems indicates that BCDs are unlikely to evolve into dwarf ellipticals without substantial loss of angular momentum. Thus, while such evolutionary scenarios may still be possible with the aid of mergers or tidal interactions, the isolated nature of BCDs suggests that the majority of BCDs will not fade to become objects similar to the present day dwarf ellipticals.Comment: 19 pages, 15 figures. To appear in A

Bistable hysteresis and resistance switching in hydrogen gold junctions

Current-voltage characteristics of H2-Au molecular junctions exhibit intriguing steps around a characteristic voltage of 40 mV. Surprisingly, we find that a hysteresis is connected to these steps with a typical time scale > 10 ms. This time constant scales linearly with the power dissipated in the junction beyond an ofset power P_s = IV_s. We propose that the hysteresis is related to vibrational heating of both the molecule in the junction and a set of surrounding hydrogen molecules. Remarkably, we can engineer our junctions such that the hysteresis' characteristic time becomes >days. We demonstrate that reliable switchable devices can be built from such junctions.Comment: Submitted to Phys. Rev. Let

Forcing Adsorption of a Tethered Polymer by Pulling

We present an analysis of a partially directed walk model of a polymer which at one end is tethered to a sticky surface and at the other end is subjected to a pulling force at fixed angle away from the point of tethering. Using the kernel method, we derive the full generating function for this model in two and three dimensions and obtain the respective phase diagrams. We observe adsorbed and desorbed phases with a thermodynamic phase transition in between. In the absence of a pulling force this model has a second-order thermal desorption transition which merely gets shifted by the presence of a lateral pulling force. On the other hand, if the pulling force contains a non-zero vertical component this transition becomes first-order. Strikingly, we find that if the angle between the pulling force and the surface is beneath a critical value, a sufficiently strong force will induce polymer adsorption, no matter how large the temperature of the system. Our findings are similar in two and three dimensions, an additional feature in three dimensions being the occurrence of a reentrance transition at constant pulling force for small temperature, which has been observed previously for this model in the presence of pure vertical pulling. Interestingly, the reentrance phenomenon vanishes under certain pulling angles, with details depending on how the three-dimensional polymer is modeled

Web assisted teaching: an undergraduate experience

The emergence of the Internet has created a number of claims as to the future of education and the possibility of dramatically changing the way in which education is delivered. Much of the attention has focussed on the adoption of teaching methods that are solely web-based. We set out to incorporate web-based teaching as support for more traditional teaching methods to improve the learning outcomes for students. This first step into web-based teaching was developed to harness the benefits of web-based teaching tools without supplanting traditional teaching methods. The aim of this paper is to report our experience with web-assisted teaching in two undergraduate courses, Accounting Information Systems and Management Accounting Services, during 2000. The paper evaluates the approach taken and proposes a tentative framework for developing future web-assisted teaching applications. We believe that web-assisted and web-based teaching are inevitable outcomes of the telecommunications and computer revolution and that academics cannot afford to become isolated from the on-line world. A considered approach is needed to ensure the integration of web-based features into the overall structure of a course. The components of the course material and the learning experiences students are exposed to need to be structured and delivered in a way that ensures they support student learning rather than replacing one form of learning with another. Therefore a careful consideration of the structure, content, level of detail and time of delivery needs to be integrated to create a course structure that provides a range of student learning experiences that are complimentary rather than competing. The feedback was positive from both extramural (distance) and internal students, demonstrating to us that web sites can be used as an effective teaching tool in support of more traditional teaching methods as well as a tool for distance education. The ability to harness the positives of the web in conjunction with more traditional teaching modes is one that should not be overlooked in the move to adopt web based instruction methods. Web-based teaching need not be seen as an all or nothing divide but can be used as a useful way of improving the range and type of learning experiences open to students. The Web challenges traditional methods and thinking but it also provides tools to develop innovative solutions to both distance and on campus learning. Further research is needed to determine how we can best meet the needs of our students while maintaining high quality learning outcomes

Igusa's p-adic local zeta function associated to a polynomial mapping and a polynomial integration measure

For p prime, we give an explicit formula for Igusa's local zeta function associated to a polynomial mapping f=(f_1,...,f_t): Q_p^n -> Q_p^t, with f_1,...,f_t in Z_p[x_1,...,x_n], and an integration measure on Z_p^n of the form |g(x)||dx|, with g another polynomial in Z_p[x_1,...,x_n]. We treat the special cases of a single polynomial and a monomial ideal separately. The formula is in terms of Newton polyhedra and will be valid for f and g sufficiently non-degenerated over F_p with respect to their Newton polyhedra. The formula is based on, and is a generalization of results of Denef - Hoornaert, Howald et al., and Veys - Zuniga-Galindo.Comment: 20 pages, 5 figures, 2 table

Helicity Amplitudes for Single-Top Production

Single top quark production at hadron colliders allows a direct measurement of the top quark charged current coupling. We present the complete tree-level helicity amplitudes for four processes involving the production and semileptonic decay of a single top quark: W-gluon fusion, flavor excitation, s-channel production and W-associated production. For the first three processes we study the quality of the narrow top width approximation. We also examine momentum and angular distributions of some of the final state particles.Comment: 27 pages, 7 figures, final versio

LeMoMaF: Lensed Mock Map Facility

We present the Lensed Mock Map Facility (LeMoMaF), a tool designed to perform mock weak lensing measurements on numerically simulated chunks of the universe. Coupling N-body simulations to a semi-analytical model of galaxy formation, LeMoMaF can create realistic lensed images and mock catalogues of galaxies, at wavelengths ranging from the UV to the submm. To demonstrate the power of such a tool we compute predictions of the source-lens clustering effect on the convergence statistics, and quantify the impact of weak lensing on galaxy counts in two different filters. We find that the source-lens clustering effect skews the probability density function of the convergence towards low values, with an intensity which strongly depends on the redshift distribution of galaxies. On the other hand, the degree of enhancement or depletion in galaxy counts due to weak lensing is independent of the source-lens clustering effect. We discuss the impact on the two-points shear statistics to be measured by future missions like SNAP and LSST. The source-lens clustering effect would bias the estimation of sigma_8 from two point statistics by 2% -5%. We conclude that accurate photometric redshifts for individual galaxies are necessary in order to quantify and isolate the source-lens clustering effect.Comment: 14 pages, 11 figures, submitted to MNRA

Scalable Task-Based Algorithm for Multiplication of Block-Rank-Sparse Matrices

A task-based formulation of Scalable Universal Matrix Multiplication Algorithm (SUMMA), a popular algorithm for matrix multiplication (MM), is applied to the multiplication of hierarchy-free, rank-structured matrices that appear in the domain of quantum chemistry (QC). The novel features of our formulation are: (1) concurrent scheduling of multiple SUMMA iterations, and (2) fine-grained task-based composition. These features make it tolerant of the load imbalance due to the irregular matrix structure and eliminate all artifactual sources of global synchronization.Scalability of iterative computation of square-root inverse of block-rank-sparse QC matrices is demonstrated; for full-rank (dense) matrices the performance of our SUMMA formulation usually exceeds that of the state-of-the-art dense MM implementations (ScaLAPACK and Cyclops Tensor Framework).Comment: 8 pages, 6 figures, accepted to IA3 2015. arXiv admin note: text overlap with arXiv:1504.0504

A simple model of a vesicle drop in a confined geometry

We present the exact solution of a two-dimensional directed walk model of a drop, or half vesicle, confined between two walls, and attached to one wall. This model is also a generalisation of a polymer model of steric stabilisation recently investigated. We explore the competition between a sticky potential on the two walls and the effect of a pressure-like term in the system. We show that a negative pressure ensures the drop/polymer is unaffected by confinement when the walls are a macroscopic distance apart