Escape of Ionizing Radiation from High Redshift Galaxies

We model the escape of ionizing radiation from high-redshift galaxies using high-resolution Adaptive Mesh Refinement N-body + hydrodynamics simulations. Our simulations include time-dependent and spatially-resolved transfer of ionizing radiation in three dimensions, including effects of dust absorption. For galaxies of total mass M > 10^11 Msun and star formation rates SFR ~ 1-5 Msun/yr, we find angular averaged escape fractions of 0.01-0.03 over the entire redshift interval studied (3<z<9). In addition, we find that the escape fraction varies by more than an order of magnitude along different lines-of-sight within individual galaxies, from the largest values near galactic poles to the smallest along the galactic disk. The escape fraction declines steeply at lower masses and SFR. We show that the low values of escape fractions are due to a small fraction of young stars located just outside the edge of HI disk. We compare our predicted escape fraction of ionizing photons with previous results, and find a general agreement with both other simulation results and available direct detection measurements at z ~ 3. We also compare our simulations with a novel method to estimate the escape fraction in galaxies from the observed distribution of neutral hydrogen column densities along the lines of sights to long duration gamma-ray bursts. Using this method we find escape fractions of the GRB host galaxies of 2-3%, consistent with our theoretical predictions. [abridged]Comment: submitted to Ap

Progressor: Social navigation support through open social student modeling

The increased volumes of online learning content have produced two problems: how to help students to find the most appropriate resources and how to engage them in using these resources. Personalized and social learning have been suggested as potential ways to address these problems. Our work presented in this paper combines the ideas of personalized and social learning in the context of educational hypermedia. We introduce Progressor, an innovative Web-based tool based on the concepts of social navigation and open student modeling that helps students to find the most relevant resources in a large collection of parameterized self-assessment questions on Java programming. We have evaluated Progressor in a semester-long classroom study, the results of which are presented in this paper. The study confirmed the impact of personalized social navigation support provided by the system in the target context. The interface encouraged students to explore more topics attempting more questions and achieving higher success rates in answering them. A deeper analysis of the social navigation support mechanism revealed that the top students successfully led the way to discovering most relevant resources by creating clear pathways for weaker students. © 2013 Taylor and Francis Group, LLC

The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems. III. Hubble Space Telescope Imaging of \lya-Absorbing Galaxies at z < 1

We present initial results of a program to obtain and analyze HST WFPC2 images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. We measure properties of 87 galaxies, of which 33 are associated with corresponding \lya absorption systems and 24 do not produce corresponding \lya absorption lines to within sensitive upper limits. Considering only galaxy and absorber pairs that are likely to be physically associated and excluding galaxy and absorber pairs within 3000 \kms of the background QSOs leaves 26 galaxy and absorber pairs and seven galaxies that do not produce corresponding \lya absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations of the galaxy and absorber pairs range from 12.4 to $157.4 h^{-1}$ kpc with a median of $62.4 h^{-1}$ kpc. The primary result of the analysis is that the amount of gas encountered along the line of sight depends on the galaxy impact parameter and B-band luminosity but does not depend strongly on the galaxy average surface brightness, disk-to-bulge ratio, or redshift. This result confirms and improves upon the anti-correlation between \lya absorption equivalent width and galaxy impact parameter found previously by Lanzetta et al. (1995). There is no evidence that galaxy interactions play an important role in distributing tenuous gas around galaxies in most cases. Galaxies might account for all \lya absorption systems with $W > 0.3$ \AA, but this depends on the unknown luminosity function and gaseous cross sections of low-luminosity galaxies as well as on the uncertainties of the observed number density of \lya absorption systems.Comment: Minor changes. Figure 1 stays intact and is available at ftp://ftp.ess.sunysb.edu/pub/lanzetta/wfpc

Near-IR studies of recurrent nova V745 Scorpii during its 2014 outburst

The recurrent nova (RN) V745 Scorpii underwent its third known outburst on 2014 February 6. Infrared monitoring of the eruption on an almost daily basis, starting from 1.3d after discovery, shows the emergence of a powerful blast wave generated by the high velocity nova ejecta exceeding 4000 kms$^{-1}$ plowing into its surrounding environment. The temperature of the shocked gas is raised to a high value exceeding 10$^{8}$K immediately after outburst commencement. The energetics of the outburst clearly surpass those of similar symbiotic systems like RS Oph and V407 Cyg which have giant secondaries. The shock does not show a free-expansion stage but rather shows a decelerative Sedov-Taylor phase from the beginning. Such strong shockfronts are known to be sites for $\gamma$ ray generation. V745 Sco is the latest nova, apart from five other known novae, to show $\gamma$ ray emission. It may be an important testbed to resolve the crucial question whether all novae are generically $\gamma$ ray emitters by virtue of having a circumbinary reservoir of material that is shocked by the ejecta rather than $\gamma$ ray generation being restricted to only symbiotic systems with a shocked red giant (RG) wind. The lack of a free-expansion stage favors V745 Sco to have a density enhancement around the white dwarf (WD), above that contributed by a RG wind. Our analysis also suggests that the WD in V745 Sco is very massive and a potential progenitor for a future SN Ia explosion.Comment: To appear in ApJ (Letters

Coherent-State Approach to Two-dimensional Electron Magnetism

We study in this paper the possible occurrence of orbital magnetim for two-dimensional electrons confined by a harmonic potential in various regimes of temperature and magnetic field. Standard coherent state families are used for calculating symbols of various involved observables like thermodynamical potential, magnetic moment, or spatialdistribution of current. Their expressions are given in a closed form and the resulting Berezin-Lieb inequalities provide a straightforward way to study magnetism in various limit regimes. In particular, we predict a paramagnetic behaviour in the thermodynamical limit as well as in the quasiclassical limit under a weak field. Eventually, we obtain an exact expression for the magnetic moment which yields a full description of the phase diagram of the magnetization.Comment: 21 pages, 6 figures, submitted to PR

Effect of interactions between a hydrophobically modified polymer and phospholipids in the rheological and frictional properties of complex gels

O/w emulsions with polymers and phospholipids are widely used for personal care products due to their thickening and emulsifying properties. However, the underlying mechanism through which the emulsion components affect rheological and tribological characteristics is not well understood. We investigate simplified systems containing a hydrophobically modified polymer and phospholipids and find that bulk rheology and interaction between components of the studied systems play a role in frictional behavior. We characterize the tribological behavior using a soft model contact consisting of polydimethylsiloxane (PDMS) on a ball-on-disk tribometer. In addition, we investigate the bulk rheology using dynamic and steady shear experiments. Rheological behavior of systems containing the hydrophobically modified polymer is consistent with concentrated microgel systems. For systems containing phospholipids and polymer, the elastic modulus shows to a two-fold increase when compared to systems without phospholipids, leading us to hypothesize that hydrophobic interactions are occurring between these two components. To further investigate these interactions, we performed Isothermal Titration Calorimetry (ITC) experiments. The results are compared to experiments with a similar polymer that does not contain the hydrophobic moiety. No interactions are observed in the latter lending credence to our notion that interactions occur only between the hydrophobically modified polymer and the phospholipids when they are present in the same system. Tribological experiments show that in the elastohydrodynamic lubrication regime, which occurs at higher speeds where the contacts are fully separated by a lubricating film, the friction coefficients increase with increasing sample viscosity. Systems containing the polymer and phospholipids show a lower friction coefficient than Newtonian fluids at the boundary regime, where the contacting asperities dictate the frictional behavior. In order to understand this behavior, friction measurements over time at a fixed low speed and adsorption studies with quartz crystal microbalance (QCM) were performed. The tribological measurements show that the friction coefficient decreases as the concentration of hydrophobically modified polymer increases. The same behavior is observed with phospholipids. In addition, QCM results show that phospholipids are being adsorbed onto the PDMS surface and their hydrated heads are causing a decrease in the friction coefficients at low entrainment speeds due to a hydration-lubrication mechanism. These results indicate that the type of polymer used and their interactions with other components present in the system strongly affect their frictional and rheological behavior, and thus, the final performance of a personal care product containing these components

Load and resistance factor design of cold-formed steel: Calibration of the AISI design provisions

In the design of steel buildings, the Allowable Stress Criteria have long been used for the design of cold-formed steel structural members in the United States and other countries.\u27 In view of the fact that the mathematical theory of probability, which has been so successfully applied in other fields of engineering, would seem to be equally applicable to cold-formed steel design by providing a more uniform degree of structural safety, the Limit State Design method based on the probabilistic concept has been used in Canada and Europe for the design of cold-formed steel structural members.\u27 In the United States, a research project on Load and Resistance Factor Design of Gold-Formed Steel Members was condudted by Rang, Supornsilaphachai, Snyder, Pan, Galambos, and Yu during the period from 1979 through 1985. The tentative load and resistance factor design (LRFD) criteria for cold-formed structural members were proposed in the Seventh Progress Report\u27 on the basis of the 1980 Edition of the AISI allowable stress design specification. In 1986, a major revision was made in the AISI Specification to reflect the results of recent research projects and improvements in design techniques. Consequently, the tentative LRFD criteria proposed in the Seventh Progress Report were revised in 1987. This progress report contains the calibrations of the AISI design provisions included in the 1986 Specification. The procedures used for calibration are summarized in Article II. Articles Ill through XI deal with (a) bending strength, (b) web crippling of beams, (c) combined bending and web crippling, (d) concentrically loaded compression members, (e) combined axial load and bending, (f) welded connections, (g) bolted connections, (h) stiffeners, and (i) wall studs and wall stud assemblies. The results of these calibrations will be used in the revision of the proposed LRFD Specification for cold-formed steel structural members

Load and resistance factor design of cold-formed steel load and resistance factor design specification for cold-formed steel structural members with commentary

FOREWORD This progress report contains the following two parts: Part I: Load and Resistance Factor Design Specification for Cold-Formed Steel Structural Members (pp. i-107). Part II: Commentary on the Load and Resistance Factor Design Specification for Cold-Formed Steel Structural Members (pp. 109-161). The load and resistance factor design specification proposed herein is the revised version of the design recommendations prepared in February 1988 and submitted to American Iron and Steel Institute as Tenth Progress Report. This document was prepared according to the 1986 edition of the AISI Specification for the Design of Cold-Formed Steel Structural Members. The selections of ϕ factors are discussed in the Commentary for various types of structural members and connections. This investigation was sponsored by American Iron and Steel Institute. The technical guidance provided by the AISI Subcommitte on Load and Resistance Factor Design and the AISI Staff is gratefully acknowledged. Members of the AISI Subcommitte are: K. H. Klippstein (Chairman), R. Bjorhovde, D. S. Ellifritt, S. J. Errera, T. V. Galambos, B. Hall, D. H. Hall, R. B. Heagler, N. Iwankiw, A. L. Johnson, D. L. Johnson, A. C. Kuentz, A. S. Nowak, T. B. Pekoz, C. W. Pinkham, R. M. Schuster, and W. W. Yu. Former members of theAISI Task Group on LRFD included R. L. Cary, N. C. Lind, R. B. Matlock, W. Mueller, F. J. Phillips, D. S. Wolford and Late Professor G. Winter. Special thanks are extended to T. V. Galambos, Consultant of the project, T. N. Rang, B. Supornsilaphachai, B. K. Snyder, L. C. Pan, and M.K. Ravindra for their contributions to the project