Milgrom's revision of cosmic dynamics: Amending Newton's laws or Keplers?

Milgrom's recent revision of Newtonian dynamics was introduced to eliminate the inference that large quantities of invisible mass exist in galaxies. Simple examples show that a Milgrom acceleration, in the form presented so far, imply other far-reaching changes in dynamics. The momentum of an isolated system is not conserved, and the usual theorem for center-of-mass motion of any system does not hold. Naive applications require extreme caution. The model fails to provide a complete description of particle dynamics and should be thought of as a revision of Kepler's laws rather than Newton's. The Milgrom acceleration also implies fundamental changes in cosmology. A quasi-Newtonian calculation adapted from Newtonian cosmology suggests that a Milgrom universe will recollapse even if the classical closure parameter theta is less than 1. The solution, however, fails to satisfy the cosmological principle. Reasons for the breakdown of this calculation are examined. A theory of gravitation needed before the behavior of a Milgrom universe can be predicted

Radio electrons and magnetic fields in the galactic halo

Radiating electron density in galactic halo and cosmic ray dat

On the detectability of infrared echo arcs around supernova 1987A

The ring-like interstellar visual echoes of radii 33 and 54 arcsec detected around SN 1987A should coincide with infrared echoes (thermal reradiation) from dust at T approximately equal to 15 to 30 K. Whether these infrared echoes are detectable at present is considered. They will be brightest at approximately 100 microns, the range of the Texas infrared photometer. Detectability depends on the ratio zeta congruent to tan(sub a)/tan(sub s)P(theta), where tan(sub a) and tan(sub s) are the visual absorption and scattering optical thicknesses of the echo layer, and P is the phase function function for small-angle scattering (theta approximately equal to 2 to 4 degrees). Zeta approximately greater than 1 is needed for a detectable signal (approximately 0.3 Jy), but zeta cannot be much less than 1; otherwise the visual echoes could not be as bright as they are. Typical dust mixtures of Mathis-Rumpl-Nordsieck type have zeta much less than 1. Zeta remains small even if a population of very small grains with power-law index as steep as approximately 5.5 is added. A population with even more small grains and/or fewer large grains could have a zeta similar to 1 and be detectable at present, but this seems unlikely. The echoes will move, but should remain accessible for many years and should be detected eventually

Coatings for directional eutectics

Eleven coating systems based on MCrAlY overlay and diffusion aluminide prototypes were evaluated to determine their capability for protecting the gamma/gamma prime-delta directionally solidified eutectic alloy (Ni-20Cb-6Cr-2.5Al) in gas turbine engine applications. Furnace oxidation and hot corrosion, Mach 0.37 burner-rig, tensile ductility, stress-rupture and thermomechanical fatigue tests were used to evaluate the coated gamma/gamma prime-delta alloy. The diffusion aluminide coatings provided adequate oxidation resistance at 1144 K (1600 F) but offered very limited protection in 114 K (1600 F) hot corrosion and 1366 K (2000 F) oxidation tests. A platinum modified NiCrAlY overlay coating exhibited excellent performance in oxidation testing and had no adverse effects upon the eutectic alloy

Nonnucleosynthetic Constraints on the Baryon Density and Other Cosmological Parameters

Because the baryon-to-photon ratio η_10 is in some doubt, we drop nucleosynthetic constraints on η_10 and fit the three cosmological parameters (h, Ω_M, η_10) to four observational constraints: Hubble parameter h_o = 0.70 ± 0.15, age of the universe t_0 =14^+7_-2 Gyr, cluster gas fraction f_o ≡ f_G h^3/2 = 0.060 ± 0.006, and effective shape parameter Γ_o = 0.255 ± 0.017. Errors quoted are 1 σ, and we assume Gaussian statistics. We experiment with a fifth constraint Ω_o = 0.2 ± 0.1 from clusters. We set the tilt parameter n = 1 and the gas enhancement factor Upsilon = 0.9. We consider cold dark matter models (open and Ω_M = 1) and flat ΛCDM models. We omit HCDM models (to which the Γ_o constraint does not apply). We test goodness of fit and draw confidence regions by the Δχ^2 method. CDM models with Ω_M = 1 (SCDM models) are accepted only because the large error on h_o allows h 0.4. The combination of the four other constraints with Ωo approx 0.2 is rejected in CDM models with 98% confidence, suggesting that light may not trace mass. ΛCDM models give similar results. In all of these models, η_10 ~> 6 is favored strongly over η_10 ~< 2. This suggests that reports of low deuterium abundances on QSO lines of sight may be correct and that observational determinations of primordial ^4He may have systematic errors. Plausible variations on n and Upsilon in our models do not change the results much. If we drop or change the crucial Γ_o constraint, lower values of Ω_M and η_10 are permitted. The constraint Γ_o = 0.15 ± 0.04, derived recently from the IRAS redshift survey, favors Ω_M approx 0.3 and η_10 approx 5 but does not exclude η_10 approx 2

The Luminosity Distribution of Local Group Galaxies

From a rediscussion of Local Group membership, and of distances to individual galaxies, we obtain $M_V$ values for 35 probable and possible Local Group members. The luminosity function of these objects is well fitted by a Schechter function with faint end slope $\alpha = -1.1 \pm 0.1$. The probability that the luminosity distribution of the Local Group is a single Schechter function with $\alpha$ steeper than -1.3 is less than 1 per cent. However, more complicated luminosity functions, such as multi-component Schechter functions with steep faint-end slopes, cannot be ruled out. There is some evidence that the luminosity distribution of dwarf spheroidal galaxies in the Local Group is steeper than that of dwarf irregular galaxies.Comment: 13 pages, 2 figures, accepted for publication in The Astronomical Journal. Figure 2 replaced, conclusion based on this figure change

The Seyfert Population in the Local Universe

The magnitude-limited catalog of the Southern Sky Redshift Survey (SSRS2), is used to characterize the properties of galaxies hosting Active Galactic Nuclei. Using emission-line ratios, we identify a total of 162 (3%) Seyfert galaxies out of the parent sample with 5399 galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1. The SSRS2 Seyfert galaxies are predominantly in spirals of types Sb and earlier, or in galaxies with perturbed appearance as the result of strong interactions or mergers. Seyfert galaxies in this sample are twice as common in barred hosts than the non-Seyferts. By assigning galaxies to groups using a percolation algorithm we find that the Seyfert galaxies in the SSRS2 are more likely to be found in binary systems, when compared to galaxies in the SSRS2 parent sample. However, there is no statistically significant difference between the Seyfert and SSRS2 parent sample when systems with more than 2 galaxies are considered. The analysis of the present sample suggests that there is a stronger correlation between the presence of the AGN phenomenon with internal properties of galaxies (morphology, presence of bar, luminosity) than with environmental effects (local galaxy density, group velocity dispersion, nearest neighbor distance).Comment: 35 pages, 13 figures, Accepted to be publised in Astronomical Journa

Toward theories of partnership praxis: an analysis of interpretive framing in literature on students as partners in teaching and learning

A body of literature on students as partners (SaP) in higher education has emerged over the last decade that documents, shares, and evaluates SaP approaches. As is typical in emerging fields of inquiry, scholars differ regarding how they see the relationship between the developments in SaP practices and the theoretical explanations that guide, illuminate, and situate such practices. In this article we explore the relationship between theory and practice in SaP work through an analysis of interpretive framing employed in scholarship of SaP in teaching and learning in higher education. Through a conceptual review of selected publications, we describe three ways of framing partnership that represent distinct but related analytical approaches: building on concepts; drawing on constructs; and imagining through metaphors. We both affirm the expansive and creative theorising in scholarship of SaP in university teaching and learning and encourage further deliberate use and thoughtful development of interpretive framings that take seriously the disruptive ethos and messy human relational processes of partnership. We argue that these developmental processes move us toward formulating theories of partnership praxis

Squeezing MOND into a Cosmological Scenario

Explaining the effects of dark matter using modified gravitational dynamics (MOND) has for decades been both an intriguing and controversial possibility. By insisting that the gravitational interaction that accounts for the Newtonian force also drives cosmic expansion, one may kinematically identify which cosmologies are compatible with MOND, without explicit reference to the underlying theory so long as the theory obeys Birkhoff's law. Using this technique, we are able to self-consistently compute a number of quantities of cosmological interest. We find that the critical acceleration a_0 must have a slight source-mass dependence (a_0 ~ M^(1/3)) and that MOND cosmologies are naturally compatible with observed late-time expansion history and the contemporary cosmic acceleration. However, cosmologies that can produce enough density perturbations to account for structure formation are contrived and fine-tuned. Even then, they may be marginally ruled out by evidence of early (z \~ 20) reionization.Comment: 11 pages revtex, 2 figure