National Science Foundation

About: National Science Foundation is a government organization based out in Alexandria, Virginia, United States. It is known for research contribution in the topics: Galaxy & Population. The organization has 4729 authors who have published 6040 publications receiving 298150 citations. The organization is also known as: NSF & N.S.F..

The effect of magnetic fields on γ-ray bursts inferred from multi-wavelength observations of the burst of 23 January 1999

Abstract: Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.

Galaxy Populations and Evolution in Clusters. I. Dynamics and the Origin of Low-Mass Galaxies in the Virgo Cluster

Abstract: Early-type dwarfs are the most common galaxy in the local universe, yet their origin and evolution remain a mystery. Various cosmological scenarios predict that dwarf-like galaxies in dense areas are the first to form and hence should be the oldest stellar systems in clusters. By using radial velocities of early-type dwarfs in the Virgo cluster we demonstrate that these galaxies are not an old cluster population but have signatures of production from the infall of field galaxies. Evidence of this includes the combined large dispersions and substructure in spatial and kinematic distributions for Virgo early-type dwarfs and a velocity dispersion ratio with giant ellipticals expected for virialized and accreted populations. We also argue that these galaxies cannot originate from accreted field dwarfs, but must have physically evolved from a precursor population, of different morphology, that fell into Virgo some time in the past.

Why Do Only Some Galaxy Clusters Have Cool Cores

Abstract: Flux-limited X-ray samples indicate that about half of rich galaxy clusters have cool cores. Why do only some clusters have cool cores while others do not? In this paper, cosmological N-body + Eulerian hydrodynamic simulations, including radiative cooling and heating, are used to address this question as we examine the formation and evolution of cool core (CC) and noncool core (NCC) clusters. These adaptive mesh refinement simulations produce both CC and NCC clusters in the same volume. They have a peak resolution of 15.6 h−1 kpc within a (256 h−1 Mpc)3 box. Our simulations suggest that there are important evolutionary differences between CC clusters and their NCC counterparts. Many of the numerical CC clusters accreted mass more slowly over time and grew enhanced CCs via hierarchical mergers; when late major mergers occurred, the CCs survived the collisions. By contrast, NCC clusters experienced major mergers early in their evolution that destroyed embryonic CCs and produced conditions that prevented CC reformation. As a result, our simulations predict observationally testable distinctions in the properties of CC and NCC beyond the core regions in clusters. In particular, we find differences between CC versus NCC clusters in the shapes of X-ray surface brightness profiles, between the temperatures and hardness ratios beyond the cores, between the distribution of masses, and between their supercluster environs. It also appears that CC clusters are no closer to hydrostatic equilibrium than NCC clusters, an issue important for precision cosmology measurements.

Vision and change in biology undergraduate education, a call for action--initial responses.

Abstract: The executive summary provides an overview of some of V&C's key recommendations regarding next steps in the effort to mobilize the biology community. It is, in essence, a call for national service. A publication discussing these recommendations and action items in more depth will be available later this year. Meanwhile, we highly recommend reading the Executive Summary of V&C, the NAS report (NAS, 2010 ), and a seminal article by Labov et al. (2010) summarizing the synergy created by these several reports on the changing nature of studies in biology and concomitant need to change biology education. Then, take action! Our hope is to see the formation of a community of biologists, similar to that forming in geology (Manduca et al., 2010 ): one that will advance biology undergraduate education so it truly reflects the discipline it serves.

A chemical abundance study of 10 open clusters based on wiyn*-hydra spectroscopy

Abstract: We present a detailed chemical abundance study of evolved stars in 10 open clusters based on Hydra multi-object echelle spectra obtained with the WIYN 3.5 m telescope. From an analysis of both equivalent widths and spectrum synthesis, abundances have been determined for the elements Fe, Na, O, Mg, Si, Ca, Ti, Ni, Zr, and for two of the 10 clusters, Al and Cr. To our knowledge, this is the first detailed abundance analysis for clusters NGC 1245, NGC 2194, NGC 2355, and NGC 2425. These 10 clusters were selected for analysis because they span a Galactocentric distance range R gc ~ 9-13 kpc, the approximate location of the transition between the inner and outer disks. Combined with cluster samples from our previous work and those of other studies in the literature, we explore abundance trends as a function of cluster R gc, age, and [Fe/H]. As found previously by us and other studies, the [Fe/H] distribution appears to decrease with increasing R gc to a distance of ~12 kpc and then flattens to a roughly constant value in the outer disk. Cluster average element [X/Fe] ratios appear to be independent of R gc, although the picture for [O/Fe] is more complicated with a clear trend of [O/Fe] with [Fe/H] and sample incompleteness. Other than oxygen, no other element [X/Fe] exhibits a clear trend with [Fe/H]; likewise, there does not appear to be any strong correlation between abundance and cluster age. We divided clusters into different age bins to explore temporal variations in the radial element distributions. The radial metallicity gradient appears to have flattened slightly as a function of time, as found by other studies. There is also some indication that the transition from the inner disk metallicity gradient to the ~constant [Fe/H] distribution of the outer disk occurs at different Galactocentric radii for different age bins. However, interpretation of the time evolution of radial abundance distributions is complicated by the unequal R gc and [Fe/H] ranges spanned by clusters in different age bins.