4,542 research outputs found
The Contribution of the First Stars to the Cosmic Infrared Background
We calculate the contribution to the cosmic infrared background from very
massive metal-free stars at high redshift. We explore two plausible
star-formation models and two limiting cases for the reprocessing of the
ionizing stellar emission. We find that Population III stars may contribute
significantly to the cosmic near-infrared background if the following
conditions are met: (i) The first stars were massive, with M > ~100 M_sun. (ii)
Molecular hydrogen can cool baryons in low-mass haloes. (iii) Pop III star
formation is ongoing, and not shut off through negative feedback effects. (iv)
Virialized haloes form stars at about 40 per cent efficiency up to the redshift
of reionization, z~7. (v) The escape fraction of the ionizing radiation into
the intergalactic medium is small. (vi) Nearly all of the stars end up in
massive black holes without contributing to the metal enrichment of the
Universe.Comment: 11 pages, 6 figures, expanded discussion, added mid-IR to Fig 6,
MNRAS in pres
Orientation TuningāA Crooked Path to the Straight and Narrow
AbstractNeurons in visual cortex are selective for the orientation of a visual stimulus, while the receptive fields of their thalamic input are circular. Cortical orientation selectivity arises from the organization of both thalamic input and local cortical circuits. In this issue of Neuron, Schummers and colleagues provide evidence that the local circuit mechanisms contributing to orientation selectivity differ depending on the local organization of the orientation map
The ecosystem and evolutionary contexts of allelopathy
Plants can release chemicals into the environment that suppress the growth and establishment of other plants in their vicinity, a process known as āallelopathyā. However, chemicals with allelopathic functions have other ecological roles, such as plant defense, nutrient chelation, and regulation of soil biota in ways that affect decomposition and soil fertility. These ecosystem-scale roles of allelopathic chemicals can augment, attenuate or modify their community-scale functions. In this review we explore allelopathy in the context of ecosystem properties, and through its role in exotic invasions consider how evolution might affect the intensity and importance of allelopathic interactions
Novel Weapons: Invasive Success and the Evolution of Increased Competitive Ability
When introduced to new habitats by humans, some plant species become much more dominant. This is primarily attributed to escape from specialist consumers. Release from these specialist enemies is also thought by some to lead to the evolution of increased competitive ability, driven by a decrease in the plant\u27s resource allocation to consumer defense and an increase in allocation to size or fecundity. Here, we discuss a new theory for invasive success ā the ānovel weapons hypothesisā. We propose that some invaders transform because they possess novel biochemical weapons that function as unusually powerful allelopathic agents, or as mediators of new plantāsoil microbial interactions. Root exudates that are relatively ineffective against their natural neighbors because of adaptation, may be highly inhibitory to newly encountered plants in invaded communities. In other words, the novel weapons of some plant invaders provide them with an advantage that may arise from differences in the regional coevolutionary trajectories of plant communities. Furthermore, the selective advantage of possessing a novel weapon may result in rapid evolution of that weapon ā for example, the production of greater quantities of allelopathic or antimicrobial root exudates. Direct selection of competitive traits provides an alternative to the āgrow versus defendā trade-offs that underpin the theory of the evolution of increased competitive ability. Read More: http://www.esajournals.org/doi/abs/10.1890/1540-9295(2004)002%5B0436%3ANWISAT%5D2.0.CO%3B
Francis Crick's Legacy for Neuroscience: Between the Ī± and the Ī©
The legacy of Francis Crick is explored by two scientists who were influenced by his wor
Phase Diagrams of Bi1-xSbx Thin Films with Different Growth Orientations
A closed-form model is developed to evaluate the band-edge shift caused by
quantum confinement for a two-dimensional non-parabolic carrier-pocket. Based
on this model, the symmetries and the band-shifts of different carrier-pockets
are evaluated for BiSb thin films that are grown along different crystalline
axes. The phase diagrams for the BiSb thin film systems with different growth
orientations are calculated and analyzed
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