573 research outputs found
The diverse hot gas content and dynamics of optically similar low-mass elliptical galaxies
The presence of hot X-ray emitting gas is ubiquitous in massive early-type
galaxies. However, much less is known about the content and physical status of
the hot X-ray gas in low-mass ellipticals. In the present paper we study the
X-ray gas content of four low-mass elliptical galaxies using archival Chandra
X-ray observations. The sample galaxies, NGC821, NGC3379, NGC4278, and NGC4697,
have approximately identical K-band luminosities, and hence stellar masses, yet
their X-ray appearance is strikingly different. We conclude that the unresolved
emission in NGC821 and NGC3379 is built up from a multitude of faint compact
objects, such as coronally active binaries and cataclysmic variables. Despite
the non-detection of X-ray gas, these galaxies may host low density, and hence
low luminosity, X-ray gas components, which undergo a Type Ia supernova (SN Ia)
driven outflow. We detect hot X-ray gas with a temperature of kT ~ 0.35 keV in
NGC4278, the component of which has a steeper surface brightness distribution
than the stellar light. Within the central 50 arcsec (~3.9 kpc) the estimated
gas mass is ~3 x 10^7 M_sun, implying a gas mass fraction of ~0.06%. We
demonstrate that the X-ray gas exhibits a bipolar morphology in the
northeast-southwest direction, indicating that it may be outflowing from the
galaxy. The mass and energy budget of the outflow can be maintained by evolved
stars and SNe Ia, respectively. The X-ray gas in NGC4697 has an average
temperature of kT ~ 0.3 keV, and a significantly broader distribution than the
stellar light. The total gas mass within 90 arcsec (~5.1 kpc) is ~2.1 x 10^8
M_sun, hence the gas mass fraction is ~0.4%. Based on the distribution and
physical parameters of the X-ray gas, we conclude that it is most likely in
hydrostatic equilibrium, although a subsonic outflow may be present.Comment: 14 pages, 8 figures, 3 tables, accepted for publication in Ap
Risk Factors Associated with the Severity of Thyroid-Associated Orbitopathy in Korean Patients
Risk Factors Associated with the Severity of Thyroid-Associated Orbitopathy in Korean Patientsope
Seasonality constraints to livestock grazing intensity
Increasing food production is essential to meet the future food demand of a growing world population. In the light of pressing sustainability challenges like climate change and the importance of the global livestock system for food security as well as GHG emissions, finding ways to increasing food production sustainably and without increasing competition for food crops is essential. Yet, many unknowns relate to livestock grazing, in particular grazing intensity, an essential variable to assess the sustainability of livestock systems. Here we explore ecological limits to grazing intensity (GI; i.e., the fraction of Net Primary Production consumed by grazing animals) by analysing the role of seasonality in natural grasslands. We estimate seasonal limitations to GI by combining monthly Net Primary Production data and a map of global livestock distribution with assumptions on the length of non-favourable periods that can be bridged by livestock (e.g., by browsing dead standing biomass, storage systems or biomass conservation). This allows us to derive a seasonality-limited potential GI, which we compare with the GI prevailing in 2000. We find that GI in 2000 lies below its potential on 39% of the total global natural grasslands, which has a potential for increasing biomass extraction of up to 181 MtC/yr. In contrast, on 61% of the area GI exceeds the potential, made possible by management. Mobilizing this potential could increase milk production by 5%, meat production by 4%, or contribute to free up to 2.8 Mio kmÂČ of grassland area at the global scale if the numerous socio-ecological constraints can be overcome. We discuss socio-ecological trade-offs, which may reduce the estimated potential considerably and require the establishment of sound monitoring systems and an improved understanding of livestock systemâs role in the Earth system
Widespread forest vertebrate extinctions induced by a mega hydroelectric dam in lowland Amazonia
Mega hydropower projects in tropical forests pose a major emergent threat to terrestrial and freshwater biodiversity worldwide. Despite the unprecedented number of existing, underconstruction and planned hydroelectric dams in lowland tropical forests, long-term effects on biodiversity have yet to be evaluated. We examine how medium and large-bodied assemblages of terrestrial and arboreal vertebrates (including 35 mammal, bird and tortoise species) responded to the drastic 26-year post-isolation history of archipelagic alteration in landscape structure and habitat quality in a major hydroelectric reservoir of Central Amazonia. The Balbina Hydroelectric Dam inundated 3,129 km2 of primary forests, simultaneously isolating 3,546 land-bridge islands. We conducted intensive biodiversity surveys at 37 of those islands and three adjacent continuous forests using a combination of four survey techniques, and detected strong forest habitat area effects in explaining patterns of vertebrate extinction. Beyond clear area effects, edge-mediated surface fire disturbance was the most important additional driver of species loss, particularly in islands smaller than 10 ha. Based on species-area models, we predict that only 0.7% of all islands now harbor a species-rich vertebrate assemblage consisting of â„80% of all species. We highlight the colossal erosion in vertebrate diversity driven by a man-made dam and show that the biodiversity impacts of mega dams in lowland tropical forest regions have been severely overlooked. The geopolitical strategy to deploy many more large hydropower infrastructure projects in regions like lowland Amazonia should be urgently reassessed, and we strongly advise that long-term biodiversity impacts should be explicitly included in pre-approval environmental impact assessments
Is it the boundaries or disorder that dominates electron transport in semiconductor `billiards'?
Semiconductor billiards are often considered as ideal systems for studying
dynamical chaos in the quantum mechanical limit. In the traditional picture,
once the electron's mean free path, as determined by the mobility, becomes
larger than the device, disorder is negligible and electron trajectories are
shaped by specular reflection from the billiard walls alone. Experimental
insight into the electron dynamics is normally obtained by magnetoconductance
measurements. A number of recent experimental studies have shown these
measurements to be largely independent of the billiards exact shape, and highly
dependent on sample-to-sample variations in disorder. In this paper, we discuss
these more recent findings within the full historical context of work on
semiconductor billiards, and offer strong evidence that small-angle scattering
at the sub-100 nm length-scale dominates transport in these devices, with
important implications for the role these devices can play for experimental
tests of ideas in quantum chaos.Comment: Submitted to Fortschritte der Physik for special issue on Quantum
Physics with Non-Hermitian Operator
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