16,608 research outputs found
Pre-Heated Isentropic Gas in Groups of Galaxies
We confirm that the standard assumption of isothermal, shock-heated gas in
cluster potentials is unable to reproduce the observed X-ray luminosity-
temperature relation of groups of galaxies. As an alternative, we construct a
physically motivated model for the adiabatic collapse of pre-heated gas into an
isothermal potential that improves upon the original work of Kaiser (1991). The
luminosity and temperature of the gas is calculated, assuming an appropriate
distribution of halo formation times and radiation due to both bremsstrahlung
and recombination processes. This model successfully reproduces the slope and
dispersion of the luminosity-temperature relation of galaxy groups. We also
present calculations of the temperature and luminosity functions for galaxy
groups under the prescription of this model. This model makes two strong
predictions for haloes with total masses M<10^13 M_sun, which are not yet
testable with current data: (1) the gas mass fraction will increase in direct
proportion to the halo mass; (2) the gas temperature will be larger than the
virial temperature of the mass. The second effect is strong enough that group
masses determined from gas temperatures will be overestimated by about an order
of magnitude if it is assumed that the gas temperature is the virial
temperature. The entropy required to match observations can be obtained by
heating the gas at the turnaround time, for example, to about 3 X 10^6 K at
z=1, which is too high to be generated by a normal rate of supernova
explosions. This model breaks down on the scale of low mass clusters, but this
is an acceptable limitation, as we expect accretion shocks to contribute
significantly to the entropy of the gas in such objects.Comment: Final, refereed version, accepted by MNRAS. One new figure and
several clarifying statements have been added. Uses mn.a4.sty (hacked
mn.sty). Also available from
http://astrowww.phys.uvic.ca/~balogh/entropy.ps.g
Inter-sensory Judgments of Signal Duration
Intersensory discrimination of signal duration using visual and auditory signal
Recommended from our members
Evolutionary processes and its environmental correlates in the cranial morphology of western chipmunks (Tamias).
The importance of the environment in shaping phenotypic evolution lies at the core of evolutionary biology. Chipmunks of the genus Tamias (subgenus Neotamias) are part of a very recent radiation, occupying a wide range of environments with marked niche partitioning among species. One open question is if and how those differences in environments affected phenotypic evolution in this lineage. Herein we examine the relative importance of genetic drift versus natural selection in the origin of cranial diversity exhibited by clade members. We also explore the degree to which variation in potential selective agents (environmental variables) are correlated with the patterns of morphological variation presented. We found that genetic drift cannot explain morphological diversification in the group, thus supporting the potential role of natural selection as the predominant evolutionary force during Neotamias cranial diversification, although the strength of selection varied greatly among species. This morphological diversification, in turn, was correlated with environmental conditions, suggesting a possible causal relationship. These results underscore that extant Neotamias represent a radiation in which aspects of the environment might have acted as the selective force driving species' divergence
A Flight Evaluation of a VTOL Jet Transport Under Visual and Simulated Instrument Conditions
Transition, approach, and vertical landing tests for VTOL transport in terminal are
Phonons in a Nanoparticle Mechanically Coupled to a Substrate
The discrete nature of the vibrational modes of an isolated nanometer-scale
solid dramatically modifies its low-energy electron and phonon dynamics from
that of a bulk crystal. However, nanocrystals are usually coupled--even if only
weakly--to an environment consisting of other nanocrystals, a support matrix,
or a solid substrate, and this environmental interaction will modify the
vibrational properties at low frequencies. In this paper we investigate the
modification of the vibrational modes of an insulating spherical nanoparticle
caused by a weak {\it mechanical} coupling to a semi-infinite substrate. The
phonons of the bulk substrate act as a bath of harmonic oscillators, and the
coupling to this reservoir shifts and broadens the nanoparticle's modes. The
vibrational density of states in the nanoparticle is obtained by solving the
Dyson equation for the phonon propagator, and we show that environmental
interaction is especially important at low frequencies. As a probe of the
modified phonon spectrum, we consider nonradiative energy relaxation of a
localized electronic impurity state in the nanoparticle, for which good
agreement with experiment is found.Comment: 10 pages, Revte
The symbolic dimensions of whale bone use in Thule winter dwellings
La maison d'os de baleine thuléenne est une des caractéristiques les plus impressionnantes du paysage arctique. Les éléments d'os de baleine boréale à l'intérieur de 18 maisons d'hiver thuléennes le long de la côte sud-est de l’île Somerset (Nunavut) ont été cartographiés et leur mode de distribution a été étudié par l'application d’un index d’utilité architecturale de la baleine boréale et par l’analyse des composants principaux. Les résultats suggèrent que même si l'os de baleine était initialement choisi pour la construction d’habitations sur la base de sa valeur architecturale, le statut socio-économique peut être reflété par le positionnement de certains éléments en os. En utilisant des analogies historiques provenant des Inupiat du Nord de l'Alaska, on apprend que le positionnement de divers éléments particulièrement dans le tunnel d'entrée semble avoir été lié au symbolisme de la baleine.The Thule whale bone house is one of the most impressive features of the arctic landscape. Bowhead whale bone elements within 18 Thule winter houses along the southeastern coast of Somerset Island (Nunavut) were mapped and the patterning of these elements was investigated through the application of a bowhead architectural utility index and through principal components analysis. The results suggest that while whale bone was initially selected for dwelling construction on the basis of architectural value, socio-economic status may be reflected in the positioning of certain bone elements. Informed by historic North Alaskan Inupiat analogies, the positioning of various elements within the entrance tunnel in particular appears to have been related to whale symbolism
On the shot-noise limit of a thermal current
The noise power spectral density of a thermal current between two macroscopic
dielectric bodies held at different temperatures and connected only at a
quantum point contact is calculated. Assuming the thermal energy is carried
only by phonons, we model the quantum point contact as a mechanical link,
having a harmonic spring potential. In the weak coupling, or weak-link limit,
we find the thermal current analog of the well-known electronic shot-noise
expression.Comment: 4 pages, 1 figur
Dissipation due to tunneling two-level systems in gold nanomechanical resonators
We present measurements of the dissipation and frequency shift in
nanomechanical gold resonators at temperatures down to 10 mK. The resonators
were fabricated as doubly-clamped beams above a GaAs substrate and actuated
magnetomotively. Measurements on beams with frequencies 7.95 MHz and 3.87 MHz
revealed that from 30 mK to 500 mK the dissipation increases with temperature
as , with saturation occurring at higher temperatures. The relative
frequency shift of the resonators increases logarithmically with temperature up
to at least 400 mK. Similarities with the behavior of bulk amorphous solids
suggest that the dissipation in our resonators is dominated by two-level
systems
Identifying Agile Requirements Engineering Patterns in Industry
Agile Software Development (ASD) is gaining in popularity in today´s business world. Industry is adopting agile methodologies both to accelerate value delivery and to enhance the ability to deal with changing requirements. However, ASD has a great impact on how Requirements Engineering (RE) is carried out in agile environments. The integration of Human-Centered Design (HCD) plays an important role due to the focus on user and stakeholder involvement. To this end, we aim to introduce agile RE patterns as main objective of this paper. On the one hand, we will describe our pattern mining process based on empirical research in literature and industry. On the other hand, we will discuss our results and provide two examples of agile RE patterns. In sum, the pattern mining process identifies 41 agile RE patterns. The accumulated knowledge will be shared by means of a web application.Ministerio de EconomÃa y Competitividad TIN2013-46928-C3-3-RMinisterio de EconomÃa y Competitividad TIN2016-76956-C3-2-RMinisterio de EconomÃa y Competitividad TIN2015-71938-RED
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