4,648 research outputs found
Morphology of galaxies with quiescent recent assembly history in a Lambda-CDM universe
The standard disc formation scenario postulates that disc forms as the gas
cools and flows into the centre of the dark matter halo, conserving the
specific angular momentum. Major mergers have been shown to be able to destroy
or highly perturb the disc components. More recently, the alignment of the
material that is accreted to form the galaxy has been pointed out as a key
ingredient to determine galaxy morphology. However, in a hierarchical scenario
galaxy formation is a complex process that combines these processes and others
in a non-linear way so that the origin of galaxy morphology remains to be fully
understood. We aim at exploring the differences in the formation histories of
galaxies with a variety of morphology, but quite recent merger histories, to
identify which mechanisms are playing a major role. We analyse when minor
mergers can be considered relevant to determine galaxy morphology. We also
study the specific angular momentum content of the disc and central spheroidal
components separately. We used cosmological hydrodynamical simulations that
include an effective, physically motivated supernova feedback that is able to
regulate the star formation in haloes of different masses. We analysed the
morphology and formation history of a sample of 15 galaxies of a cosmological
simulation. We performed a spheroid-disc decomposition of the selected galaxies
and their progenitor systems. The angular momentum orientation of the merging
systems as well as their relative masses were estimated to analyse the role
played by orientation and by minor mergers in the determination of the
morphology. We found the discs to be formed by conserving the specific angular
momentum in accordance with the classical disc formation model. The specific
angular momentum of the stellar central spheroid correlates with the dark
matter halo angular momentum and determines a power law. AbridgedComment: 10 pages, 9 figures, A&A in pres
Transient features of quantum open maps
We study families of open chaotic maps that classically share the same
asymptotic properties -- forward and backwards trapped sets, repeller
dimensions, escape rate -- but differ in their short time behavior. When these
maps are quantized we find that the fine details of the distribution of
resonances and the corresponding eigenfunctions are sensitive to the initial
shape and size of the openings. We study phase space localization of the
resonances with respect to the repeller and find strong delocalization effects
when the area of the openings is smaller than .Comment: 7 pages, 7 figure
Quantum chaotic resonances from short periodic orbits
We present an approach to calculating the quantum resonances and resonance
wave functions of chaotic scattering systems, based on the construction of
states localized on classical periodic orbits and adapted to the dynamics.
Typically only a few of such states are necessary for constructing a resonance.
Using only short orbits (with periods up to the Ehrenfest time), we obtain
approximations to the longest living states, avoiding computation of the
background of short living states. This makes our approach considerably more
efficient than previous ones. The number of long lived states produced within
our formulation is in agreement with the fractal Weyl law conjectured recently
in this setting. We confirm the accuracy of the approximations using the open
quantum baker map as an example.Comment: 4 pages, 4 figure
The gas metallicity gradient and the star formation activity of disc galaxies
We study oxygen abundance profiles of the gaseous disc components in
simulated galaxies in a hierarchical universe. We analyse the disc metallicity
gradients in relation to the stellar masses and star formation rates of the
simulated galaxies. We find a trend for galaxies with low stellar masses to
have steeper metallicity gradients than galaxies with high stellar masses at z
~0. We also detect that the gas-phase metallicity slopes and the specific star
formation rate (sSFR) of our simulated disc galaxies are consistent with
recently reported observations at z ~0. Simulated galaxies with high stellar
masses reproduce the observed relationship at all analysed redshifts and have
an increasing contribution of discs with positive metallicity slopes with
increasing redshift. Simulated galaxies with low stellar masses a have larger
fraction of negative metallicity gradients with increasing redshift. Simulated
galaxies with positive or very negative metallicity slopes exhibit disturbed
morphologies and/or have a close neighbour. We analyse the evolution of the
slope of the oxygen profile and sSFR for a gas-rich galaxy-galaxy encounter,
finding that this kind of events could generate either positive and negative
gas-phase oxygen profiles depending on their state of evolution. Our results
support claims that the determination of reliable metallicity gradients as a
function of redshift is a key piece of information to understand galaxy
formation and set constrains on the subgrid physics.Comment: 12 pages, 8 figures, accepted MNRA
Oral tradition as a worldwide phenomenon
It is my view that oral tradition is the source of all of the world's literary traditions. Two principal concerns have driven my work (1995, 2000, 2002): the collection of examples of living oral literature (ballads, songs, proverbs, legends, stories, oral history, and epics) in different countries and traditions (Spain, Hispanoamerica, central Africa), and the comparative study of world literary traditions from Homer to Garcia Marquez.Not
Metallicity dependence of HMXB populations
High-mass X-ray binaries (HMXBs) might have contributed a non-negligible
fraction of the energy feedback to the interstellar and intergalactic media at
high redshift, becoming important sources for the heating and ionization
history of the Universe. However, the importance of this contribution depends
on the hypothesized increase in the number of HMXBs formed in low-metallicity
galaxies and in their luminosities. In this work we test the aforementioned
hypothesis, and quantify the metallicity dependence of HMXB population
properties. We compile from the literature a large set of data on the sizes and
X-ray luminosities of HMXB populations in nearby galaxies with known
metallicities and star formation rates. We use Bayesian inference to fit simple
Monte Carlo models that describe the metallicity dependence of the size and
luminosity of the HMXB populations. We find that HMXBs are typically ten times
more numerous per unit star formation rate in low-metallicity galaxies (12 +
log(O/H) < 8, namely < 20% solar) than in solar-metallicity galaxies. The
metallicity dependence of the luminosity of HMXBs is small compared to that of
the population size. Our results support the hypothesis that HMXBs are more
numerous in low-metallicity galaxies, implying the need to investigate the
feedback in the form of X-rays and energetic mass outflows of these high-energy
sources during cosmic dawn.Comment: 9 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Chemistry Textbook Approaches to Chemical Equilibrium and Student Alternative Conceptions
In the field of chemistry there has been considerable interest in discovering the reasons
why many students have great difficulty in successfully developing a scientifically accepted
understanding of chemical equilibrium. Research projects influenced and inspired by constructivism
have been invaluable for identifying alternative conceptions that may turn into obstacles to
meaningful learning of fundamental ideas regarding chemical equilibrium. Research findings provide
evidence that misunderstandings of and related to chemical equilibrium are widespread at various
levels of education, including prospective chemistry teachers. Language used in textbooks may give
rise, or reinforce student alternative conceptions and research findings point to a direct relationship
between language used in textbooks and some of the alternative conceptions students develop at
various levels of school chemistry. Some of the students’ alternative conceptions already identified,
and available in literature, were the basis of an instrument devised to analyse chemistry textbooks
used in secondary and tertiary education. This paper presents and discusses typical problematic
language found in student textbooks in the light of students’ current frameworks of understanding as
described in literature
Zonal flows and long-distance correlations during the formation of the edge shear layer in the TJ-II stellarator
A theoretical interpretation is given for the observed long-distance
correlations in potential fluctuations in TJ-II. The value of the correlation
increases above the critical point of the transition for the emergence of the
plasma edge shear flow layer. Mean (i.e. surface averaged, zero-frequency)
sheared flows cannot account for the experimental results. A model consisting
of four envelope equations for the fluctuation level, the mean flow shear, the
zonal flow amplitude shear, and the averaged pressure gradient is proposed. It
is shown that the presence of zonal flows is essential to reproduce the main
features of the experimental observations.Comment: 19 pages, 7 figure
Estudio comparativo con microscopio electrónico de barrido de la degradación de una lámina impermeable de cloruro de polivinilo plastificado en diferentes condiciones
This paper discusses the analysis of several samples of a plasticized polyvinyl chloride (PVC-P) waterproofing membrane. The samples were extracted from different areas of the same flat roof, which was in service for over 12 years. An original sample of an identical PVC-P membrane that was not installed on the roof was also analyzed. The analysis of the materials was carried out using a scanning electron microscope (SEM). An elemental analysis of every sample was also performed by energy dispersive X-ray spectroscopy (EDS). Micrographs and the elemental composition of the samples were compared with the data obtained in the analysis of the original sample. The results show dehydrochlorination of the polymer in two of the samples studied and great deterioration that was not visible to the naked eye in the sample that was totally exposed to the weather.Este estudio discute el análisis de varias muestras de una membrana impermeable de cloruro de polivinilo plastificado (PVC-P). Las muestras fueron extraídas de diferentes áreas de la misma cubierta plana, que estuvo en servicio durante más de 12 años. Una muestra original de idéntica membrana de PVC-P que no estuvo instalada en la cubierta fue también analizada. El análisis de los materiales se llevó a cabo utilizando un microscopio electrónico de barrido (MEB). Además se realizó un análisis elemental sobre cada una de las muestras mediante espectroscopía de dispersión de energía de rayos X (EDS). Las micrografías, y la composición elemental de las muestras, se comparan con los datos obtenidos en el análisis de la muestra original. Los resultados muestran deshidrocloración del polímero en dos de las muestras estudiadas, y un gran deterioro de la muestra totalmente expuesta a la intemperie, que no era visible a simple vista
Membranas sintéticas impermeables y capas auxiliares separadoras en la cubierta plana invertida. Efecto de la presión sobre la degradación
Flat roofs are inverted when the insulation material is placed over the waterproofing membrane. This widely-used constructive system sets materials which may interact easily with each other causing their deterioration. The incompatibilities and the materials that may interact with each other are not completely described, the elements currently used to separate the materials are not totally efficient to avoid deterioration. This paper intends to enlarge the knowledge regarding the materials that are incompatible for inverted flat roofs, the conditions were interactions take place and the efficiency of some alternative solutions to separate the materials. The ethylene propylene diene monomer (EPDM) waterproofing membranes are incompatible with the extruded polystyrene (XPS). The interactions between incompatible materials increase with the pressure.Las cubiertas planas son invertidas cuando el material aislante se coloca sobre la membrana impermeable. Este sistema constructivo ampliamente utilizado emplea materiales que pueden interactuar con facilidad entre ellos, produciendo deterioro. Las incompatibilidades y los materiales que pueden interactuar no se han descrito completamente, los elementos utilizados actualmente para separar los materiales no son totalmente eficientes para evitar el deterioro. Este artículo pretende aumentar el conocimiento acerca de que materiales son incompatibles en la cubierta invertida plana, las condiciones para que las interacciones se den, y la eficiencia de algunas alternativas para separar materiales. Las membranas impermeables de etileno propileno monómero dieno (EPDM) son incompatibles con el poliestireno extrusionado (XPS). Las interacciones entre materiales incompatibles aumentan con la presión
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