Gaseous drag on a gravitational perturber in Modified Newtonian Dynamics and the structure of the wake

We calculate the structure of a wake generated by, and the dynamical friction force on, a gravitational perturber travelling through a gaseous medium of uniform density and constant background acceleration g_ext, in the context of Modified Newtonian Dynamics (MOND). The wake is described as a linear superposition of two terms. The dominant part displays the same structure as the wake generated in Newtonian gravity scaled up by a factor mu^{-1}(g_ext/a_0), where a_{0} is the constant MOND acceleration and mu the interpolating function. The structure of the second term depends greatly on the angle between g_{ext} and and the velocity of the perturber. We evaluate the dynamical drag force numerically and compare our MOND results with the Newtonian case. We mention the relevance of our calculations to orbit evolution of globular clusters and satellites in a gaseous proto-galaxy. Potential differences in the X-ray emission of gravitational galactic wakes in MOND and in Newtonian gravity with a dark halo are highlighted.Comment: 13 pages, 7 figures, accepted for publication in MNRA

PONENCIA XV JORNADAS EUBD. Tecnología de apoyo y sociedad de la información. Proyectos europeos

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Nuevos datos sobre Juan de Solís, pintor escenógrafo y decorador en la Corte de Felipe IV.

Sin resume

Dynamical Friction of Double Perturbers in a Gaseous Medium

In many astrophysical situations, as in the coalescence of supermassive black hole pairs at gas rich galactic nuclei, the dynamical friction experienced by an object is a combination of its own wake as well as the wakes of its companions. Using a semi-analytic approach, we investigate the composite wake due to, and the resulting drag forces on, double perturbers that are placed at the opposite sides of the orbital center and move on a circular orbit in a uniform gaseous medium. The circular orbit makes the wake of each perturber asymmetric, creating an overdense tail at the trailing side. The tail not only drags the perturber backward but it also exerts a positive torque on the companion. For equal-mass perturbers, the positive torque created by the companion wake is, on average, a fraction ~40-50% of the negative torque created by its own wake, but this fraction may be even larger for perturbers moving subsonically. This suggests that the orbital decay of a perturber in a double system, especially in the subsonic regime, can take considerably longer than in isolation. We provide the fitting formulae for the forces due to the companion wake and discuss our results in light of recent numerical simulations for mergers of binary black holes.Comment: 4 pages, 3 figures, accepted for publication in ApJ

Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks

High--resolution 2D--MHD numerical simulations have been carried out to investigate the effects of continuing infall of clumpy gas in extended HI galactic disks. Given a certain accretion rate, the response of the disk depends on its surface gas density and temperature. For Galactic conditions at a galactocentric distance of ~20 kpc, and for mass accretion rates consistent with current empirical and theoretical determinations in the Milky Way, the rain of compact high velocity clouds onto the disk can maintain transonic turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only mechanism of driving turbulence. Some statistical properties of the resulting forcing flow are shown in this Letter. The radial dependence of the gas velocity dispersion is also discussed.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letter

Relaciones y vida laboral de tres generaciones de profesionales en Cali-Colombia

Sociedad & Economía #20. Año 2011Trayectorias Laborales, Estrategias Relacionales, Clases Medias Profesionales, Generaciones

Cassoni italianos en los Museos Lázaro Galdíano y Arqueológico Nacional.

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Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi dSph) against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 1000 solar masses at a halo density in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses >3000 solar masses improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses $\sim 10^{5}$ solar masses can comprise no more than a halo mass fraction $\sim 0.01$. Better determinations of the velocity dispersion of old overdense regions in dSphs may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed, compact objects with masses above $\sim 100$ solar masses could be excluded from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter

The survival of dynamical fossils in dwarf spheroidal galaxies in conventional and modified dynamics

The survival of unbound density substructure against orbital mixing imposes strong constraints on the slope of the underlying gravitational potential and provides a new test on modified gravities. Here we investigate whether the interpretation that the stellar clump in Ursa Minor (UMi) dwarf spheroidal galaxy is a `dynamical fossil' is consistent with Modified Newtonian dynamics (MOND). For UMi mass models inferred by fitting the velocity dispersion profile, the stellar clump around the second peak of UMi is erased very rapidly, within 1.25 Gyr (6.5 orbits), even with the inclusion of self-gravity. We find that the clump can hardly survive for more than 2 Gyr even under more generous conditions. Alternative scenarios which could lead to a kinematically cold clump are discussed but, so far, none of them were found to be fully satisfactory. Our conclusion is that the cold clump in UMi poses a challenge for both LambdaCDM and MOND.Comment: 14 pages, 13 figures, accepted for publication in MNRA

Brechas de género en salarios y puestos directivos: ¿contribuyen las políticas de conciliación laboral y familiar a alcanzar la igualdad de género entre los titulados universitarios europeos?

This article analyzes the gender gap in wages and access to managerial positions among university graduates in 12 European countries and explores the capability of work-family balance policies to close these gaps. Using the REFLEX database, we apply the coarsened exact matching algorithm to construct a balanced sample of women and men with the same academic characteristics (field of study, internships, and academic achievement, among others). The analysis reveals that the academic program characteristics play a relevant role in labor market outcomes as the gender gaps diminish when controlling for academic features. We find that gender differences in hourly wages and access to top wages are smaller in countries with longer paid paternity leaves and larger enrollment rates of children aged 0-3 years in preschools. In contrast, work-family reconciliation policies have little effect on the constraints women face in accessing high-level positions that require strong commitment and availability.Este artículo analiza la brecha salarial de género y en el acceso a puestos directivos entre titulados universitarios de 12 países europeos y explora la capacidad de las políticas de conciliación laboral y familiar para reducir dichas brechas. Utilizando la base de datos REFLEX y aplicando el algoritmo coarsened exact matching para construir una muestra equilibrada de hombres y mujeres con las mismas características académicas (área de estudio, realización de prácticas, resultados académicos, etc.). Los resultados muestran que las características del programa de estudios ejercen un papel importante en los resultados laborales de los titulados y que las brechas de género tienden a estrecharse una vez incorporadas en las estimaciones. Encontramos que las diferencias de género en el salario por hora y en el acceso a empleos con salarios muy altos son menores en aquellos países con mayor duración del permiso de paternidad y tasas más elevadas de matriculación de niños de 0-3 años en guarderías. Por el contrario, los resultados muestran que las políticas de conciliación laboral y familiar ejercen escaso efecto para superar las limitaciones a las que se enfrentan las mujeres para acceder a puestos de alto nivel, caracterizados por requerir elevados niveles de compromiso y disponibilidad