18,603 research outputs found
The Role of Cold Flows in the Assembly of Galaxy Disks
We use high resolution cosmological hydrodynamical simulations to demonstrate
that cold flow gas accretion, particularly along filaments, modifies the
standard picture of gas accretion and cooling onto galaxy disks. In the
standard picture, all gas is initially heated to the virial temperature of the
galaxy as it enters the virial radius. Low mass galaxies are instead dominated
by accretion of gas that stays well below the virial temperature, and even when
a hot halo is able to develop in more massive galaxies there exist dense
filaments that penetrate inside of the virial radius and deliver cold gas to
the central galaxy. For galaxies up to ~L*, this cold accretion gas is
responsible for the star formation in the disk at all times to the present.
Even for galaxies at higher masses, cold flows dominate the growth of the disk
at early times. Within this modified picture, galaxies are able to accrete a
large mass of cold gas, with lower initial gas temperatures leading to shorter
cooling times to reach the disk. Although star formation in the disk is
mitigated by supernovae feedback, the short cooling times allow for the growth
of stellar disks at higher redshifts than predicted by the standard model.Comment: accepted to Ap
The (In)Stability of Planetary Systems
We present results of numerical simulations which examine the dynamical
stability of known planetary systems, a star with two or more planets. First we
vary the initial conditions of each system based on observational data. We then
determine regions of phase space which produce stable planetary configurations.
For each system we perform 1000 ~1 million year integrations. We examine
upsilon And, HD83443, GJ876, HD82943, 47UMa, HD168443, and the solar system
(SS). We find that the resonant systems, 2 planets in a first order mean motion
resonance, (HD82943 and GJ876) have very narrow zones of stability. The
interacting systems, not in first order resonance, but able to perturb each
other (upsilon And, 47UMa, and SS) have broad regions of stability. The
separated systems, 2 planets beyond 10:1 resonance, (we only examine HD83443
and HD168443) are fully stable. Furthermore we find that the best fits to the
interacting and resonant systems place them very close to unstable regions. The
boundary in phase space between stability and instability depends strongly on
the eccentricities, and (if applicable) the proximity of the system to perfect
resonance. In addition to million year integrations, we also examined stability
on ~100 million year timescales. For each system we ran ~10 long term
simulations, and find that the Keplerian fits to these systems all contain
configurations which may be regular on this timescale.Comment: 37 pages, 49 figures, 13 tables, submitted to Ap
Exchange and Correlation Corrections to the Response Functions of a Spin-Polarized Electron Gas
We analyze the spin and charge responses induced in a spin-polarized electron gas by a weak electromagnetic field. The coupled spin-charge response is derived from the equation of motion of the particle distribution function in the presence of the perturbation. To obtain the correct frequency and the wave-vector dependence we introduce the spin-dependent local-field factors, G±σ=Gxσ±Gcσ, which give the exchange (x) and correlation (c) corrections to the random phase approximation. For an arbitrary degree of polarization of the electron gas, we derive the exact analytical expressions for G±σ(q→,ω) in the limit of high frequency or large wave vectors. The results for q→→∞ are expressed in terms of the two-particle correlation function, g(r→) at r=0
Effects of Extreme Obliquity Variations on the Habitability of Exoplanets
We explore the impact of obliquity variations on planetary habitability in
hypothetical systems with high mutual inclination. We show that large
amplitude, high frequency obliquity oscillations on Earth-like exoplanets can
suppress the ice-albedo feedback, increasing the outer edge of the habitable
zone. We restrict our exploration to hypothetical systems consisting of a
solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We
verify that these systems are stable for years with N-body simulations,
and calculate the obliquity variations induced by the orbital evolution of the
Earth-mass planet and a torque from the host star. We run a simplified energy
balance model on the terrestrial planet to assess surface temperature and ice
coverage on the planet's surface, and we calculate differences in the outer
edge of the habitable zone for planets with rapid obliquity variations. For
each hypothetical system, we calculate the outer edge of habitability for two
conditions: 1) the full evolution of the planetary spin and orbit, and 2) the
eccentricity and obliquity fixed at their average values. We recover previous
results that higher values of fixed obliquity and eccentricity expand the
habitable zone, but also find that obliquity oscillations further expand
habitable orbits in all cases. Terrestrial planets near the outer edge of the
habitable zone may be more likely to support life in systems that induce rapid
obliquity oscillations as opposed to fixed-spin planets. Such planets may be
the easiest to directly characterize with space-borne telescopes.Comment: 46 pages, 12 Figures, 5 Table
Collective Excitations in an Asymmetrically Spin-Polarized Quantum Well
We present a phenomenological picture for the many-body excitations of a two-dimensional electron gas in a quantum well spin polarized by a dc magnetic field at an angle with the axis of the well. In the framework of the Landau theory of charged Fermi liquids, we determine the frequencies of collective modes by solving a transport equation for quasiparticles in the local electromagnetic field associated with the charge- and spin-density fluctuations. In the long-wavelength limit, analytic solutions for (qជ) are obtained as functions of the degree of spin polarization and of the angle
Coupling of Spin Waves with Charge- and Spin Density Excitations in Spin-Polarized Quantum Wells
The coupling of spin waves with charge- and spin-density waves is shown to be induced by a spin-dependent interaction in a quantum well, which is spin polarized by a dc magnetic field at an angle to the symmetry axis. The mixing of the plasmonic and magnonic modes, which occurs for both intra- and intersubband transitions, depends on the coupling constant of the spin-spin interaction, the tilt angle , and the initial spin polarization . ͓S0163-1829͑99͒08247-8
Fos co-operation with PTEN loss elicits keratoacanthoma not carcinoma due to p53/p21<sup>WAF</sup>-induced differentiation triggered by GSK3b inactivation and reduced AKT activity
To investigate gene synergism in multistage skin carcinogenesis, the RU486-inducible cre/lox system was employed to ablate PTEN function [K14.cre/D5PTENflx] in mouse epidermis expressing activated v-fos [HK1.fos]. RU486-treated HK1.fos/D5PTENflx mice exhibited hyperplasia, hyperkeratosis and tumours that progressed to highly differentiated keratoacanthomas rather than carcinomas, due to re-expression of high p53 and p21WAF levels. Despite elevated MAP kinase activity, cyclin D1/E2 over expression and increased AKT activity forming areas of highly proliferative, papillomatous keratinocytes, increasing levels of GSK3b inactivation exceeded a threshold that induced p53/p21WAF expression to halt proliferation and accelerate differentiation, giving the hallmark keratosis of keratoacanthomas. A pivotal facet to this GSK3b-triggered mechanism centred on increasing p53 expression in basal layer keratinocytes. This reduced activated AKT expression and released inhibition of p21WAF, which accelerated keratinocyte differentiation, as indicated by unique basal layer expression of differentiation-specific keratin K1, alongside premature filaggrin and loricrin expression. Thus, fos synergism with PTEN loss elicited a benign tumour context where GSK3b-induced, p53/p21WAF expression continually switched AKT-associated proliferation into one of differentiation, preventing further progression. This putative compensatory mechanism required the critical availability of normal p53 and/or p21WAF otherwise deregulated fos, Akt and GSK3b associate with malignant progression
Persistent Decadal-Scale Rainfall Variability in the Tropical South Pacific Convergence Zone Through the Past Six Centuries
Modern Pacific decadal variability (PDV) has global impacts; hence records of PDV from the pre-instrumental period are needed to better inform models that are used to project future climate variability. We focus here on reconstructing rainfall in the western tropical Pacific (Solomon Islands; similar to 9.5 degrees S, similar to 160 degrees E), a region directly influenced by PDV, using cave deposits (stalagmite). A relationship is developed between delta O-18 variations in the stalagmite and local rainfall amount to produce a 600 yr record of rainfall variability from the South Pacific Convergence Zone (SPCZ). We present evidence for large (similar to 1.5 m), abrupt, and periodic changes in total annual rainfall amount on decadal to multidecadal timescales since 1423 +/- 5 CE (Common Era) in the Solomon Islands. The timing of the decadal changes in rainfall inferred from the 20th-century portion of the stalagmite delta O-18 record coincides with previously identified decadal shifts in PDV-related Pacific ocean-atmosphere behavior (Clement et al., 2011; Deser et al., 2004). The Solomons record of PDV is not associated with variations in external forcings, but rather results from internal climate variability. The 600 yr Solomon Islands stalagmite delta O-18 record indicates that decadal oscillations in rainfall are a persistent characteristic of SPCZ-related climate variability.Taiwan ROC NSCNTU 101-2116-M-002-009, 102-2116-M-002-016, 101R7625Geological Science
Hierarchical formation of bulgeless galaxies II: Redistribution of angular momentum via galactic fountains
Within a fully cosmological hydrodynamical simulation, we form a galaxy which
rotates at 140 km/s, and is characterised by two loose spiral arms and a bar,
indicative of a Hubble Type SBc/d galaxy. We show that our simulated galaxy has
no classical bulge, with a pure disc profile at z=1, well after the major
merging activity has ended. A long-lived bar subsequently forms, resulting in
the formation of a secularly-formed "pseudo" bulge, with the final
bulge-to-total light ratio B/T=0.21. We show that the majority of gas which
loses angular momentum and falls to the central region of the galaxy during the
merging epoch is blown back into the hot halo, with much of it returning later
to form stars in the disc. We propose that this mechanism of redistribution of
angular momentum via a galactic fountain, when coupled with the results from
our previous study which showed why gas outflows are biased to have low angular
momentum, can solve the angular momentum/bulgeless disc problem of the cold
dark matter paradigm.Comment: 9 Pages, 10 Figures, accepted MNRAS version. Comments welcom
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