135 research outputs found
INFLUENCE OF VARIOUS ANOMALIES IN THE VEHICLE AND TRACK PARAMETERS ON THE POSSIBILITY OF DERAILMENT
A detailed non linear model of the four-axle real vehicle, considering both vertical and lateral
vibrations has been proposed. Inertial and elastic-viscous properties of the track have
been taken into account. Comparison of the theoretical investigations and on-track tests
has confirmed the rather good authenticity of the model. The calculations for anomalies
either in track or in vehicle parameters as well as for their combinations have been
performed
Fast relaxation of photo-excited carriers in 2-D transition metal dichalcogenides
We predict a fast relaxation of photo-excited carriers in monolayer transition metal dichalcogenides, which is mediated by the emission of longitudinal optical (LO) and homopolar (HP) phonons. By evaluating Born effective charges for MoS2, MoSe2, WS2, and WSe2, we find that, due to the polar coupling of electrons with LO phonons, and the HP phonons lattice deformation potential, the cooling times for hot electrons and holes from excitation energies of several hundred meV are at ps-scale
Localized interlayer complexes in heterobilayer transition metal dichalcogenides
We present theoretical results for the radiative rates and doping-dependent photoluminescence spectrum of interlayer excitonic complexes localized by donor impurities in MoSe2/WSe2 twisted heterobilayers, supported by quantum Monte Carlo calculations of binding energies and wave-function overlap integrals. For closely aligned layers, radiative decay is made possible by the momentum spread of the localized complexes' wave functions, resulting in radiative rates of a few μs−1. For strongly misaligned layers, the short-range interaction between the carriers and impurity provides a finite radiative rate with a strong asymptotic twist angle dependence ∝θ−8. Finally, phonon-assisted recombination is considered, with emission of optical phonons in both layers resulting in additional, weaker emission lines, redshifted by the phonon energy
Gas Accretion in Star-Forming Galaxies
Cold-mode gas accretion onto galaxies is a direct prediction of LCDM
simulations and provides galaxies with fuel that allows them to continue to
form stars over the lifetime of the Universe. Given its dramatic influence on a
galaxy's gas reservoir, gas accretion has to be largely responsible for how
galaxies form and evolve. Therefore, given the importance of gas accretion, it
is necessary to observe and quantify how these gas flows affect galaxy
evolution. However, observational data have yet to conclusively show that gas
accretion ubiquitously occurs at any epoch. Directly detecting gas accretion is
a challenging endeavor and we now have obtained a significant amount of
observational evidence to support it. This chapter reviews the current
observational evidence of gas accretion onto star-forming galaxies.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springer. This chapter includes 22 pages with 7 Figure
The Birth of a Galaxy. II. The Role of Radiation Pressure
Massive stars provide feedback that shapes the interstellar medium of
galaxies at all redshifts and their resulting stellar populations. Here we
present three adaptive mesh refinement radiation hydrodynamics simulations that
illustrate the impact of momentum transfer from ionising radiation to the
absorbing gas on star formation in high-redshift dwarf galaxies. Momentum
transfer is calculated by solving the radiative transfer equation with a ray
tracing algorithm that is adaptive in spatial and angular coordinates. We find
that momentum input partially affects star formation by increasing the
turbulent support to a three-dimensional rms velocity equal to the circular
velocity of early haloes. Compared to a calculation that neglects radiation
pressure, the star formation rate is decreased by a factor of five to 1.8 x
10^{-2} Msun/yr in a dwarf galaxy with a dark matter and stellar mass of 2.0 x
10^8 and 4.5 x 10^5 solar masses, respectively, when radiation pressure is
included. Its mean metallicity of 10^{-2.1} Z_sun is consistent with the
observed dwarf galaxy luminosity-metallicity relation. However, what one may
naively expect from the calculation without radiation pressure, the central
region of the galaxy overcools and produces a compact, metal-rich stellar
population with an average metallicity of 0.3 Z_sun, indicative of an incorrect
physical recipe. In addition to photo-heating in HII regions, radiation
pressure further drives dense gas from star forming regions, so supernovae
feedback occurs in a warmer and more diffuse medium, launching metal-rich
outflows. Capturing this aspect and a temporal separation between the start of
radiative and supernova feedback are numerically important in the modeling of
galaxies to avoid the "overcooling problem". We estimate that dust in early
low-mass galaxies is unlikely to aid in momentum transfer from radiation to the
gas.Comment: 18 pages, 11 figures, replaced with accepted version, MNRAS. Minor
changes with the conclusions unaffecte
The growth of the central region by acquisition of counterrotating gas in star-forming galaxies
Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Here, based on new measurements with integral field spectroscopy of a large representative galaxy sample, we find an appreciable fraction of counterrotators among blue galaxies (9 out of 489 galaxies). The central regions of blue counterrotators show younger stellar populations and more intense, ongoing star formation than their outer parts, indicating ongoing growth of the central regions. The result offers observational evidence that the acquisition of external gas in blue galaxies is possible; the interaction with pre-existing gas funnels the gas into nuclear regions (<1 kpc) to form new stars
The mysterious morphology of MRC0943-242 as revealed by ALMA and MUSE
© 2016 ESO. We present a pilot study of the z = 2.923 radio galaxy MRC0943-242, where we combine information from ALMA and MUSE data cubes for the first time. Even with modest integration times, we disentangle the AGN and starburst dominated components. These data reveal a highly complex morphology as the AGN, starburst, and molecular gas components show up as widely separated sources in dust continuum, optical continuum, and CO line emission observations. CO(1-0) and CO(8-7) line emission suggest that there is a molecular gas reservoir offset from both the dust and the optical continuum that is located ~90 kpc from the AGN. The UV line emission has a complex structure in emission and absorption. The line emission is mostly due to a large scale ionisation cone energised by the AGN, and a Lya emitting bridge of gas between the radio galaxy and a heavily star-forming set of components. Strangely, the ionisation cone has no Lya emission. We find this is due to an optically thick layer of neutral gas with unity covering fraction spread out over a region of at least ~100 kpc from the AGN. Other less thick absorption components are associated with Lya emitting gas within a few tens of kpc from the radio galaxy and are connected by a bridge of emission. We speculate that this linear structure of dust, Lya and CO emission, and the redshifted absorption seen in the circum nuclear region may represent an accretion flow feeding gas into this massive AGN host galaxy
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