1,176 research outputs found
Field-free two-direction alignment alternation of linear molecules by elliptic laser pulses
We show that a linear molecule subjected to a short specific elliptically
polarized laser field yields postpulse revivals exhibiting alignment
alternatively located along the orthogonal axis and the major axis of the
ellipse. The effect is experimentally demonstrated by measuring the optical
Kerr effect along two different axes. The conditions ensuring an optimal
field-free alternation of high alignments along both directions are derived.Comment: 5 pages, 4 color figure
Estimation non paramétrique des quantiles de crue par la méthode des noyaux
La détermination du débit de crue d'une période de retour donnée nécessite l'estimation de la distribution des crues annuelles. L'utilisation des distributions non paramétriques - comme alternative aux lois statistiques - est examinée dans cet ouvrage. Le principal défi dans l'estimation par la méthode des noyaux réside dans le calcul du paramètre qui détermine le degré de lissage de la densité non paramétrique. Nous avons comparé plusieurs méthodes et avons retenu la méthode plug-in et la méthode des moindres carrés avec validation croisée comme les plus prometteuses.Plusieurs conclusions intéressantes ont été tirées de cette étude. Entre autres, pour l'estimation des quantiles de crue, il semble préférable de considérer des estimateurs basés directement sur la fonction de distribution plutôt que sur la fonction de densité. Une comparaison de la méthode plug-in à l'ajustement de trois lois statistiques a permis de conclure que la méthode des noyaux représente une alternative intéressante aux méthodes paramétriques traditionnelles.Traditional flood frequency analysis involves the fitting of a statistical distribution to observed annual peak flows. The choice of statistical distribution is crucial, since it can have significant impact on design flow estimates. Unfortunately, it is often difficult to determine in an objective way which distribution is the most appropriate.To avoid the inherent arbitrariness associated with the choice of distribution in parametric frequency analysis, one can employ a method based on nonparametric density estimation. Although potentially subject to larger standard error of quantile estimates, the use of nonparametric densities eliminates the need for selecting a particular distribution and the potential bias associated with a wrong choice.The kernel method is a conceptually simple approach, similar in nature to a smoothed histogram. The critical parameter in kernel estimation is the smoothing parameter that determines the degree of smoothing. Methods for estimating the smoothing parameter have already been compared in a number of statistical papers. The novelty of our work is the particular emphasis on quantile estimation, in particular the estimation of quantiles outside the range of observed data. The flood estimation problem is unique in this sense and has been the motivating factor for this study.Seven methods for estimating the smoothing parameter are compared in the paper. All methods are based on some goodness-of-fit measures. More specifically, we considered the least-squares cross-validation method, the maximum likelihood cross-validation method, Adamowski's (1985) method, a plug-in method developed by Altman and Leger (1995) and modified by the authors (Faucher et al., 2001), Breiman's goodness-of-fit criterion method (Breiman, 1977), the variable-kernel maximum likelihood method, and the variable-kernel least-squares cross-validation method.The estimation methods can be classified according to whether they are based on fixed or variable kernels, and whether they are based on the goodness-of-fit of the density function or cumulative distribution function.The quality of the different estimation methods was explored in a Monte Carlo study. Hundred (100) samples of sizes 10, 20, 50, and 100 were simulated from an LP3 distribution. The nonparametric estimation methods were then applied to each of the simulated samples, and quantiles with return period 10, 20, 50, 100, 200, and 1000 were estimated. Bias and root-mean square error of quantile estimates were the key figures used to compare methods. The results of the study can be summarized as follows :1. Comparison of kernels. The literature reports that the kernel choice is relatively unimportant compared to the choice of the smoothing parameter. To determine whether this assertion also holds in the case of the estimation of large quantiles outside the range of data, we compared six different kernel candidates. We found no major differences between the biweight, the Normal, the Epanechnikov, and the EV1 kernels. However, the rectangular and the Cauchy kernel should be avoided.2. Comparison of sample size. The quality of estimates, whether parametric or nonparametric, deteriorates as sample size decreases. To examine the degree of sensitivity to sample size, we compared estimates of the 200-year event obtained by assuming a GEV distribution and a nonparametric density estimated by maximum likelihood cross-validation. The main conclusion is that the root mean square error for the parametric model (GEV) is more sensitive to sample size than the nonparametric model. 3. Comparison of estimators of the smoothing parameter. Among the methods considered in the study, the plug-in method, developed by Altman and Leger (1995) and modified by the authors (Faucher et al. 2001), turned out to perform the best along with the least-squares cross-validation method which had a similar performance. Adamowski's method had to be excluded, because it consistently failed to converge. The methods based on variable kernels generally did not perform as well as the fixed kernel methods.4. Comparison of density-based and cumulative distribution-based methods. The only cumulative distribution-based method considered in the comparison study was the plug-in method. Adamowski's method is also based on the cumulative distribution function, but was rejected for the reasons mentioned above. Although the plug-in method did well in the comparison, it is not clear whether this can be attributed to the fact that it is based on estimation of the cumulative distribution function. However, one could hypothesize that when the objective is to estimate quantiles, a method that emphasizes the cumulative distribution function rather than the density should have certain advantages. 5. Comparison of parametric and nonparametric methods. Nonparametric methods were compared with conventional parametric methods. The LP3, the 2-parameter lognormal, and the GEV distributions were used to fit the simulated samples. It was found that nonparametric methods perform quite similarly to the parametric methods. This is a significant result, because data were generated from an LP3 distribution so one would intuitively expect the LP3 model to be superior which however was not the case. In actual applications, flood distributions are often irregular and in such cases nonparametric methods would likely be superior to parametric methods
Cosmic ray feedback in the FIRE simulations: constraining cosmic ray propagation with GeV gamma ray emission
We present the implementation and the first results of cosmic ray (CR)
feedback in the Feedback In Realistic Environments (FIRE) simulations. We
investigate CR feedback in non-cosmological simulations of dwarf, sub-
starburst, and galaxies with different propagation models, including
advection, isotropic and anisotropic diffusion, and streaming along field lines
with different transport coefficients. We simulate CR diffusion and streaming
simultaneously in galaxies with high resolution, using a two moment method. We
forward-model and compare to observations of -ray emission from nearby
and starburst galaxies. We reproduce the -ray observations of dwarf and
galaxies with constant isotropic diffusion coefficient . Advection-only and streaming-only
models produce order-of-magnitude too large -ray luminosities in dwarf
and galaxies. We show that in models that match the -ray
observations, most CRs escape low-gas-density galaxies (e.g.\ dwarfs) before
significant collisional losses, while starburst galaxies are CR proton
calorimeters. While adiabatic losses can be significant, they occur only after
CRs escape galaxies, so they are only of secondary importance for -ray
emissivities. Models where CRs are ``trapped'' in the star-forming disk have
lower star formation efficiency, but these models are ruled out by -ray
observations. For models with constant that match the -ray
observations, CRs form extended halos with scale heights of several kpc to
several tens of kpc.Comment: 31 pages, 26 figures, accepted for publication in MNRA
Fermi Gamma-ray Haze via Dark Matter and Millisecond Pulsars
We study possible astrophysical and dark matter (DM) explanations for the
Fermi gamma-ray haze in the Milky Way halo. As representatives of various DM
models, we consider DM particles annihilating into W+W-, b-bbar, and e+e-. In
the first two cases, the prompt gamma-ray emission from DM annihilations is
significant or even dominant at E > 10 GeV, while inverse Compton scattering
(ICS) from annihilating DM products is insignificant. For the e+e- annihilation
mode, we require a boost factor of order 100 to get significant contribution to
the gamma-ray haze from ICS photons. Possible astrophysical sources of high
energy particles at high latitudes include type Ia supernovae (SNe) and
millisecond pulsars (MSPs). Based on our current understanding of Ia SNe rates,
they do not contribute significantly to gamma-ray flux in the halo of the Milky
Way. As the MSP population in the stellar halo of the Milky Way is not well
constrained, MSPs may be a viable source of gamma-rays at high latitudes
provided that there are ~ 20 000 - 60 000 of MSPs in the Milky Way stellar
halo. In this case, pulsed gamma-ray emission from MSPs can contribute to
gamma-rays around few GeV's while the ICS photons from MSP electrons and
positrons may be significant at all energies in the gamma-ray haze. The
plausibility of such a population of MSPs is discussed. Consistency with the
Wilkinson Microwave Anisotropy Probe (WMAP) microwave haze requires that either
a significant fraction of MSP spin-down energy is converted into e+e- flux or
the DM annihilates predominantly into leptons with a boost factor of order 100.Comment: 18 pages, 1 table, 5 figures; v2: references and a few discussions
added, v3: minor change
The origin of ultra diffuse galaxies: stellar feedback and quenching
We test if the cosmological zoom-in simulations of isolated galaxies from the
FIRE project reproduce the properties of ultra diffuse galaxies. We show that
stellar feedback-generated outflows that dynamically heat galactic stars,
together with a passively aging stellar population after imposed quenching
(from e.g. infall into a galaxy cluster), naturally reproduce the observed
population of red UDGs, without the need for high spin halos or dynamical
influence from their host cluster. We reproduce the range of surface
brightness, radius and absolute magnitude of the observed z=0 red UDGs by
quenching simulated galaxies at a range of different times. They represent a
mostly uniform population of dark matter-dominated galaxies with M_star ~1e8
Msun, low metallicity and a broad range of ages. The most massive simulated
UDGs require earliest quenching and are therefore the oldest. Our simulations
provide a good match to the central enclosed masses and the velocity
dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the
simulated UDGs remain largely fixed across a broad range of quenching times
because the central regions of their dark matter halos complete their growth
early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The
most massive red UDG in our sample requires quenching at z~3 when its halo
reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0
its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If
our simulated dwarfs are not quenched, they evolve into bluer low-surface
brightness galaxies with mass-to-light ratios similar to observed field dwarfs.
While our simulation sample covers a limited range of formation histories and
halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy
type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio
Strongly Time-Variable Ultra-Violet Metal Line Emission from the Circum-Galactic Medium of High-Redshift Galaxies
We use cosmological simulations from the Feedback In Realistic Environments
(FIRE) project, which implement a comprehensive set of stellar feedback
processes, to study ultra-violet (UV) metal line emission from the
circum-galactic medium of high-redshift (z=2-4) galaxies. Our simulations cover
the halo mass range Mh ~ 2x10^11 - 8.5x10^12 Msun at z=2, representative of
Lyman break galaxies. Of the transitions we analyze, the low-ionization C III
(977 A) and Si III (1207 A) emission lines are the most luminous, with C IV
(1548 A) and Si IV (1394 A) also showing interesting spatially-extended
structures. The more massive halos are on average more UV-luminous. The UV
metal line emission from galactic halos in our simulations arises primarily
from collisionally ionized gas and is strongly time variable, with
peak-to-trough variations of up to ~2 dex. The peaks of UV metal line
luminosity correspond closely to massive and energetic mass outflow events,
which follow bursts of star formation and inject sufficient energy into
galactic halos to power the metal line emission. The strong time variability
implies that even some relatively low-mass halos may be detectable. Conversely,
flux-limited samples will be biased toward halos whose central galaxy has
recently experienced a strong burst of star formation. Spatially-extended UV
metal line emission around high-redshift galaxies should be detectable by
current and upcoming integral field spectrographs such as the Multi Unit
Spectroscopic Explorer (MUSE) on the Very Large Telescope and Keck Cosmic Web
Imager (KCWI).Comment: 16 pages, 8 figures, accepted for publication in MNRA
Stability of Relativistic Matter with Magnetic Fields for Nuclear Charges up to the Critical Value
We give a proof of stability of relativistic matter with magnetic fields all
the way up to the critical value of the nuclear charge .Comment: LaTeX2e, 12 page
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