2,266 research outputs found
Non-steady relaxation and critical exponents at the depinning transition
We study the non-steady relaxation of a driven one-dimensional elastic
interface at the depinning transition by extensive numerical simulations
concurrently implemented on graphics processing units (GPUs). We compute the
time-dependent velocity and roughness as the interface relaxes from a flat
initial configuration at the thermodynamic random-manifold critical force.
Above a first, non-universal microscopic time-regime, we find a non-trivial
long crossover towards the non-steady macroscopic critical regime. This
"mesoscopic" time-regime is robust under changes of the microscopic disorder
including its random-bond or random-field character, and can be fairly
described as power-law corrections to the asymptotic scaling forms yielding the
true critical exponents. In order to avoid fitting effective exponents with a
systematic bias we implement a practical criterion of consistency and perform
large-scale (L~2^{25}) simulations for the non-steady dynamics of the continuum
displacement quenched Edwards Wilkinson equation, getting accurate and
consistent depinning exponents for this class: \beta = 0.245 \pm 0.006, z =
1.433 \pm 0.007, \zeta=1.250 \pm 0.005 and \nu=1.333 \pm 0.007. Our study may
explain numerical discrepancies (as large as 30% for the velocity exponent
\beta) found in the literature. It might also be relevant for the analysis of
experimental protocols with driven interfaces keeping a long-term memory of the
initial condition.Comment: Published version (including erratum). Codes and Supplemental
Material available at https://bitbucket.org/ezeferrero/qe
Representative galaxy age-metallicity relationships
The ongoing surveys of galaxies and those for the next generation of
telescopes will demand the execution of high-CPU consuming machine codes for
recovering detailed star formation histories (SFHs) and hence age-metallicity
relationships (AMRs). We present here an expeditive method which provides
quick-look AMRs on the basis of representative ages and metallicities obtained
from colour-magnitude diagram (CMD) analyses. We have tested its perfomance by
generating synthetic CMDs for a wide variety of galaxy SFHs. The representative
AMRs turn out to be reliable down to a magnitude limit with a photometric
completeness factor higher than 85 per cent, and trace the chemical
evolution history for any stellar population (represented by a mean age and an
intrinsic age spread) with a total mass within ~ 40 per cent of the more
massive stellar population in the galaxy.Comment: 12 pages, 11 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The age-metallicity relationship in the Fornax spheroidal dwarf galaxy
We produce a comprehensive field star age-metallicity relationship (AMR) from
the earliest epoch until ~ 1 Gyr ago for three fields in the Fornax dSph galaxy
by using VI photometric data obtained with FORS1 at the VLT. We find that the
innermost one does not contains dominant very old stars (age > 12 Gyr), whereas
the relatively outer field does not account for representative star field
populations younger than ~ 3 Gyr. When focusing on the most prominent stellar
populations, we find that the derived AMRs are engraved by the evidence of a
outside-in star formation process. The studied fields show bimodal metallicity
distributions peaked at [Fe/H] = (-0.95 +- 0.15) dex and (-1.15 or -1.25 +-
0.05) dex, respectively, but only during the first half of the entire galaxy
lifetime. Furthermore, the more metal-rich population appears to be more
numerous in the outer fields, while in the innermost Fornax field the
contribution of both metallicity populations seems to be similar. We also find
that the metallicity spread ~ 6 Gyr ago is remarkable large, while the
intrinsic metallicity dispersion at ~ 1-2 Gyr results smaller than that for the
relatively older generations of stars. We interpret these outcomes as a result
of a possible merger of two galaxies that would have triggered a star formation
bursting process that peaked between ~ 6 and 9 Gyr ago, depending on the
position of the field in the galaxy.Comment: 7 pages, 5 figures, MNRAS, in pres
Serendipitous discovery of a strong-lensed galaxy in integral field spectroscopy from MUSE
2MASX J04035024-0239275 is a bright red elliptical galaxy at redshift 0.0661
that presents two extended sources at 2\arcsec~to the north-east and
1\arcsec~to the south-west. The sizes and surface brightnesses of the two blue
sources are consistent with a gravitationally-lensed background galaxy. In this
paper we present MUSE observations of this galaxy from the All-weather MUse
Supernova Integral-field Nearby Galaxies (AMUSING) survey, and report the
discovery of a background lensed galaxy at redshift 0.1915, together with other
15 background galaxies at redshifts ranging from 0.09 to 0.9, that are not
multiply imaged. We have extracted aperture spectra of the lens and all the
sources and fit the stellar continuum with STARLIGHT to estimate their stellar
and emission line properties. A trace of past merger and active nucleus
activity is found in the lensing galaxy, while the background lensed galaxy is
found to be star-forming. Modeling the lensing potential with a singular
isothermal ellipsoid, we find an Einstein radius of 1\farcs450\farcs04,
which corresponds to 1.9 kpc at the redshift of the lens and it is much smaller
than its effective radius ( 9\arcsec). Comparing the Einstein
mass and the STARLIGHT stellar mass within the same aperture yields a dark
matter fraction of \% within the Einstein radius. The advent of
large surveys such as the Large Synoptic Survey Telescope (LSST) will discover
a number of strong-lensed systems, and here we demonstrate how wide-field
integral field spectroscopy offers an excellent approach to study them and to
precisely model lensing effects.Comment: 12 pages, 12 Figures, 4 Tables. Accepted in MNRA
Magnetic domain wall creep and depinning: A scalar field model approach
Magnetic domain wall motion is at the heart of new magnetoelectronic technologies and hence the need for a deeper understanding of domain wall dynamics in magnetic systems. In this context, numerical simulations using simple models can capture the main ingredients responsible for the complex observed domain wall behavior. We present a scalar field model for the magnetization dynamics of quasi-two-dimensional systems with a perpendicular easy axis of magnetization which allows a direct comparison with typical experimental protocols, used in polar magneto-optical Kerr effect microscopy experiments. We show that the thermally activated creep and depinning regimes of domain wall motion can be reached and the effect of different quenched disorder implementations can be assessed with the model. In particular, we show that the depinning field increases with the mean grain size of a Voronoi tessellation model for the disorder.Fil: Caballero, Nirvana Belén. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Ferrero, Ezequiel E.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Kolton, Alejandro Benedykt. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Curiale, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Jeudy, Vincent. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; Francia. Université Paris–Saclay; FranciaFil: Bustingorry, Sebastián. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin
A feature selection method based on Shapley values robust to concept shift in regression
Feature selection is one of the most relevant processes in any methodology
for creating a statistical learning model. Generally, existing algorithms
establish some criterion to select the most influential variables, discarding
those that do not contribute any relevant information to the model. This
methodology makes sense in a classical static situation where the joint
distribution of the data does not vary over time. However, when dealing with
real data, it is common to encounter the problem of the dataset shift and,
specifically, changes in the relationships between variables (concept shift).
In this case, the influence of a variable cannot be the only indicator of its
quality as a regressor of the model, since the relationship learned in the
traning phase may not correspond to the current situation. Thus, we propose a
new feature selection methodology for regression problems that takes this fact
into account, using Shapley values to study the effect that each variable has
on the predictions. Five examples are analysed: four correspond to typical
situations where the method matches the state of the art and one example
related to electricity price forecasting where a concept shift phenomenon has
occurred in the Iberian market. In this case the proposed algorithm improves
the results significantly
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