780 research outputs found
Modelling and interpreting spectral energy distributions of galaxies with BEAGLE
We present a new-generation tool to model and interpret spectral energy
distributions (SEDs) of galaxies, which incorporates in a consistent way the
production of radiation and its transfer through the interstellar and
intergalactic media. This flexible tool, named BEAGLE (for BayEsian Analysis of
GaLaxy sEds), allows one to build mock galaxy catalogues as well as to
interpret any combination of photometric and spectroscopic galaxy observations
in terms of physical parameters. The current version of the tool includes
versatile modeling of the emission from stars and photoionized gas, attenuation
by dust and accounting for different instrumental effects, such as
spectroscopic flux calibration and line spread function. We show a first
application of the BEAGLE tool to the interpretation of broadband SEDs of a
published sample of galaxies at redshifts . We find that the constraints derived on photometric redshifts
using this multi-purpose tool are comparable to those obtained using public,
dedicated photometric-redshift codes and quantify this result in a rigorous
statistical way. We also show how the post-processing of BEAGLE output data
with the Python extension PYP-BEAGLE allows the characterization of systematic
deviations between models and observations, in particular through posterior
predictive checks. The modular design of the BEAGLE tool allows easy extensions
to incorporate, for example, the absorption by neutral galactic and
circumgalactic gas, and the emission from an active galactic nucleus, dust and
shock-ionized gas. Information about public releases of the BEAGLE tool will be
maintained on http://www.jacopochevallard.org/beagle.Comment: added missing term in equation 4.1 (Erratum submitted to MNRAS
Enseñar matemáticas en la sociedad de mañana: alegato a favor de un contraparadigma emergente
El análisis histórico de la enseñanza de las matemáticas en secundaria muestra cómo se han ido sucediendo en el tiempo diferentes paradigmas escolares. En este trabajo se describe y se analiza un nuevo paradigma didáctico, todavía en su infancia, el paradigma del “cuestionamiento del mundo”, que se basa principalmente en cuatro conceptos interrelacionados, el de la indagación y el de ser “herbartiano”, “procognitivo”, y “exotérico”. La ambición del autor es mostrar, aunque sea de manera sucinta, cómo la actual crisis en la educación matemática se podría con suerte resolver según estas líneas, descartando el recurso a estrategias que solo buscan poner parches al viejo paradigma, todavía dominante, “de la visita de las obras”
The [OIII]H Equivalent Width Distribution at z7: Implications for the Contribution of Galaxies to Reionization
We quantify the distribution of [OIII]+H line strengths at z7
using a sample of 20 bright (M 21) galaxies. We
select these systems over wide-area fields (2.3 deg total) using a new
colour-selection which precisely selects galaxies at z6.636.83, a
redshift range where blue Spitzer/IRAC [3.6][4.5] colours unambiguously
indicate strong [OIII]H emission. These 20 galaxies suggest a
log-normal [OIII]H EW distribution with median EW =
759 and standard deviation =
0.26 dex. We find no evidence for strong variation in this EW
distribution with UV luminosity. The typical [OIII]+H EW at z7
implied by our sample is considerably larger than that in massive star forming
galaxies at z2, consistent with a shift toward larger average sSFR (4.4
Gyr) and lower metallicities (0.16 Z). We also find evidence for
the emergence of a population with yet more extreme nebular emission
([OIII]+H EW1200 ) that is rarely seen at
lower redshifts. These objects have extremely large sSFR (30 Gyr), as
would be expected for systems undergoing a burst or upturn in star formation.
While this may be a short-lived phase, our results suggest that 20% of the
z7 population has such extreme nebular emission, implying that galaxies
likely undergo intense star formation episodes regularly at z6. We argue
that this population may be among the most effective ionizing agents in the
reionization era, both in terms of photon production efficiency and escape
fraction. We furthermore suggest that galaxies passing through this large sSFR
phase are likely to be very efficient in forming bound star clusters.Comment: 20 pages, 11 figures. Accepted in MNRAS with minor revision
On using the ATD: Some clarifications and comments
The main purpose of this opening lecture is to clarify and comment on a number of aspects of the genesis, structure and use of the ATD that I believe deserve clarification. The current weight of common opinion in didactics, in fact, has sometimes led either to the forgetting of certain key elements of the anthropological theory of the didactic, or to a somewhat limited understanding of what it implies. The most visible effect of this phenomenon is the fact that the theoretical and practical use of the theory does not always seem optimal. In the following, I have therefore tried to identify and cope with a selection of these difficulties of reception, that I felt it was possible to address in a meaningful way in the context of this presentation
La pédagogie de l'enquête dans l'éducation au développement durable
International audienceThis study highlights the difficulties that assail a pedagogy based on knowing-throughinquiry, which is so essential to the citizen today, notably in the field of sustainable development education. In a world shaped by an incomplete intellectual education, retrocognitive habitus hinder the necessary adoption of the proactive mode of study, crucial to free intellectual activity.Cette étude met en avant les difficultés auxquelles se heurte une pédagogie de la connaissance par l'enquête, essentielle au citoyen aujourd'hui, notamment dans le domaine de l'éducation au développement durable. Dans un monde façonné par une éducation intellectuelle incomplète, les habitus rétrocognitifs font obstacle à l'adoption nécessaire du mode d'étude proactif, crucial dans l'activité intellectuelle libre
Effect of primordial non-Gaussianities on the far-UV luminosity function of high-redshift galaxies: implications for cosmic reionization
[Abridged] Understanding how the intergalactic medium (IGM) was reionized at
z > 6 is one of the big challenges of current high redshift astronomy. It
requires modelling the collapse of the first astrophysical objects (Pop III
stars, first galaxies) and their interaction with the IGM, while at the same
time pushing current observational facilities to their limits. The
observational and theoretical progress of the last few years have led to the
emergence of a coherent picture in which the budget of hydrogen-ionizing
photons is dominated by low-mass star-forming galaxies, with little
contribution from Pop III stars and quasars. The reionization history of the
Universe therefore critically depends on the number density of low-mass
galaxies at high redshift. In this work, we explore how changes in the
statistical properties of initial density fluctuations affect the formation of
early galaxies. Following Habouzit et al. (2014), we run 5 N-body simulations
with Gaussian and (scale-dependent) non-Gaussian initial conditions, all
consistent with Planck constraints. By appealing to a galaxy formation model
and to a population synthesis code, we compute the far-UV galaxy luminosity
function down to M_UV = -14 at redshift 7 < z < 15. We find that models with
strong primordial non-Gaussianities on < Mpc scales show a far-UV luminosity
function significantly enhanced in low-mass galaxies. We adopt a reionization
model calibrated from state-of-the-art hydrodynamical simulations and show that
such non-Gaussianities leave a clear imprint on the Universe reionization
history and electron Thomson scattering optical depth tau_E. Although current
uncertainties in the physics of reionization and on the determination of tau_E
still dominate the signatures of non-Gaussianities, our results suggest that
tau_E could ultimately be used to constrain the statistical properties of
initial density fluctuations.Comment: 18 pages, 12 figures, accepted for publication in MNRA
Testing primordial non-Gaussianities on galactic scales at high redshift
Primordial non-Gaussianities provide an important test of inflationary
models. Although the Planck CMB experiment has produced strong limits on
non-Gaussianity on scales of clusters, there is still room for considerable
non-Gaussianity on galactic scales. We have tested the effect of local
non-Gaussianity on the high redshift galaxy population by running five
cosmological N-body simulations down to z=6.5. For these simulations, we adopt
the same initial phases, and either Gaussian or scale-dependent non-Gaussian
primordial fluctuations, all consistent with the constraints set by Planck on
clusters scales. We then assign stellar masses to each halo using the halo -
stellar mass empirical relation of Behroozi et al. (2013). Our simulations with
non-Gaussian initial conditions produce halo mass functions that show clear
departures from those obtained from the analogous simulations with Gaussian
initial conditions at z>~10. We observe a >0.3 dex enhancement of the low-end
of the halo mass function, which leads to a similar effect on the galaxy
stellar mass function, which should be testable with future galaxy surveys at
z>10. As cosmic reionization is thought to be driven by dwarf galaxies at high
redshift, our findings may have implications for the reionization history of
the Universe.Comment: 6 pages, 3 figures, 1 table, MNRAS (Letters) in pres
Zig-zag instability of an Ising wall in liquid crystals
We present a theoretical explanation for the interfacial zigzag instability
that appears in anisotropic systems. Such an instability has been
experimentally highlighted for an Ising wall formed in a nematic liquid crystal
cell under homeotropic anchoring conditions. From an envelope equation,
relevant close to the Freedericksz transition, we have derived an asymptotic
equation describing the interface dynamics in the vicinity of its bifurcation.
The asymptotic limit used accounts for a strong difference between two of the
elastic constants. The model is characterized by a conservative order parameter
which satisfies a Cahn-Hilliard equation. It provides a good qualitative
understanding of the experiments.Comment: 4 pagess, 4 figures, lette
Single polymer dynamics: coil-stretch transition in a random flow
By quantitative studies of statistics of polymer stretching in a random flow
and of a flow field we demonstrate that the stretching of polymer molecules in
a 3D random flow occurs rather sharply via the coil-stretch transition at the
value of the criterion close to theoretically predicted.Comment: 4 pages, 5 figure
Atomic-scale expressions for viscosity and fragile-strong behavior in metal alloys based on the Zwanzig-Mountain formula
We combine the shoving model of -dependent viscosity of supercooled
liquids with the Zwanzig-Mountain formula for the high-frequency shear modulus,
using the of MD simulations of metal alloys as the input. This scheme
leads to a semi-analytical expression for the viscosity as a function of
temperature, which provides a three-parameter model fitting of experimental
data of viscosity for the same alloy for which was calculated. The model
provides direct access to the influence of atomic-scale physical quantities
such as the interatomic potential , on the viscosity and
fragile-strong behavior. In particular, it is established that a steeper
interatomic repulsion leads to fragile liquids, or, conversely, that "soft
atoms make strong liquids"
- …