8,411 research outputs found
High Sensitivity Array Observations of the QSO BRI 1335-0417
We present sensitive phase-referenced VLBI results on the radio continuum
emission from the QSO BRI 1335--0417. The observations were carried out
at 1.4 GHz using the High Sensitivity Array (HSA). Our sensitive VLBI image at
mas ( kpc) resolution shows continuum
emission in BRI 1335--0417 with a total flux density of Jy,
consistent with the flux density measured with the VLA. The size of the source
at FWHM is mas ( kpc) and the derived
intrinsic brightness temperature is K. No continuum
emission is detected at the full VLBI resolution ( mas, pc), with a 4 point source upper limit of 34 Jy
beam, or an upper limit to the intrinsic brightness temperature of
K. The highest angular resolution with at least a 4.5
detection of the radio continuum emission is mas ( kpc). At this resolution, the image shows a continuum feature in BRI
1335--0417 with a size of mas ( kpc) at FWHM,
and intrinsic brightness temperature of K. The extent of
the observed continuum sources at 1.4 GHz and the derived brightness
temperatures show that the radio emission (and thus presumably the far-infrared
emission) in BRI 1335--0417 is powered by a major starburst, with a massive
star formation rate of order a few thousand M_{\odot} {\rm yr}^{-1}z=4.4$ QSO.Comment: 13 pages, 3 figures, AJ accepte
Wigner-Crystal Formulation of Strong-Coupling Theory for Counter-ions Near Planar Charged Interfaces
We present a new analytical approach to the strong electrostatic coupling
regime (SC), that can be achieved equivalently at low temperatures, high
charges, low dielectric permittivity etc. Two geometries are analyzed in
detail: one charged wall first, and then, two parallel walls at small
distances, that can be likely or oppositely charged. In all cases, one type of
mobile counter-ions only is present, and ensures electroneutrality (salt free
case). The method is based on a systematic expansion around the ground state
formed by the two-dimensional Wigner crystal(s) of counter-ions at the
plate(s). The leading SC order stems from a single-particle theory, and
coincides with the virial SC approach that has been much studied in the last 10
years. The first correction has the functional form of the virial SC
prediction, but the prefactor is different. The present theory is free of
divergences and the obtained results, both for symmetrically and asymmetrically
charged plates, are in excellent agreement with available data of Monte-Carlo
simulations under strong and intermediate Coulombic couplings. All results
obtained represent relevant improvements over the virial SC estimates. The
present SC theory starting from the Wigner crystal and therefore coined Wigner
SC, sheds light on anomalous phenomena like the counter-ion mediated
like-charge attraction, and the opposite-charge repulsion
Remittances, exchange rate regimes, and the Dutch disease: a panel data analysis
Using disaggregated sectorial data, this study shows that rising levels of remittances have spending effects that lead to real exchange rate appreciation and resource movement effects that favor the nontradable sector at the expense of tradable goods production. These characteristics are two aspects of the phenomenon known as Dutch disease. The results further indicate that these effects operate more strongly under fixed nominal exchange rate regimes.Emigrant remittances ; Foreign exchange rates
An informational approach to the global optimization of expensive-to-evaluate functions
In many global optimization problems motivated by engineering applications,
the number of function evaluations is severely limited by time or cost. To
ensure that each evaluation contributes to the localization of good candidates
for the role of global minimizer, a sequential choice of evaluation points is
usually carried out. In particular, when Kriging is used to interpolate past
evaluations, the uncertainty associated with the lack of information on the
function can be expressed and used to compute a number of criteria accounting
for the interest of an additional evaluation at any given point. This paper
introduces minimizer entropy as a new Kriging-based criterion for the
sequential choice of points at which the function should be evaluated. Based on
\emph{stepwise uncertainty reduction}, it accounts for the informational gain
on the minimizer expected from a new evaluation. The criterion is approximated
using conditional simulations of the Gaussian process model behind Kriging, and
then inserted into an algorithm similar in spirit to the \emph{Efficient Global
Optimization} (EGO) algorithm. An empirical comparison is carried out between
our criterion and \emph{expected improvement}, one of the reference criteria in
the literature. Experimental results indicate major evaluation savings over
EGO. Finally, the method, which we call IAGO (for Informational Approach to
Global Optimization) is extended to robust optimization problems, where both
the factors to be tuned and the function evaluations are corrupted by noise.Comment: Accepted for publication in the Journal of Global Optimization (This
is the revised version, with additional details on computational problems,
and some grammatical changes
Dynamics of a tracer granular particle as a non-equilibrium Markov process
The dynamics of a tracer particle in a stationary driven granular gas is
investigated. We show how to transform the linear Boltzmann equation describing
the dynamics of the tracer into a master equation for a continuous Markov
process. The transition rates depend upon the stationary velocity distribution
of the gas. When the gas has a Gaussian velocity probability distribution
function (pdf), the stationary velocity pdf of the tracer is Gaussian with a
lower temperature and satisfies detailed balance for any value of the
restitution coefficient . As soon as the velocity pdf of the gas
departs from the Gaussian form, detailed balance is violated. This
non-equilibrium state can be characterized in terms of a Lebowitz-Spohn action
functional defined over trajectories of time duration . We
discuss the properties of this functional and of a similar functional
which differs from the first for a term which is non-extensive
in time. On the one hand we show that in numerical experiments, i.e. at finite
times , the two functionals have different fluctuations and
always satisfies an Evans-Searles-like symmetry. On the other hand we cannot
observe the verification of the Lebowitz-Spohn-Gallavotti-Cohen (LS-GC)
relation, which is expected for at very large times . We give
an argument for the possible failure of the LS-GC relation in this situation.
We also suggest practical recipes for measuring and
in experiments.Comment: 16 pages, 3 figures, submitted for publicatio
"GiGa": the Billion Galaxy HI Survey -- Tracing Galaxy Assembly from Reionization to the Present
In this paper, we review the Billion Galaxy Survey that will be carried out
at radio--optical wavelengths to micro--nanoJansky levels with the telescopes
of the next decades. These are the Low-Frequency Array, the Square Kilometer
Array and the Large Synoptic Survey Telescope as survey telescopes, and the
Thirty Meter class Telescopes for high spectral resolution+AO, and the James
Webb Space Telescope (JWST) for high spatial resolution near--mid IR follow-up.
With these facilities, we will be addressing fundamental questions like how
galaxies assemble with super-massive black-holes inside from the epoch of First
Light until the present, how these objects started and finished the
reionization of the universe, and how the processes of star-formation, stellar
evolution, and metal enrichment of the IGM proceeded over cosmic time. We also
summarize the high-resolution science that has been done thus far on high
redshift galaxies with the Hubble Space Telescope (HST). Faint galaxies have
steadily decreasing sizes at fainter fluxes and higher redshifts, reflecting
the hierarchical formation of galaxies over cosmic time. HST has imaged this
process in great structural detail to z<~6. We show that ultradeep
radio-optical surveys may slowly approach the natural confusion limit, where
objects start to unavoidably overlap because of their own sizes, which only SKA
can remedy with HI redshifts for individual sub-clumps. Finally, we summarize
how the 6.5 meter James Webb Space Telescope (JWST) will measure first light,
reionization, and galaxy assembly in the near--mid-IR.Comment: 8 pages, LaTeX2e requires 'aip' style (included), 8 postscript
figures. To appear in the proceedings of the `The Evolution of Galaxies
through the Neutral Hydrogen Window' conference, Arecibo Observatory Feb 1-3,
2008; Eds. R. Minchin & E. Momjian, AIP Conf Pro
Genes, Economics, and Happiness
We explore the influence of genetic variation on subjective well-being by employing a twin design and genetic association study. In a nationally-representative twin sample, we first show that about 33% of the variation in life satisfaction is explained by genetic variation. Although previous studies have shown that baseline happiness is significantly heritable, little research has considered molecular genetic associations with subjective well-being. We study the relationship between a functional polymorphism on the serotonin transporter gene (5-HTTLPR) and life satisfaction. We initially find that individuals with the longer, transcriptionally more efficient variant of this genotype report greater life satisfaction (n=2,545, p=0.012). However, our replication attempts on independent samples produce mixed results indicating that more work needs to be done to better understand the relationship between this genotype and subjective well-being. This work has implications for how economists think about the determinants of utility, and the extent to which exogenous shocks might affect individual well-being.life satisfaction, twin study, genetic association, serotonin transporter gene, 5-HTTLPR, rs2020933
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