3,280 research outputs found
Halpha surface photometry of galaxies in the Virgo cluster. IV: the current star formation in nearby clusters of galaxies
Halpha+[NII] imaging observations of 369 late-type galaxies in the Virgo
cluster and in the Coma/A1367 supercluster are analyzed. They constitute an
optically selected sample (m_p<16.0) observed with 60% c.a. completeness.These
observations provide us with the current (T<10^7 yrs) star formation properties
of galaxies. The expected decrease of the star formation rate (SFR),as traced
by the Halpha E.W., with decreasing clustercentric projected distance is found
only when galaxies brighter than M_p=-19.5 are considered. We also include in
our analysis Near Infrared data, providing us with informations on the old
(T>10^9yrs) stars. Put together, the young and the old stellar indicators give
the ratio of currently formed stars over the stars formed in the past, or
"birthrate" parameter b. We also determine the "global gas content" combining
HI with CO observations. We define the "gas deficiency" parameter as the
logarithmic difference between the gas content of isolated galaxies of a given
Hubble type and the measured gas content.For the isolated objects we find that
b decreases with increasing NIR luminosity. The gas-deficient objects,
primarily members to the Virgo cluster, have their birthrate significantly
lower than the isolated objects with normal gas content and of similar NIR
luminosity. This indicates that the current star formation is regulated by the
gaseous content of spirals.Whatever mechanism (most plausibly ram-pressure
stripping) is responsible for the pattern of gas deficiency observed in spiral
galaxies members to rich clusters, it also produces the observed quenching of
the current star formation.Comment: 22 pages,14 figures,3 figures available in jpeg format.To be
published in A&
A Holographic Path to the Turbulent Side of Gravity
We study the dynamics of a 2+1 dimensional relativistic viscous conformal
fluid in Minkowski spacetime. Such fluid solutions arise as duals, under the
"gravity/fluid correspondence", to 3+1 dimensional asymptotically anti-de
Sitter (AAdS) black brane solutions to the Einstein equation. We examine
stability properties of shear flows, which correspond to hydrodynamic
quasinormal modes of the black brane. We find that, for sufficiently high
Reynolds number, the solution undergoes an inverse turbulent cascade to long
wavelength modes. We then map this fluid solution, via the gravity/fluid
duality, into a bulk metric. This suggests a new and interesting feature of the
behavior of perturbed AAdS black holes and black branes, which is not readily
captured by a standard quasinormal mode analysis. Namely, for sufficiently
large perturbed black objects (with long-lived quasinormal modes), nonlinear
effects transfer energy from short to long wavelength modes via a turbulent
cascade within the metric perturbation. As long wavelength modes have slower
decay, this lengthens the overall lifetime of the perturbation. We also discuss
various implications of this behavior, including expectations for higher
dimensions, and the possibility of predicting turbulence in more general
gravitational scenarios.Comment: 24 pages, 10 figures; v2: references added, and several minor change
Modelling for Pest Risk Analysis: Spread and Economic Impacts
The introduction of invasive pests beyond their natural range is one of the main
causes of the loss of biodiversity and leads to severe costs. Bioeconomic models that
integrate biological invasion spread theory, economic impacts and invasion
management would be of great help to increase the transparency of pest risk
analysis (PRA) and provide for more effective and efficient management of invasive
pests.
In this thesis, bioeconomic models of management of invasive pests are developed.
The models are applied to three cases of study. The main case looks at the invasion
in Europe by the western corn rootworm (WCR), Diabrotica virgifera ssp. virgifera
LeConte (Coleoptera: Chrysomelidae). A range of quantitative modelling approaches
was employed: (i) dispersal kernels fitted to mark-release-recapture experimental
data; (ii) optimal control models combined with info-gap theory; (iii) spatially explicit
stochastic simulation models; and (iv) agent-based models.
As a result of the application of the models new insights on the management of
invasive pests and the links between spread and economic impacts were gained: (i)
current official management measures to eradicate WCR were found to be
ineffective; (ii) eradication and containment programmes that are economically
optimal under no uncertainty were found out to be also the most robustly immune
policy to unacceptable outcomes under severe uncertainty; (iii) PRA focusing on
single invasive pests might lead to management alternatives that dot not correspond
to the optimal economic allocation if the rest of the invasive pests sharing the same management budget are considered; (iv) the control of satellite colonies of an
invasion occurring by stratified dispersal is ineffective when a strong propagule
pressure is generated from the main body of the invasion and this effect is increased
by the presence of human-assisted long-distance dispersal; and (v) agent-based
models were shown to be an adequate tool to integrate biological invasion spread
models with economic analysis models
SOLUBILIZATION OF P-ALKYLPHENOLS IN PLURONICS F-68 AND F-127 MICELLES: PARTITION COEFFICIENTS AND EFFECT OF SOLUTE ON THE AGGREGATE STRUCTURE
Indexación: Web of Science; Scielo.The partition of a series of substituted phenols between water and polymer micelles formed by Pluronic F-68 and F-127 has been studied. The formation of micelles by these block copolymers has been evidenced by steady-state fluorescence and dynamic light scattering. The data show that micelles and larger aggregates are formed above the critical micelle concentration. In presence of p-alkylphenols a micelle rearrangement occurs that leads to the formation of just one and large aggregate. This effect depends both on polymer and phenols structures. The partition coefficients and the standard free energies of transfer from the aqueous to the micellar phases were determined. The incremental free energy for a methylene group are 2.88 kJ mol-1 and 1.65 kJ mol-1 for F-127 and F-68, respectively. This difference is explained in terms of the core sizes that are determined by the length of the PPO block.http://ref.scielo.org/xfqmx
Inhomogeneous nucleation in quark hadron phase transition
The effect of subcritical hadron bubbles on a first-order quark-hadron phase
transition is studied. These subcritical hadron bubbles are created due to
thermal fluctuations, and can introduce a finite amount of phase mixing (quark
phase mixed with hadron phase) even at and above the critical temperature. For
reasonable choices of surface tension and correlation length, as obtained from
the lattice QCD calculations, we show that the amount of phase mixing at the
critical temperature remains below the percolation threshold. Thus, as the
system cools below the critical temperature, the transition proceeds through
the nucleation of critical-size hadron bubbles from a metastable quark-gluon
phase (QGP), within an inhomogeneous background populated by an equilibrium
distribution of subcritical hadron bubbles. The inhomogeneity of the medium
results in a substantial reduction of the nucleation barrier for critical
bubbles. Using the corrected nucleation barrier, we estimate the amount of
supercooling for different parameters controlling the phase transition, and
briefly discuss its implications to cosmology and heavy-ion collisions.Comment: LaTeX, 14 pages with 8 Postscript figures. Discussion added in
introduction and conclusion, Fig. 8 added, few more references added,
Typographical errors corrected. Version to appear in Phys. Rev.
- …