827 research outputs found
Wavelength flattened directional couplers for mirrorsymmetric interferometers
In the context of guided optics, we derive, analytically and geometrically, a
rigorous general criterion to design wavelength insensitive interferometers
with mirror symmetry, which are needed for wavelength
multiplexing/demultiplexing. The criterion is applied to a practical case,
resulting in an interferometer that works on a band wider than 70 nm.Comment: 21 pages, 9 figure
Adding Long Wavelength Modes to an -Body Simulation
We present a new method to add long wavelength power to an evolved -body
simulation, making use of the Zel'dovich (1970) approximation to change
positions and velocities of particles. We describe the theoretical framework of
our technique and apply it to a PM cosmological simulation performed on a
cube of Mpc on a side, obtaining a new ``simulation'' of Mpc on a
side. We study the effect of the power added by long waves by mean of several
statistics of the density and velocity field, and suggest possible applications
of our method to the study of the large-scale structure of the universe.Comment: Revised version, shortened. 15 pages without figures. Accepted for
publication in the Astrophysical Journal. Paper and 11 Figures available as
.ps.gz files by anonymous ftp at ftp://ftp.mpa-garching.mpg.de/pub/bepi/MA
Constraining the distribution of dark matter in dwarf spheroidal galaxies with stellar tidal streams
We use high-resolution N-body simulations to follow the formation and
evolution of tidal streams associated to dwarf spheroidal galaxies (dSphs). The
dSph models are embedded in dark matter (DM) haloes with either a
centrally-divergent 'cusp', or an homogeneous-density 'core'. In agreement with
previous studies, we find that as tides strip the galaxy the evolution of the
half-light radius and the averaged velocity dispersion follows well-defined
tracks that are mainly controlled by the amount of mass lost. Crucially, the
evolutionary tracks behave differently depending on the shape of the DM
profile: at a fixed remnant mass, dSphs embedded in cored haloes have larger
sizes and higher velocity dispersions than their cuspy counterparts. The
divergent evolution is particularly pronounced in galaxies whose stellar
component is strongly segregated within their DM halo and becomes more
disparate as the remnant mass decreases. Our analysis indicates that the DM
profile plays an important role in defining the internal dynamics of tidal
streams. We find that stellar streams associated to cored DM models have
velocity dispersions that lie systematically above their cuspy counterparts.
Our results suggest that the dynamics of streams with known dSph progenitors
may provide strong constraints on the distribution of DM on the smallest
galactic scales.Comment: 5 pages, 4 figure
Survival of Substructure within Dark Matter Haloes
Using high resolution cosmological N-body simulations, we investigate the
survival of dark matter satellites falling into larger haloes. Satellites
preserve their identity for some time after merging. We compute their loss of
mass, energy and angular momentum as dynamical friction, tidal forces and
collisions with other satellites dissolve them. We also analyse the evolution
of their internal structure. Satellites with less than a few per cent the mass
of the main halo may survive for several billion years, whereas larger
satellites rapidly sink into the center of the main halo potential well and
lose their identity. Penetrating encounters between satellites are frequent and
may lead to significant mass loss and disruption. Only a minor fraction of
cluster mass (10 per cent on average) is bound to substructure at most
redshifts of interest. We discuss the application of these results to the
survival and extent of dark matter haloes associated with cluster galaxies, and
to interactions between galaxies in clusters. We find that 35-40 per cent of
galaxy dark matter haloes are disrupted by the present time. The fraction of
satellites undergoing close encounters is similar to the fraction of
interacting or merging galaxies in clusters at moderate redshift.Comment: 16 pages, Latex, 14 Postscript figures. Submitted to MNRAS.
Postscript version also available at http://www.mpa-garching.mpg.de/~bep
Formation times and masses of dark matter haloes
The most commonly used definition of halo formation is the time when a halo's
most massive progenitor first contains at least half the final mass of its
parent. Reasonably accurate formulae for the distribution of formation times of
haloes of fixed mass have been available for some time. We use numerical
simulations of hierarchical gravitational clustering to test the accuracy of
formulae for the mass at formation. We also derive and test a formula for the
joint distribution of formation masses and times. The structure of a halo is
expected to be related to its accretion history. Our tests show that our
formulae for formation masses and times are reasonably accurate, so we expect
that they will aid future analytic studies of halo structure.Comment: 5 pages, 5 figures, MNRAS, in pres
The importance of the merging activity for the kinetic polarization of the Sunyaev-Zel'dovich signal from galaxy clusters
The polarization sensitivity of the upcoming millimetric observatories will
open new possibilities for studying the properties of galaxy clusters and for
using them as powerful cosmological probes. For this reason it is necessary to
investigate in detail the characteristics of the polarization signals produced
by their highly ionized intra-cluster medium (ICM). This work is focussed on
the polarization effect induced by the ICM bulk motions, the so-called kpSZ
signal, which has an amplitude proportional to the optical depth and to the
square of the tangential velocity. In particular we study how this polarization
signal is affected by the internal dynamics of galaxy clusters and what is its
dependence on the physical modelling adopted to describe the baryonic
component. This is done by producing realistic kpSZ maps starting from the
outputs of two different sets of high-resolution hydrodynamical N-body
simulations. The first set (17 objects) follows only non-radiative
hydrodynamics, while for each of 9 objects of the second set we implement four
different kinds of physical processes. Our results shows that the kpSZ signal
turns out to be a very sensitive probe of the dynamical status of galaxy
clusters. We find that major merger events can amplify the signal up to one
order of magnitude with respect to relaxed clusters, reaching amplitude up to
about 100 nuK. This result implies that the internal ICM dynamics must be taken
into account when evaluating this signal because simplicistic models, based on
spherical rigid bodies, may provide wrong estimates. Finally we find that the
dependence on the physical modelling of the baryonic component is relevant only
in the very inner regions of clusters.Comment: 13 pages, 7 figures, submitted to A&
Ellipsoidal halo finders and implications for models of triaxial halo formation
We describe an algorithm for identifying ellipsoidal haloes in numerical
simulations, and quantify how the resulting estimates of halo mass and shape
differ with respect to spherical halo finders. Haloes become more prolate when
fit with ellipsoids, the difference being most pronounced for the more
aspherical objects. Although the ellipsoidal mass is systematically larger,
this is less than 10% for most of the haloes. However, even this small
difference in mass corresponds to a significant difference in shape. We
quantify these effects also on the initial mass and deformation tensors, on
which most models of triaxial collapse are based. By studying the properties of
protohaloes in the initial conditions, we find that models in which protohaloes
are identified in Lagrangian space by three positive eigenvalues of the
deformation tensor are tenable only at the masses well-above . The
overdensity within almost any protohalo is larger than the critical
value associated with spherical collapse (increasing as mass decreases); this
is in good qualitative agreement with models which identify haloes requiring
that collapse have occured along all three principal axes, each axis having
turned around from the universal expansion at a different time. The
distributions of initial values are in agreement with the simplest predictions
associated with ellipsoidal collapse, assuming initially spherical protohaloes,
collapsed around random positions which were sufficiently overdense. However,
most protohaloes are not spherical and departures from sphericity increase as
protohalo mass decreases. [Abridged]Comment: 18 pages, 17 figures, accepted for publication in MNRA
Sunyaev-Zel'dovich profiles and scaling relations: modelling effects and observational biases
We use high-resolution hydrodynamic re-simulations to investigate the
properties of the thermal Sunyaev-Zel'dovich (SZ) effect from galaxy clusters.
We compare results obtained using different physical models for the
intracluster medium (ICM), and show how they modify the SZ emission in terms of
cluster profiles and scaling relations. We also produce realistic mock
observations to verify whether the results from hydrodynamic simulations can be
confirmed. We find that SZ profiles depend marginally on the modelled physical
processes, while they exhibit a strong dependence on cluster mass. The central
and total SZ emission strongly correlate with the cluster X-ray luminosity and
temperature. The logarithmic slopes of these scaling relations differ from the
self-similar predictions by less than 0.2; the normalization of the relations
is lower for simulations including radiative cooling. The observational test
suggests that SZ cluster profiles are unlikely to be able to probe the ICM
physics. The total SZ decrement appears to be an observable much more robust
than the central intensity, and we suggest using the former to investigate
scaling relations.Comment: 13 pages, 12 figures, accepted by MNRA
Extração, Caracterização de Substâncias Húmicas E Avaliação da Capacidade em Agregar Pireno em Meio Aquoso
TCC (graduação) - Universidade Federal de Santa Catarina. Centro de Ciências Físicas e Matemáticas. Curso de Química.As substâncias húmicas (SH), por representarem a maior parte da matéria orgânica dissolvida, são as principais responsáveis na regulação dos processos de degradação e disponibilidade de poluentes orgânicos hidrofóbicos aos ecossistemas. Neste trabalho, SH de diferentes ambientes foram extraídas e caracterizadas por análise elementar e espectroscópica, e através da técnica de supressão de fluorescência, foi determinada sua capacidade de agregar pireno em meio aquoso. Os resultados mostram que as SH têm características bem definidas com a origem da MO e chegam a agregar mais de 80 % do pireno
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