18,496 research outputs found
Spherical Earth analysis and modeling of lithospheric gravity and magnetic anomalies
A comprehensive approach to the lithospheric analysis of potential field anomalies in the spherical domain is provided. It has widespread application in the analysis and design of satellite gravity and magnetic surveys for geological investigation
On bubble clustering and energy spectra in pseudo-turbulence
3D-Particle Tracking (3D-PTV) and Phase Sensitive Constant Temperature
Anemometry in pseudo-turbulence--i.e., flow solely driven by rising bubbles--
were performed to investigate bubble clustering and to obtain the mean bubble
rise velocity, distributions of bubble velocities, and energy spectra at dilute
gas concentrations (%). To characterize the clustering the pair
correlation function was calculated. The deformable bubbles with
equivalent bubble diameter mm were found to cluster within a radial
distance of a few bubble radii with a preferred vertical orientation. This
vertical alignment was present at both small and large scales. For small
distances also some horizontal clustering was found. The large number of
data-points and the non intrusiveness of PTV allowed to obtain well-converged
Probability Density Functions (PDFs) of the bubble velocity. The PDFs had a
non-Gaussian form for all velocity components and intermittency effects could
be observed. The energy spectrum of the liquid velocity fluctuations decayed
with a power law of -3.2, different from the found for
homogeneous isotropic turbulence, but close to the prediction -3 by
\cite{lance} for pseudo-turbulence
Breaking the self-averaging properties of spatial galaxy fluctuations in the Sloan Digital Sky Survey - Data Release Six
Statistical analyses of finite sample distributions usually assume that
fluctuations are self-averaging, i.e. that they are statistically similar in
different regions of the given sample volume. By using the scale-length method,
we test whether this assumption is satisfied in several samples of the Sloan
Digital Sky Survey Data Release Six. We find that the probability density
function (PDF) of conditional fluctuations, filtered on large enough spatial
scales (i.e., r>30 Mpc/h), shows relevant systematic variations in different
sub-volumes of the survey. Instead for scales r<30 Mpc/h the PDF is
statistically stable, and its first moment presents scaling behavior with a
negative exponent around one. Thus while up to 30 Mpc/h galaxy structures have
well-defined power-law correlations, on larger scales it is not possible to
consider whole sample average quantities as meaningful and useful statistical
descriptors. This situation is due to the fact that galaxy structures
correspond to density fluctuations which are too large in amplitude and too
extended in space to be self-averaging on such large scales inside the sample
volumes: galaxy distribution is inhomogeneous up to the largest scales, i.e. r
~ 100 Mpc/h, probed by the SDSS samples. We show that cosmological corrections,
as K-corrections and standard evolutionary corrections, do not qualitatively
change the relevant behaviors. Finally we show that the large amplitude galaxy
fluctuations observed in the SDSS samples are at odds with the predictions of
the standard LCDM model of structure formation.(Abridged version).Comment: 32 pages, 28 figures, accepted for publication in Astronomy and
Astrophysics. A higher resolution version is available at
http://pil.phys.uniroma1.it/~sylos/fsl_highlights.html . Version v2 has been
corrected to match the published on
Dynamical flows through Dark Matter Haloes II: one and two points statistics at the virial radius
In a serie of three papers, the dynamical interplay between environments and
dark matter haloes is investigated, while focussing on the dynamical flows
through their virial sphere. Our method relies on both cosmological
simulations, to constrain the environments, and an extension to the classical
matrix method to derive the response of the halo (see Pichon & Aubert (2006),
paper I).
The current paper focuses on the statistical characterisation of the
environments surrounding haloes, using a set of large scale simulations. Our
description relies on a `fluid' halocentric representation where the
interactions between the halo and its environment are investigated in terms of
a time dependent external tidal field and a source term characterizing the
infall. The method is applied to 15000 haloes, with masses between 5 x 10^12 Ms
and 10^14 Ms evolving between z = 1 and z = 0.
The net accretion at the virial radius is found to decrease with time,
resulting from both an absolute decrease of infall and from a growing
contribution of outflows. Infall is found to be mainly radial and occurring at
velocities ~ 0.75 V200. Outflows are also detected through the virial sphere
and occur at lower velocities ~ 0.6 V200 on more circular orbits. The external
tidal field is found to be strongly quadrupolar and mostly stationnary,
possibly reflecting the distribution of matter in the halo's near environment.
The coherence time of the small scale fluctuations of the potential hints a
possible anisotropic distribution of accreted satellites. The flux density of
mass on the virial sphere appears to be more clustered than the potential while
the shape of its angular power spectrum seems stationnary.Comment: 34 pages, 29 figures, accepted for publication in MNRA
The Toroidal Obscuration of Active Galactic Nuclei
Observations give strong support for the unification scheme of active
galactic nuclei. The scheme is premised on toroidal obscuration of the central
engine by dusty clouds that are individually very optically thick. These
lectures summarize the torus properties, describe the handling and implications
of its clumpy nature and present speculations about its dynamic origin.Comment: To appear in proceedings of "Active Galactic Nuclei at the highest
angular resolution: theory and observations", 2007 summer school, Torun,
Polan
Magnetic Fields and Faraday Rotation in Clusters of Galaxies
We present a numerical approach to investigate the relationship between
magnetic fields and Faraday rotation effects in clusters of galaxies. We can
infer the structure and strength of intra-cluster magnetic fields by comparing
our simulations with the observed polarization properties of extended cluster
radio sources such as radio galaxies and halos. We find the observations
require a magnetic field which fluctuates over a wide range of spatial scales
(at least one order of magnitude). If several polarized radio sources are
located at different projected positions in a galaxy cluster, as is the case
for A119, detailed Faraday rotation images allow us to constrain both the
magnetic field strength and the slope of the power spectrum. Our results show
that the standard analytic expressions applied in the literature overestimate
the cluster magnetic field strengths by a factor of about 2. We investigate the
possible effects of our models on beam depolarization of radio sources whose
radiation traverses the magnetized intracluster medium. Finally, we point out
that radio halos may provide important information about the spatial power
spectrum of the magnetic field fluctuations on large scales. In particular,
different values of the index of the power spectrum produce very different
total intensity and polarization brightness distributions.Comment: 19 pages, 11 figures. Accepted for publication in A&
A universal angular momentum profile for galactic halos
[Abridged] We study the angular-momentum profiles of a statistical sample of
halos drawn from a high-resolution N-body simulation of the LCDM cosmology. We
find that the cumulative mass distribution of specific angular momentum, j, in
a halo of mass Mv is well fit by a universal function, M(<j) = Mv \mu
j/(j_0+j). This profile is defined by one shape parameter (\mu or j_0) in
addition to the global spin parameter \lambda. It follows a power-law over most
of the mass, and flattens at large j, with the flattening more pronounced for
small values of \mu. Compared to a uniform sphere in solid-body rotation, most
halos have a higher fraction of their mass in the low- and high-j tails of the
distribution. The spatial distribution of angular momentum in halos tends to be
cylindrical and is well-aligned within each halo for ~80% of the halos. We
investigate two ideas for the origin of this profile. The first is based on a
revised version of linear tidal-torque theory combined with extended
Press-Schechter mass accretion, and the second focuses on j transport in minor
mergers. Finally, we briefly explore implications of the M(<j) profile on the
formation of galactic disks assuming that j is conserved during an adiabatic
baryonic infall. The implied gas density profile deviates from an exponential
disk, with a higher density at small radii and a tail extending to large radii.
The steep central density profiles may imply disk scale lengths that are
smaller than observed. This is reminiscent of the "angular-momentum problem"
seen in hydrodynamic simulations, even though we have assumed perfect j
conservation. A possible solution is to associate the central excesses with
bulge components and the outer regions with extended gaseous disks.Comment: 19 pages LaTeX, uses emulateapj5, 22 embedded figures, 1 separate
figure, Submitted to ApJ, version with higher quality figures available at
http://www.astronomy.ohio-state.edu/~james/PAPER/parts.htm
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