28,852 research outputs found
The behaviour of shape and velocity anisotropy in dark matter haloes
Dark matter haloes from cosmological N-body simulations typically have
triaxial shapes and anisotropic velocity distributions. Recently it has been
shown that the velocity anisotropy, beta, of cosmological haloes and major
merger remnants depends on direction in such a way that beta is largest along
the major axis and smallest along the minor axis. In this work we use a wide
range of non-cosmological N-body simulations to examine halo shapes and
direction-dependence of velocity anisotropy profiles. For each of our simulated
haloes we define 48 cones pointing in different directions, and from the
particles inside each cone we compute velocity anisotropy profiles. We find
that elongated haloes can have very distinct velocity anisotropies. We group
the behaviour of haloes into three different categories, that range from
spherically symmetric profiles to a much more complex behaviour, where
significant differences are found for beta along the major and minor axes. We
encourage future studies of velocity anisotropies in haloes from cosmological
simulations to calculate beta-profiles in cones, since it reveals information,
which is hidden from a spherically averaged profile. Finally, we show that
spherically averaged profiles often obey a linear relation between beta and the
logarithmic density slope in the inner parts of haloes, but this relation is
not necessarily obeyed, when properties are calculated in cones.Comment: 23 pages, 14 figures. Accepted for publication in JCA
Asymmetric velocity anisotropies in remnants of collisionless mergers
Dark matter haloes in cosmological N-body simulations are affected by
processes such as mergers, accretion and the gravitational interaction with
baryonic matter. Typically the analysis of dark matter haloes is performed in
spherical or elliptical bins and the velocity distributions are often assumed
to be constant within those bins. However, the velocity anisotropy, which
describes differences between the radial and tangential velocity dispersion,
has recently been show to have a strong dependence on direction in the triaxial
halos formed in cosmological simulations. In this study we derive properties of
particles in cones parallel or perpendicular to the collision axis of merger
remnants. We find that the velocity anisotropy has a strong dependence on
direction. The finding that the direction-dependence of the velocity anisotropy
of a halo depends on the merger history, explains the existence of such trends
in cosmological simulations. It also explains why a large diversity is seen in
the velocity anisotropy profiles in the outer parts of high-resolution
simulations of cosmological haloes.Comment: 19 pages, 15 figures, Resubmitted to JCAP after referee comment
Observational constraints on the inflaton potential combined with flow-equations in inflaton space
Direct observations provide constraints on the first two derivatives of the
inflaton potential in slow roll models. We discuss how present day
observations, combined with the flow equations in slow roll parameter space,
provide a non-trivial constraint on the third derivative of the inflaton
potential. We find a lower bound on the third derivative of the inflaton
potential V'''/V > -0.2. We also show that unless the third derivative of the
inflaton potential is unreasonably large, then one predicts the tensor to
scalar ratio, r, to be bounded from below r > 3 * 10^{-6}.Comment: 4 pages, 2 figures. Important sign mistake corrected. Conclusions,
abstract and discussion change
On the extraction of spectral densities from lattice correlators
Hadronic spectral densities are important quantities whose non-perturbative
knowledge allows for calculating phenomenologically relevant observables, such
as inclusive hadronic cross-sections and non-leptonic decay-rates. The
extraction of spectral densities from lattice correlators is a notoriously
difficult problem because lattice simulations are performed in Euclidean time
and lattice data are unavoidably affected by statistical and systematic
uncertainties. In this paper we present a new method for extracting hadronic
spectral densities from lattice correlators. The method allows for choosing a
smearing function at the beginning of the procedure and it provides results for
the spectral densities smeared with this function together with reliable
estimates of the associated uncertainties. The same smearing function can be
used in the analysis of correlators obtained on different volumes, such that
the infinite volume limit can be studied in a consistent way. While the method
is described by using the language of lattice simulations, in reality it is
completely general and can profitably be used to cope with inverse problems
arising in different fields of research.Comment: 15 pages, 14 figures. Updated to match published versio
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