10,965 research outputs found
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
Vorticity generation in large-scale structure caustics
A fundamental hypothesis for the interpretation of the measured large-scale
line-of-sight peculiar velocities of galaxies is that the large-scale cosmic
flows are irrotational. In order to assess the validity of this assumption, we
estimate, within the frame of the gravitational instability scenario, the
amount of vorticity generated after the first shell crossings in large-scale
caustics. In the Zel'dovich approximation the first emerging singularities form
sheet like structures. Here we compute the expectation profile of an initial
overdensity under the constraint that it goes through its first shell crossing
at the present time. We find that this profile corresponds to rather oblate
structures in Lagrangian space. Assuming the Zel'dovich approximation is still
adequate not only at the first stages of the evolution but also slightly after
the first shell crossing, we calculate the size and shape of those caustics and
their vorticity content as a function of time and for different cosmologies.
The average vorticity created in these caustics is small: of the order of one
(in units of the Hubble constant). To illustrate this point we compute the
contribution of such caustics to the probability distribution function of the
filtered vorticity at large scales. We find that this contribution that this
yields a negligible contribution at the 10 to 15 Mpc scales. It becomes
significant only at the scales of 3 to 4 Mpc, that is, slightly above
the galaxy cluster scales.Comment: 25 pages 16 figures; accepted for publication by A&A vol 342 (1999
Dynamical flows through Dark Matter Haloes: Inner perturbative dynamics, secular evolution, and applications
We investigate statistically the dynamical consequences of cosmological
fluxes of matter and related moments on progenitors of today's dark matter
haloes. Their dynamics is described via canonical perturbation theory which
accounts for two types of perturbations: the tidal field corresponding to
fly-bys and accretion of dark matter through the halo's outer boundary. he
dynamical equations are solved linearly, order by order, projecting on a
biorthogonal basis to consistently satisfy the field equation. Since our
solution of the Boltzmann Poisson equations is explicit, it allows statistical
predictions for the ensemble distribution of the inner dynamical features of
haloes. The secular evolution of open galactic haloes is investigated: we
derive the kinetic equation which governs the quasi-linear evolution of dark
matter profile induced by infall and its corresponding gravitational
correlations. This yields a Fokker Planck-like equation for the angle-averaged
underlying distribution function. We show how these extensions to the classical
theory could be used to (i) observationally constrain the statistical nature of
the infall (ii) predict the observed distribution and correlations of
substructures in upcoming surveys, (iii) predict the past evolution of the
observed distribution of clumps, and finally (iv) weight the relative
importance of the intrinsic (via the unperturbed distribution function) and
external (tidal and/ or infall) influence of the environment in determining the
fate of galaxies.Comment: 35 pages, 12 Postscript figures, accepted for publication by MNRA
Lipschitz normal embedding among superisolated singularities
Any germ of a complex analytic space is equipped with two natural metrics:
the outer metric induced by the hermitian metric of the ambient space and the
inner metric, which is the associated riemannian metric on the germ. A complex
analytic germ is said Lipschitz normally embedded (LNE) if its outer and inner
metrics are bilipschitz equivalent. LNE seems to be fairly rare among surface
singularities; the only known LNE surface germs outside the trivial case
(straight cones) are the minimal singularities. In this paper, we show that a
superisolated hypersurface singularity is LNE if and only if its projectivized
tangent cone has only ordinary singularities. This provides an infinite family
of LNE singularities which is radically different from the class of minimal
singularities.Comment: 17 pages, 8 figures. Minor errors and misprints corrected. Comments
are welcome
Non parametric reconstruction of distribution functions from observed galactic disks
A general inversion technique for the recovery of the underlying distribution
function for observed galactic disks is presented and illustrated. Under the
assumption that these disks are axi-symmetric and thin, the proposed method
yields the unique distribution compatible with all the observables available.
The derivation may be carried out from the measurement of the azimuthal
velocity distribution arising from positioning the slit of a spectrograph along
the major axis of the galaxy. More generally, it may account for the
simultaneous measurements of velocity distributions corresponding to slits
presenting arbitrary orientations with respect to the major axis. The approach
is non-parametric, i.e. it does not rely on a particular algebraic model for
the distribution function. Special care is taken to account for the fraction of
counter-rotating stars which strongly affects the stability of the disk. An
optimisation algorithm is devised -- generalising the work of Skilling & Bryan
(1984) -- to carry this truly two-dimensional ill-conditioned inversion
efficiently. The performances of the overall inversion technique with respect
to the noise level and truncation in the data set is investigated with
simulated data. Reliable results are obtained up to a mean signal to noise
ratio of~5 and when measurements are available up to . A discussion of
the residual biases involved in non parametric inversions is presented.
Prospects of application to observed galaxies and other inversion problems are
discussed.Comment: 11 pages, 13 figures; accepted for publication by MNRA
Curve segmentation using directional information, relation to pattern detection
©2005 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or distribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder.Presented at the 2005 International Conference on Image Processing (ICIP)September 11-14, 2005, Genova, Italy.DOI: 10.1109/ICIP.2005.1530175We propose an extension of the conformal (or geodesic) active contour framework in which the conformal factor depends not only on the position of the curve but also on the direction of its tangent. We describe several properties for variational curve segmentation schemes that justify the construction of optimal conformal factors (i.e., learning) in strong connection with pattern matching. The determination of optimal curves (i.e., segmentation) can be performed using either the calculus of variations or dynamic programming. The technique is illustrated on a road detection problem for different signal to noise ratios
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