2,489 research outputs found
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
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
The rich cluster of galaxies ABCG~85. IV. Emission line galaxies, luminosity function and dynamical properties
This paper is the fourth of a series dealing with the cluster of galaxies
ABCG 85. Using our two extensive photometric and spectroscopic catalogues (with
4232 and 551 galaxies respectively), we discuss here three topics derived from
optical data. First, we present the properties of emission line versus
non-emission line galaxies, showing that their spatial distributions somewhat
differ; emission line galaxies tend to be more concentrated in the south region
where groups appear to be falling onto the main cluster, in agreement with the
hypothesis (presented in our previous paper) that this infall may create a
shock which can heat the X-ray emitting gas and also enhance star formation in
galaxies. Then, we analyze the luminosity function in the R band, which shows
the presence of a dip similar to that observed in other clusters at comparable
absolute magnitudes; this result is interpreted as due to comparable
distributions of spirals, ellipticals and dwarfs in these various clusters.
Finally, we present the dynamical analysis of the cluster using parametric and
non-parametric methods and compare the dynamical mass profiles obtained from
the X-ray and optical data.Comment: accepted for publication in A&
Encircling the dark: constraining dark energy via cosmic density in spheres
The recently published analytic probability density function for the mildly
non-linear cosmic density field within spherical cells is used to build a
simple but accurate maximum likelihood estimate for the redshift evolution of
the variance of the density, which, as expected, is shown to have smaller
relative error than the sample variance. This estimator provides a competitive
probe for the equation of state of dark energy, reaching a few percent accuracy
on wp and wa for a Euclid-like survey. The corresponding likelihood function
can take into account the configuration of the cells via their relative
separations. A code to compute one-cell density probability density functions
for arbitrary initial power spectrum, top-hat smoothing and various spherical
collapse dynamics is made available online so as to provide straightforward
means of testing the effect of alternative dark energy models and initial
power-spectra on the low-redshift matter distribution.Comment: 7 pages, replaced to match the MNRAS accepted versio
The three dimensional skeleton: tracing the filamentary structure of the universe
The skeleton formalism aims at extracting and quantifying the filamentary
structure of the universe is generalized to 3D density fields; a numerical
method for computating a local approximation of the skeleton is presented and
validated here on Gaussian random fields. This method manages to trace well the
filamentary structure in 3D fields such as given by numerical simulations of
the dark matter distribution on large scales and is insensitive to monotonic
biasing. Two of its characteristics, namely its length and differential length,
are analyzed for Gaussian random fields. Its differential length per unit
normalized density contrast scales like the PDF of the underlying density
contrast times the total length times a quadratic Edgeworth correction
involving the square of the spectral parameter. The total length scales like
the inverse square smoothing length, with a scaling factor given by 0.21 (5.28+
n) where n is the power index of the underlying field. This dependency implies
that the total length can be used to constrain the shape of the underlying
power spectrum, hence the cosmology. Possible applications of the skeleton to
galaxy formation and cosmology are discussed. As an illustration, the
orientation of the spin of dark halos and the orientation of the flow near the
skeleton is computed for dark matter simulations. The flow is laminar along the
filaments, while spins of dark halos within 500 kpc of the skeleton are
preferentially orthogonal to the direction of the flow at a level of 25%.Comment: 17 pages, 11 figures, submitted to MNRA
On the filamentary environment of galaxies
The correlation between the large-scale distribution of galaxies and their
spectroscopic properties at z=1.5 is investigated using the Horizon MareNostrum
cosmological run.
We have extracted a large sample of 10^5 galaxies from this large
hydrodynamical simulation featuring standard galaxy formation physics. Spectral
synthesis is applied to these single stellar populations to generate spectra
and colours for all galaxies. We use the skeleton as a tracer of the cosmic web
and study how our galaxy catalogue depends on the distance to the skeleton. We
show that galaxies closer to the skeleton tend to be redder, but that the
effect is mostly due to the proximity of large haloes at the nodes of the
skeleton, rather than the filaments themselves.
This effects translate into a bimodality in the colour distribution of our
sample. The origin of this bimodality is investigated and seems to follow from
the ram pressure stripping of satellite galaxies within the more massive
clusters of the simulation.
The virtual catalogues (spectroscopical properties of the MareNostrum
galaxies at various redshifts) are available online at
http://www.iap.fr/users/pichon/MareNostrum/cataloguesComment: 18 pages, 27 figures, accepted for publication in MNRA
Non-parametric reconstruction of distribution functions from observed galactic discs
A general inversion technique for the recovery of the underlying distribution function for observed galactic discs is presented and illustrated. Under the assumption that these discs are axisymmetric and thin, the proposed method yields a 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 disc. An optimization algorithm is devised â generalizing the work of Skilling &38; Bryan â to carry this truly two-dimensional ill-conditioned inversion efficiently. The performance 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 4 Re. A discussion of the residual biases involved in non-parametric inversions is presented. The prospects of application of the algorithm to observed galaxies and other inversion problems are discusse
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