14 research outputs found
Detailed theoretical predictions of the outskirts of dark matter halos
In the present work we describe the formalism necessary to derive the
properties of dark matter halos beyond two virial radius using the spherical
collapse model (without shell crossing), and provide the framework for the
theoretical prediction presented in Prada et al. (2005). We show in detail how
to obtain within this model the probability distribution for the
spherically-averaged enclosed density at any radii P(delta,r). Using this
probability distribution, we compute the most probable and mean density
profiles, which turns out to differ considerably from each other. We also show
how to obtain the typical profile, as well as the probability distribution and
mean profile for the spherically averaged radial velocity. Two probability
distributions are obtained: a first one is derived using a simple assumption,
that is, if Q is the virial radius in Lagrangian coordinates, then the enclosed
linear contrast delta_l(q,Q) must satisfy the condition that delta_l(q=Q) =
delta_vir, where delta_vir is the linear density contrast within the virial
radius Rvir at the moment of virialization. Then we introduce an additional
constraint to obtain a more accurate P(delta,r) which reproduces to a higher
degree of precision the distribution of the spherically averaged enclosed
density found in the simulations. This new constraint is delta_l(q,Q) <
delta_vir for all q > Q, which means that there are no radii larger than Rvir
where the density contrast is larger than that used to define the virial
radius. Finally, we compare in detail our theoretical predictions for the
probability distributions with the results found in the simulations.Comment: 12 pages, 8 figures, 1 table, replaced to match the published versio
Statistics of Voids in the 2dF Galaxy Redshift Survey
We present a statistical analysis of voids in the 2dF galaxy redshift survey
(2dFGRS). In order to detect the voids, we have developed two robust
algorithms. We define voids as non-overlapping maximal spheres empty of halos
or galaxies with mass or luminosity above a given one. We search for voids in
cosmological -Body simulations to test the performance of our void finders.
We obtain and analyze the void statistics for several volume-limited samples
for the North Galactic Strip (NGP) and the South Galactic Strip (SGP)
constructed from the 2dFGRS full data release. We find that the results
obtained from the NGP and the SGP are statistically compatible. From the
results of several statistical tests we conclude that voids are essentially
uncorrelated, with at most a mild anticorrelation and that there is a
dependence of the void number density on redshift at least at the 99.5%
confidence level. We develop a technique to correct the distortion caused by
the fact that we use the redshift as the radial coordinate. We calibrate this
technique with mock catalogues and find that the correction might be of some
relevance to carry out accurate inferences from void statistics. We study the
statistics of the galaxies inside nine nearby voids. We find that galaxies in
voids are not randomly distributed: they form structures like filaments. We
also obtain the galaxy number density profile in voids. This profile follow a
similar but steeper trend to that follow by halos in voids.Comment: 16 pages, 10 figures, replaced to match the published version in
MNRA
A prescription for the conditional mass function of dark matter haloes
[ABRIDGED] The unconditional mass function (UMF) of dark matter haloes has
been determined accurately in the literature, showing excellent agreement with
high resolution numerical simulations. However, this is not the case for the
conditional mass function (CMF). We propose a simple analytical procedure to
derive the CMF by rescaling the UMF to the constrained environment using the
appropriate mean and variance of the density field at the constrained point.
This method introduces two major modifications with respect to the standard
re-scaling procedure. First of all, rather than using in the scaling procedure
the properties of the environment averaged over all the conditioning region, we
implement the re-scaling locally. We show that for high masses this
modification may lead to substantially different results. Secondly, we modify
the (local) standard re-scaling procedure in such a manner as to force
normalisation, in the sense that when one integrates the CMF over all possible
values of the constraint multiplied by their corresponding probability
distribution, the UMF is recovered. In practise, we do this by replacing in the
standard procedure the value delta_c (the linear density contrast for collapse)
by certain adjustable effective parameter delta_eff. In order to test the
method, we compare our prescription with the results obtained from numerical
simulations in voids (Gottlober et al. 2003), finding a very good agreement.
Based on these results, we finally present a very accurate analytical fit to
the (accumulated) conditional mass function obtained with our procedure, which
may be useful for any theoretical treatment of the large scale structure.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
Halo concentrations in the standard LCDM cosmology
We study the concentration of dark matter halos and its evolution in N-body
simulations of the standard LCDM cosmology. The results presented in this paper
are based on 4 large N-body simulations with about 10 billion particles each:
the Millennium-I and II, Bolshoi, and MultiDark simulations. The MultiDark (or
BigBolshoi) simulation is introduced in this paper. This suite of simulations
with high mass resolution over a large volume allows us to compute with
unprecedented accuracy the concentration over a large range of scales (about
six orders of magnitude in mass), which constitutes the state-of-the-art of our
current knowledge on this basic property of dark matter halos in the LCDM
cosmology. We find that there is consistency among the different simulation
data sets. We confirm a novel feature for halo concentrations at high
redshifts: a flattening and upturn with increasing mass. The concentration
c(M,z) as a function of mass and the redshift and for different cosmological
parameters shows a remarkably complex pattern. However, when expressed in terms
of the linear rms fluctuation of the density field sigma(M,z), the halo
concentration c(sigma) shows a nearly-universal simple U-shaped behaviour with
a minimum at a well defined scale at sigma=0.71. Yet, some small dependences
with redshift and cosmology still remain. At the high-mass end (sigma < 1) the
median halo kinematic profiles show large signatures of infall and highly
radial orbits. This c-sigma(M,z) relation can be accurately parametrized and
provides an analytical model for the dependence of concentration on halo mass.
When applied to galaxy clusters, our estimates of concentrations are
substantially larger -- by a factor up to 1.5 -- than previous results from
smaller simulations, and are in much better agreement with results of
observations. (abridged)Comment: Submitted to MNRA