22 research outputs found
Fluctuations of K-band galaxy counts
We measure the variance in the distribution of off-plane (|b|>20 deg.)
galaxies with m_K<13.5 from the 2MASS K-band survey in circles of diameter
between 0.344 deg. and 57.2 deg. The use of a near-infrared survey makes
negligible the contribution of Galactic extinction to these fluctuations. We
calculate these variances within the standard Lambda-CDM model assuming that
the sources are distributed like halos of the corresponding mass, and it
reproduces qualitatively the galaxy counts variance. Therefore, we test that
the counts can be basically explained in terms only of the large scale
structure. A second result of this paper is a new method to determine the two
point correlation function obtained by forcing agreement between model and
data. This method does not need the knowledge of the two-point angular
correlation function, allows an estimation of the errors (which are low with
this method), and can be used even with incomplete surveys.
Using this method we get xi(z=0, r<10 h^{-1}Mpc)=(29.8+/-0.3)
(r/h^{-1}Mpc)^{-1.79+/-0.02}, which is the first measure of the amplitude of xi
in the local Universe for the K-band. It is more or less in agreement with
those obtained through red optical filters selected samples, but it is larger
than the amplitude obtained for blue optical filters selected samples.Comment: 7 pages, 5 figures, accepted to be published in A&
Generation of galactic disc warps due to intergalactic accretion flows onto the disc
A new method is developed to calculate the amplitude of the galactic warps
generated by a torque due to external forces. This takes into account that the
warp is produced as a reorientation of the different rings which constitute the
disc in order to compensate the differential precession generated by the
external force, yielding a uniform asymptotic precession for all rings.
Application of this method to gravitational tidal forces in the Milky Way due
to the Magellanic Clouds leads to a very low amplitude of the warp. If the
force were due to an extragalactic magnetic field, its intensity would have to
be very high, to generate the observed warps. An alternative hypothesis is
explored: the accretion of the intergalactic medium over the disk. A cup-shaped
distortion is expected, due to the transmission of the linear momentum; but,
this effect is small and the predominant effect turns out to be the
transmission of angular momentum, i.e. a torque giving an integral-sign shape
warp. The torque produced by a flow of velocity ~100 km/s and baryon density
\~10^{-25} kg/m^3 is enough to generate the observed warps and this mechanism
offers quite a plausible explanation. First, because this order of accretion
rate is inferred from other processes observed in the Galaxy, notably its
chemical evolution. The inferred rate of infall of matter, ~1 solar-mass/yr, to
the Galactic disc that this theory predicts agrees with the quantitative
predictions of this chemical evolution resolving key issues, notably the
G-dwarf problem. Second, because the required density of the intergalactic
medium is within the range of values compatible with observation. By this
mechanism, we can explain the warp phenomenon in terms of intergalactic
accretion flows onto the disk of the galaxy.Comment: 18 pages, 11 figures, accepted to be published in A&
Virial theorem in clusters of galaxies with MOND
A specific modification of Newtonian dynamics known as MOND has been shown to
reproduce the dynamics of most astrophysical systems at different scales
without invoking non-baryonic dark matter (DM). There is, however, a
long-standing unsolved problem when MOND is applied to rich clusters of
galaxies in the form of a deficit (by a factor around two) of predicted
dynamical mass derived from the virial theorem with respect to observations. In
this article we approach the virial theorem using the velocity dispersion of
cluster members along the line of sight rather than using the cluster
temperature from X-ray data and hydrostatic equilibrium. Analytical
calculations of the virial theorem in clusters for Newtonian gravity+DM and
MOND are developed, applying pressure (surface) corrections for non-closed
systems. Recent calibrations of DM profiles, baryonic ratio and baryonic
( model or others) profiles are used, while allowing free parameters to
range within the observational constraints. It is shown that solutions exist
for MOND in clusters that give similar results to Newton+DM -- particularly in
the case of an isothermal model for and core radii
between 0.1 and 0.3 times (in agreement with the known data).
The disagreements found in previous studies seem to be due to the lack of
pressure corrections (based on inappropriate hydrostatic equilibrium
assumptions) and/or inappropriate parameters for the baryonic matter profiles.Comment: accepted to be published in MNRA
Biases in galaxy cluster velocity dispersion and mass estimates in the small number of galaxies regime
We present a study of the statistical properties of three velocity dispersion
and mass estimators, namely biweight, gapper and standard deviation, in the
small number of galaxies regime ().
Using a set of 73 numerically simulated galaxy clusters, we characterise the
statistical bias and the variance for the three estimators, both in the
determination of the velocity dispersion and the dynamical mass of the clusters
via the relation. The results are used to define a new set of
unbiased estimators, that are able to correct for those statistical biases with
a minimal increase of the associated variance. The numerical simulations are
also used to characterise the impact of velocity segregation in the selection
of cluster members, and the impact of using cluster members within different
physical radii from the cluster centre.
The standard deviation is found to be the lowest variance estimator. The
selection of galaxies within the sub-sample of the most massive galaxies in the
cluster introduces a \% bias in the velocity dispersion estimate when
calculated using a quarter of the most massive cluster members. We also find a
dependence of the velocity dispersion estimate on the aperture radius as a
fraction of , consistent with previous results.
The proposed set of unbiased estimators effectively provides a correction of
the velocity dispersion and mass estimates from all those effects in the small
number of cluster members regime. This is tested by applying the new estimators
to a subset of simulated observations. Although for a single galaxy cluster the
statistical and physical effects discussed here are comparable or slightly
smaller than the bias introduced by interlopers, they will be of relevance when
dealing with ensemble properties and scaling relations for large cluster
samples (Abridged).Comment: accepted for publication in A&
Statistical Tests for CHDM and \LambdaCDM Cosmologies
We apply several statistical estimators to high-resolution N-body simulations
of two currently viable cosmological models: a mixed dark matter model, having
contributed by two massive neutrinos (C+2\nuDM), and a Cold
Dark Matter model with Cosmological Constant (\LambdaCDM) with
and h=0.7. Our aim is to compare simulated galaxy samples with the
Perseus-Pisces redshift survey (PPS). We consider the n-point correlation
functions (n=2-4), the N-count probability functions P_N, including the void
probability function P_0, and the underdensity probability function U_\epsilon
(where \epsilon fixes the underdensity threshold in percentage of the average).
We find that P_0 (for which PPS and CfA2 data agree) and P_1 distinguish
efficiently between the models, while U_\epsilon is only marginally
discriminatory. On the contrary, the reduced skewness and kurtosis are,
respectively, S_3\simeq 2.2 and S_4\simeq 6-7 in all cases, quite independent
of the scale, in agreement with hierarchical scaling predictions and estimates
based on redshift surveys. Among our results, we emphasize the remarkable
agreement between PPS data and C+2\nuDM in all the tests performed. In
contrast, the above \LambdaCDM model has serious difficulties in reproducing
observational data if galaxies and matter overdensities are related in a simple
way.Comment: 12 pages, 10 figures, LaTeX (aaspp4 macro), in press on ApJ, Vol.
479, April 199
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