134 research outputs found
Complete Zeldovich approximation
We have developed a generalization of the Zeldovich approximation (ZA) that
is exact in a wide variety of situations, including plannar, spherical and
cilyndrical symmetries. We have shown that this generalization, that we call
complete Zeldovich approximation (CZA), is exact to second order at an
arbitrary point within any field. For gaussian fields, the third order error
have been obtained and shown to be very small. For statistical purposes, the
CZA leads to results exact to the third order.Comment: 11 pages+1 figure, accepted in ApJ Letter
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&
Probability distribution of density fluctuations in the non-linear regime
We present a general procedure for obtaining the present density fluctuation
probability distribution given the statistics of the initial conditions. The
main difficulties faced with regard to this problem are those related to the
non-linear evolution of the density fluctuations and those posed by the fact
that the fields we are interested in are the result of filtering an underlying
field with structure down to scales much smaller than that of filtering. The
solution to the latter problem is discussed here in detail and the solution to
the former is taken from a previous work.
We have checked the procedure for values of the rms density fluctuation as
large as 3/2 and several power spectra and found that it leads to results in
excellent agreement with those obtained in numerical simulations. We also
recover all available exact results from perturbation theory.Comment: Accepted to be published in Ap
The spherical collapse model with shell crossing
In this work, we study the formation and evolution of dark matter halos by
means of the spherical infall model with shell-crossing. We present a framework
to tackle this effect properly based on the numerical follow-up, with time, of
that individual shell of matter that contains always the same fraction of mass
with respect to the total mass. In this first step, we do not include angular
momentum, velocity dispersion or triaxiality. Within this framework - named as
the Spherical Shell Tracker (SST) - we investigate the dependence of the
evolution of the halo with virial mass, with the adopted mass fraction of the
shell, and for different cosmologies. We find that our results are very
sensitive to a variation of the halo virial mass or the mass fraction of the
shell that we consider. However, we obtain a negligible dependence on
cosmology. Furthermore, we show that the effect of shell-crossing plays a
crucial role in the way that the halo reaches the stabilization in radius and
the virial equilibrium. We find that the values currently adopted in the
literature for the actual density contrast at the moment of virialization,
delta_vir, may not be accurate enough. In this context, we stress the problems
related to the definition of a virial mass and a virial radius for the halo.
The question of whether the results found here may be obtained by tracking the
shells with an analytic approximation remains to be explored.Comment: 15 pages, 4 figures, 9 tables, replaced to match the published MNRAS
versio
Azimuthal dependence of the density distribution in outer galactic discs accreting intergalactic flows
AIMS. The amplitude and scaleheight of the Galactic gas disc density are not
axisymmetric against expectations in a self-gravity axisymmetric disc. However,
this lopsidedness can be explained in terms of intergalactic accretion flows,
which produce non-axisymmetric pressure on the disc. This mechanism could be
also responsible for the formation of a warp.
METHODS. We analytically derive the relationship between the disc density and
the self-gravity and external pressure.
RESULTS. The same scenario of accretion as we proposed years ago to explain
the formation of the warp explains the azimuthal dependence of the density and
its scaleheight, with minimum/maximum in the positions of maximum amplitude of
the warp (phi=95 deg. and 275 deg.), as expected from its pressure
distribution.Comment: 4 pages, accepted to be published in A&A-letter
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
Cosmic Voids and Galaxy Bias in the Halo Occupation Framework
(Abridged) We investigate the power of void statistics to constrain galaxy
bias and the amplitude of dark matter fluctuations. We use the halo occupation
distribution (HOD) framework to describe the relation between galaxies and dark
matter. After choosing HOD parameters that reproduce the mean space density
n_gal and projected correlation function w_p measured for galaxy samples with
M_r<-19 and M_r<-21 from the Sloan Digital Sky Survey (SDSS), we predict the
void probability function (VPF) and underdensity probability function (UPF) of
these samples by populating the halos of a large, high-resolution N-body
simulation. If we make the conventional assumption that the HOD is independent
of large scale environment at fixed halo mass, then models constrained to match
n_gal and w_p predict nearly identical void statistics, independent of the
scatter between halo mass and central galaxy luminosity or uncertainties in HOD
parameters. Models with sigma_8=0.7 and sigma_8=0.9 also predict very similar
void statistics. However, the VPF and UPF are sensitive to environmental
variations of the HOD in a regime where these variations have little impact on
w_p. For example, doubling the minimum host halo mass in regions with large
scale (5 Mpc/h) density contrast delta<-0.65 has a readily detectable impact on
void probabilities of M_r<-19 galaxies, and a similar change for delta<-0.2
alters the void probabilities of M_r<-21 galaxies at a detectable level. The
VPF and UPF provide complementary information about the onset and magnitude of
density- dependence in the HOD. By detecting or ruling out HOD changes in low
density regions, void statistics can reduce systematic uncertainties in the
cosmological constraints derived from HOD modeling, and, more importantly,
reveal connections between halo formation history and galaxy properties.Comment: emulateapj, 16 pages, 13 figure
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