5,483 research outputs found
Toward an Improved Analytical Description of Lagrangian Bias
We carry out a detailed numerical investigation of the spatial correlation
function of the initial positions of cosmological dark matter halos. In this
Lagrangian coordinate system, which is especially useful for analytic studies
of cosmological feedback, we are able to construct cross-correlation functions
of objects with varying masses and formation redshifts and compare them with a
variety of analytical approaches. For the case in which both formation
redshifts are equal, we find good agreement between our numerical results and
the bivariate model of Scannapieco & Barkana (2002; SB02) at all masses,
redshifts, and separations, while the model of Porciani et al. (1998) does well
for all parameters except for objects with different masses at small
separations. We find that the standard mapping between Lagrangian and Eulerian
bias performs well for rare objects at all separations, but fails if the
objects are highly-nonlinear (low-sigma) peaks. In the Lagrangian case in which
the formation redshifts differ, the SB02 model does well for all separations
and combinations of masses, apart from a discrepancy at small separations in
situations in which the smaller object is formed earlier and the difference
between redshifts or masses is large. As this same limitation arises in the
standard approach to the single-point progenitor distribution developed by
Lacey & Cole (1993), we conclude that a more complete understanding of the
progenitor distribution is the most important outstanding issue in the analytic
modeling of Lagrangian bias.Comment: 22 pages, 8 figures, ApJ, in pres
Predictions from Star Formation in the Multiverse
We compute trivariate probability distributions in the landscape, scanning
simultaneously over the cosmological constant, the primordial density contrast,
and spatial curvature. We consider two different measures for regulating the
divergences of eternal inflation, and three different models for observers. In
one model, observers are assumed to arise in proportion to the entropy produced
by stars; in the others, they arise at a fixed time (5 or 10 billion years)
after star formation. The star formation rate, which underlies all our observer
models, depends sensitively on the three scanning parameters. We employ a
recently developed model of star formation in the multiverse, a considerable
refinement over previous treatments of the astrophysical and cosmological
properties of different pocket universes. For each combination of observer
model and measure, we display all single and bivariate probability
distributions, both with the remaining parameter(s) held fixed, and
marginalized. Our results depend only weakly on the observer model but more
strongly on the measure. Using the causal diamond measure, the observed
parameter values (or bounds) lie within the central of nearly all
probability distributions we compute, and always within . This success
is encouraging and rather nontrivial, considering the large size and dimension
of the parameter space. The causal patch measure gives similar results as long
as curvature is negligible. If curvature dominates, the causal patch leads to a
novel runaway: it prefers a negative value of the cosmological constant, with
the smallest magnitude available in the landscape.Comment: 68 pages, 19 figure
Effective Screening due to Minihalos During the Epoch of Reionization
We show that the gaseous halos of collapsed objects introduce a substantial
cumulative opacity to ionizing radiation, even after the smoothly distributed
hydrogen in the intergalactic medium has been fully reionized. This opacity
causes a delay of around unity in redshift between the time of the overlap of
ionized bubbles in the intergalactic medium and the lifting of complete
Gunn-Peterson Lyman alpha absorption. The minihalos responsible for this
screening effect are not resolved by existing numerical simulations of
reionization.Comment: 24 pages, 5 figures, submitted to Ap
Mass of Clusters in Simulations
We show that dark matter haloes, in n--body simulations, have a boundary
layer (BL) with precise features. In particular, it encloses all dynamically
stable mass while, outside it, dynamical stability is lost soon. Particles can
pass through such BL, which however acts as a confinement barrier for dynamical
properties. BL is set by evaluating kinetic and potential energies (T(r) and
W(r)) and calculating R=-2T/W. Then, on BL, R has a minimum which closely
approaches a maximum of w= -dlog W/dlog r. Such ``requirement'' is
consistent with virial equilibrium, but implies further regularities. We test
the presence of a BL around haloes in spatially flat CDM simulations, with or
without cosmological constant. We find that the mass M_c, enclosed within the
radius r_c, where the requirement is fulfilled, closely approaches the
mass M_{dyn}, evaluated from the velocities of all particles within r_c,
according to the virial theorem. Using r_c we can then determine an individual
density contrast Delta_c for each virialized halo, which can be compared with
the "virial" density contrast (Omega_m: matter
density parameter) obtained assuming a spherically symmetric and unperturbed
fluctuation growth. The spread in Delta_c is wide, and cannot be neglected when
global physical quantities related to the clusters are calculated, while the
average Delta_c is ~25 % smaller than the corresponding Delta_v; moreover if
is defined from the radius linked to Delta_v, we have a much worse
fit with particle mass then starting from {\it Rw} requirement.Comment: 4 pages, 5 figures, contribution to the XXXVIIth Rencontres de
Moriond, The Cosmological Model, Les Arc March 16-23 2002, to appear in the
proceeding
Formation time distribution of dark matter haloes: theories versus N-body simulations
This paper uses numerical simulations to test the formation time distribution
of dark matter haloes predicted by the analytic excursion set approaches. The
formation time distribution is closely linked to the conditional mass function
and this test is therefore an indirect probe of this distribution. The
excursion set models tested are the extended Press-Schechter (EPS) model, the
ellipsoidal collapse (EC) model, and the non-spherical collapse boundary (NCB)
model. Three sets of simulations (6 realizations) have been used to investigate
the halo formation time distribution for halo masses ranging from dwarf-galaxy
like haloes (, where is the characteristic non-linear mass
scale) to massive haloes of . None of the models can match the
simulation results at both high and low redshift. In particular, dark matter
haloes formed generally earlier in our simulations than predicted by the EPS
model. This discrepancy might help explain why semi-analytic models of galaxy
formation, based on EPS merger trees, under-predict the number of high redshift
galaxies compared with recent observations.Comment: 7 pages, 5 figures, accepted for publication in MNRA
The mass function
We present the mass functions for different mass estimators for a range of
cosmological models. We pay particular attention to how universal the mass
function is, and how it depends on the cosmology, halo identification and mass
estimator chosen. We investigate quantitatively how well we can relate observed
masses to theoretical mass functions.Comment: 14 pages, 12 figures, to appear in ApJ
Dynamical Evolution of Galaxies in Clusters
Tidal forces acting on galaxies in clusters lead to a strong dynamical
evolution. In order to quantify the amount of evolution, I run self-consistent
N-body simulations of disk galaxies for a variety of models in the
hierarchically forming clusters. The tidal field along the galactic orbits is
extracted from the simulations of cluster formation in the Omega_0=1;
Omega_0=0.4; and Omega_0=0.4, Omega_Lambda=0.6 cosmological scenarios. For
large spiral galaxies with the rotation speed of 250 km/s, tidal interactions
truncate massive dark matter halos at 30 +- 6 kpc, and thicken stellar disks by
a factor 2 to 3, increasing Toomre's parameter to Q > 2 and halting star
formation. Low density galaxies, such as the dwarf spheroidals with the
circular velocity of 20 km/s and the extended low surface brightness galaxies
with the scale length of 10-15 kpc, are completely disrupted by tidal shocks.
Their debris contribute to the diffuse intracluster light. The tidal effects
are significant not only in the core but throughout the cluster and can be
parametrized by the critical tidal density. The tidally-induced evolution
results in the transformation of the infalling spirals into S0 galaxies and in
the depletion of the LSB population. In the low Omega_0 cosmological models,
clusters form earlier and produce stronger evolution of galaxies.Comment: accepted to Ap
Conditional Mass Functions and Merger Rates of Dark Matter Halos in the Ellipsoidal Collapse Model
Analytic models based on spherical and ellipsoidal gravitational collapse
have been used to derive the mass functions of dark matter halos and their
progenitors (the conditional mass function). The ellipsoidal model generally
provides a better match to simulation results, but there has been no simple
analytic expression in this model for the conditional mass function that is
accurate for small time steps, a limit that is important for generating halo
merger trees and computing halo merger rates. We remedy the situation by
deriving accurate analytic formulae for the first-crossing distribution, the
conditional mass function, and the halo merger rate in the ellipsoidal collapse
model in the limit of small look-back times. We show that our formulae provide
a closer match to the Millennium simulation results than those in the spherical
collapse model and the ellipsoidal model of Sheth & Tormen (2002).Comment: 5 pages, 3 figures, accepted by MNRAS letter
Nonlinear stochastic biasing from the formation epoch distribution of dark halos
We propose a physical model for nonlinear stochastic biasing of one-point
statistics resulting from the formation epoch distribution of dark halos. In
contrast to previous works on the basis of extensive numerical simulations, our
model provides for the first time an analytic expression for the joint
probability function. Specifically we derive the joint probability function of
halo and mass density contrasts from the extended Press-Schechter theory. Since
this function is derived in the framework of the standard gravitational
instability theory assuming the random-Gaussianity of the primordial density
field alone, we expect that the basic features of the nonlinear and stochastic
biasing predicted from our model are fairly generic. As representative
examples, we compute the various biasing parameters in cold dark matter models
as a function of a redshift and a smoothing length. Our major findings are (1)
the biasing of the variance evolves strongly as redshift while its
scale-dependence is generally weak and a simple linear biasing model provides a
reasonable approximation roughly at R\simgt 2(1+z)\himpc, and (2) the
stochasticity exhibits moderate scale-dependence especially on R\simlt
20\himpc, but is almost independent of . Comparison with the previous
numerical simulations shows good agreement with the above behavior, indicating
that the nonlinear and stochastic nature of the halo biasing is essentially
understood by taking account of the distribution of the halo mass and the
formation epoch.Comment: 34 pages, 11 figures, ApJ (2000) in pres
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