147 research outputs found
The Halo Mass Function from Excursion Set Theory. II. The Diffusing Barrier
In excursion set theory the computation of the halo mass function is mapped
into a first-passage time process in the presence of a barrier, which in the
spherical collapse model is a constant and in the ellipsoidal collapse model is
a fixed function of the variance of the smoothed density field. However, N-body
simulations show that dark matter halos grow through a mixture of smooth
accretion, violent encounters and fragmentations, and modeling halo collapse as
spherical, or even as ellipsoidal, is a significant oversimplification. We
propose that some of the physical complications inherent to a realistic
description of halo formation can be included in the excursion set theory
framework, at least at an effective level, by taking into account that the
critical value for collapse is not a fixed constant , as in the
spherical collapse model, nor a fixed function of the variance of the
smoothed density field, as in the ellipsoidal collapse model, but rather is
itself a stochastic variable, whose scatter reflects a number of complicated
aspects of the underlying dynamics. Solving the first-passage time problem in
the presence of a diffusing barrier we find that the exponential factor in the
Press-Schechter mass function changes from to
, where and is the
diffusion coefficient of the barrier. The numerical value of , and
therefore the corresponding value of , depends among other things on the
algorithm used for identifying halos. We discuss the physical origin of the
stochasticity of the barrier and we compare with the mass function found in
N-body simulations, for the same halo definition.[Abridged]Comment: 7 pages, 5 figures. v3: significant conceptual improvements. More
detailed comparison with N-body simulations. References adde
Characterizing the Shapes of Galaxy Clusters Using Moments of the Gravitational Lensing Shear
We explore the use of the tangential component of weak lensing shear to
characterize the ellipticity of clusters of galaxies. We introduce an
ellipticity estimator, and quantify its properties for isolated clusters from
LCDM N-body simulations. We compare the N-body results to results from smooth
analytic models. The expected distribution of the estimator for mock
observations is presented, and we show how this distribution is impacted by
contaminants such as noise, line of sight projections, and misalignment of the
central galaxy used to determine the orientation of the triaxial halo. We
examine the radial profile of the estimator and discuss tradeoffs in the
observational strategy to determine cluster shape.Comment: 17 pages, 6 figures; added references, corrected typos, matches
published versio
The overdensity and masses of the friends-of-friends halos and universality of the halo mass function
The friends-of-friends algorithm (hereafter, FOF) is a percolation algorithm
which is routinely used to identify dark matter halos from N-body simulations.
We use results from percolation theory to show that the boundary of FOF halos
does not correspond to a single density threshold but to a range of densities
close to a critical value that depends upon the linking length parameter, b. We
show that for the commonly used choice of b = 0.2, this critical density is
equal to 81.62 times the mean matter density. Consequently, halos identified by
the FOF algorithm enclose an average overdensity which depends on their density
profile (concentration) and therefore changes with halo mass contrary to the
popular belief that the average overdensity is ~180. We derive an analytical
expression for the overdensity as a function of the linking length parameter b
and the concentration of the halo. Results of tests carried out using simulated
and actual FOF halos identified in cosmological simulations show excellent
agreement with our analytical prediction. We also find that the mass of the
halo that the FOF algorithm selects crucially depends upon mass resolution. We
find a percolation theory motivated formula that is able to accurately correct
for the dependence on number of particles for the mock realizations of
spherical and triaxial Navarro-Frenk-White halos. However, we show that this
correction breaks down when applied to the real cosmological FOF halos due to
presence of substructures. Given that abundance of substructure depends on
redshift and cosmology, we expect that the resolution effects due to
substructure on the FOF mass and halo mass function will also depend on
redshift and cosmology and will be difficult to correct for in general.
Finally, we discuss the implications of our results for the universality of the
mass function.Comment: 19 pages, 17 figures, submitted to ApJ supplemen
The Large Scale Bias of Dark Matter Halos: Numerical Calibration and Model Tests
We measure the clustering of dark matter halos in a large set of
collisionless cosmological simulations of the flat LCDM cosmology. Halos are
identified using the spherical overdensity algorithm, which finds the mass
around isolated peaks in the density field such that the mean density is Delta
times the background. We calibrate fitting functions for the large scale bias
that are adaptable to any value of Delta we examine. We find a ~6% scatter
about our best fit bias relation. Our fitting functions couple to the halo mass
functions of Tinker et. al. (2008) such that bias of all dark matter is
normalized to unity. We demonstrate that the bias of massive, rare halos is
higher than that predicted in the modified ellipsoidal collapse model of Sheth,
Mo, & Tormen (2001), and approaches the predictions of the spherical collapse
model for the rarest halos. Halo bias results based on friends-of-friends halos
identified with linking length 0.2 are systematically lower than for halos with
the canonical Delta=200 overdensity by ~10%. In contrast to our previous
results on the mass function, we find that the universal bias function evolves
very weakly with redshift, if at all. We use our numerical results, both for
the mass function and the bias relation, to test the peak-background split
model for halo bias. We find that the peak-background split achieves a
reasonable agreement with the numerical results, but ~20% residuals remain,
both at high and low masses.Comment: 11 pages, submitted to ApJ, revised to include referee's coment
Detection of lensing substructure using ALMA observations of the dusty galaxy SDP.81
We study the abundance of substructure in the matter density near galaxies
using ALMA Science Verification observations of the strong lensing system
SDP.81. We present a method to measure the abundance of subhalos around
galaxies using interferometric observations of gravitational lenses. Using
simulated ALMA observations, we explore the effects of various systematics,
including antenna phase errors and source priors, and show how such errors may
be measured or marginalized. We apply our formalism to ALMA observations of
SDP.81. We find evidence for the presence of a
subhalo near one of the images, with a significance of in a joint
fit to data from bands 6 and 7; the effect of the subhalo is also detected in
both bands individually. We also derive constraints on the abundance of dark
matter subhalos down to , pushing down to the
mass regime of the smallest detected satellites in the Local Group, where there
are significant discrepancies between the observed population of luminous
galaxies and predicted dark matter subhalos. We find hints of additional
substructure, warranting further study using the full SDP.81 dataset
(including, for example, the spectroscopic imaging of the lensed carbon
monoxide emission). We compare the results of this search to the predictions of
CDM halos, and find that given current uncertainties in the host halo
properties of SDP.81, our measurements of substructure are consistent with
theoretical expectations. Observations of larger samples of gravitational
lenses with ALMA should be able to improve the constraints on the abundance of
galactic substructure.Comment: 18 pages, 13 figures, Comments are welcom
Mass Function Predictions Beyond LCDM
The mass distribution of halos, as specified by the halo mass function, is a
key input for several cosmological probes. The sizes of -body simulations
are now such that, for the most part, results need no longer be
statistics-limited, but are still subject to various systematic uncertainties.
We investigate and discuss some of the reasons for these differences.
Quantifying error sources and compensating for them as appropriate, we carry
out a high-statistics study of dark matter halos from 67 -body simulations
to investigate the mass function and its evolution for a reference CDM
cosmology and for a set of CDM cosmologies. For the reference CDM
cosmology (close to WMAP5), we quantify the breaking of universality in the
form of the mass function as a function of redshift, finding an evolution of as
much as 10% away from the universal form between redshifts and . For
cosmologies very close to this reference we provide a fitting formula to our
results for the (evolving) CDM mass function over a mass range of
M to an estimated accuracy of about
2%. The set of CDM cosmologies is taken from the Coyote Universe simulation
suite. The mass functions from this suite (which includes a CDM
cosmology and others with ) are described by the fitting formula for
the reference CDM case at an accuracy level of 10%, but with clear
systematic deviations. We argue that, as a consequence, fitting formulae based
on a universal form for the mass function may have limited utility in high
precision cosmological applications.Comment: 19 pages; 18 figures; accepted for publication in the Ap
Mass functions and bias of dark matter halos
We revisit the study of the mass functions and the bias of dark matter halos.
Focusing on the limit of rare massive halos, we point out that exact analytical
results can be obtained for the large-mass tail of the halo mass function. This
is most easily seen from a steepest-descent approach, that becomes
asymptotically exact for rare events. We also revisit the traditional
derivation of the bias of massive halos, associated with overdense regions in
the primordial density field. We check that the theoretical large-mass cutoff
agrees with the mass functions measured in numerical simulations. For halos
defined by a nonlinear threshold this corresponds to using a
linear threshold instead of the traditional value . We also provide a fitting formula that matches simulations over all
mass scales and obeys the exact large-mass tail. Next, paying attention to the
Lagrangian-Eulerian mapping (i.e. corrections associated with the motions of
halos), we improve the standard analytical formula for the bias of massive
halos. We check that our prediction, which contains no free parameter, agrees
reasonably well with numerical simulations. In particular, it recovers the
steepening of the dependence on scale of the bias that is observed at higher
redshifts, which published fitting formulae did not capture. This behavior
mostly arises from nonlinear biasing.Comment: 15 page
Collapse Barriers and Halo Abundance: Testing the Excursion Set Ansatz
Our heuristic understanding of the abundance of dark matter halos centers
around the concept of a density threshold, or "barrier", for gravitational
collapse. If one adopts the ansatz that regions of the linearly evolved density
field smoothed on mass scale M with an overdensity that exceeds the barrier
will undergo gravitational collapse into halos of mass M, the corresponding
abundance of such halos can be estimated simply as a fraction of the mass
density satisfying the collapse criterion divided by the mass M. The key
ingredient of this ansatz is therefore the functional form of the collapse
barrier as a function of mass M or, equivalently, of the variance sigma^2(M).
Several such barriers based on the spherical, Zel'dovich, and ellipsoidal
collapse models have been extensively discussed. Using large scale cosmological
simulations, we show that the relation between the linear overdensity and the
mass variance for regions that collapse to form halos by the present epoch
resembles expectations from dynamical models of ellipsoidal collapse. However,
we also show that using such a collapse barrier with the excursion set ansatz
predicts a halo mass function inconsistent with that measured directly in
cosmological simulations. This inconsistency demonstrates a failure of the
excursion set ansatz as a physical model for halo collapse. We discuss
implications of our results for understanding the collapse epoch for halos as a
function of mass, and avenues for improving consistency between analytical
models for the collapse epoch and the results of cosmological simulations.Comment: Version accepted by ApJ, scheduled for May 2009, v696. High-res
version available at
http://kicp.uchicago.edu/~brant/astro-ph/excursion_set_ansatz/robertson_excursion_set_ansatz.pd
The evolution of substructure II: linking dynamics to environment
We present results from a series of high-resolution N-body simulations that
focus on the formation and evolution of eight dark matter halos, each of order
a million particles within the virial radius. We follow the time evolution of
hundreds of satellite galaxies with unprecedented time resolution, relating
their physical properties to the differing halo environmental conditions. The
self-consistent cosmological framework in which our analysis was undertaken
allows us to explore satellite disruption within live host potentials, a
natural complement to earlier work conducted within static potentials. Our host
halos were chosen to sample a variety of formation histories, ages, and
triaxialities; despite their obvious differences, we find striking similarities
within the associated substructure populations. Namely, all satellite orbits
follow nearly the same eccentricity distribution with a correlation between
eccentricity and pericentre. We also find that the destruction rate of the
substructure population is nearly independent of the mass, age, and triaxiality
of the host halo. There are, however, subtle differences in the velocity
anisotropy of the satellite distribution. We find that the local velocity bias
at all radii is greater than unity for all halos and this increases as we move
closer to the halo centre, where it varies from 1.1 to 1.4. For the global
velocity bias we find a small but slightly positive bias, although when we
restrict the global velocity bias calculation to satellites that have had at
least one orbit, the bias is essentially removed.Comment: 14 pages, 14 figures, MNRAS in pres
The Effect of Environment on Shear in Strong Gravitational Lenses
Using new photometric and spectroscopic data in the fields of nine strong
gravitational lenses that lie in galaxy groups, we analyze the effects of both
the local group environment and line-of-sight galaxies on the lens potential.
We use Monte Carlo simulations to derive the shear directly from measurements
of the complex lens environment, providing the first detailed independent check
of the shear obtained from lens modeling. We account for possible tidal
stripping of the group galaxies by varying the fraction of total mass
apportioned between the group dark matter halo and individual group galaxies.
The environment produces an average shear of gamma = 0.08 (ranging from 0.02 to
0.17), significant enough to affect quantities derived from lens observables.
However, the direction and magnitude of the shears do not match those obtained
from lens modeling in three of the six 4-image systems in our sample (B1422,
RXJ1131, and WFI2033). The source of this disagreement is not clear, implying
that the assumptions inherent in both the environment and lens model approaches
must be reconsidered. If only the local group environment of the lens is
included, the average shear is gamma = 0.05 (ranging from 0.01 to 0.14),
indicating that line-of-sight contributions to the lens potential are not
negligible. We isolate the effects of various theoretical and observational
uncertainties on our results. Of those uncertainties, the scatter in the
Faber-Jackson relation and error in the group centroid position dominate.
Future surveys of lens environments should prioritize spectroscopic sampling of
both the local lens environment and objects along the line of sight,
particularly those bright (I < 21.5) galaxies projected within 5' of the lens.Comment: Accepted for publication in The Astrophysical Journal; 28 pages, 9
figures, 5 table
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