1,699 research outputs found
Galaxy-Induced Transformation of Dark Matter Halos
We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the
effect of the assembly of a central galaxy on the shape and mass profile of its
dark halo. Two series of simulations are compared; one that follows only the
evolution of the dark matter component and a second one where a baryonic
component is added. These simulations include radiative cooling but neglect
star formation and feedback, leading most baryons to collect at the halo center
in a disk which is too small and too massive when compared with typical spiral.
This unrealistic model allows us, nevertheless, to gauge the maximum effect
that galaxies may have in transforming their dark halos. We find that the shape
of the halo becomes more axisymmetric: halos are transformed from triaxial into
essentially oblate systems, with well-aligned isopotential contours of roughly
constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy
assembly, but the effect is substantially less pronounced than predicted by the
"adiabatic contraction" hypothesis. The reduced contraction helps to reconcile
LambdaCDM halos with constraints on the dark matter content inside the solar
circle and should alleviate the long-standing difficulty of matching
simultaneously the scaling properties of galaxy disks and the luminosity
function. The halo contraction is also less pronounced than found in earlier
simulations, a disagreement that suggests that halo contraction is not solely a
function of the initial and final distribution of baryons. Not only how much
baryonic mass has been deposited at the center of a halo matters, but also the
mode of its deposition. It might prove impossible to predict the halo response
without a detailed understanding of a galaxy's assembly history. (Abriged)Comment: 11 pages and 9 figure
On DDO154 and Cold Dark Matter halo profiles
We investigate the claim by Burkert and Silk (1997) that the observed
rotation curve of the dwarf irregular galaxy DDO154 cannot be reconciled with
the universal CDM halo profile of Navarro, Frenk & White (1996,1997) even when
allowance is made for the effect of violent gas outflow events on the structure
of the galaxy. By means of N-body simulations we show that under certain
conditions it is possible to obtain a reasonable fit to the observed rotation
curve without invoking Burkert & Silk's proposed spheroidal MACHO component. We
are able to best reproduce the observed decline in the rotation curve by
postulating additional hidden disc mass, in an amount that is compatible with
disc stability requirements. In the process we improve upon the results of
Navarro, Eke & Frenk (1996) on the formation of halo cores by mass loss by
using actual haloes from Cold Dark Matter simulations instead of Hernquist
(1990) distributions.Comment: LaTeX (mn.sty), 8 pages, 6 figures included; updated to match final
version to appear in MNRA
Saddle-point entropy states of equilibrated self-gravitating systems
In this Letter, we investigate the stability of the statistical equilibrium
of spherically symmetric collisionless self-gravitating systems. By calculating
the second variation of the entropy, we find that perturbations of the relevant
physical quantities should be classified as long- and short-range
perturbations, which correspond to the long- and short-range relaxation
mechanisms, respectively. We show that the statistical equilibrium states of
self-gravitating systems are neither maximum nor minimum, but complex
saddle-point entropy states, and hence differ greatly from the case of ideal
gas. Violent relaxation should be divided into two phases. The first phase is
the entropy-production phase, while the second phase is the entropy-decreasing
phase. We speculate that the second-phase violent relaxation may just be the
long-wave Landau damping, which would work together with short-range
relaxations to keep the system equilibrated around the saddle-point entropy
states.Comment: 5 pages, 1 figure, MNRAS Letter, in the pres
On the streaming motions of haloes and galaxies
A simple model of how objects of different masses stream towards each other
as they cluster gravitationally is described. The model shows how the mean
streaming velocity of dark matter particles is related to the motions of the
parent dark matter haloes. It also provides a reasonably accurate description
of how the pairwise velocity dispersion of dark matter particles differs from
that of the parent haloes. The analysis is then extended to describe the
streaming motions of galaxies. This shows explicitly that the streaming motions
measured in a given galaxy sample depend on how the sample was selected, and
shows how to account for this dependence on sample selection. In addition,we
show that the pairwise dispersion should also depend on sample type. Our model
predicts that, on small scales, redshift space distortions should affect red
galaxies more strongly than blue.Comment: 10 pages, submitted to MNRA
Identikit 2: An Algorithm for Reconstructing Galactic Collisions
Using a combination of self-consistent and test-particle techniques,
Identikit 1 provided a way to vary the initial geometry of a galactic collision
and instantly visualize the outcome. Identikit 2 uses the same techniques to
define a mapping from the current morphology and kinematics of a tidal
encounter back to the initial conditions. By requiring that various regions
along a tidal feature all originate from a single disc with a unique
orientation, this mapping can be used to derive the initial collision geometry.
In addition, Identikit 2 offers a robust way to measure how well a particular
model reproduces the morphology and kinematics of a pair of interacting
galaxies. A set of eight self-consistent simulations is used to demonstrate the
algorithm's ability to search a ten-dimensional parameter space and find
near-optimal matches; all eight systems are successfully reconstructed.Comment: 14 pages, 8 figures. Accepted for publication in MNRAS. To get a copy
with high-resolution figures, use the web interface, or download the
Identikit software, visit
http://www.ifa.hawaii.edu/faculty/barnes/research/identikit
Assembly bias and the dynamical structure of dark matter halos
Based on the Millennium Simulation we examine assembly bias for the halo
properties: shape, triaxiality, concentration, spin, shape of the velocity
ellipsoid and velocity anisotropy. For consistency we determine all these
properties using the same set of particles, namely all gravitationally
self-bound particles belonging to the most massive sub-structure of a given
friends-of-friends halo. We confirm that near-spherical and high-spin halos
show enhanced clustering. The opposite is true for strongly aspherical and
low-spin halos. Further, below the typical collapse mass, M*, more concentrated
halos show stronger clustering whereas less concentrated halos are less
clustered which is reversed for masses above M*. Going beyond earlier work we
show that: (1) oblate halos are more strongly clustered than prolate ones; (2)
the dependence of clustering on the shape of the velocity ellipsoid coincides
with that of the real-space shape, although the signal is stronger; (3) halos
with weak velocity anisotropy are more clustered, whereas radially anisotropic
halos are more weakly clustered; (4) for all highly clustered subsets we find
systematically less radially biased velocity anisotropy profiles. These
findings indicate that the velocity structure of halos is tightly correlated
with environment.Comment: 5 pages, 2 figures, accepted for publication in Ap
Where can we really find the First Stars' Remnants today?
A number of recent numerical investigations concluded that the remnants of
rare structures formed at very high redshift, such as the very first stars and
bright redshift z~6 QSOs, are preferentially located at the center of the most
massive galaxy clusters at redshift z=0. In this paper we readdress this
question using a combination of cosmological simulations of structure formation
and extended Press-Schechter formalism and we show that the typical remnants of
Population III stars are instead more likely to be found in a group
environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun.
Similarly, the descendants of the brightest z~6 QSOs are expected to be in
medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the
most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1
Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims
that the most massive clusters preferentially host these remnants is rooted in
the numerical method used to initialize their numerical simulations: Only a
small region of the cosmological volume of interest was simulated with
sufficient resolution to identify low-mass halos at early times, and this
region was chosen to host the most massive halo in the cosmological volume at
late times. The conclusion that the earliest structures formed in the entire
cosmological volume evolve into the most massive halo at late times was thus
arrived at by construction. We demonstrate that, to the contrary, the first
structures to form in a cosmological region evolve into relatively typical
objects at later times. We propose alternative numerical methods for simulating
the earliest structures in cosmological volumes.Comment: 18 pages, 5 figures, ApJ accepted, high resolution version of the
paper available at http://www.stsci.edu/~trenti/papers/halo_evolution.pd
Scale-dependent bias and the halo model
We use a simplified version of the halo model with a power law power spectrum
to study scale dependence in galaxy bias at the very large scales relevant to
baryon oscillations. In addition to providing a useful pedagogical explanation
of the scale dependence of galaxy bias, the model provides an analytic tool for
studying how changes in the Halo Occupation Distribution (HOD) impact the scale
dependence of galaxy bias on scales between 10 and 1000 Mpc/h, which is useful
for interpreting the results of complex N-body simulations. We find that
changing the mean number of galaxies per halo of a given mass will change the
scale dependence of the bias, but that changing the way the galaxies are
distributed within the halo has a smaller effect on the scale dependence of
bias at large scales. We use the model to explain the decay in amplitude of the
baryon oscillations as k increases, and generalize the model to make
predictions about scale dependent galaxy bias when redshift space distortions
are introduced.Comment: 13 pages, 2 figures; corrected typos, extended discussion of redshift
space distortions, matches published versio
CIV Absorption From Galaxies in the Process of Formation
We investigate the heavy element QSO absorption systems caused by gas
condensations at high redshift which evolve into galaxies with circular
velocity of 100 to 200 km/s at the present epoch. Artificial QSO spectra were
generated for a variety of lines-of-sight through regions of the universe
simulated with a hydrodynamics code. The CIV and HI absorption features in
these spectra closely resemble observed CIV and HI absorption systems over a
wide range in column density. CIV absorption complexes with multiple-component
structure and velocity spreads up to about 600 km/s are found. The broadest
systems are caused by lines-of-sight passing through groups of protogalactic
clumps with individual velocity dispersions of less than 150 km/s aligned along
filamentary structures. The temperature of most of the gas does not take the
photoionization equilibrium value. This invalidates density and size estimates
derived from thermal equilibrium models. Consequences for metal abundance
determinations are briefly discussed. We predict occasional exceptionally large
ratios of CIV to HI column density (up to a third) for lines-of-sight passing
through compact halos of hot gas with temperature close to 3 10^5 K. Our model
may be able to explain both high-ionization multi-component heavy-element
absorbers and damped Lyman alpha systems as groups of small protogalactic
clumps.Comment: 13 pages, uuencoded postscript file, 4 figures included submitted to
ApJ (Letters); complete version also available at
http://www.mpa-garching.mpg.de/Galaxien/prep.htm
Preheating by Previrialization and its Impact on Galaxy Formation
We use recent observations of the HI-mass function to constrain galaxy
formation. The data conflicts with the standard model where most of the gas in
a low-mass dark matter halo is assumed to settle into a disk of cold gas that
is depleted by star formation and supernova-driven outflows until the disk
becomes gravitationally stable. A consistent model can be found if low-mass
haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev
cm^2. Such a model simultaneously matches the faint-end slope of the galaxy
luminosity function. We propose a preheating model where the medium around
low-mass haloes is preheated by gravitational pancaking. Since gravitational
tidal fields suppress the formation of low-mass haloes while promoting that of
pancakes, the formation of massive pancakes precedes that of the low-mass
haloes within them. We demonstrate that the progenitors of present-day dark
matter haloes with M<10^{12}h^{-1}\msun were embedded in pancakes of masses
~5x10^{12}h^{-1}\msun at z~2. The formation of such pancakes heats the gas to
a temperature of 5x10^5K and compresses it to an overdensity of ~10. Such gas
has a cooling time that exceeds the age of the Universe at z~2, and has a
specific entropy of ~15Kev cm^2, almost exactly the amount required to explain
the stellar and HI mass functions. (Abridged)Comment: 13 pages, 3 figures. Accepted for publication in MNRA
- …