49 research outputs found
On Physical Scales of Dark Matter Halos
It is common practice to describe formal size and mass scales of dark matter
halos as spherical overdensities with respect to an evolving density threshold.
Here, we critically investigate the evolutionary effects of several such
commonly used definitions and compare them to the halo evolution within fixed
physical scales as well as to the evolution of other intrinsic physical
properties of dark matter halos. It is shown that, in general, the traditional
way of characterizing sizes and masses of halos dramatically overpredicts the
degree of evolution in the last 10 Gyr, especially for low-mass halos. This
pseudo-evolution leads to the illusion of growth even though there are no major
changes within fixed physical scales. Such formal size definitions also serve
as proxies for the virialized region of a halo in the literature. In general,
those spherical overdensity scales do not coincide with the virialized region.
A physically more precise nomenclature would be to simply characterize them by
their very definition instead of calling such formal size and mass definitions
'virial'. In general, we find a discrepancy between the evolution of the
underlying physical structure of dark matter halos seen in cosmological
structure formation simulations and pseudo-evolving formal virial quantities.
We question the importance of the role of formal virial quantities currently
ubiquitously used in descriptions, models and relations that involve properties
of dark matter structures. Concepts and relations based on pseudo-evolving
formal virial quantities do not properly reflect the actual evolution of dark
matter halos and lead to an inaccurate picture of the physical evolution of our
universe.Comment: 17 pages, 14 figures, 1 table, ApJ accepte
Revisiting The First Galaxies: The effects of Population III stars on their host galaxies
We revisit the formation and evolution of the first galaxies using new
hydrodynamic cosmological simulations with the ART code. Our simulations
feature a recently developed model for H2 formation and dissociation, and a
star formation recipe that is based on molecular rather than atomic gas. Here,
we develop and implement a recipe for the formation of metal-free Population
III stars in galaxy-scale simulations that resolve primordial clouds with
sufficiently high density. We base our recipe on the results of prior zoom-in
simulations that resolved the protostellar collapse in pre-galactic objects. We
find the epoch during which Pop III stars dominated the energy and metal budget
of the first galaxies to be short-lived. Galaxies which host Pop III stars do
not retain dynamical signatures of their thermal and radiative feedback for
more than 10^8 yr after the lives of the stars end in pair-instability
supernovae, even when we consider the maximum reasonable efficiency of the
feedback. Though metals ejected by the supernovae can travel well beyond the
virial radius of the host galaxy, they typically begin to fall back quickly,
and do not enrich a large fraction of the intergalactic medium. Galaxies with
total mass in excess of 3 x 10^6 Msun re-accrete most of their baryons and
transition to metal-enriched Pop II star formation.Comment: 13 pages, 9 figures, published in Ap
Revisiting The First Galaxies: The epoch of Population III stars
We investigate the transition from primordial Population III (Pop III) star
formation to normal Pop II star formation in the first galaxies using new
cosmological hydrodynamic simulations. We find that while the first stars seed
their host galaxies with metals, they cannot sustain significant outflows to
enrich the intergalactic medium, even assuming a top-heavy initial mass
function. This means that Pop III star formation could potentially continue
until z~6 in different unenriched regions of the universe, before being
ultimately shut off by cosmic reionization. Within an individual galaxy, the
metal production and stellar feedback from Pop II stars overtake Pop III stars
in 20-200 Myr, depending on galaxy mass.Comment: 9 pages, 7 figures, published in Ap
Massive Black Hole Recoil in High Resolution Hosts
The final inspiral and coalescence of a black hole binary can produce highly
beamed gravitational wave radiation. To conserve linear momentum, the black
hole remnant can recoil with "kick" velocity as high as 4000 km/s. We present
two sets of full N-body simulations of recoiling massive black holes (MBH) in
high-resolution, non-axisymmetric potentials. The host to the first set of
simulations is the main halo of the Via Lactea I simulation (Diemand et al.
2007). The nature of the resulting orbits is investigated through a numerical
model where orbits are integrated assuming an evolving, triaxial NFW potential,
and dynamical friction is calculated directly from the velocity dispersion
along the major axes of the main halo of Via Lactea I. By comparing the
triaxial case to a spherical model, we find that the wandering time spent by
the MBH is significantly increased due to the asphericity of the halo. For
kicks larger than 200 km/s, the remnant MBH does not return to the inner 200 pc
within 1 Gyr, a timescale an order of magnitude larger than the upper limit of
the estimated QSO lifetime. The second set of simulations is run using the
outcome of a high-resolution gas-rich merger (Mayer et al. 2007) as host
potential. In this case, a recoil velocity of 500 km/s cannot remove the MBH
from the nuclear region.Comment: 4 pages, 4 figures. Proceedings of the conference Galactic & Stellar
Dynamics In the Era of High Resolution Survey
An optimum time-stepping scheme for N-body simulations
We present a new time-stepping criterion for N-body simulations that is based on the true dynamical time of a particle. This allows us to follow the orbits of particles correctly in all environments since it has better adaptivity than previous time-stepping criteria used in N-body simulations. Furthermore, it requires far fewer force evaluations in low-density regions of the simulation and has no dependence on artificial parameters such as, for example, the softening length. This can be orders of magnitude faster than conventional ad hoc methods that employ combinations of acceleration and softening and is ideally suited for hard problems, such as obtaining the correct dynamics in the very central regions of dark matter haloes. We also derive an eccentricity correction for a general leapfrog integration scheme that can follow gravitational scattering events for orbits with eccentricity e→ 1 with high precision. These new approaches allow us to study a range of problems in collisionless and collisional dynamics from few body problems to cosmological structure formation. We present tests of the time-stepping scheme in N-body simulations of two-body orbits with eccentricity e→ 1 (elliptic and hyperbolic), equilibrium haloes and a hierarchical cosmological structure formation ru
Cusps in cold dark matter haloes
We resolve the inner region of a massive cluster forming in a cosmological Λ cold dark matter (CDM) simulation with a mass resolution of 2 × 106 M⊙ and before z= 4.4 even 3 × 105 M⊙. This is a billion times less than the cluster's final virial mass and a substantial increase over current ΛCDM simulations. We achieve this resolution using a new multimass refinement procedure and are now able to probe a dark matter halo density profile down to 0.1 per cent of the virial radius. The inner density profile of this cluster halo is well fitted by a power law ρ∝r−γ down to the smallest resolved scale. An inner region with roughly constant logarithmic slope is now resolved, which suggests that cuspy profiles describe the inner profile better than recently proposed profiles with a core. The cluster studied here is one out of a sample of six high-resolution cluster simulations, and its inner slope of about γ= 1.2 lies close to the sample averag
Globular clusters, satellite galaxies and stellar haloes from early dark matter peaks
The Milky Way contains several distinct old stellar components that provide a fossil record of its formation. We can understand their spatial distribution and kinematics in a hierarchical formation scenario by associating the protogalactic fragments envisaged by Searle & Zinn (1978) with the rare peaks able to cool gas in the cold dark matter density field collapsing at redshift z > 10. We use hierarchical structure formation simulations to explore the kinematics and spatial distribution of these early star-forming structures in galaxy haloes today. Most of the protogalaxies rapidly merge, their stellar contents and dark matter becoming smoothly distributed and forming the inner Galactic halo. The metal-poor globular clusters and old halo stars become tracers of this early evolutionary phase, centrally biased and naturally reproducing the observed steep fall off with radius. The most outlying peaks fall in late and survive to the present day as satellite galaxies. The observed radial velocity dispersion profile and the local radial velocity anisotropy of Milky Way halo stars are successfully reproduced in this model. If this epoch of structure formation coincides with a suppression of further cooling into lower sigma peaks then we can reproduce the rarity, kinematics and spatial distribution of satellite galaxies as suggested by Bullock, Kravtsov & Weinberg (2000). Reionization at z= 12 ± 2 provides a natural solution to the missing satellites problem. Measuring the distribution of globular clusters and halo light on scales from galaxies to clusters could be used to constrain global versus local reionization models. If reionization occurs contemporary, our model predicts a constant frequency of blue globulars relative to the host halo mass, except for dwarf galaxies where the average relative frequencies become smalle
Globular clusters, satellite galaxies and stellar haloes from early dark matter peaks
The Milky Way contains several distinct old stellar components that provide a
fossil record of its formation. We can understand their spatial distribution
and kinematics in a hierarchical formation scenario by associating the
proto-galactic fragments envisaged by Searle and Zinn (1978) with the rare
peaks able to cool gas in the cold dark matter density field collapsing at
redshift z>10. We use hierarchical structure formation simulations to explore
the kinematics and spatial distribution of these early star-forming structures
in galaxy haloes today. Most of the proto-galaxies rapidly merge, their stellar
contents and dark matter becoming smoothly distributed and forming the inner
Galactic halo. The metal-poor globular clusters and old halo stars become
tracers of this early evolutionary phase, centrally biased and naturally
reproducing the observed steep fall off with radius. The most outlying peaks
fall in late and survive to the present day as satellite galaxies. The observed
radial velocity dispersion profile and the local radial velocity anisotropy of
Milky Way halo stars are successfully reproduced in this model. If this epoch
of structure formation coincides with a suppression of further cooling into
lower sigma peaks then we can reproduce the rarity, kinematics and spatial
distribution of satellite galaxies as suggested by Bullock et al. (2000).
Reionisation at z=12+/-2 provides a natural solution to the missing satellites
problem. Measuring the distribution of globular clusters and halo light on
scales from galaxies to clusters could be used to constrain global versus local
reionisation models. If reionisation occurs contemporary, our model predicts a
constant frequency of blue globulars relative to the host halo mass, except for
dwarf galaxies where the average relative frequencies become smaller.Comment: Submitted to MNRA
Does the Fornax dwarf spheroidal have a central cusp or core?
The dark matter dominated Fornax dwarf spheroidal has five globular clusters orbiting at ∼1 kpc from its centre. In a cuspy cold dark matter halo the globulars would sink to the centre from their current positions within a few Gyr, presenting a puzzle as to why they survive undigested at the present epoch. We show that a solution to this timing problem is to adopt a cored dark matter halo. We use numerical simulations and analytic calculations to show that, under these conditions, the sinking time becomes many Hubble times; the globulars effectively stall at the dark matter core radius. We conclude that the Fornax dwarf spheroidal has a shallow inner density profile with a core radius constrained by the observed positions of its globular clusters. If the phase space density of the core is primordial then it implies a warm dark matter particle and gives an upper limit to its mass of ∼0.5 keV, consistent with that required to significantly alleviate the substructure proble