661 research outputs found
Accurate halo mass functions from the simplest excursion set theory
Excursion set theory is a powerful and widely used tool for describing the
distribution of dark matter haloes, but it is normally applied with simplifying
approximations. We use numerical sampling methods to study the mass functions
predicted by the theory without approximations. With a spherical top-hat window
and a constant threshold, the theory accurately predicts mass
functions with the mass definition, both unconditional and
conditional, in simulations of a range of matter-dominated cosmologies. For
CDM at the present epoch, predictions lie between the
and mass functions. In contrast, with the
same window function, a nonconstant threshold based on ellipsoidal collapse
predicts uniformly too few haloes. This work indicates a new way to simply and
accurately evaluate halo mass functions, clustering bias, and assembly
histories for a range of cosmologies. We provide a simple fitting function that
accurately represents the predictions of the theory for a wide range of
parameters.Comment: 16 pages, 14 figures. Accepted by MNRAS; includes more estimates of
statistical uncertaint
Can prompt cusps of WIMP dark matter be detected as individual gamma-ray sources?
Prompt density cusps are the densest and most abundant
dark matter systems. If the dark matter is a weakly interacting massive
particle (WIMP), recent studies have shown that prompt cusps dominate the
aggregate dark matter annihilation rate. This article explores whether
individual prompt cusps could be detected as gamma-ray sources. At the Fermi
telescope's point-source sensitivity, WIMPs with the canonical annihilation
cross section could form detectable prompt cusps if the particle mass is of
order 10 GeV. These objects could be 10-100 pc away and weigh under a solar
mass; they would subtend around 0.1 degrees on the sky. For GeV-scale dark
matter particles with below-canonical cross sections, searches for individual
prompt cusps can be more sensitive than searches for the annihilation signals
from galactic dark matter halos.Comment: 8 pages, 5 figures; accepted by Physical Review
Massive prompt cusps: A new signature of warm dark matter
Every dark matter halo and subhalo is expected to have a prompt central density cusp, which is a relic of its condensation out of the
smooth mass distribution of the early universe. The sizes of these prompt cusps
are linked to the scales of the peaks in the initial density field from which
they formed. In warm dark matter (WDM) models, the smoothing scale set by free
streaming of the dark matter can result in prompt cusps with masses of order
M. We show that WDM models with particle masses ranging from 2
to 6 keV predict prompt cusps that could detectably alter the observed
kinematics of Local Group dwarf galaxies. Thus, prompt cusps present a viable
new probe of WDM. A prompt cusp's properties are highly sensitive to when it
formed, so prospects can be improved with a better understanding of when the
haloes of the Local Group dwarfs originally formed. Tidal stripping can also
affect prompt cusps, so constraints on satellite galaxy orbits can further
tighten WDM inferences.Comment: 5 pages, 6 figures; accepted by MNRAS Letters. Includes more detail
on the sampling of prompt cusp
Simulations of Gravitational Heating Due to Early Matter Domination
In cosmologies with an early matter-dominated era (EMDE) prior to Big Bang
nucleosynthesis, the boosted growth of small-scale matter perturbations during
the EMDE leads to microhalo formation long before halos would otherwise begin
to form. For a range of models, halos can even form during the EMDE itself.
These halos would dissipate at the end of the EMDE, releasing their
gravitationally heated dark matter and thereby imprinting a free-streaming
cut-off on the matter power spectrum. We conduct the first cosmological
-body simulations of the formation and evaporation of halos during and after
an EMDE. We show that in these scenarios, the free-streaming cut-off after the
EMDE can be predicted accurately from the linear matter power spectrum.
Although the free streaming can erase much of the EMDE-driven boost to density
perturbations, we use our findings to show that the (re-)formation of halos
after the EMDE nevertheless proceeds before redshift . Early-forming
microhalos are a key observational signature of an EMDE, and our prescription
for the impact of gravitational heating will allow studies of the observational
status and prospects of EMDE scenarios to cover a much wider range of
parameters.Comment: 33 pages, 16 figures. Comments welcom
Lensing constraints on ultradense dark matter halos
Cosmological observations precisely measure primordial variations in the
density of the Universe at megaparsec and larger scales, but much smaller
scales remain poorly constrained. However, sufficiently large initial
perturbations at small scales can lead to an abundance of ultradense dark
matter minihalos that form during the radiation epoch and survive into the
late-time Universe. Because of their early formation, these objects can be
compact enough to produce detectable microlensing signatures. We investigate
whether the EROS, OGLE, and HSC surveys can probe these halos by fully
accounting for finite source size and extended lens effects. We find that
current data may already constrain the amplitudes of primordial curvature
perturbations in a new region of parameter space, but this conclusion is
strongly sensitive to yet undetermined details about the internal structures of
these ultradense halos. Under optimistic assumptions, current and future HSC
data would constrain a power spectrum that features an enhancement at scales , and an amplitude as low as may be accessible. This is a particularly interesting regime because
it connects to primordial black hole formation in a portion of the
LIGO/Virgo/Kagra mass range and the production of scalar-induced gravitational
waves in the nanohertz frequency range reachable by pulsar timing arrays. These
prospects motivate further study of the ultradense halo formation scenario to
clarify their internal structures.Comment: 17 pages, 10 figures. v2: matching published versio
Inner cusps of the first dark matter haloes: Formation and survival in a cosmological context
We use very high resolution cosmological zoom simulations to follow the early
evolution of twelve first-generation haloes formed from gaussian initial
conditions with scale-free power spectra truncated on small scales by a
gaussian. Initial collapse occurs with a diverse range of sheet- or
filament-like caustic morphologies, but in almost all cases it gives rise to a
numerically converged density cusp with and total mass
comparable to that of the corresponding peak in the initial linear density
field. The constant can be estimated to within about 10 per cent from the
properties of this peak. This outcome agrees with earlier work on the first
haloes in cold and warm dark matter universes. Within central cusps, the
velocity dispersion is close to isotropic, and equidensity surfaces tend to
align with those of the main body of the halo at larger radii. As haloes grow,
their cusps are often (but not always) overlaid with additional material at
intermediate radii to produce profiles more similar to the Einasto or NFW forms
typical of more massive haloes. Nevertheless, to the extent that we can resolve
them, cusps survive at the smallest radii. Major mergers can disturb them, but
the effect is quite weak in the cases that we study. The cusps extend down to
the resolution limits of our simulations, which are typically a factor of
several larger than the cores that would be produced by phase-space
conservation if the initial power spectrum cutoff arises from free streaming.Comment: 23 pages, 28 figures; to be submitted to MNRA
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