390 research outputs found
Relic density and CMB constraints on dark matter annihilation with Sommerfeld enhancement
We calculate how the relic density of dark matter particles is altered when
their annihilation is enhanced by the Sommerfeld mechanism due to a Yukawa
interaction between the annihilating particles. Maintaining a dark matter
abundance consistent with current observational bounds requires the
normalization of the s-wave annihilation cross section to be decreased compared
to a model without enhancement. The level of suppression depends on the
specific parameters of the particle model, with the kinetic decoupling
temperature having the most effect. We find that the cross section can be
reduced by as much as an order of magnitude for extreme cases. We also compute
the mu-type distortion of the CMB energy spectrum caused by energy injection
from such Sommerfeld-enhanced annihilation. Our results indicate that in the
vicinity of resonances, associated with bound states, distortions can be large
enough to be excluded by the upper limit |mu|<9.0x10^(-5) found by the
COBE/FIRAS experiment.Comment: 10 pages, 6 figures, accepted for publication in Physical Review D.
Corrections to eqs. 9,10,14 and 16. Figures updated accordingly. No major
changes to previous results. Website with online tools for Sommerfeld-related
calculations can be found at
http://www.mpa-garching.mpg.de/~vogelsma/sommerfeld
The Missing Satellite Problem in 3D
It is widely believed that the large discrepancy between the observed number
of satellite galaxies and the predicted number of dark subhalos can be resolved
via a variety of baryonic effects which suppress star formation in low mass
halos.Supporting this hypothesis, numerous high resolution simulations with
star formation, and associated feedback have been shown to reproduce the
satellite luminosity function around Milky Way-mass simulated galaxies at
redshift zero. However, a more stringent test of these models is their ability
to simultaneously match the satellite luminosity functions of a range of host
halo masses and redshifts. In this work we measure the luminosity function of
faint (sub-Small Magellanic Cloud luminosity) satellites around hosts with
stellar masses 10.5M/M to an unprecedented
redshift of 1.5. This new measurement of the satellite luminosity function
provides powerful new constraining power; we compare these results with
predictions from four different simulations and show that although the models
perform similarly over-all, no one model reproduces the satellite luminosity
function reliably at all redshifts and host stellar masses. This result
highlights the continued need for improvement in understanding the fundamental
physics that governs satellite galaxy evolution.Comment: 9 pages, 2 figures, submitted to MNRA
Bound and unbound substructures in Galaxy-scale Dark Matter haloes
We analyse the coarse-grained phase-space structure of the six Galaxy-scale
dark matter haloes of the Aquarius Project using a state-of-the-art 6D
substructure finder. Within r_50, we find that about 35% of the mass is in
identifiable substructures, predominantly tidal streams, but including about
14% in self-bound subhaloes. The slope of the differential substructure mass
function is close to -2, which should be compared to around -1.9 for the
population of self-bound subhaloes. Near r_50 about 60% of the mass is in
substructures, with about 30% in self-bound subhaloes. The inner 35 kpc of the
highest resolution simulation has only 0.5% of its mass in self-bound
subhaloes, but 3.3% in detected substructure, again primarily tidal streams.
The densest tidal streams near the solar position have a 3-D mass density about
1% of the local mean, and populate the high velocity tail of the velocity
distribution.Comment: Submitted to MNRAS on 12/10/2010, 11 pages, 10 figure
Dark Matter Caustics
Caustics are a generic feature of the nonlinear growth of structure in the
dark matter distribution. If the dark matter were absolutely cold, its mass
density would diverge at caustics, and the integrated annihilation probability
would also diverge for individual particles participating in them. For
realistic dark matter candidates, this behaviour is regularised by small but
non-zero initial thermal velocities. We present a mathematical treatment of
evolution from Hot, Warm or Cold Dark Matter initial conditions which can be
directly implemented in cosmological N-body codes. It allows the identification
of caustics and the estimation of their annihilation radiation in fully general
simulations of structure formation.Comment: 6 pages, 1 figure, Accepted for publication in MNRAS, minor edit
Ultra-fine dark matter structure in the Solar neighbourhood
The direct detection of dark matter on Earth depends crucially on its density
and its velocity distribution on a milliparsec scale. Conventional N-body
simulations are unable to access this scale, making the development of other
approaches necessary. In this paper, we apply the method developed in Fantin et
al. 2008 to a cosmologically-based merger tree, transforming it into a useful
instrument to reproduce and analyse the merger history of a Milky Way-like
system. The aim of the model is to investigate the implications of any
ultra-fine structure for the current and next generation of directional dark
matter detectors. We find that the velocity distribution of a Milky Way-like
Galaxy is almost smooth, due to the overlap of many streams of particles
generated by multiple mergers. Only the merger of a 10^10 Msun analyse can
generate significant features in the ultra-local velocity distribution,
detectable at the resolution attainable by current experiments.Comment: 9 pages, 6 figures, accepted for publication in MNRA
A Deep Learning Approach to Galaxy Cluster X-ray Masses
We present a machine-learning approach for estimating galaxy cluster masses
from Chandra mock images. We utilize a Convolutional Neural Network (CNN), a
deep machine learning tool commonly used in image recognition tasks. The CNN is
trained and tested on our sample of 7,896 Chandra X-ray mock observations,
which are based on 329 massive clusters from the IllustrisTNG simulation. Our
CNN learns from a low resolution spatial distribution of photon counts and does
not use spectral information. Despite our simplifying assumption to neglect
spectral information, the resulting mass values estimated by the CNN exhibit
small bias in comparison to the true masses of the simulated clusters (-0.02
dex) and reproduce the cluster masses with low intrinsic scatter, 8% in our
best fold and 12% averaging over all. In contrast, a more standard core-excised
luminosity method achieves 15-18% scatter. We interpret the results with an
approach inspired by Google DeepDream and find that the CNN ignores the central
regions of clusters, which are known to have high scatter with mass.Comment: 10 pages, 6 figures, accepted for publication in The Astrophysical
Journa
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Reproducing the kinematics of damped Lyman α systems
We examine the kinematic structure of Damped Lyman-alpha Systems (DLAs) in a
series of cosmological hydrodynamic simulations using the AREPO code. We are
able to match the distribution of velocity widths of associated low ionisation
metal absorbers substantially better than earlier work. Our simulations produce
a population of DLAs dominated by halos with virial velocities around 70 km/s,
consistent with a picture of relatively small, faint objects. In addition, we
reproduce the observed correlation between velocity width and metallicity and
the equivalent width distribution of SiII. Some discrepancies of moderate
statistical significance remain; too many of our spectra show absorption
concentrated at the edge of the profile and there are slight differences in the
exact shape of the velocity width distribution. We show that the improvement
over previous work is mostly due to our strong feedback from star formation and
our detailed modelling of the metal ionisation state
Nonspherical similarity solutions for dark halo formation
We carry out fully 3-dimensional simulations of evolution from self-similar,
spherically symmetric linear perturbations of a Cold Dark Matter dominated
Einstein-de Sitter universe. As a result of the radial orbit instability, the
haloes which grow from such initial conditions are triaxial with major-to-minor
axis ratios of order 3:1. They nevertheless grow approximately self-similarly
in time. In all cases they have power-law density profiles and near-constant
velocity anisotropy in their inner regions. Both the power-law index and the
value of the velocity anisotropy depend on the similarity index of the initial
conditions, the former as expected from simple scaling arguments. Halo
structure is thus not "universal" but remembers the initial conditions. On
larger scales the density and anisotropy profiles show two characteristic
scales, corresponding to particles at first pericentre and at first apocentre
after infall. They are well approximated by the NFW model only for one value of
the similarity index. In contrast, at all radii within the outer caustic the
pseudo phase-space density can be fit by a single power law with an index which
depends only very weakly on the similarity index of the initial conditions.
This behaviour is very similar to that found for haloes formed from LCDM
initial conditions and so can be considered approximately universal.Comment: 8 pages, 7 figures, submitted to MNRA
Quenched fractions in the IllustrisTNG simulations: Comparison with observations and other theoretical models
We make an in-depth comparison of the IllustrisTNG cosmological simulations with observed quenched fractions of central and satellite galaxies, for Mstars = 109-12 M⊙ at 0 ≤ z ≤ 3. We show how measurement choices [aperture, quenched definition, and star formation rate (SFR) indicator time-scale], as well as sample selection issues (projection effects, satellite/central misclassification, and host mass distribution sampling), impact this comparison. The quenched definition produces differences of up to 70 (30) percentage points for centrals (satellites) above ∼1010.5 M⊙. At z Z 2, a larger aperture within which SFR is measured suppresses the quenched fractions by up to ∼50 percentage points. Proper consideration of the stellar and host mass distributions is crucial: Naive comparisons to volume-limited samples from simulations lead to misinterpretation of the quenched fractions as a function of redshift by up to 20 percentage points. Including observational uncertainties to theoretical values of Mstars and SFR changes the quenched fraction values and their trend and/or slope with mass. Taking projected rather than three-dimensional distances for satellites decreases the quenched fractions by up to 10 per cent. TNG produces quenched fractions for both centrals and satellites broadly consistent with observations and predicts up to ∼80 (90) per cent of quenched centrals at z = 0 (z = 2), in line with recent observations, and higher than other theoretical models. The quantitative agreement of TNG and Sloan Digital Sky Survey for satellite quenched fractions in groups and clusters depends strongly on the galaxy and host mass range. Our mock comparison highlights the importance of properly accounting for observational effects and biases
Constraining SIDM with halo shapes: Revisited predictions from realistic simulations of early-type galaxies
We study the effect of self-interacting dark matter (SIDM) and baryons on the shape of early-type galaxies (ETGs) and their dark matter haloes, comparing them to the predictions of the cold dark matter (CDM) scenario. We use five hydrodynamical zoom-in simulations of haloes hosting ETGs (Mvir sim 10 13 , M ⊙ and M ∗ ∼ 10 11 , M ⊙), simulated in CDM and a SIDM model with constant cross-section of σT/mχ = 1 cm2g-1. We measure the 3D and projected shapes of the dark matter haloes and their baryonic content using the inertia tensor and compare our measurements to the results of three HST samples of gravitational lenses and Chandra and XMM-Newton X-ray observations. We find that the inclusion of baryons greatly reduces the differences between CDM and a SIDM, together with the ability to draw constraints based on shapes. Lensing measurements reject the predictions of CDM dark-matter-only simulations and prefer one of the hydro scenarios. When we consider the total sample of lenses, observational data prefer the CDM hydro scenario. The shapes of the X-ray emitting gas are compatible with observational results in both hydro runs, with CDM predicting higher elongations only in the very centre. Contrary to previous claims at the scale of elliptical galaxies, we conclude that both CDM and our SIDM model can still explain observed shapes once we include baryons in the simulations. Our results demonstrate that this is essential to derive realistic constraints and that new simulations are needed to confirm and extend our findings
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