17,813 research outputs found
The density profile of equilibrium and non-equilibrium dark matter halos
We study the diversity of the density profiles of dark matter halos based on
a large set of high-resolution cosmological simulations of 256^3 particles. The
cosmological models include four scale-free models and three representative
cold dark matter models. The simulations have good force resolution, and there
are about 400 massive halos with more than 10^4 particles within the virial
radius in each cosmological model. Our unbiased selection of all massive halos
enables to quantify how well the bulk of dark matter halos can be described by
the Navarro, Frenk & White (NFW) profile which was established for equilibrium
halos. We find that about seventy percent of the halos can be fitted by the NFW
profile with a fitting residual dvi_{max} less than 30% in Omega_0=1 universes.
This percentage is higher in lower density cosmological models. The rest of the
halos exhibits larger deviations from the NFW profile for more significant
internal substructures. There is a considerable amount of variation in the
density profile even for the halos which can be fitted by the NFW profile (i.e.
dvi_{max}<0.30). The distribution of the profile parameter, the concentration
, can be well described by a lognormal function with the mean value \bar c
slightly smaller (15%) than the NFW result and the dispersion \sigma_c in \ln c
about 0.25. The more virialized halos with dvi_{max}<0.15 have the mean value
\bar c in good agreement with the NFW result and a slightly smaller dispersion
\sigma_c (about 0.2). Our results can alleviate some of the conflicts found
recently between the theoretical NFW profile and observational results.
Implications for theoretical and observational studies of galaxy formation are
discussed.Comment: The final version accepted for publication in ApJ; one figure and one
paragraph added to demonstrate that all the conclusions of the first version
are solid to the resoltuion effects; 19 pages with 6 figure
Quantum superchemistry in an output coupler of coherent matter waves
We investigate the quantum superchemistry or Bose-enhanced atom-molecule
conversions in a coherent output coupler of matter waves, as a simple
generalization of the two-color photo-association. The stimulated effects of
molecular output step and atomic revivals are exhibited by steering the rf
output couplings. The quantum noise-induced molecular damping occurs near a
total conversion in a levitation trap. This suggests a feasible two-trap scheme
to make a stable coherent molecular beam.Comment: 3 figures, accepted by Phys.Rev.A (submitted to prl in July,
transferred to pra in Sep. and accepted in Nov.
Coherent atom-trimer conversion in a repulsive Bose-Einstein condensate
We show that the use of a generalized atom-molecule dark state permits the
enhanced coherent creation of triatomic molecules in a repulsive atomic
Bose-Einstein condensate, with further enhancement being possible in the case
of heteronuclear trimers via the constructive interference between two chemical
reaction channels.Comment: 3 figure
Quantum Noise in the Collective Abstraction Reaction A+B AB+B
We demonstrate theoretically that the collective abstraction reaction A+B AB+B can be realized efficiently with degenerate bosonic or fermionic
matter waves. We show that this is dominated by quantum fluctuations, which are
critical in triggering its initial stages with the appearance of macroscopic
non-classical correlations of the atomic and molecular fields as a result. This
study opens up a promising new regime of quantum degenerate matter-wave
chemistry.Comment: 4 pages, 3 figures, publishe
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Improving LMOF luminescence quantum yield through guest-mediated rigidification
Rotation of a specific pyridyl ring in LMOF-236 is locked by loading guest molecules into the MOF's pore, improving quantum yield by nearly 400%–an example of a generalizable strategy for maximizing quantum yield via guest-packing rigidification
Environmental Dependence of Cold Dark Matter Halo Formation
We use a high-resolution -body simulation to study how the formation of
cold dark matter (CDM) halos is affected by their environments, and how such
environmental effects produce the age-dependence of halo clustering observed in
recent -body simulations. We estimate, for each halo selected at redshift
, an `initial' mass defined to be the mass enclosed by the
largest sphere which contains the initial barycenter of the halo particles and
within which the mean linear density is equal to the critical value for
spherical collapse at . For halos of a given final mass, , the
ratio has large scatter, and the scatter is larger for
halos of lower final masses. Halos that form earlier on average have larger
, and so correspond to higher peaks in the initial density
field than their final masses imply. Old halos are more strongly clustered than
younger ones of the same mass because their initial masses are larger. The
age-dependence of clustering for low-mass halos is entirely due to the
difference in the initial/final mass ratio. Low-mass old halos are almost
always located in the vicinity of big structures, and their old ages are
largely due to the fact that their mass accretions are suppressed by the hot
environments produced by the tidal fields of the larger structure. The
age-dependence of clustering is weaker for more massive halos because the
heating by large-scale tidal fields is less important.Comment: 18 pages,19 figures, accepted by MNRA
Accurate determination of the Lagrangian bias for the dark matter halos
We use a new method, the cross power spectrum between the linear density
field and the halo number density field, to measure the Lagrangian bias for
dark matter halos. The method has several important advantages over the
conventional correlation function analysis. By applying this method to a set of
high-resolution simulations of 256^3 particles, we have accurately determined
the Lagrangian bias, over 4 magnitudes in halo mass, for four scale-free models
with the index n=-0.5, -1.0, -1.5 and -2.0 and three typical CDM models. Our
result for massive halos with ( is a characteristic non-linear
mass) is in very good agreement with the analytical formula of Mo & White for
the Lagrangian bias, but the analytical formula significantly underestimates
the Lagrangian clustering for the less massive halos $M < M_*. Our simulation
result however can be satisfactorily described, with an accuracy better than
15%, by the fitting formula of Jing for Eulerian bias under the assumption that
the Lagrangian clustering and the Eulerian clustering are related with a linear
mapping. It implies that it is the failure of the Press-Schechter theories for
describing the formation of small halos that leads to the inaccuracy of the Mo
& White formula for the Eulerian bias. The non-linear mapping between the
Lagrangian clustering and the Eulerian clustering, which was speculated as
another possible cause for the inaccuracy of the Mo & White formula, must at
most have a second-order effect. Our result indicates that the halo formation
model adopted by the Press-Schechter theories must be improved.Comment: Minor changes; accepted for publication in ApJ (Letters) ; 11 pages
with 2 figures include
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