4,324 research outputs found
Rhapsody. I. Structural Properties and Formation History From a Statistical Sample of Re-simulated Cluster-size Halos
We present the first results from the Rhapsody cluster re-simulation project:
a sample of 96 "zoom-in" simulations of dark matter halos of 10^14.8 +- 0.05
Msun/h, selected from a 1 (Gpc/h)^3 volume. This simulation suite is the first
to resolve this many halos with ~5x10^6 particles per halo in the cluster-mass
regime, allowing us to statistically characterize the distribution of and
correlation between halo properties at fixed mass. We focus on the properties
of the main halos and how they are affected by formation history, which we
track back to z=12, over five decades in mass. We give particular attention to
the impact of the formation history on the density profiles of the halos. We
find that the deviations from the Navarro-Frenk-White (NFW) model and the
Einasto model depend on formation time. Late-forming halos tend to have
considerable deviations from both models, partly due to the presence of massive
subhalos, while early-forming halos deviate less but still significantly from
the NFW model and are better described by the Einasto model. We find that the
halo shapes depend only moderately on formation time. Departure from spherical
symmetry impacts the density profiles through the anisotropic distribution of
massive subhalos. Further evidence of the impact of subhalos is provided by
analyzing the phase-space structure. A detailed analysis of the properties of
the subhalo population in Rhapsody is presented in a companion paper.Comment: 20 pages, 13 figures, replaced to match published versio
Phenomenological study on the significance of the scalar potential and Lamb shift
We indicated in our previous work that for QED the contributions of the
scalar potential which appears at the loop level is much smaller than that of
the vector potential and in fact negligible. But the situation may be different
for QCD, one reason is that the loop effects are more significant because
is much larger than , and secondly the non-perturbative QCD
effects may induce the scalar potential. In this work, we phenomenologically
study the contribution of the scalar potential to the spectra of charmonia.
Taking into account both vector and scalar potentials, by fitting the well
measured charmonia spectra, we re-fix the relevant parameters and test them by
calculating other states of the charmonia family. We also consider the role of
the Lamb shift and present the numerical results with and without involving the
Lamb shift
Rhapsody. II. Subhalo Properties and the Impact of Tidal Stripping From a Statistical Sample of Cluster-Size Halos
We discuss the properties of subhalos in cluster-size halos, using a
high-resolution statistical sample: the Rhapsody simulations introduced in Wu
et al. (2012). We demonstrate that the criteria applied to select subhalos have
significant impact on the inferred properties of the sample, including the
scatter in the number of subhalos, the correlation between the subhalo number
and formation time, and the shape of subhalos' spatial distribution and
velocity structure. We find that the number of subhalos, when selected using
the peak maximum circular velocity in their histories (a property expected to
be closely related to the galaxy luminosity), is uncorrelated with the
formation time of the main halo. This is in contrast to the previously reported
correlation from studies where subhalos are selected by the current maximum
circular velocity; we show that this difference is a result of the tidal
stripping of the subhalos. We also find that the dominance of the main halo and
the subhalo mass fraction are strongly correlated with halo concentration and
formation history. These correlations are important to take into account when
interpreting results from cluster samples selected with different criteria. Our
sample also includes a fossil cluster, which is presented separately and placed
in the context of the rest of the sample.Comment: 15 pages, 10 figures; Paper I: arXiv:1209.3309; replaced to match
published versio
Capturing Nucleation at 4D Atomic Resolution
Nucleation plays a critical role in many physical and biological phenomena
ranging from crystallization, melting and evaporation to the formation of
clouds and the initiation of neurodegenerative diseases. However, nucleation is
a challenging process to study in experiments especially in the early stage
when several atoms/molecules start to form a new phase from its parent phase.
Here, we advance atomic electron tomography to study early stage nucleation at
4D atomic resolution. Using FePt nanoparticles as a model system, we reveal
that early stage nuclei are irregularly shaped, each has a core of one to few
atoms with the maximum order parameter, and the order parameter gradient points
from the core to the boundary of the nucleus. We capture the structure and
dynamics of the same nuclei undergoing growth, fluctuation, dissolution,
merging and/or division, which are regulated by the order parameter
distribution and its gradient. These experimental observations differ from
classical nucleation theory (CNT) and to explain them we propose the order
parameter gradient (OPG) model. We show the OPG model generalizes CNT and
energetically favours diffuse interfaces for small nuclei and sharp interfaces
for large nuclei. We further corroborate this model using molecular dynamics
simulations of heterogeneous and homogeneous nucleation in liquid-solid phase
transitions of Pt. We anticipate that the OPG model is applicable to different
nucleation processes and our experimental method opens the door to study the
structure and dynamics of materials with 4D atomic resolution.Comment: 42 pages, 5 figures, 12 supplementary figures and one supplementary
tabl
Local-Scale Drivers of Tree Survival in a Temperate Forest
Tree survival plays a central role in forest ecosystems. Although many factors such as tree size, abiotic and biotic neighborhoods have been proposed as being important in explaining patterns of tree survival, their contributions are still subject to debate. We used generalized linear mixed models to examine the relative importance of tree size, local abiotic conditions and the density and identity of neighbors on tree survival in an old-growth temperate forest in northeastern China at three levels (community, guild and species). Tree size and both abiotic and biotic neighborhood variables influenced tree survival under current forest conditions, but their relative importance varied dramatically within and among the community, guild and species levels. Of the variables tested, tree size was typically the most important predictor of tree survival, followed by biotic and then abiotic variables. The effect of tree size on survival varied from strongly positive for small trees (1–20 cm dbh) and medium trees (20–40 cm dbh), to slightly negative for large trees (>40 cm dbh). Among the biotic factors, we found strong evidence for negative density and frequency dependence in this temperate forest, as indicated by negative effects of both total basal area of neighbors and the frequency of conspecific neighbors. Among the abiotic factors tested, soil nutrients tended to be more important in affecting tree survival than topographic variables. Abiotic factors generally influenced survival for species with relatively high abundance, for individuals in smaller size classes and for shade-tolerant species. Our study demonstrates that the relative importance of variables driving patterns of tree survival differs greatly among size classes, species guilds and abundance classes in temperate forest, which can further understanding of forest dynamics and offer important insights into forest management
Large Scale Cavities Surrounding Microquasars Inferred from Evolution of their Relativistic Jets
The black hole X-ray transient XTE J1550-564 has undergone a strong outburst
in 1998 and two relativistic X-ray jets have been detected years later with the
X-ray observatory; the eastern jet was found previously to have
decelerated after its first detection. Here we report a full analysis of the
evolution of the western jet; significant deceleration is also detected in the
western side. Our analysis indicates that there is a cavity outside the central
source and the jets first traveled with constant velocity and then were slowed
down by the interactions between the jets and the interstellar medium (ISM).
The best fitted radius of the cavity is 0.31 pc on the eastern side and
0.44 pc on the western side, and the densities also show asymmetry, of
0.034 cm on the east to 0.12 cm on the west. The best
fitted magnetic fields on both sides are 0.5 mG. Similar analysis is also
applied to another microquasar system, H 1743-322, and a large scale low
density region is also found. Based on these results and the comparison with
other microquasar systems, we suggest a generic scenario for microquasar jets,
classifying the observed jets into three main categories, with different jet
morphologies (and sizes) corresponding to different scales of vacuous
environments surrounding them. We also suggest that either continuous jets or
accretion disk winds, or both may be responsible for creating these cavities.
Therefore X-ray jets from microquasars provide us with a promising method of
probing the environment of accreting black holes.Comment: 29 pages, 5 tables, 7 figures. Accepted for publication in Ap
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