5,430 research outputs found
On the origin of cold dark matter halo density profiles
N-body simulations predict that CDM halo-assembly occurs in two phases: 1) a
fast accretion phase with a rapidly deepening potential well; and 2) a slow
accretion phase characterised by a gentle addition of mass to the outer halo
with little change in the inner potential well. We demonstrate, using
one-dimensional simulations, that this two-phase accretion leads to CDM halos
of the NFW form and provides physical insight into the properties of the mass
accretion history that influence the final profile. Assuming that the
velocities of CDM particles are effectively isotropised by fluctuations in the
gravitational potential during the fast accretion phase, we show that
gravitational collapse in this phase leads to an inner profile rho(r) ~ r^{-1}.
Slow accretion onto an established potential well leads to an outer profile
with rho(r) ~ r^{-3}. The concentration of a halo is determined by the fraction
of mass that is accreted during the fast accretion phase. Using an ensemble of
realistic mass accretion histories, we show that the model predictions of the
dependence of halo concentration on halo formation time, and hence the
dependence of halo concentration on halo mass, and the distribution of halo
concentrations all match those found in cosmological N-body simulations. Using
a simple analytic model that captures much of the important physics we show
that the inner r^{-1} profile of CDM halos is a natural result of hierarchical
mass assembly with a initial phase of rapid accretion.Comment: Accepted for publication in MNRAS, references added, 11 pages, 8
figure
Dark matter halo response to the disk growth
We consider the sensitivity of the circular-orbit adiabatic contraction
approximation to the baryon condensation rate and the orbital structure of dark
matter halos in the CDM paradigm. Using one-dimensional hydrodynamic
simulations including the dark matter halo mass accretion history and gas
cooling, we demonstrate that the adiabatic approximation is approximately valid
even though halos and disks may assemble simultaneously. We further demonstrate
the validity of the simple approximation for CDM halos with isotropic
velocity distributions using three-dimensional N-body simulations. This result
is easily understood: an isotropic velocity distribution in a cuspy halo
requires more circular orbits than radial orbits. Conversely, the approximation
is poor in the extreme case of a radial orbit halo. It overestimates the
response a core dark matter halo, where radial orbit fraction is larger.
Because no astronomically relevant models are dominated by low-angular momentum
orbits in the vicinity of the disk and the growth time scale is never shorter
than a dynamical time, we conclude that the adiabatic contraction approximation
is useful in modeling the response of dark matter halos to the growth of a
disk.Comment: 7 pages, 6 figures, accepted for publication in MNRA
Bayesian inferences of galaxy formation from the K-band luminosity and HI mass functions of galaxies: constraining star formation and feedback
We infer mechanisms of galaxy formation for a broad family of semi-analytic
models (SAMs) constrained by the K-band luminosity function and HI mass
function of local galaxies using tools of Bayesian analysis. Even with a broad
search in parameter space the whole model family fails to match to constraining
data. In the best fitting models, the star formation and feedback parameters in
low-mass haloes are tightly constrained by the two data sets, and the analysis
reveals several generic failures of models that similarly apply to other
existing SAMs. First, based on the assumption that baryon accretion follows the
dark matter accretion, large mass-loading factors are required for haloes with
circular velocities lower than 200 km/s, and most of the wind mass must be
expelled from the haloes. Second, assuming that the feedback is powered by
Type-II supernovae with a Chabrier IMF, the outflow requires more than 25% of
the available SN kinetic energy. Finally, the posterior predictive
distributions for the star formation history are dramatically inconsistent with
observations for masses similar to or smaller than the Milky-Way mass. The
inferences suggest that the current model family is still missing some key
physical processes that regulate the gas accretion and star formation in
galaxies with masses below that of the Milky Way.Comment: 17 pages, 9 figures, 1 table, accepted for publication in MNRA
Coerced Mechanical Coarsening of Nanoparticle Assemblies
Coarsening is a ubiquitous phenomenon [1-3] that underpins countless processes in nature, including epitaxial growth [1,3,4], the phase separation of alloys, polymers and binary fluids [2], the growth of bubbles in foams5, and pattern formation in biomembranes6. Here we show, in the first real-time experimental study of the evolution of an adsorbed colloidal nanoparticle array, that tapping-mode atomic force microscopy (TM-AFM) can drive the coarsening of Au nanoparticle assemblies on silicon surfaces. Although the growth exponent has a strong dependence on the initial sample morphology, our observations are largely consistent with modified Ostwald ripening processes [7-9]. To date, ripening processes have been exclusively considered to be thermally activated, but we show that nanoparticle assemblies can be mechanically coerced towards equilibrium, representing a new approach to directed coarsening. This strategy enables precise control over the evolution of micro- and nanostructures
Star Formation and Stellar Mass Assembly in Dark Matter Halos: From Giants to Dwarfs
The empirical model of Lu et al. 2014 is updated with recent data and used to
study galaxy star formation and assembly histories. At , the predicted
galaxy stellar mass functions are steep, and a significant amount of star
formation is hosted by low-mass haloes that may be missed in current
observations. Most of the stars in cluster centrals formed earlier than
but have been assembled much later. Milky Way mass galaxies have
had on-going star formation without significant mergers since , and
are thus free of significant (classic) bulges produced by major mergers. In
massive clusters, stars bound in galaxies and scattered in the halo form a
homogeneous population that is old and with solar metallicity. In contrast, in
Milky Way mass systems the two components form two distinct populations, with
halo stars being older and poorer in metals by a factor of . Dwarf
galaxies in haloes with have experienced a
star formation burst accompanied by major mergers at , followed by a
nearly constant star formation rate after . The early burst leaves a
significant old stellar population that is distributed in spheroids.Comment: 17 pages, 17 figure
Standardised Evaluation of Shanghai-Hangzhou High-Speed Maglev Project
In recent years, high-speed maglev systems have received renewed attention once again. However, a systematic and transparent approach to evaluate high-speed maglev projects does not currently exist, which could be an obstacle for their application, even with technical success. In Germany, the Standardised Evaluation is applied as a basis for decision making regarding the public funding of projects. It should be implemented for all investments of urban public transport projects with a value of more than € 25 million. In this paper, the economic evaluation for the Shanghai-Hangzhou maglev project is carried out with the Standardised Evaluation. One of the most important contributions of this work is to demonstrate the applicability of Standardised Evaluation for high-speed maglev projects. With the Standardised Evaluation, the evidence of macroand microeconomic benefit can be presented in a transparent and systematic way. The result can be used to prove the project’s profitability and to rank different projects or project alternatives.</p
The Weak Clustering of Gas-Rich Galaxies
We examine the clustering properties of HI-selected galaxies through an
analysis of the HI Parkes All-Sky Survey Catalogue (HICAT) two-point
correlation function. Various sub-samples are extracted from this catalogue to
study the overall clustering of HI-rich galaxies and its dependence on
luminosity, HI gas mass and rotational velocity. These samples cover the entire
southern sky Dec < 0 deg, containing up to 4,174 galaxies over the radial
velocity range 300-12,700 km/s. A scale length of r_0 = 3.45 +/- 0.25 Mpc/h and
slope of gamma = 1.47 +/- 0.08 is obtained for the HI-rich galaxy real-space
correlation function, making gas-rich galaxies among the most weakly clustered
objects known. HI-selected galaxies also exhibit weaker clustering than
optically selected galaxies of comparable luminosities. Good agreement is found
between our results and those of synthetic HI-rich galaxy catalogues generated
from the Millennium Run CDM simulation. Bisecting HICAT using different
parameter cuts, clustering is found to depend most strongly on rotational
velocity and luminosity, while the dependency on HI mass is marginal. Splitting
the sample around v_rot = 108 km/s, a scale length of r_0 = 2.86 +/- 0.46 Mpc/h
is found for galaxies with low rotational velocities compared to r_0 = 3.96 +/-
0.33 Mpc/h for the high rotational velocity sample.Comment: Accepted for publication in the Astrophysical Journa
Reacting to Perceived Overqualification: Uniting Strain-Based and Self-Regulatory Adjustment Reactions and the Moderating Role of Formal Work Arrangements
Thus far, research on perceived overqualification has focused on either maladaptive, strain-based versus more adaptive, self-regulatory reactions in isolation. Following person-environment fit theory, we seek to advance this one-sided focus by uniting both types of adjustment reactions and to consider their implications for perceived person-job fit, and performance and wellbeing outcomes. In line with theory, we also examine contextual boundary conditions in the form of indicators of formal work arrangements (i.e., permanent vs. temporary employment contract and job tenure). Utilizing three-wave data from 453 employees, we found that perceived overqualification indirectly and sequentially related to decreases in task performance, organizational citizenship behavior and job satisfaction via anger toward employment situation and lower levels of perceived person-job fit-thus reflecting the strain-based pathway. For the self-regulatory pathway, findings did not align with our initial proposition that the positive relationship between perceived overqualification and work organization (a form of structural job crafting whereby employees improve their work processes) would be weaker among temporary employees and those with longer tenure. Instead, having a temporary employment contract or having longer job tenure resulted in a negative relationship between perceived overqualification and work organization, which further contributed to a decrease in performance and satisfaction via lower levels of perceived person-job fit. Our study highlights the demotivating role of a temporary employment contract and long job tenure for overqualified employees to reorganize their work. In discussing our findings, we point to the importance of job stage and develop recommendations for managing overqualified employees
Quantifying the improvement of surrogate indices of hepatic insulin resistance using complex measurement techniques
We evaluated the ability of simple and complex surrogate-indices to identify individuals from an overweight/obese cohort with hepatic insulin-resistance (HEP-IR). Five indices, one previously defined and four newly generated through step-wise linear regression, were created against a single-cohort sample of 77 extensively characterised participants with the metabolic syndrome (age 55.6±1.0 years, BMI 31.5±0.4 kg/m2; 30 males). HEP-IR was defined by measuring endogenous-glucose-production (EGP) with [6–62H2] glucose during fasting and euglycemic-hyperinsulinemic clamps and expressed as EGP*fasting plasma insulin. Complex measures were incorporated into the model, including various non-standard biomarkers and the measurement of body-fat distribution and liver-fat, to further improve the predictive capability of the index. Validation was performed against a data set of the same subjects after an isoenergetic dietary intervention (4 arms, diets varying in protein and fiber content versus control). All five indices produced comparable prediction of HEP-IR, explaining 39–56% of the variance, depending on regression variable combination. The validation of the regression equations showed little variation between the different proposed indices (r2 = 27–32%) on a matched dataset. New complex indices encompassing advanced measurement techniques offered an improved correlation (r = 0.75, P<0.001). However, when validated against the alternative dataset all indices performed comparably with the standard homeostasis model assessment for insulin resistance (HOMA-IR) (r = 0.54, P<0.001). Thus, simple estimates of HEP-IR performed comparable to more complex indices and could be an efficient and cost effective approach in large epidemiological investigations
Scale-dependent bias and the halo model
We use a simplified version of the halo model with a power law power spectrum
to study scale dependence in galaxy bias at the very large scales relevant to
baryon oscillations. In addition to providing a useful pedagogical explanation
of the scale dependence of galaxy bias, the model provides an analytic tool for
studying how changes in the Halo Occupation Distribution (HOD) impact the scale
dependence of galaxy bias on scales between 10 and 1000 Mpc/h, which is useful
for interpreting the results of complex N-body simulations. We find that
changing the mean number of galaxies per halo of a given mass will change the
scale dependence of the bias, but that changing the way the galaxies are
distributed within the halo has a smaller effect on the scale dependence of
bias at large scales. We use the model to explain the decay in amplitude of the
baryon oscillations as k increases, and generalize the model to make
predictions about scale dependent galaxy bias when redshift space distortions
are introduced.Comment: 13 pages, 2 figures; corrected typos, extended discussion of redshift
space distortions, matches published versio
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