1,403 research outputs found
The MUSIC of CLASH: predictions on the concentration-mass relation
We present the results of a numerical study based on the analysis of the
MUSIC-2 simulations, aimed at estimating the expected concentration-mass
relation for the CLASH cluster sample. We study nearly 1400 halos simulated at
high spatial and mass resolution, which were projected along many
lines-of-sight each. We study the shape of both their density and
surface-density profiles and fit them with a variety of radial functions,
including the Navarro-Frenk-White, the generalised Navarro-Frenk-White, and the
Einasto density profiles. We derive concentrations and masses from these fits
and investigate their distributions as a function of redshift and halo
relaxation. We use the X-ray image simulator X-MAS to produce simulated Chandra
observations of the halos and we use them to identify objects resembling the
X-ray morphologies and masses of the clusters in the CLASH X-ray selected
sample. We also derive a concentration-mass relation for strong-lensing
clusters. We find that the sample of simulated halos which resemble the X-ray
morphology of the CLASH clusters is composed mainly by relaxed halos, but it
also contains a significant fraction of un-relaxed systems. For such a sample
we measure an average 2D concentration which is ~11% higher than found for the
full sample of simulated halos. After accounting for projection and selection
effects, the average NFW concentrations of CLASH clusters are expected to be
intermediate between those predicted in 3D for relaxed and super-relaxed halos.
Matching the simulations to the individual CLASH clusters on the basis of the
X-ray morphology, we expect that the NFW concentrations recovered from the
lensing analysis of the CLASH clusters are in the range [3-6], with an average
value of 3.87 and a standard deviation of 0.61. Simulated halos with X-ray
morphologies similar to those of the CLASH clusters are affected by a modest
orientation bias.Comment: 21 pages, 16 figures, 3 tables, submitted to Ap
Molecular Hydrogen and Global Star Formation Relations in Galaxies
(ABRIDGED) We use hydrodynamical simulations of disk galaxies to study
relations between star formation and properties of the molecular interstellar
medium (ISM). We implement a model for the ISM that includes low-temperature
(T<10^4K) cooling, directly ties the star formation rate to the molecular gas
density, and accounts for the destruction of H2 by an interstellar radiation
field from young stars. We demonstrate that the ISM and star formation model
simultaneously produces a spatially-resolved molecular-gas surface density
Schmidt-Kennicutt relation of the form Sigma_SFR \propto Sigma_Hmol^n_mol with
n_mol~1.4 independent of galaxy mass, and a total gas surface density -- star
formation rate relation Sigma_SFR \propto Sigma_gas^n_tot with a power-law
index that steepens from n_tot~2 for large galaxies to n_tot>~4 for small dwarf
galaxies. We show that deviations from the disk-averaged Sigma_SFR \propto
Sigma_gas^1.4 correlation determined by Kennicutt (1998) owe primarily to
spatial trends in the molecular fraction f_H2 and may explain observed
deviations from the global Schmidt-Kennicutt relation.Comment: Version accepted by ApJ, high-res version available at
http://kicp.uchicago.edu/~brant/astro-ph/molecular_ism/rk2007.pd
Single neutral pion production by charged-current interactions on hydrocarbon at 3.6 GeV
Single neutral pion production via muon antineutrino charged-current
interactions in plastic scintillator (CH) is studied using the \minerva
detector exposed to the NuMI low-energy, wideband antineutrino beam at
Fermilab. Measurement of this process constrains models of neutral pion
production in nuclei, which is important because the neutral-current analog is
a background for appearance oscillation experiments. The
differential cross sections for momentum and production angle, for
events with a single observed and no charged pions, are presented and
compared to model predictions. These results comprise the first measurement of
the kinematics for this process.Comment: 6 pages, 5 figures, submitted to Physics Letters
Evaluación de un modelo estadístico para estimar la radiación solar en Magdalena, Colombia
The global solar radiation is an important meteorological parameter for understanding the biochemical processes in the environment. Their measurement is complex and requires a high cost of installation, operation and maintenance, but this can be estimated by empirical equations based primarily on their geolocation. However, the accuracy of the results obtained is not suitable for understanding the processes evaluated. The aim of this study was to establish a statistical correlation model for estimating global solar radiation from alternative meteorological parameter. The meteorological information was provided by the Institute of Hydrology, Meteorology and Environmental Studies of Colombia, which provided the measurement of solar radiation, temperature, relative humidity and sunshine of the day for a year. Simple regressions between variables and solar radiation were raised; and multiple regressions between the possible combinations of each variables and solar radiation. The results show a good statistical correlation between variables solar radiation, temperature and relative humidity and little relation to the hours of sunshine, where relative humidity has the higher correlation and therefore is the parameter that more influence has on the model. The multivariate models (R2 average 11.91%) have better fit than simple models (R2 average 5.66%). The best model is multivariate regression in which all variables were included (R2 of 13.75%) although not the simplest model estimation.La radiación solar global se establece como una variable meteorológica necesaria para en el entendimiento de los procesos bioquímicos del ambiente; su medición es compleja y requiere un alto costo de instalación, operación y mantenimiento. Su estimación se puede realizar mediante fórmulas empíricas basadas fundamentalmente en su geo-posicionamiento, no obstante la precisión de los resultados obtenidos no es adecuada para el entendimiento de los procesos evaluados. El objetivo del presente trabajo fue establecer un modelo estadístico de correlación para estimar la radiación solar global a partir de los datos meteorológicos alternos. La información meteorológica fue suministrada por el Instituto de Hidrología, Meteorología y Estudios Ambientales de Colombia, que proporcionó la medición de radiación solar, temperatura, humedad relativa y horas de sol. Se plantearon regresiones simples entre las variables y la radiación solar; y regresiones múltiples entre las posibles combinaciones de las variables y la radiación solar. Se evidenció una buena correlación estadística entre las variables radiación solar, temperatura y humedad relativa, y poca relación con las horas de sol; siendo la humedad relativa la que guarda una mayor correlación y por tanto influencia en el modelo. Los modelos multivariados (R2 promedio 11,91%) presentaron mejor ajuste comparado con los modelos simples (R2 promedio 5,66%). El mejor modelo lo establece la regresión múltiple en la que se incluyen todas las variables (R2 de 13,75%), no siendo este el modelo más simple para su estimación
Density profiles of dark matter haloes: diversity and dependence on environment
(Abridged) We study the outer density profiles of dark matter haloes
predicted by a generalized secondary infall model and observed in a N-body
cosmological simulation of a \Lambda CDM model. We find substantial systematic
variations in shapes and concentrations of the halo profiles as well as a
strong correlation of the profiles with the environment. In the N-body
simulation, the average outer slope of the density profiles, \beta (\rho\propto
r^{-\beta}), of isolated haloes is \approx 2.9; 68% of these haloes have values
of \beta between 2.5 and 3.8. Haloes in dense environments of clusters are more
concentrated and exhibit a broad distribution of \beta with values larger than
for isolated haloes . Contrary to what one may expect, the haloes contained
within groups and galaxy systems are less concentrated and have flatter outer
density profiles than the isolated haloes. The concentration decreases with
M_h, but its scatter for a given mass is substantial. The mass and circular
velocity of the haloes are strongly correlated: M_h \propto V_m^{\alpha} with
\alpha ~ 3.3 (isolated) and ~3.5 (haloes in clusters). For M_h=10^12M_sun the
rms deviations from these relations are \Delta logM_h=0.12 and 0.18,
respectively. Approximately 30% of the haloes are contained within larger
haloes or have massive companions (larger than ~0.3 the mass of the current
halo) within 3 virial radii. The remaining 70% of the haloes are isolated
objects. The distribution of \beta as well as the concentration-mass and
M_h-V_m relations for the isolated haloes agree very well with the predictions
of our seminumerical approach which is based on a generalization of the
secondary infall model and on the extended Press-Schechter formalism.Comment: 14 pages, 11 figures included, uses mn.sty, accepted by MNRAS. Minor
modifications, new and updated reference
Galaxy cluster mass bias from projected mass maps: The Three Hundred-NIKA2 LPSZ twin samples
The determination of the mass of galaxy clusters from observations is subject
to systematic uncertainties. Beyond the errors due to instrumental and
observational systematic effects, in this work we investigate the bias
introduced by modelling assumptions. In particular, we consider the
reconstruction of the mass of galaxy clusters from convergence maps employing
spherical mass density models. We make use of The Three Hundred simulations,
selecting clusters in the same redshift and mass range as the NIKA2
Sunyaev-Zel'dovich Large Program sample: and . We study different
modelling and intrinsic uncertainties that should be accounted for when using
the single cluster mass estimates for scaling relations. We confirm that the
orientation of clusters and the radial ranges considered for the fit have an
important impact on the mass bias. The effect of the projection adds
uncertainties to the order of to to the mass estimates. We also
find that the scatter from cluster to cluster in the mass bias when using
spherical mass models is less than of the true mass of the clusters
Analysis of Star Formation in Galaxy-like Objects
Using cosmological hydrodynamical simulations, we investigate the effects of
hierarchical aggregation on the triggering of star formation in galactic-like
objects. We include a simple star formation model to transform the cold gas in
dense regions into stars. Simulations with different parameters have been
performed in order to quantify the dependence of the results on the parameters.
We then resort to stellar population synthesis models to trace the color
evolution of each object with red-shift and in relation to their merger
histories. We find that, in a hierarchical clustering scenario, the process of
assembling of the structure is one natural mechanism that may trigger star
formation.
The resulting star formation rate history for each individual galactic object
is composed of a continuous one () and a series of
star bursts.
We find that even the accretion of a small satellite can be correlated with a
stellar burst. Massive mergers are found to be more efficient at transforming
gas into starsComment: 11 postscript figures. 2000, ApJ, accepte
Isolation of Flow and Nonflow Correlations by Two- and Four-Particle Cumulant Measurements of Azimuthal Harmonics in 200 GeV Au+Au Collisions
A data-driven method was applied to measurements of Au+Au collisions at
200 GeV made with the STAR detector at RHIC to isolate
pseudorapidity distance -dependent and -independent
correlations by using two- and four-particle azimuthal cumulant measurements.
We identified a component of the correlation that is -independent,
which is likely dominated by anisotropic flow and flow fluctuations. It was
also found to be independent of within the measured range of
pseudorapidity . The relative flow fluctuation was found to be for particles of transverse momentum
less than GeV/. The -dependent part may be attributed to
nonflow correlations, and is found to be relative to the
flow of the measured second harmonic cumulant at
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