16842883
A Comparison of Simple Mass Estimators for Galaxy Clusters
High-resolution N-body simulations are used to investigate systematic trends in the mass profiles and total masses of clusters as derived from three simple estimators: (1) the weak gravitational lensing shear field, under the assumption of an isothermal cluster potential; (2) the dynamical mass obtained from the measured velocity dispersion, under the assumption of an isothermal cluster potential; and (3) the classical virial estimator. The clusters used for the analysis were obtained from simulations of a standard cold dark matter universe at z = 0.5 and consist of order ? 105 particles of mass mp 1010 M?. The clusters are not smooth and spherically symmetric but, rather, the mass distribution is triaxial, and significant substructure exists within the clusters. Not surprisingly, the level of agreement between the cluster mass profiles obtained from the various estimators and the actual mass profiles is found to be scale-dependent. We define the total cluster mass to be the mass contained within a three-dimensional radius r200 of the cluster centers, where r200 is the radius inside which the mean interior overdensity is equal to 200. Under this definition the classical virial estimator yields a good measurement of the total cluster mass, although it is systematically underestimated by ~10%. This result suggests that, at least in the limit of ideal data, the virial estimator is quite robust to deviations from pure spherical symmetry and the presence of substructure within a cluster. The dynamical mass estimate based upon a measurement of the cluster velocity dispersion and an assumption of an isothermal potential yields a poor measurement of the total cluster mass, underestimating it by ~40% for the case in which ?v is computed from an average over the entire cluster. The weak lensing estimate yields a very good measurement of the total cluster mass, provided that the mean shear used to determine the equivalent cluster velocity dispersion is computed from an average of the lensing signal over the entire cluster (i.e., the mean shear is computed interior to a projected cluster radius of R200).
118163991
Anisotropic Galaxy-Galaxy Lensing
We investigate the weak lensing shear due to dark matter galaxy halos whose mass distributions, as projected on the sky, are nearly elliptical. The shear pattern due to these halos is anisotropic about the lens centers and we quantify the level of anisotropy by comparing the mean shear experienced by sources located closest to the major axes of the lenses $ _{major}$, to that experienced by sources located closest to the minor axes, $ _{minor}$. We also compute the degree to which the anisotropy in the lensing signal is degraded due to a noisy determination of the position angles of the lens galaxies and find that provided the typical error on the orientation of the lenses is less than $15^\circ$, more than 90% of the true lensing signal is recovered in the mean. We discuss our our results in the context of detecting anisotropic galaxy-galaxy lensing in large, ground-based data sets and conclude that a modest net flattening of dark matter halos should be detectable at a statistically significant level. The forthcoming Sloan Digital Sky Survey (SDSS) data will necessarily provide a very useful data set for this analysis, but a detection of anisotropic galaxy-galaxy lensing is not dependent upon the very large sky coverage of the SDSS. Rather, we argue that a significant detection of this effect can also be obtained from an imaging survey that is of order two magnitudes fainter than SDSS and which covers only a relatively small area of sky, of order one square degree. [Abridged]
7233194
Gravitational Lensing by NFW Halos
We investigate the gravitational lensing properties of dark matter halos with Navarro, Frenk & White (NFW) density profiles and derive an analytic expression for the radial dependence of the shear, γ(x), due to these objects. In addition, we derive an expression for the mean shear interior to a given radius, (x), and use this to quantify systematic errors that will arise in weak-lensing mass estimates of astronomical objects in cases where the mass estimate is based on an a priori assumption that the underlying potential is that of a singular isothermal sphere when in fact the potential is that of an NFW-type object. On mass scales of 1011 M☉ M200 1015 M☉, the assumption of an isothermal sphere potential results in an overestimate of the halo mass, and the amount by which the mass is overestimated increases linearly with the value of the NFW concentration parameter. Considerable overestimates of the mass (~60%) can occur for galaxy-sized halos, but for rich clusters the mass overestimate is small. The degree to which the mass is systematically in error depends on the cosmology adopted, with a COBE-normalized standard cold dark matter (CDM) model yielding the largest systematic errors for a given true value of the halo mass.
110675795
Architecture of the MEGA detector trigger
Abstract The trigger for the MEGA detector system is based on signals from single, high-energy photons interacting in one of the three MEGA pair spectrometers. The trigger is divided into a fast and a slow stage. The first stage produces a fast output if a specific pattern of detector hits is observed in the scintillators and high-speed wire chambers of a pair spectrometer. The second, slow-stage interrogates drift chamber hit patterns and provides a veto when the pattern fails a minimal requirement for reconstruction of the hits into a pair of circular orbits. The trigger interacts with the photon detector electronics by gating limited sections of the detector during the read-out of an event. This paper describes the two stage trigger system, the photon detector electronics, and the implementation of the trigger outputs to strobe the data acquisition system. The performance of the trigger is compared to Monte Carlo simulations of the photon detector response.
17282332
Disks around the Young Stars TW Hydrae and HD 100546 Imaged at 3.4 Millimeters with the Australia Telescope Compact Array
We present observations of the young stars TW Hya and HD 100546 made with the Australia Telescope Compact Array at 89 GHz with ~2'' resolution and ~3 mJy continuum sensitivity. Compact thermal dust continuum emission is detected from disks surrounding both stars. HD 100546 also shows hints of extended emission, presumably a residual protostellar envelope, which is also visible in scattered light at optical wavelengths. For TW Hya, HCO+ J = 1-0 line emission from the circumstellar disk is detected and spatially resolved. The observed size and intensity are in good agreement with model calculations based on an irradiated disk with substantial depletions derived previously from single-dish observations of higher J HCO+ transitions.
94022512
SOLVING DYNAMIC EQUATIONS FOR THE COLLECTION AND EVAPORATION OF AN AEROSOL
We investigate the coalescence of drops undergoing random Brownian motion and evaporation. The evolution of the drop size spectrum is determined by solving a partial integro-differential equation with a kernel that describes the pairwise coalescence of drops. Numerical results for fuel injection with n-octane suggest that the size distribution of drops is determined by two competing processes, namely coalescence and evaporation. 
 
 
 
Nous etudions ici la coalescence de gouttes dans des conditions de deplacement brownien aleatoire et d'evaporation. L'evolution du spectre de la taille des gouttes est determinee par la resolution d'une equation integro-differentielle partielle avec un noyau qui decrit la coalescence par paire des gouttes. Les resultats numeriques pour l'injection du fuel avec du n-octane suggkrent que la distribution de taille des gouttes est determinee par deux processus se faisant concurrence, a savoir la coalescence et l'evaporation.