Gas Bulk Velocities Research Articles

The 3D Direct Simulation Monte Carlo Study of Europa’s Gas Plume

Europa has been spotted as having water outgassing activities by space- and ground-based telescopes as well as reanalysis of the Galileo data. We adopt a 3D Direct Simulation Monte Carlo (DSMC) model to investigate the observed plume characteristics of Europa assuming that supersonic expansion originated from the subsurface vent. With a parametric study of the total gas production rate and initial gas bulk velocity, the gas number density, temperature and velocity information of the outgassing plumes from various case studies were derived. Our results show that the plume gases experience acceleration through mutual collisions and adiabatic cooling when exiting from the surface. The central part of the plume with relatively large gas production rates (1029 and 1030 H2O s−1) was found to sustain thermal equilibrium and near continuum condition. Column density maps integrated along two different viewing angles are presented to demonstrate the importance of the projection effect on remote sensing diagnostics. Finally, the density profiles at different altitudes are provided to prepare for observations of Europa’s plumes including upcoming spacecraft missions such as JUICE and Europa Clipper.

Lyα emitters in a cosmological volume – I. The impact of radiative transfer

Lyman-{\alpha} emitters (LAEs) are a promising target to probe the large scale structure of the Universe at high redshifts, $z\gtrsim 2$. However, their detection is sensitive to radiative transfer effects that depend on local astrophysical conditions. Thus, modeling the bulk properties of this galaxy population remains challenging for theoretical models. Here we develop a physically-motivated scheme to predict LAEs in cosmological simulations. The escape of Ly{\alpha} photons is computed using a Monte Carlo radiative transfer code which outputs a Ly{\alpha} escape fraction. To speed-up the process of assigning escape fractions to individual galaxies, we employ fitting formulae that approximate the full Monte Carlo results within an accuracy of 10% for a broad range of column densities, gas metallicities and gas bulk velocities. We apply our methodology to the semi-analytical model GALFORM on a large N-body simulation. The Ly{\alpha} photons escape through an outflowing neutral gas medium, implemented assuming different geometries. This results in different predictions for the typical column density and outflow velocities of the LAE population. To understand the impact of radiative transfer on our predictions, we contrast our models against a simple abundance matching assignment. Our full models populate LAEs in less massive haloes than what is obtained with abundance matching. Overall, radiative transfer effects result in better agreement when confronting the properties of LAEs against observational measurements. This suggest that incorporating the effects of Ly{\alpha} radiative transfer in the analysis of this galaxy population, including their clustering, can be important for obtaining an unbiased interpretation of future datasets.

Search for gas bulk motions in eight nearby clusters of galaxies with Suzaku

Abstract To search for bulk motions of the intracluster medium, we analyzed X-ray spectra taken with the Suzaku satellite, and measured the Doppler shift of the Fe-K line emission from eight nearby clusters of galaxies with various X-ray morphologies. In the cores of the Centaurus and Perseus clusters, the gas bulk velocity does not exceed the sound velocity, which confirms the results of previous researchs. For the Cen 45 subcluster, we found that the radial velocity relative to the Centaurus core, <780 km s−1, is significantly smaller than that reported in the optical band at the 3.9 σ level, which suggests an offset between the gas and galaxy distributions along the line of sight due to the subcluster merger. In A 2199, A 2142, A 3667, and A 133, no significant bulk motion was detected, indicating an upper limit on the radial velocity of 3000–4000 km s−1. A sign of large bulk velocity in excess of the instrumental calibration uncertainty was found near the center of the cool-core cluster A 2029 and in a subcluster of the merging cluster A 2255, suggesting that the nonthermal pressure support is not negligible in estimating the total gravitational mass of not only merging clusters, but also relaxed clusters, as predicted by numerical simulations. To improve the significance of the detection, however, a further examination by follow-up observations is required. The present study provides a pilot survey prior to future high-resolution spectroscopy with ASTRO-H, which is expected to play a critical role in revealing the dynamical evolutions of clusters.

Numerical Study on the Penetration of Droplets in a Zigzag Demister

Abstract A zigzag demister model is built to investigate the penetration of droplets in a zigzag demister. The rationality of the model is verified by comparing with the published experimental data. The flue gas and droplets are one-way coupled to observe the motion of droplets, and the effect of gas velocity and size grading on the separation efficiency was studied. Then, the coalescence of droplets is considered for the overall separation efficiency and grade separation efficiency in the model. The results show that under a certain gas bulk velocity, the coalescence of droplets remarkably affects the overall separation efficiency, and the influence of coalescence on the grade separation efficiency becomes manifested for the droplets between 7.5 and 22.5 μm. The effect of demister on the number concentration is investigated by counting the number of droplets at the entrance and exit. The results show that the separation efficiency based on the number concentration is lower than that based on the mass con...

Efficient Simulations of Early Structure Formation and Reionization

Detailed theoretical studies of the high-redshift universe, and especially reionization, are generally forced to rely on time-consuming N-body codes and/or radiative transfer algorithms. We present a method to construct seminumerical simulations, which can efficiently generate realizations of halo distributions and ionization maps at high redshifts. Our procedure combines an excursion-set approach with first-order Lagrangian perturbation theory and operates directly on the linear density and velocity fields. As such, the achievable dynamic range with our algorithm surpasses the current practical limit of N-body codes by orders of magnitude. This is particularly significant in studies of reionization, where the dynamic range is the principal limiting factor, because ionized regions reach scales of tens of comoving Mpc. We test our halo-finding and ionization-mapping algorithms separately against N-body simulations with radiative transfer and obtain excellent agreement. We compute the size distributions of ionized and neutral regions in our maps. We find even larger ionized bubbles than do purely analytic models at the same volume-weighted mean hydrogen neutral fraction, H I, especially early in reionization. We also generate maps and power spectra of 21 cm brightness temperature fluctuations, which for the first time include corrections due to gas bulk velocities. We find that velocities widen the tails of the temperature distributions and increase small-scale power, although these effects quickly diminish as reionization progresses. We also include some preliminary results from a simulation run with the largest dynamic range to date: a 250 Mpc box that resolves halos with masses M ≥ 2.2 × 108 M☉. We show that accurately modeling the late stages of reionization, H I 0.5, requires such large scales. The speed and dynamic range provided by our seminumerical approach will be extremely useful in the modeling of early structure formation and reionization.

Direct Measurements of Gas Bulk Flows in the Intracluster Medium of the Centaurus Cluster with theChandraSatellite

We present the analysis of the velocity structure of the intracluster gas near the core of Abell 3526 obtained with two off-center Chandra observations, specifically designed to eliminate errors due to spatial variations of the instrumental gain. We detected a significant velocity gradient along the northeast-southwest direction, roughly perpendicular to the direction of the incoming subgroup Cen 45, in agreement with previous ASCA SIS measurements. The presence of gas bulk velocities is observed both with and without the inclusion of the Fe K line complex in the spectral fittings. The configuration and magnitude of the velocity gradient is consistent with near transonic circulatory motion, either bulk or eddylike. The velocity difference obtained using the best calibrated central regions of ACIS-S3 is found to be (2.4 ± 1.0) × 103 km s-1 for rectangular regions 24 × 3' roughly diametrically opposed around the cluster's core. There are also indications of a high-velocity zone toward the southern region with similar magnitudes. The detection of velocity gradients is significant at >99.4% confidence, and simulations show that intrachip gain fluctuations >1800 km s-1 are required to explain the velocity gradient by chance. The measurements suggest that >1% of the total merger energy can still be bulk kinetic 0.4 Gyr after the merging event. This is the first direct confirmation of velocity gradients in the intracluster gas with independent instruments and indicates that strong departure from hydrostatic equilibrium is possible even for cool clusters that do not show obvious signs of merging.

Prospects for Detection of Intracluster Gas Bulk Velocities through the Sunyaev‐Zeldovich Effect

Intracluster gas velocity gradients have been recently detected by the authors in the Centaurus Cluster (A3526) using the Doppler shift of X-ray spectral lines with ASCA Solid-State Imaging Spectrometers. The velocity gradient was found to be maximum along a line roughly perpendicular to the direction of the incoming subgroup Cen 45 and has a correspondent velocity difference of ~(3.4 ± 1.1) × 103 km s-1 within a ~10' diameter region centered on the cD galaxy NGC 4696. Such bulk velocities should Comptonize the cosmic microwave background radiation (CMBR) producing variations of intensity and temperature that can be detectable in the near future with bolometers such as BOLOCAM. In this paper we realistically estimate the expected CMBR Comptonization for the central region of A3526, using ASCA and ROSAT data to constrain the Sunyaev-Zeldovich parameter expectations.

Detection of Bulk Motions in the Intracluster Medium of the Centaurus Cluster

Several recent numerical simulations of off-center cluster mergers predict that significant angular momentum with associated velocities of a few ?103 km?s-1 can be imparted to the resulting cluster. Such gas bulk velocities can be detected by the Doppler shift of X-ray spectral lines with ASCA spectrometers. Using two ASCA observations of the Centaurus cluster (Abell 3526), we produced a velocity map for the gas in the cluster's central regions. We also detected radial and azimuthal gradients in temperature and metal abundance distributions, which seem to be associated with the infalling subgroup centered at NGC 4709 (Cen 45). More importantly, we found a significant (>99.8% confidence level) velocity gradient along a line near-perpendicular to the direction of the incoming subgroup and with a maximum velocity difference of ~3.4 ? 1.1 ? 103 km s-1. It is unlikely (P < 0.002) that the observed velocity gradient is generated by gain fluctuations across the detectors. While the observed azimuthal temperature and abundance variations can be attributed to the interaction with Cen 45, we argue that the intracluster gas velocity gradient is more likely due to a previous off-center merging event in the main body of the Centaurus cluster.