Neutral Gas (neutral + gas)

Distribution by Scientific Domains

Selected Abstracts

Effect of Cathode and Anode Voltage on an Ion Sheath Thickness in a Magnetically Confined Diffusion Plasma

M. Kr.
Abstract This article reports about the ion sheath thickness variation occurring in front of a negatively biased plate immersed in the target plasma region of a double plasma device. The target plasma is produced due to the local ionization of neutral gas by the high energetic electrons coming from the source region (main discharge region). It is observed that for an increase in cathode voltage (filament bias voltage) in the source region, the ion flux into the plate increases. As a result, the sheath at the plate contracts. Again, for an increase in source anode voltage (magnetic cage bias), the ion flux to the plate decreases. As a result, the sheath expands at the plate. The ion sheath formed at the separation grid of the device is found to expand for an increase in cathode voltage and it contracts for an increase in the anode voltage of the main discharge region. One important observation is that the applied anode bias can control the Bohm speed of the ions towards the separation grid. Furthermore, it is observed that the ion current collected by the separation grid is independent of changes in plasma density in the diffusion region but is highly dependent on the source plasma parameters. ( 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Monte Carlo Simulations of the Electron Currents Collected by Electrostatic Probes

D. Trunec
Abstract We have carried out Monte Carlo simulation of the motion of electrons in the space charge sheath surrounding a cylindrical Langmuir (electrostatic) probe. The electron currents to the probe have been calculated from these simulations for different conditions (pressure of neutral gas, presence of magnetic field). The results of the simulations have been compared with recent Langmuir probe measurements made in cylindrical DC magnetron plasmas. ( 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Probing feedback in protogalaxies: multiphase gas in a DLA at z, 2.4

N. Lehner
ABSTRACT We investigate the physical processes occurring in the multiphase gas of a damped Ly, absorber (DLA). We base our analysis on a high-quality Keck High-Resolution Echelle Spectrometer (HIRES) spectrum of the QSO J1211+0422 in which a DLA is detected at z, 2.377. There is little contamination of the high-ion (O vi, N v, C iv, Si iv) absorption, allowing us to explore the properties of the highly ionized gas and its connection to other gas phases. The metallicity ([Z/H]=,1.41 0.08), H i column density [], full-width velocity (,vneut, 70 km s,1) and relative abundances ([Si/Fe]=+0.23 0.05 and [N/Si]=,0.88 0.07) of this DLA are not unusual. However, we derive the lowest C ii* cooling rate in a DLA, lc < 10,27.8 erg s,1 per H atom (3,). Using this stringent limit, we show that the neutral gas (confined at |v| < +39 km s,1) must be warm and the star formation rate is <7.1 10,3 M, yr,1 kpc,2. Surprisingly, the gas shows strong, complex absorption profiles from highly ionized gas whose kinematics appear connected to each other and the low ions. The total amount of highly and weakly ionized gas is very large with ,1.5. At |v| ,+39 km s,1, the gas is fully and highly ionized []. Based on ionization models, O vi and N v are generally difficult to produce by hard photons, while Si iv and C iv can be photoionized to a large extent. There is, however, no evidence of O vi -bearing gas at T, 106 K associated with this DLA. In contrast, there is some evidence for narrow O vi, N v and C iv components (unexplained by photoionization), implying too low temperatures (T < 105 K) for simple collisional ionization models to produce their observed column densities. Stellar feedback is a possible source for producing the high ions, but we cannot rule out accretion of non-pristine material on to the protogalaxy. [source]

A high-resolution radio study of neutral gas in the starburst galaxy NGC 520

R. J. Beswick
ABSTRACT We present subarcsec angular resolution observations of the neutral gas in the nearby starburst galaxy NGC 520. The central kpc region of NGC 520 contains an area of significantly enhanced star formation. The radio continuum structure of this region resolves into ,10 continuum components. By comparing the flux densities of the brightest of these components at 1.4 GHz with published 15-GHz data we infer that these components detected at 1.4 and 1.6 GHz are related to the starburst and are most likely to be collections of several supernova remnants within the beam. None of these components is consistent with emission from an active galactic nuclei. Both neutral hydrogen (H i) and hydroxyl (OH) absorption lines are observed against the continuum emission, along with a weak OH maser feature probably related to the star formation activity in this galaxy. Strong H i absorption (NH, 1022 atoms cm,2) traces a velocity gradient of 0.5 km s,1 pc,1 across the central kpc of NGC 520. The H i absorption velocity structure is consistent with the velocity gradients observed in both the OH absorption and in CO emission observations. The neutral gas velocity structure observed within the central kpc of NGC 520 is attributed to a kpc-scale ring or disc. It is also noted that the velocity gradients observed for these neutral gas components appear to differ with the velocity gradients observed from optical ionized emission lines. This apparent disagreement is discussed and attributed to the extinction of the optical emission from the actual centre of this source hence implying that optical ionized emission lines are only detected from regions with significantly different radii to those sampled by the observations presented here. [source]

Post-starburst galaxies and the transformation of blue into red galaxies

S. De Rijcke
Abstract We present deep single-dish radio observations of a sample of nearby post-starburst galaxies (0.05 < z < 0.1). About 50% of these post-starburst galaxies are detected at 21 cm, with HI masses of ,109 M,, up to ,1010 M,. These post-starburst galaxies are as gas-rich as spiral galaxies with comparable luminosities. There appears to exist no direct correlation between the amount of H I present in a post-starburst galaxy and its star formation rate as traced by radio continuum emission. Moreover, the end of the starburst clearly does not necessarily require the complete exhaustion of the neutral gas reservoir. High-resolution radio observations of one post-starburst binary system suggest that most of the neutral gas resides outside the stellar bodies of the galaxies. Most likely, the gas was expelled by supernova and/or AGN feedback. This effectively stops star formation, even though copious amounts of diffuse neutral gas remain in the immediate vicinity. This remaining H I reservoir may eventually lead to further episodes of star formation. This may indicate that some post-starbursts are observed in the inactive phase ofthe star formation duty cycle ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The evolution of the gas content of galaxy groups

E.M. Wilcots
Abstract We examine multiple facets of the evolution of the gas content of galaxy groups. Complementing building evidence that a tremendous amount of galaxy transformation takes place in the group environment we find evidence of similar transformation of the gas content. In dynamically young groups galaxy-galaxy interactions appear to be responsible for depositing large quantities of neutral gas into the intergalactic medium. The gas content of dynamically evolved groups, however, is characterized by extended halos of diffuse hot gas. We also find that groups may harbor significant quantities of heretofore undetected baryons and that feedback from AGN may be responsible for heating the intragroup medium ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]