scholarly journals High Velocity Clouds in the Galaxy

1995 ◽  
Vol 164 ◽  
pp. 129-132
Author(s):  
Felix J. Lockman
Keyword(s):  
High Velocity ◽  
Large Scale ◽  
Galactic Plane ◽  
Spectral Lines ◽  
Galactic Rotation ◽  
The Sun ◽  
The Galaxy ◽  
High Velocity Clouds ◽  
Major Review ◽  
Lower Limits

Early observers measuring 21 cm HI profiles away from the Galactic plane found not only the emission near zero velocity expected from gas in the immediate vicinity of the Sun, but also occasional emission at velocities reaching several hundred km s−1. It seemed unlikely that these spectral lines could come from gas in normal galactic rotation (they are sometimes found at |b| > 45°), and so began the puzzle of “high-velocity clouds” (HVCs). The early result that all HVCs had negative velocity implying that they were infalling was soon shown to be incorrect with the discovery of many positive velocity clouds in the southern hemisphere. Attempts to determine the distance to HVCs by searching for them in absorption against stars yielded only lower limits, typically > 1 kpc. By 1984 several large-scale surveys had established that a significant fraction of the sky was covered with high velocity HI (e.g., Oort, 1966; Giovanelli, 1980). A recent major review is by Wakker (1991a; see also van Woerden, 1993). For this brief presentation to a specialized audience, I will concentrate on issues that may be relevant to the topic of stellar populations.

scholarly journals Analysis of Low-and High-Resolution Observations of High-Velocity Clouds

1989 ◽  
Vol 120 ◽  
pp. 416-423
Author(s):  
Bart P. Wakker
Keyword(s):  
High Velocity ◽  
Galactic Center ◽  
Neutral Hydrogen ◽  
Current Status ◽  
Galactic Rotation ◽  
Single Model ◽  
Neutral Gas ◽  
The Galaxy ◽  
High Velocity Clouds ◽  
Distance Problem

For almost three decades neutral hydrogen moving at velocities unexplicable by galactic rotation has been observed. These so-called high-velocity clouds (HVCs) have been invoked as evidence for infall of neutral gas to the galaxy, as manifestations of a galactic fountain, as energy source for the formation of supershells, etc. No general consensus about their origin has presently been reached. However, it is becoming clear that no single model will suffice to explain all HVCs. A number of clouds may consist of material streaming toward the galactic center, as Mirabel (this conference) has advocated for several years, though their origin still remains unclear. A better understanding is mainly hampered by the fact that the distance remains unknown. An overview of the current status of the distance problem is given by van Woerden elsewhere in this volume.

scholarly journals Filamentary Structures Near the Galactic Center

1989 ◽  
Vol 136 ◽  
pp. 243-263 ◽  
Author(s):  
F. Yusef-Zadeh
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Galactic Center ◽  
Galactic Plane ◽  
Galactic Rotation ◽  
Physical Interaction ◽  
Magnetic Structures ◽  
The Galaxy ◽  
Field Lines ◽  
The Magnetic Field

Recent studies of the Galactic center environment have revealed a wealth of new thermal and nonthermal features with unusual characteristics. A system of nonthermal filamentary structures tracing magnetic field lines are found to extend over 200pc in the direction perpendicular to the Galactic plane. Ionized structures, like nonthermal features, appear filamentary and show forbidden velocity fields in the sense of Galactic rotation and large line widths. Faraday rotation characteristics and the flat spectral index distributions of the nonthermal filaments suggest a mixture of thermal and nonthermal gas. Furthermore, the relative spatial distributions of the magnetic structures with respect to those of the ionized and molecular gas suggest a physical interaction between these two systems. In spite of numerous questions concerning the origin of the large-scale organized magnetic structures, the mechanism by which particles are accelerated to relativistic energies, and the source or sources of heating the dust and gas, recent studies have been able to distinguish the inner 200pc of the nucleus from the disk of the Galaxy in at least two more respects: (1) the recognition that the magnetic field has a large-scale structure and is strong, uniform and dynamically important; and (2) the physics of interstellar matter may be dominated by the poloidal component of the magnetic field.

Maser effects in the interstellar hydrogen clouds of the galactic halo

1967 ◽  
Vol 31 ◽  
pp. 291-293
Author(s):  
I. S. Šklovskij
Keyword(s):  
High Velocity ◽  
Population Inversion ◽  
Galactic Halo ◽  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Collisional Excitation ◽  
Hydrogen Atoms ◽  
Hydrogen Density ◽  
The Galaxy ◽  
High Velocity Clouds

It is suggested that the neutral hydrogen atoms in clouds with high negative velocities observed at high galactic latitudes may, when moving towards a galactic H 11 region, be excited by radiation in the red wing of the Lyman-α profile. The steepness of this wing may cause a population inversion of the hyperfine-structure levels. Consequently, estimates of the hydrogen density in the high-velocity clouds, and of the flow of matter towards the galactic plane (or into the Galaxy), when based on the assumption of collisional excitation, may be too high by two orders of magnitude.

scholarly journals High Velocity Neutral Hydrogen Clouds

1974 ◽  
Vol 60 ◽  
pp. 599-616
Author(s):  
R. D. Davies
Keyword(s):  
High Velocity ◽  
Spiral Structure ◽  
Neutral Hydrogen ◽  
Magellanic Clouds ◽  
Galactic Rotation ◽  
Velocity Pattern ◽  
Tidal Interactions ◽  
The Galaxy ◽  
High Velocity Clouds

A review is given of the observations of neutral hydrogen high velocity clouds (|ν| > 80 km s−1) in and near the Galaxy. The positive and negative clouds are seen to have different distributions in the sky, following roughly the velocity pattern of galactic rotation. A characteristic of the majority of the clouds is their distribution in elongated bands or strings. The various theories of origin of HVCs are discussed; the possible role of the tidal interactions between the Magellanic Clouds and the Galaxy is emphasized. Tests are suggested to distinguish between the Oort theory of the infall of intergalactic material and theories which envisage the HVCs as originating in the outermost spiral structure.

scholarly journals The High-Velocity Clouds: Galactic or Extragalactic?

1997 ◽  
Vol 166 ◽  
pp. 467-470 ◽  
Author(s):  
Hugo van Woerden ◽  
Bart P. Wakker ◽  
Ulrich J. Schwarz ◽  
Reynier F. Peletier ◽  
Peter M.W. Kalberla
Keyword(s):  
High Velocity ◽  
Milky Way ◽  
Local Group ◽  
Galactic Plane ◽  
Vertical Distance ◽  
High Velocity Clouds ◽  
Lower Limits ◽  
Firm Evidence ◽  
Milky Way Halo

AbstractWe present firm evidence that one of the major high velocity clouds (HVCs), Complex A, lies in the Milky Way Halo, at a vertical distance z = 3 - 7 kpc from the Galactic plane. For clouds MII/MIII, Danly et al. and Keenan et al. had already found z < 5 kpc. We further report that the metallicity in the largest HVC, Complex C, is at least 0.1 solar. Call/Hi ratios in 6 HVCs, ranging from 0.002 to 0.07 times solar, set lower limits to their metallicities.Blitz et al. have recently suggested that most of the HVCs are relatively unprocessed, extragalactic remnants of the gas which formed the Local Group of galaxies. However, the results mentioned above indicate that several major HVC complexes are neither primordial nor extragalactic. For the smaller HVCs, some of which have much higher velocities, a location in the Local Group remains a possibility.

scholarly journals High-velocity Clouds

1991 ◽  
Vol 144 ◽  
pp. 27-40 ◽  
Author(s):  
B.P. Wakker
Keyword(s):  
Energy Source ◽  
High Velocity ◽  
Neutral Hydrogen ◽  
Galactic Rotation ◽  
Single Model ◽  
The Galaxy ◽  
High Velocity Clouds

This contribution describes high-velocity clouds (HVCs), neutral hydrogen moving with velocities inexplicable by differential galactic rotation. They have been invoked as evidence for infall of gas to the Galaxy, as manifestations of a galactic fountain, as energy source for the formation of supershells, etc. It is becoming clear that a single model will not suffice to explain all HVCs. A better understanding is mainly hampered by the fact that the distance remains unknown. Many aspects to the study of HVCs will be discussed here.

scholarly journals The Hydrogen Clouds in the Galactic Halo

2004 ◽  
Vol 217 ◽  
pp. 130-135
Author(s):  
Felix J. Lockman
Keyword(s):  
High Velocity ◽  
Galactic Halo ◽  
Significant Fraction ◽  
Galactic Rotation ◽  
The Galaxy ◽  
Diffuse Cloud ◽  
High Velocity Clouds ◽  
Two Phases ◽  
Green Bank Telescope

New 21cm observations with the Green Bank Telescope show that a significant fraction of the HI in the inner Galaxy's halo ~ 1 kpc from the midplane exists in the form of discrete clouds. Some look very much like a Spitzer (1968) “standard” diffuse cloud but with their HI in two phases. They mark the transition between the neutral disk and the highly ionized halo. The dominant motion of the clouds is Galactic rotation, but some have random velocities of as much as 50 km s−1. They are part of the Galaxy and are not related to high-velocity clouds, yet their origin is obscure.

Galactic orbits of stars

1966 ◽  
Vol 25 ◽  
pp. 93-97
Author(s):  
Richard Woolley
Keyword(s):  
Globular Cluster ◽  
Potential Field ◽  
High Velocity ◽  
Velocity Vector ◽  
Variable Stars ◽  
The Sun ◽  
Proper Motions ◽  
Rr Lyrae ◽  
The Galaxy

It is now possible to determine proper motions of high-velocity objects in such a way as to obtain with some accuracy the velocity vector relevant to the Sun. If a potential field of the Galaxy is assumed, one can compute an actual orbit. A determination of the velocity of the globular clusterωCentauri has recently been completed at Greenwich, and it is found that the orbit is strongly retrograde in the Galaxy. Similar calculations may be made, though with less certainty, in the case of RR Lyrae variable stars.

scholarly journals Abundances in the Gaseous Galactic Halo

1998 ◽  
Vol 11 (1) ◽  
pp. 86-89
Author(s):  
Ulysses J. Sofia
Keyword(s):  
Interstellar Medium ◽  
Gas Phase ◽  
High Velocity ◽  
Galactic Halo ◽  
Solar Neighborhood ◽  
Local Interstellar Medium ◽  
The Galaxy ◽  
Metal Abundances ◽  
High Velocity Clouds

Abstract The well measured gas-phase abundances in the low halo suggest that this region of the Galaxy has total (gas plus dust) metal abundances which are close to those in the solar neighborhood. The gas-phase abundances in the halo are generally higher than those seen in the disk, however, this affect is likely due to the destruction of dust in the halo clouds. Observations of high velocity clouds (HVCs) in the halo suggest that these clouds have metal abundances which are substantially lower than those measured for the local interstellar medium. These determinations, however, are often of lower quality than those for the low halo because of uncertainties in the hydrogen abundances along the sightlines, in the incorporation of elements into dust, and in the partial ionization of the clouds.

scholarly journals An Interpretation of the Distribution of Metre Wavelength Radio Emission

1968 ◽  
Vol 21 (2) ◽  
pp. 167 ◽  
Author(s):  
KW Yates
Keyword(s):  
High Latitude ◽  
Large Scale ◽  
Galactic Halo ◽  
Galactic Plane ◽  
Absolute Calibration ◽  
Fundamental Difficulty ◽  
The Galaxy ◽  
Calibration Accuracy ◽  
Limit Estimate ◽  
Spiral Arm

A recent 85 MHz survey of the southern sky had an absolute calibration accuracy and resolution comparable with a number of surveys made for the northern skies. By combining the results of these surveys in both hemispheres a complete sky map has been produced, and in this paper an analysis is made of the distribution of the medium and high latitude emission. A fundamental difficulty encountered is the identification and isolation of the spurs of emission projecting from the galactic plane. Two hypotheses are proposed. The first attributes the spurs to a large-scale feature associated with the galactic core and the remaining emission to a galactic halo. The second postulates the origin of the spurs within the local spiral arm, which is itself considered to contribute significantly to the high latitude background. An upper-limit estimate of the emissivity of the local arm is made from currently available independent data. Using this result a model local arm is proposed, which, together with an isotropic component from beyond the Galaxy and a small additional galactic component, explains the observed distribution.

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