scholarly journals High Velocity Gas in the Halos of Spiral Galaxies

2004 ◽  
Vol 217 ◽  
pp. 136-141 ◽  
Author(s):  
Filippo Fraternali ◽  
Tom Oosterloo ◽  
Rense Boomsma ◽  
Rob Swaters ◽  
Renzo Sancisi
Keyword(s):  
High Velocity ◽  
Spiral Galaxies ◽  
High Sensitivity ◽  
Neutral Gas ◽  
Ngc 6946 ◽  
Inclination Angles ◽  
High Velocity Clouds ◽  
Halo Gas ◽  
Very High ◽  
Thick Layers

Recent, high sensitivity, HI observations of nearby spiral galaxies show that their thin ‘cold’ disks are surrounded by thick layers (halos) of neutral gas with anomalous kinematics. We present results for three galaxies viewed at different inclination angles: NGC 891 (edge-on), NGC 2403 (i=60°), and NGC 6946 (almost face-on). These studies show the presence of halo gas up to distances of 10-15 kpc from the plane. Such gas has a mean rotation 25-50 km s−1 lower than that of the gas in the plane, and some complexes are detected at very high velocities, up to 200-300 km s−1. The nature and origin of this halo gas are poorly understood. It can either be the result of a galactic fountain or of accretion from the intergalactic medium. It is probably analogous to some of the High Velocity Clouds (HVCs) of the Milky Way.

scholarly journals High-Velocity Clouds and the Local Group

2004 ◽  
Vol 217 ◽  
pp. 2-11 ◽  
Author(s):  
B. P. Wakker
Keyword(s):  
High Velocity ◽  
Milky Way ◽  
Local Group ◽  
Current Data ◽  
The Other ◽  
Hot Gas ◽  
Neutral Gas ◽  
Hα Emission ◽  
M 31 ◽  
High Velocity Clouds

I examine some of the evidence relevant to the idea that high-velocity clouds (HVCs) are gas clouds distributed throughout the Local Group, as proposed by Blitz et al. (1999) and Braun & Burton (1999). This model makes several predictions: a) the clouds have low metallicities; b) there should be no detectable Hα emission; c) analogues near other galaxies should exist; and d) many faint HVCs in the region around M 31 can be found. Low metallicities are indeed found in several HVCs, although they are also expected in several other models. Hα emission detected in most HVCs and, when examined more closely, distant (D>200 kpc) HVCs should be almost fully ionized, implying that most HVCs with H I must lie near the Milky Way. No clear extragalactic analogues have been found, even though the current data appear sensitive enough. The final prediction (d) has not yet been tested. on balance there appears to be no strong evidence for neutral gas clouds distributed throughout the Local Group, but there may be many such clouds within 100 or so kpc from the Milky Way (and M31). on the other hand, some (but not all) of the high-velocity O VI recently discovered may originate in hot gas distributed throughout the Local Group.

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 Probing ISM Models with Hα Observations

1998 ◽  
Vol 15 (1) ◽  
pp. 19-23
Author(s):  
Andrea Ferrara
Keyword(s):  
Energy Dissipation ◽  
Interstellar Medium ◽  
High Velocity ◽  
Spiral Galaxies ◽  
Turbulent Energy ◽  
Ionising Radiation ◽  
Supernova Explosions ◽  
High Velocity Clouds ◽  
Viable Model ◽  
Crucial Experiments

AbstractI review the capabilities of Hα observations to constrain some aspects of the current models of the interstellar medium. In particular, it is shown that turbulence is a necessary ingredient of any viable model, since most of the energy produced by supernova explosions and ionising radiation is stored in kinetic form in the ISM. Various forms of turbulent energy dissipation, including cloud collisions, are analysed. Two additional aspects, concerning the existence of galactic fountains and their relation with high-velocity Clouds, and the extended ionised layer of spiral galaxies are discussed; some crucial experiments are suggested.

scholarly journals Neutral Gas Outside the Disks of Local Group Galaxies

2016 ◽  
Vol 11 (S321) ◽  
pp. 223-225
Author(s):  
Felix J. Lockman
Keyword(s):  
High Velocity ◽  
Local Group ◽  
Dwarf Galaxies ◽  
Neutral Gas ◽  
High Velocity Clouds ◽  
Circumgalactic Medium ◽  
Magellanic Stream ◽  
Green Bank Telescope

AbstractOf the three kinds of neutral gas found outside the stellar disks of Local Group galaxies, only the products of interaction, like the Magellanic Stream, have a clearly understandable origin. Both the high-velocity clouds and the faint H I between M31 and M33 remain a mystery. New observations of the region between M31 and M33 with the Green Bank Telescope show that the H I there resides in clouds with a size and mass similar to that of dwarf galaxies, but without stars. These clouds might be products of an interaction, or condensations in the hot circumgalactic medium of M31, but both these models have difficulties. The prevalence of clouds like this in the Local Group remains to be determined.

scholarly journals High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way

2012 ◽  
Vol 424 (4) ◽  
pp. 2896-2913 ◽  
Author(s):  
N. Lehner ◽  
J. C. Howk ◽  
C. Thom ◽  
A. J. Fox ◽  
J. Tumlinson ◽  
...  
Keyword(s):  
High Velocity ◽  
Milky Way ◽  
Neutral Gas ◽  
High Velocity Clouds

scholarly journals HI holes and high-velocity clouds in the spiral galaxy NGC 6946

2008 ◽  
Vol 490 (2) ◽  
pp. 555-570 ◽  
Author(s):  
R. Boomsma ◽  
T. A. Oosterloo ◽  
F. Fraternali ◽  
J. M. van der Hulst ◽  
R. Sancisi
Keyword(s):  
Spiral Galaxy ◽  
High Velocity ◽  
Ngc 6946 ◽  
High Velocity Clouds

scholarly journals A sensitive high-velocity HI survey

1979 ◽  
Vol 84 ◽  
pp. 541-544
Author(s):  
Riccardo Giovanelli
Keyword(s):  
High Velocity ◽  
Observational Studies ◽  
Local Group ◽  
Neutral Hydrogen ◽  
Velocity Range ◽  
Sensitivity Limit ◽  
Local Supercluster ◽  
High Velocity Clouds ◽  
Very High

The observational studies of the neutral hydrogen high velocity clouds (HVC's) in the last decade have for the most part concentrated on the detailed study of the prominent complexes discovered by the surveys of the late 1960's. More recently, several HVC's were discovered, often by accident, with very high velocities, which are typically outside of the velocity range and below the sensitivity limit of the old surveys. The tendency to associate these clouds with the closest galaxy at hand became a rather common practice. The idea that the Local Group, or the Local Supercluster, is populated with such “intergalactic clouds” was fed by these discoveries, and generalized to the whole field of the HVC's. I have repeatedly argued against the latter generalization (Giovanelli 1977, 1978). In this paper I shall report on the progress of a new survey that intends to constitute a test of the intergalactic approach itself.

scholarly journals Metals and Molecules in Halo Clouds

1991 ◽  
Vol 144 ◽  
pp. 161-164 ◽  
Author(s):  
Klaas S. de Boer ◽  
Uwe Herbstmeier ◽  
Ulrich Mebold
Keyword(s):  
High Velocity ◽  
Milky Way ◽  
High Velocity Clouds ◽  
Halo Gas

The abundance of the elements in clouds of halo gas, as determined from observations, is an important parameter for the test of the validity of models explaining the existence of, e.g., the high latitutude high-velocity clouds (HVCs) of the Milky Way. Individual HVCs have been detected in absorption only on very few lines of sight so that the distance of the HVCs, another important parameter for the models, stays ill determined as well. We will follow here the more or less established convention by calling HVCs those with /v/ >100 km s−1 and IVCs (intermediate-velocity clouds) those with 50 </v/ <100 km s−1. We will define halo as the space with /z/> 1 kpc, although for /b/> 45 also /z/> 0.5 kpc is used.

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