scholarly journals Observations of high velocity HI clouds in the Local Group

1978 ◽  
Vol 79 ◽  
pp. 49-49
Author(s):  
W.K. Huchtmeier
Keyword(s):  
Radio Telescope ◽  
High Velocity ◽  
Local Group ◽  
Neutral Hydrogen ◽  
Beam Width ◽  
Late Type ◽  
M 31 ◽  
High Velocity Clouds ◽  
Half Power ◽  
Magellanic Stream

High velocity clouds (HVC) of neutral hydrogen in or around our galaxy and the observations of intergalactic HI in the Local group: Magellanic stream (Mathewson et al., 1974, Astrophys. J. 190, p. 291), M 31 (Davies, R.D., 1975, Mon. Not. R. astr. Soc., 170, p. 45P), and in the Sculptor group of galaxies (Mathewson et al., 1975, Astrophys. J. 195, p. L97) motivated us to search for HVC-phenomena in a number of nearby late-type galaxies with the 100 m Effelsberg radio telescope which has a half power beam width of 8.5′ at the wavelength of 21 cm.

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 Hα Distances to High Velocity Clouds

2004 ◽  
Vol 217 ◽  
pp. 12-19
Author(s):  
Joss Bland-Hawthorn ◽  
Mary Putman
Keyword(s):  
High Velocity ◽  
Local Group ◽  
Solid Angle ◽  
Galactic Disk ◽  
Distance Constraint ◽  
Upper Limits ◽  
The Galaxy ◽  
Ionization Model ◽  
High Velocity Clouds ◽  
Magellanic Stream

Several observing teams have now obtained deep Hα spectroscopy towards high-velocity clouds (HVCs) which vary in structure from compact (CHVCs) to the Magellanic Stream. Our team has observed clouds which range from being bright (~640 mR) to having upper limits on the order of 30 to 70 mR. The Hα measurements can be interpreted as a distance constraint if we adopt a halo ionization model based on fesc ≈ 6% of the ionizing photons escaping normal to the Galactic disk (fesc ≈ 1 − 2% when averaged over solid angle). The results suggest that many HVCs and CHVCs are within a ~40 kpc radius from the Galaxy and are not members of the Local Group at megaparsec distances. We refer the reader to Putman et al. (2003) for the full version of the paper presented here.

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 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 Morphological Characteristics of Compact High-Velocity Clouds Revealed by High-Resolution WSRT Imaging

2000 ◽  
Vol 174 ◽  
pp. 136-147
Author(s):  
W. B. Burton ◽  
R. Braun
Keyword(s):  
Dark Matter ◽  
High Resolution ◽  
High Velocity ◽  
Physical State ◽  
Local Group ◽  
Morphological Characteristics ◽  
Kinetic Temperature ◽  
High Resolution Imaging ◽  
Resolution Imaging ◽  
High Velocity Clouds

AbstractA class of compact, isolated high–velocity clouds which plausibly represents a homogeneous subsample of the HVC phenomenon in a single physical state was objectively identified by Braun and Burton (1999). Six examples of the CHVCs, unresolved in single–dish data, have been imaged with the Westerbork Synthesis Radio Telescope. The high–resolution imaging reveals the morphology of these objects, including a core/halo distribution of fluxes, signatures of rotation indicating dark matter, and narrow linewidths constraining the kinetic temperature of several opaque cores. In these regards, as well as in their kinematic and spatial deployment on the sky, the CHVC objects are evidently a dynamically cold ensemble of dark–matter–dominated H ɪ clouds accreting onto the Local Group in a continuing process of galactic evolution.

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.

The Magellanic Stream: New Observations

1989 ◽  
Vol 8 (2) ◽  
pp. 195-203 ◽  
Author(s):  
S. R. Wayte
Keyword(s):  
High Velocity ◽  
Complicated Structure ◽  
Bulk Motion ◽  
High Velocity Clouds ◽  
Magellanic Stream

AbstractNew HI observations of the Magellanic Stream have been made using the Parkes 64 m telescope. These observations highlight in detail its complicated structure and uncover new features of the Stream. The extreme velocity clouds (EVCs) are morphologically linked to the Stream indicating that these HI clouds are very likely truly part of the Stream. Also it is suggested that many of the high velocity clouds seen around the Stream are indeed part of the Magellanic Stream itself, and that the stream consists of many different velocity components in bulk motion.

scholarly journals A Neutral Gas Jet in a Low Velocity Shock Front at the Boundary of the Draco Nebula

1984 ◽  
Vol 81 ◽  
pp. 243-247 ◽  
Author(s):  
P.W.M. Kalberla ◽  
U. Herbstmeier ◽  
U. Mebold
Keyword(s):  
Shock Front ◽  
Radio Telescope ◽  
High Velocity ◽  
Neutral Hydrogen ◽  
Gas Jet ◽  
Low Velocity ◽  
X Ray ◽  
Neutral Gas ◽  
Thermal Bremsstrahlung

Abstract21-cm line observations with the Westerbork Synthesis Radio Telescope of a dust and molecular filament at the boundary of the Draco Nebula (l≈ 91°, b≈ 38°) reveal a jet-like neutral hydrogen feature funneling through an outlet in the low velocity shock front at the interface between the Draco Nebula and the surrounding gas. The jet-like feature is apparently connected with a high velocity filament at VLSR = -180 km s-1. We suggest that the soft x-ray emission observed in the area is thermal bremsstrahlung produced by the deceleration of high velocity gas in galactic gas.

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.

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