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 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 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 A population of high-velocity absorption-line systems residing in the Local Group

2019 ◽  
Vol 627 ◽  
pp. A20 ◽  
Author(s):  
S. J. D. Bouma ◽  
P. Richter ◽  
C. Fechner
Keyword(s):  
Spectral Data ◽  
High Velocity ◽  
Entire Range ◽  
Milky Way ◽  
Local Group ◽  
Galactic Halo ◽  
Trace Gas ◽  
Low Density ◽  
Wide Range ◽  
High Velocity Clouds

Aims. We investigated the ionisation conditions and distances of Galactic high-velocity clouds (HVCs) in the Galactic halo and beyond in the direction of the Local Group (LG) barycentre and anti-barycentre, by studying spectral data of 29 extragalactic background sources obtained with the Cosmic Origins Spectropgraph (COS) installed on the Hubble Space Telescope (HST). Methods. We model column-densities of low, intermediate, and high ions such as Si II, C II, Si III, Si VI, and C IV, and use these data to construct a set of Cloudy ionisation models. Results. In total, we found 69 high-velocity absorption components along the 29 lines of sight. The components in the direction of the LG barycentre span the entire range of studied velocities, 100 ≲ |vLSR| ≲ 400 km s−1, while those in the anti-barycentre sample have velocities up to about 300 km s−1. For 49 components, we infer the gas densities. In the direction of the LG barycentre, the gas densities exhibit a wide range from log nH = −3.96 to −2.55, while in the anti-barycentre direction the densities are systematically higher, log nH >  −3.25. The barycentre absorbers can be split into two groups based on their density: a high-density group with log nH >  −3.54, which can be affected by the Milky Way radiation field, and a low-density group (log nH ≤ −3.54). The latter has very low thermal pressures of P/k <  7.3 K cm−3. Conclusions. Our study shows that part of the absorbers in the LG barycentre direction trace gas at very low gas densities and thermal pressures. These properties indicate that the absorbers are located beyond the virial radius of the Milky Way. Our study also confirms results from earlier, single-sightline studies, suggesting the presence of a metal-enriched intragroup medium filling the LG near its barycentre.

scholarly journals H I observations of three compact high-velocity clouds around the Milky Way

2014 ◽  
Vol 563 ◽  
pp. A99 ◽  
Author(s):  
S. Faridani ◽  
L. Flöer ◽  
J. Kerp ◽  
T. Westmeier
Keyword(s):  
High Velocity ◽  
Milky Way ◽  
High Velocity Clouds

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 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 The Milky Way: a halo, a corona, or both?

1985 ◽  
Vol 106 ◽  
pp. 415-420
Author(s):  
Klaas S. De Boer
Keyword(s):  
Transition Zone ◽  
Equilibrium Model ◽  
High Velocity ◽  
Milky Way ◽  
Galactic Plane ◽  
Hydrostatic Equilibrium ◽  
Hot Gas ◽  
Halo Stars ◽  
Intergalactic Space ◽  
Gaseous Halo

The detection in absorption lines of gas clouds outside the galactic plane at high velocities by Münch and Zirin (1961), high velocities then defined as velocities differing by more than 20 km/s from the LSR, showed that the space outside the Milky-Way disk contains not just stars. Of course, from a continuity argument it had been all along clear that some transition zone had to exist between the dense (relatively speaking) gas of the Milky-Way plane and the vast (almost) emptiness of intergalactic space. The presence of these clouds requires a mechanism to prevent their evaporation, and Spitzer (1956) proposed that dilute hot gas had to exist outside the Milky-Way disk reaching, in his hydrostatic-equilibrium model, temperatures of a few million K at several tens of kpc. These high temperatures led him to name these gases the Galactic Corona. Observational confirmation of the abundance of these cool clouds came from the measurements of 21-cm HI emission, but no one-to-one correspondence with clouds detected in the visual did appear (Habing 1969). For the majority of the high-velocity (HV) clouds (Hulsbosch 1978) no distances are known, and all of those are believed to exist as a gaseous halo with the halo stars. Thus our Milky Way appears to have outside the disk: a halo, a gaseous halo, and a corona.

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.

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