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Topic: High Velocity Clouds

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	Interstellar cloud - Wikipedia, the free encyclopedia
		Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies.
		Analysing the composition of interstellar clouds is achieved by studying electromagnetic radiation that we receive from them.
		High velocity clouds are identified with a HVC prefix, as with HVC 127-41-330.
	en.wikipedia.org /wiki/High_velocity_cloud (488 words)

	High-Velocity Clouds Press Release
		However, new observations of high-velocity clouds in other galaxies suggest that they are at an intermediate distance, according to a team of researchers presenting a paper today to the American Astronomical Society meeting in Washington, DC.
		Schulman and his team avoided the problem of uncertain distances by studying high-velocity clouds in other galaxies, where the high-velocity clouds and the bulk of the atomic hydrogen in the galaxies are at the approximately the same distance from us.
		The researchers found evidence for high-velocity clouds in ten of the fifteen galaxies they studied; the mass in high-velocity atomic hydrogen for the galaxies with high-velocity clouds is consistent with Galactic high-velocity clouds having a typical distance of about ten thousand light years from us.
	members.bellatlantic.net /~vze3fs8i/astro/hvcpr.html (560 words)

	Clouds Dominate the Galactic Halo
		Though the initial studies by Lockman revealed the presence of these clouds, the data were insufficient to conclusively show that they were present throughout the entire halo.
		High-velocity clouds are vagabond clumps of intergalactic gas, possibly left over from the formation of the Milky Way and other nearby galaxies.
		According to the researcher, the ubiquitous nature and dynamics of these newly discovered clouds support the theory that they are condensing out of the hot gas that is lifted into the halo through supernova explosions.
	www.nrao.edu /pr/2003/honeclouds (853 words)

	[No title] (Site not responding. Last check: 2007-10-29)
		The High Velocity Clouds, first detected in 1963, have velocities too high to have the nearly circular orbits about the center of the Milky Way that most of the rest of the atomic gas has, and in some cases are very large, even going from horizon to horizon as detected from observatories on earth.
		Most important among the properties of the High Velocity Clouds explained by Blitz and Spergel are the motions of the clouds as measured by their velocities along the line of sight, and their positions in the sky, which have previously never been adequately explained.
		The High Velocity Clouds have long been known to have internal velocities that are too high to be the result of the gravitational effects of the visible matter observed within them, and they might be held together by dark matter which would explain both their stability and their large internal velocities.
	www.astro.umd.edu /~teuben/hvc/press97.txt (1524 words)

	The high-velocity clouds and the Magellanic Clouds (Site not responding. Last check: 2007-10-29)
		The orbits of the Large and Small Magellanic Clouds have been traced backwards in time to estimate the position and velocity of the Clouds at the time of the collision between the two Clouds, and to infer the initial conditions of the HVCs.
		The initial velocities of the HVCs were the result of velocities of expansion that permitted the escape of the HVCs from the Magellanic Clouds plus the systemic velocity of the Magellanic Clouds at the time of the collision.
		The eccentric position of the Sun within the cloud of HVCs explains the asymmetries between the sky distributions of the HVCs of the northern Galactic hemisphere and those of the southern Galactic hemisphere.
	www.edpsciences.org /articles/aa/abs/2004/33/aa4102/aa4102.html (484 words)

	01.14.00 - High velocity clouds between galaxies identified as remnants of early universe and seeds of the Milky Way ...
		Discovered by radio astronomers in 1963, the gas clouds are moving at high velocity through space in orbits that don't conform to the nicely circular orbits of most other objects in the disk of the Milky Way.
		A second prediction was that, if the high velocity clouds are part of the Local Group, they should show little of the fluorescence (hydrogen alpha emission) seen in relatively nearby clouds of atomic hydrogen.
		Rather than postulate that all the galaxies we see are in the last third of their lives and have run out of fuel, the theory suggests instead that high velocity clouds fall in and continually replenish the gas needed to fuel star formation.
	www.berkeley.edu /news/media/releases/2000/01/01-14-2000.html (1182 words)

	High Energy Astrophysics Group
		decay occurs during high energy nuclei interaction with the interstellar medium and this is an emission process that is possible only for nuclei (mostly for protons because of their higher concentration).
		In the case of SNRs the initial energy release is produced by a supernova explosion, in the case of HVCs an energy emission is produced by the interaction of the clouds with gas in the Galaxy halo and in the Solar wind shocks energy is supplied by the energetic processes on the Sun.
		These clouds were first observed by the emission of neutral hydrogen as gas clouds falling on the Galaxy plain with the velocities of about 100-150 km/s.
	www.ioffe.rssi.ru /astro/HEA/ourwork.html (2398 words)

	Cosmos: The SAO Encyclopedia: high velocity cloud (Site not responding. Last check: 2007-10-29)
		High Velocity Clouds are interstellar gas clouds that are moving at speeds substantially different (up to several hundred km/s) to the rotation of the disk of the Milky Way galaxy.
		For example, it is thought that many of the southern high velocity clouds originated in the Magellanic stream - a trail of gas torn out of the Magellanic Clouds in an interaction with the Milky Way hundreds of millions of years ago.
		This suggests that these clouds could previously have been ejected from the Galaxy (in a galactic fountain) by supernova explosions, and are only now returning to the Milky Way.
	www.cosmos.swin.edu.au /entries/highvelocitycloud/highvelocitycloud.html (291 words)

	MPIA-PR_000608 (Site not responding. Last check: 2007-10-29)
		The gas clouds have velocities exceeding 60 miles or 100 km per second relative to the Sun, similar to a group of objects known as high-velocity clouds.
		However, unlike the well-known high-velocity clouds, which tend to span many tens of degrees on the sky and form diffuse extended complexes, the clouds identified by Braun and Burton are compact, isolated objects.
		While most high-velocity clouds are relatively nearby at distances of 30,000 to 300,000 light years in the outskirts and immediate surroundings of our parent galaxy, the Milky Way, compact high-velocity clouds appear to be situated much farther away at distances of 1.5 to 3 million light years.
	www.astro.unibas.ch /~grebel/aas196_pressrel_2b.html (729 words)

	GBT Reveals Satellite of Milky Way
		High velocity clouds are essentially what their name implies, fast-moving clouds of predominately neutral atomic hydrogen.
		Earlier studies of Complex H were hindered because the cloud currently is passing almost exactly behind the outer disk of the Galaxy.
		Among the most prominent of these objects are the Magellanic Clouds, which also are being affected by their interaction with the Milky Way, and are shedding their gas in a long stream.
	www.nrao.edu /pr/2003/complexh (824 words)

	SPACE.com -- Fog Lifts on Source of High-Speed Galactic Clouds
		Clouds of gas -- sans any stars -- would be scattered throughout the sphere, too, many moving faster than the general galactic rotation.
		If a cloud is made mostly of hydrogen atoms, researchers figure it is probably left over from the early days of galaxy formation.
		If, however, a cloud was formed from gas ejected by an exploding star (a supernova) some of the hydrogen would have been converted to heavier elements during the star's life and those elements should still be present.
	www.space.com /science/astronomy/galactic_clouds_991123.html (1304 words)

	3 Numerical simulations of high velocity clouds (Site not responding. Last check: 2007-10-29)
		Outside the cloud, plasma and neutral gas densities are comparable such that the neutral gas acquires the same velocity as the streaming plasma.
		The center of the neutral gas cloud is now moving with a small velocity in the negative y-direction as a result of the continuous impact of plasma on the cloud.
		At the front of the HVC, the magnetic field strength is increased by a factor 10, in the tail by a factor 4.
	www.edpsciences.org /articles/aa/abs/2002/32/aah3470/node3.html (3432 words)

	HubbleSite - Release Text about "High Velocity Clouds Found to Dwell in Milky Way's Halo"
		Danly reported that the cloud's distance, estimated to be between 5,000 and 14,000 light years from Earth, can be used to calculate the cloud's mass as between 2,600 and 22,000 times the mass our sun.
		Previously detected absorption line features, seen when a cloud lies in front of a more distant object and filters some light, have not given distance information because most background sources, such as quasars, are very far away, she explained.
		The cloud is falling toward us at 200,000 miles per hour and will eventually crash into the plane of our galaxy, if nothing acts to slow it down," she said.
	hubblesite.org /newscenter/newsdesk/archive/releases/1993/04/text (604 words)

	Puzzle of galactic evolution solved (Nov 24, 1999)
		Massive clouds of gas, discovered long ago but only recently identified as being within the margins of the Milky Way, play a key role in the ability of the galaxy to churn out new stars by raining gas onto the plane of the galaxy, a new report suggests.
		Discovered 35 years ago, the clouds were an enigma because they behave differently than most galactic objects -- coursing through space at high speeds and not neatly rotating along with the rest of the galaxy.
		The new evidence, said Wakker, strongly suggests that some of the clouds play a key role in the chemical evolution of the galaxy by showering it with metal-poor gas that counteracts a buildup of heavy elements within the stars and gas found in the disk of the Milky Way.
	www.news.wisc.edu /3438.html (860 words)

	High Velocity Cloud Science Summary Presentation Page (Site not responding. Last check: 2007-10-29)
		Clouds of gas raining into the galaxy from the outside are heated as they pass through this corona, much like a meteor heats up as it passes through the earth's atmosphere.
		It is this heating of the clouds that is observed with the FUSE satellite.
		Since wavelength of light is related to velocity (through the Doppler formula), we show a wavelength scale on the top (in nanometers) and a velocity scale on the bottom (in kilometers per second).
	fuse.pha.jhu.edu /wpb/sci_HVC.html (952 words)

	High-Velocity Clouds in Nearby Disk Galaxies
		A galactic fountain, in which hot gas is created in associations of massive starts, forms superbubbles which break out of the Galactic disk, cools radiatively as it rises upward into the halo, and eventually recombines and returns to the disk ballistically.
		The distance to high-velocity clouds in the Galaxy is very difficult to determine, which means that we can't determine their masses, linear diameters, or densities.
		The velocity extent perpendicular to the disk is successfully represented by a constant Gaussian velocity dispersion of 9 km/s.
	home1.gte.net /~vze3fs8i/astro/hvc.html (504 words)

	HIGH CLOUDS (Site not responding. Last check: 2007-10-29)
		The type of clouds most often found in the high level etage are cirrus, cirrocumulus and cirrostratus.
		These clouds are composed primarily of ice crystals and usually present a bright white, or mostly white appearance except near sunrise or sunset.
		Cirrus are detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands.
	www.met.tamu.edu /class/Metr304/Exer10dir/highclouds.html (422 words)

	High Velocity Clouds: Remnants of Local Group Formation (Site not responding. Last check: 2007-10-29)
		We re-evaluate the evidence for the nature of the High Velocity HI clouds (HVCs) and show that they are most plausibly explained as members of the Local Group of galaxies.
		We show further that if the clouds are stable entities, with formation and destruction timescales long compared to their crossing times, that upper and lower distance limits placed by gravitational boundedness and tidal stability place the clouds, on average, at distances consistent with membership in the Local Group.
		We show also that the clouds exhibit a relation between their angular sizes and their velocities relative to the Local Group Standard of Rest, with clouds inferred to be closer to the Milky Way having larger angular sizes.
	www.aas.org /publications/baas/v28n4/aas189/abs/S061001.html (325 words)

	CAS - Research - 2005 Colloquia
		High-velocity clouds (HVCs) are defined by their high radial velocities which are incompatible with a participation in the normal rotation of our Galaxy.
		We found several HVCs near the disk of M31, but not a single HVC was found beyond a projected distance of about 50 kpc, confirming that HVCs are located in the vicinity of the large galaxies.
		High spectral resolution observations are critical to understanding the morphological structure of nearby interstellar material.
	astronomy.swin.edu.au /research/colloquia_2005.html (3290 words)

	Cloud Distances and Associations
		In the case of high-velocity clouds, distances are so poorly known in most cases that the debate over whether the clouds are galactic or extragalactic entities continues nearly 40 years after their discovery in 1963.
		The method depends on the availability of stars with known distances where some fall in front of the cloud and some lie beyond the cloud and are hence seen in absorption.
		There are many instances where galactic emission nebulae or dust clouds are being heated by known stellar sources often in close association with gas or dust.
	www.rssd.esa.int /SA-general/Projects/GAIA_files/LATEX2HTML/node56.html (492 words)

	ATNF Science Highlights
		Because HVCs are not in simple Galactic rotation and have no associated stars, it has been impossible to determine their distances and, hence, their masses.
		Because of its association with the Magellanic Clouds, the Stream is known to be located at a distance of approximately 55,000 parsecs.
		The non-detection of HVC analogues in these three groups allows constraints to be placed on the masses of the clouds.
	www.atnf.csiro.au /research/highlights/2003/pisano/pisano.html (1218 words)

	RedOrbit NEWS \| Cosmic Dandruff Mystery Solved (Site not responding. Last check: 2007-10-29)
		Small 'high-velocity' clouds of hydrogen gas seen outside our Galaxy are mostly scraps shed by satellite galaxies orbiting the Milky Way, researchers say - a kind of cosmic dandruff.
		The high-velocity clouds were discovered 40 years ago, but it has been very difficult to determine exactly how far away and massive they are.
		Some of the high-velocity clouds are associated with the Magellanic Stream, the researchers found.
	www.redorbit.com /modules/news/tools.php?tool=print&id=1337 (420 words)

	ING Scientific Highlights in 1999
		From recent echelle spectroscopy of 21, apparently normal, high Galactic latitude, early-type stars of solar metallicity drawn from the Palomar-Green survey, astronomers concluded that distances, ages, and velocities are consistent with escape from the Galactic plane.
		High signal-to-noise ratio spectropolarimetry using ISIS spectrograph of 4C 74.26 has revealed that in polarized light the H-alpha emission line is redshifted by about 2000 km/s.
		High signal-to-noise ratio spectra taken with ISIS on the WHT of star cluster F in M82 showed that this cluster is the brightest known.
	www.ing.iac.es /PR/AR1999/high_99.html (8191 words)

	Eric D. Miller - High Velocity Clouds in M 83 and M 51
		The gas clouds superimposed on the spiral arms of the galaxy are extended and likely related to the ongoing star formation in those regions of the galaxy, as stellar explosions drive material out of the disk of the galaxy.
		The small feature identified by the crosshairs is a distinct cloud of neutral hydrogen moving 80 km/s faster than the bulk of the gas at that point in the disk.
		This velocity is similar to what we see for gas clouds near our own Galaxy, and the presence of such features in M 83 and M 51 suggests that the gas clouds in the Milky Way are nearby, probably produced by material ejected from the Galaxy by star formation regions.
	space.mit.edu /~milleric/research/thesis/hvc_press_release.html (445 words)

	Caralyn Flack's REU homepage, Summer 2004
		However, clouds of mostly hydrogen have been detected in the halo of the Milky Way that travel at velocities greater than can be explained by the rotation of the galaxy.
		When a cloud is in front of a background source, the elements in the cloud absorb some of the light passing through it.
		The cloud's velocity is 102km/s and its abundances are 0.72 solar.
	www.astro.wisc.edu /~flack (2761 words)

	School of Physics Colloquium: Monday 4th June 2001 (Site not responding. Last check: 2007-10-29)
		Their velocities do not conform to the rotation of the Galaxy which has encouraged wide ranging speculation as to their nature and origin.
		Early models proposed that the clouds are associated with ancient supernova explosions within a few thousand light years.
		The clouds have even been claimed to be primordial in origin, in orbit about the Galaxy more than a million light years away.
	www.physics.usyd.edu.au /coll/2001bland-hawthorn.html (127 words)

	[116.03] High Velocity Clouds in M 83 and M 51 (Site not responding. Last check: 2007-10-29)
		The varying nature of the detected HI clouds requires the presence of multiple formation mechanisms, with a galactic fountain responsible for the extended anomalous disk and inner-disk clouds, and tidal effects responsible for off-disk clouds.
		The mass and kinetic energy of the clouds are consistent with the expected mass exchange rate under the galactic fountain model.
		If the clouds in M 83 and M 51 are drawn from a similar population as the Galactic HVCs, then the distances to the latter must be less than about 25 kpc.
	www.aas.org /publications/baas/v35n5/aas203/182.htm (378 words)

	'Cosmic dandruff' mystery solved, scientists say (Site not responding. Last check: 2007-10-29)
		The clouds are not associated with the 'missing' clumps of dark matterpredicted by the 'cold dark matter' theory of galaxy formation, the teamconcludes.
		The high-velocity clouds were discovered 40 years ago but it has beenvery difficult to determine exactly how far away and how massive theyare.
		Instead, the clouds are probably shreds of a small satellite galaxy thathas now been swallowed by the Milky Way, she said.
	www.atlasaerospace.net /eng/newsi-r.htm?id=464 (864 words)

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