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High-velocity clouds (HVCs) are large collections of gas found throughout the galactic halo of the Milky Way. Their bulk motions in the local standard of rest have velocities which are measured in excess of 70–90 km s−1. These clouds of gas can be massive in size, some on the order of millions of times the mass of the Sun (), and cover large portions of the sky. They have been observed in the Milky Way's halo and within other nearby galaxies. HVCs are important to the understanding of Galactic evolution because they account for a large amount of baryonic matter in the Galactic halo. In addition, as these clouds fall into the disk of the Galaxy, they add material that can form stars in addition to the dilute star forming material already present in the disk. This new material aids in maintaining the star formation rate (SFR) of the Galaxy. The origins of the HVCs are still in question. No one theory explains all of the HVCs in the Galaxy. However, it is known that some HVCs are probably spawned by interactions between the Milky Way and satellite galaxies, such as the Large and Small Magellanic Clouds (LMC and SMC, respectively) which produce a well-known HVC called the Magellanic Stream. Because of the various possible mechanisms that could potentially produce an HVC, there are still many questions surrounding HVCs for researchers to study. ==Observational history== In the mid-1950s, dense pockets of gas were first discovered outside of the galactic plane. This was quite notable because the models of the Milky Way showed the density of gas decreasing with distance from the galactic plane, rendering this a striking exception. According to the prevailing galactic models, the dense pockets should have dissipated long ago, making their very existence in the halo quite puzzling. In 1956 the solution was proposed that the dense pockets were stabilized by a hot, gaseous corona that surrounds the Milky Way. Inspired by this proposal, Jan Oort, of Leiden University, Netherlands, proposed that cold gas clouds might be found in the galactic halo, far away from the galactic plane. They were soon located, in 1963, via their neutral hydrogen radio emission. They were traveling toward the galactic disk at a very high velocity relative to other entities in the galactic disk. The first two clouds that were located were named Complex A and Complex C. Due to their anomalous velocities, these objects were dubbed "high-velocity clouds", distinguishing them from both gas at normal local standard of rest velocities as well as their slower-moving counterparts known as intermediate-velocity clouds (IVCs). Several astronomers proposed hypotheses (which later proved to be inaccurate) regarding the nature of HVCs, but their models were further complicated in the early 1970s by the discovery of the Magellanic Stream, which behaves like a string of HVCs. In 1988, a northern-sky survey of neutral hydrogen radio emissions was completed using the Dwingeloo radio telescope in the Netherlands. From this survey, astronomers were able to detect more HVCs. In 1997, a map of the Milky Way's neutral hydrogen was largely complete, again allowing astronomers to detect more HVCs. In the late 1990s, using data from the La Palma Observatory in the Canary Islands, the Hubble Space Telescope, and, later, the Far Ultraviolet Spectroscopic Explorer (FUSE), the distance to an HVC was gauged for the first time. Around the same time, the chemical composition of HVCs was first measured. Additionally, in 2000, a southern hemisphere survey of neutral hydrogen radio emissions was completed using the Villa Elisa radio telescope in Argentina from which yet more HVCs were discovered.〔 Later observations of Complex C showed that the cloud, originally thought to be deficient in heavy elements (also known as low metallicity), contains some sections with a higher metallicity compared to the bulk of the cloud, indicating that it has begun to mix with other gas in the halo. Using observations of highly ionized oxygen and other ions astronomers were able to show that hot gas in Complex C is an interface between hot and cold gas.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「High-velocity cloud」の詳細全文を読む スポンサード リンク
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