Thick Disk

Features believed to exist in some, but not all disk galaxies; it clear whether the thick disk of the Milky Way Galaxy is typical of thick disks in other spiral galaxies. The thick disk contains about 10% of the stellar mass of the Milky Way and has a scale height of ~1,000 light years (the thin disk has a scale height of only ~400 light years). The greater scale height of the thick disk results from it being ‘kinematically hotter’ than the thin disk; thick disk stars are, on average, moving ~20 km/sec faster in a vertical direction with respect to the galactic plane than thin disk stars. In contrast, thick disk stars rotate about the Galactic center more slowly than the thin disk stars by ~40 km/sec.

The stars in thick disks are almost all older than 10 Ga. Metallicities of thick disk stars range from ~1/10–1/2 that of the Sun, with a typical value of ~1/4 solar (thin disk values range from 1/3–3 times solar. Both the differing age and chemical properties of the stellar populations suggests that the formation scenarios for the thin and thick disks were different. The thick disk probably formed early in the Galaxy’s history and then ceased all star formation. The thin disk formed later and continues to form stars to this day.

The thick disk may have formed as the result of a significant merger event early in the Milky Way’s history. If this is the case, the event must have taken place before the thin disk had fully formed, as both observations and N-body modeling indicate such events disrupt the disk and consume a significant fraction of the cold gas in a burst of new star formation. The presence of a substantial, uniform disk in modern times therefore indicates that any major merger must have taken place long ago. As an alternative, the thick disk formed relatively slowly by the actions of multiple minor mergers. The gas would collapse back into the galactic plane forming a thin disk, but the stars would retain the larger scale height of a thick disk. The Milky Way is currently undergoing at least two minor mergers, with the Sagittarius and Canis Major dwarf galaxies, suggesting that this could be a real possibility. However, the lack of young stars in the thick disk would again suggest that such processes must have stopped, or at least significantly slowed, a long time ago.

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