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Concept
Galactic disc
Summary
A galactic disc (or galactic disk) is a component of disc galaxies, such as spiral galaxies, lenticular galaxies, and the Milky Way. Galactic discs consist of a stellar component (composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust). The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center. Discs can be fairly thin because the disc material's motion lies predominantly on the plane of the disc (very little vertical motion). The Milky Way's disc, for example, is approximately 1 kly thick, but thickness can vary for discs in other galaxies.
Galactic discs have surface brightness profiles that very closely follow exponential functions in both the radial and vertical directions.
The surface brightness of the galactic disc of a typical disc galaxy (viewed face-on) roughly follows an exponential function:
where is the galaxy's central brightness and is the scale length. The scale length is the radius at which the galaxy is a factor of e (~2.7) less bright than it is at its center. Due to the diversity in the shapes and sizes of galaxies, not all galactic discs follow this simple exponential form in their brightness profiles. Some galaxies have been found to have discs with profiles that become truncated in the outermost regions.
When viewed edge-on, the vertical surface brightness profiles of galactic discs follow a very similar exponential profile that is proportional to the disc's radial profile:
where is the scale height. Although exponential profiles serve as a useful first approximations, vertical surface brightness profiles can also be more complicated. For example, the scale height , although assumed to be a constant above, can in some cases increase with the radius.
Most of a disc galaxy's gas lies within the disc. Both cool atomic hydrogen (HI) and warm molecular hydrogen (HII) make up most of the disc's gaseous component.
Official source
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Ontological neighbourhood