Galactic disc
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.
Galactic discs have surface brightness profiles that very closely follow exponential functions in both the radial and vertical directions.
The surface brightness radial profile of the galactic disc of a typical disc galaxy (viewed face-on) roughly follows an exponential function:
[2] 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.
[3][4] Some galaxies have been found to have discs with profiles that become truncated in the outermost regions.
, although assumed to be a constant above, can in some cases increase with the radius.
This gas serves as the fuel for the formation of new stars in the disc.
[8] 21 cm emission by HI also reveals that the gaseous component can flare out at the outer regions of the galaxy.
[9] The abundance of molecular hydrogen makes it a great candidate to help trace the dynamics within the disc.
Like the stars within the disc, clumps or clouds of gas follow approximately circular orbits about the galactic center.
The circular velocity of the gas in the disc is strongly correlated with the luminosity of the galaxy (see Tully–Fisher relation).
[10] This relationship becomes stronger when the stellar mass is also taken into consideration.
[12] The young thin disc is a region in which star formation is taking place and contains the MW's youngest stars and most of its gas and dust.
The scale height of this component is roughly 100 pc.
[13] The metal-rich stars in the thin disc have metallicities close to that of the sun (
These distinct ages and metallicities in the different stellar components of the disc point to a strong relationship between the metallicities and ages of stars.
