Disk galaxys are one of the two main types of galaxy observed in the universe, the other type being elliptical galaxies. We live in a disk galaxy called the Milky Way, it is a spiral disk galaxy.

Disk galaxies, as their name suggests , are disk like. what this means is that they are flat. A disk galaxy can have three or four components these being the disk, the bulge the halo and the dark matter halo.

The typical disk galaxy contains a mass of about 10^12 M sun ( i.e. 10^12 times the mass of the Sun), with variations of an order in magnitude above or below this common. This does not mean that there are 10^12 stars in such galaxies, the galaxy will have many non-stellar components such as molecular clouds, dust, neutral gas and dark matter.

the disk
The typical radius of the disk can be up to 10 Kpc (Kilo-parsecs) while the scale height of the disk is only about 50 pc (parsecs). The stellar population of the disk will tend to be young. This is because there is a lot of neutral and cold gas in the disk from which star formation can occur. The disk can exhibit strong spiral arms in which the disk galaxy is said to be a spiral galaxy. many disk galaxies also have bars in them, these are large horizontal features which frequently will have spiral arms extending from their ends. Sometimes there will be no obvious large features on the disk but there will be a wave like fluctuation. Such galaxies are called flocculent. Galaxies are labeled according to the Hubble classification scheme. Some disk galaxies are not perfectly flat but will have a bend in them, these are referred to as warps.

the halo
There is an older stellar population in a spherical distribution around our galaxy. We call this the halo. The amount of mass in the halo is several orders of magnitude less than in the disk. This makes it practically impossible to observe in galaxies other than our own, but there is no reason to believe that other disk galaxies should not have such a component. it is a prediction of some models of disk formation, but the question of how disks formed is still open.

the bulge
Many (and perhaps all, though this is currently being argued in the community) disk galaxies have a central bulge that looks like a small elliptical galaxy. The stellar population is always older than that of the disk.

dark matter halo
An estimate of the mass of a galaxy is obtained by measuring the rotation speed of parts of the galaxy and applying a simple dynamical argument based on Newton's laws of motion. The circular velocity is a function of the mass enclosed within the radius of revolution. This is one measurement that requires the presence of dark matter. Disk galaxies have a rotation velocity profile which is inconstant with the amount of luminous matter that is seen in the galaxy. The way to get the answer to come out right is to place the disk in a sphere of dark matter whose radius is at least as large as the radius of the disk. This was first suggested for individual galaxies in the 1974 (the year i was born) by Ostriker and Einasto independently. Fritz Zwicky made the suggestion in 1933 based on data from clusters of galaxies but no one took him seriously. Zwicky's was the first ever comment on dark matter.

Examples of disk galaxies

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