s are one of the
two main types of galaxy observed in
, the other type being
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.
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.
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.
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.
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