(Submitted September 15, 2006)
What exactly determines the size of a galaxy?
Galaxy formation is a very active area of scientific research today
and the exact means by which they form and hence their size is
determined is not exactly known. The generally accepted view of the
process by which a galaxy forms is that dust and gas come together
within a region, are gravitationally attracted and coalesce into stars
and other celestial bodies inside the galaxy. The amount of material
and the angular momentum that material has (that is to say how much
spinning movement the particles of dust and gas have) are believed to
be the main contributers to the ultimate size of the galaxy.
The story doesn't quite end here however. In recent years there has
been observations made about the mergers of galaxies whereby an even
larger galaxy can be formed as material from one galaxy is added to
another. Whether or not there may be other factors or events in the
formation or evolution of a galaxy that can contribute to its size is
still being studied.
Some links you might find useful that discuss galaxy formation and
Hope this helps
Jason and Koji
for "Ask an Astrophysicist"