The molecular material seems to be more confined to the inner regions of the galaxy. It ranges from about 1.5 kpc out to at most 12 kpc, with its peak around 6 kpc. This information is illustrated in Figure 20-5 of the text. For observations of atomic gas, astronomers use the 21-cm line, as mentioned in problem 2. CO is observed at radio wavelengths and its distribution is believed to be representative of the H 2 distribution.
Spiral galaxies are ones in which star formation is still occuring.
They tend to be brightest in blue light from the bright, hot young stars that have just formed. In order to have their high level of star formation, they need to have a lot of dust present. Since the dust resides in the disk, this is the reason that stars are formed in the galactic disk. Elliptical galaxies tend to have a reddish color - being relatively devoid of young stars. As such, little or no star formation is occuring in elliptical galaxies, mainly because of a lack of dust in these galaxies.