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. From the Hubble Law, we know that galaxies that are located farther away from us will have a higher redshift. We expect most galaxies to have a spectrum that will resemble that of a K-giant star. Due to the redshift, a large portion of the light can be moved to lower wavelengths. A filter that is designed to observe K stars will thus detect lower emission.