We saw in Chapter 
that the scale length ratios in the
bands clearly indicate that a larger proportion of radio
galaxies than the control sample galaxies tend to have bluer
colors near the center.
The color gradients are also indicative
of this behavior. We further showed using extrapolated profiles
near the centers of the radio and control galaxies that the radio
galaxies tend to have an excess emission near the center more
frequently than the control galaxies.
We noted that the excess is over a few kpc, and is likely to
be due to recent star formation rather than an AGN.
In this chapter we attempt to understand the possible origin of this effect. We begin by describing a number of similar cases observed in different types of galaxies and models which are used to estimate the strength, i.e., the gas mass undergoing the burst, and the age of the burst at the time of the observation. We will see what factors are involved in the estimation and how accurately one can expect to obtain these two important parameters. We especially note how the age dating technique can be used to constrain the age and strength with limited data.
Just as the interpretation of the color gradients is made difficult by the age-metallicity degeneracy, so also the interpretation related to the excess blue color is made difficult by the age-strength degeneracy: an old but strong burst can produce features similar to what a young, weak burst can. We describe techniques that are being developed by different groups to break this degeneracy using line ratios in different parts of the spectrum.
Some recent observations have indicated that Wolf-Rayet stars can play a very important role in the kind of study we are presenting here. The signature of these stars is visible only for a few million years and hence they are indicative of recent activity. We discuss the possibility of Wolf-Rayet stars giving rise to the blue color we see in the radio galaxies.