Until now we have discussed mean colors and SBF magnitudes, but these quantities vary within galaxies, most notably because of radial gradients in stellar population properties. SBF gradients were known from ground-based data for a few galaxies (Tonry 1991; Sodemann & Thomsen 1995, 1996; Tonry et al. 2001), but until recently there were no systematic studies. NGC 205 was the first galaxy where an SBF gradient was noted. It is unusual in that its center is blue and it becomes redder outward, the reverse of typical gradients. This is because the gradient is due to relatively recent central star formation, and therefore an age effect. The slope of the dependence of I on (V - I) is also shallower than that of the mean dependence for galaxies.
Cantiello et al. (2005, 2007a) have used the ACS/WFC to make the first studies of SBF gradients in significant samples of galaxies (see Figure 4). They show that the size of the internal SBF vs color slope for multiple annuli within a galaxy can reveal whether the gradients are due to age or metallicity. In most cases, they are metallicity gradients, but in some cases an age gradient appears equally or more important. The galaxies with age gradients tend to be bluer and show some evidence for past mergers (e.g., NGC 1344). At these optical wavelengths, age and metallicity variations cause qualitatively similar SBF vs color relations, though with different slopes. It would be extremely interesting to study internal SBF gradients in the near-IR where pure age or metallicity variations would translate to very different dependences of SBF magnitude on color. With improvements in near-IR instruments, this is now becoming possible.
Figure 4. (B - I) colors and I-band SBF measurements in a series of concentric annuli are shown for seven early-type galaxies studied by Cantiello et al. (2005) with the ACS/WFC. In all of these galaxies, the color gets bluer and I brighter with increasing radial distance from the galaxy center. The vertical shifts are due to the different galaxy distances (see Cantiello et al. 2005, 2007a for details).