6.6 Opt-IR Surface Brightness
The half-light "effective" size comparison is shown in Figure 25. Now there is little difference across T-types, with the intra-type scatter completely dominating the distribution, holding steady at roughly 80% for the ratio between the NIR to blue half-light diameters. It is interesting to note that although the NIR is better suited to tracing the older stellar population (and accordingly, the bulge population), the "effective" size is still somewhat smaller than that measured at optical wavelengths.
The half-light surface brightness comparison reveals population differences (Figure 26). The "effective" B-K color is now much redder than that seen in the mean colors (compare with Figure 23), ranging from 5 to 6 mag for early-types and 3 to 5 for late-types. The effect of nuclear and large-scale bars are now bettter seen (lower panel)ùbarred galaxies have bluer colors for early-types (S0, Sa, Sab, Sb)ùhinting at a younger (more massive, hotter) population of stars within or associated with the barred structures. This effect is not seen in the later-types, suggesting that the contrast in stellar populations between bar and spiral arm is much less for late-types--late-type galaxies are dominated by young massive stars in both the arm and bar structures. We should note that these results are complicated by the fact that the optical and NIR stellar distributions, as scaled by the half-light, are somewhat different (see Figure 25).