Ages of galaxy bulges and disks from optical and near-infrared colors

We compare optical and near-infrared colors of disks and bulges in a diameter-limited sample of inclined, bright, nearby, early-type spirals. Color profiles along wedge apertures at 15 degrees from the major axis and on the minor axis on the side of the galaxy opposite to the dust lane are used to assign nominal colors for the inner disks (at 2 scale length) and for the bulges (similar to 0.5r(eff)), respectively. We estimate that the effects of dust reddening and the cross-talk between the colors of the two components is negligible. We find that color differences (bulge - disk) are very small: Delta(U-R)=0.126+/-0.165, Delta(R-K)=0.078+/-0.165. Disks tend to be bluer by an amount three times smaller than that reported by Bothun & Gregg [ApJ, 350, 73 (1990)] for S0s. Color variations from galaxy to galaxy are much larger than color differences between disk and bulge in each galaxy. Probably, the underlying old population of disks and bulges is much more similar than the population paradigm would lead us to believe. Implied age differences, assuming identical metallicities, are less than 30%. (C) 1996 American Astronomical Society