6.4 Optical-Infrared Color

The mean B-K color is shown in Figure 23, where the "total" measurement is used for each band (note that we are not matching apertures since the B-band and NIR-band "total" apertures can be quite different, see below). Here again we see clear statistical trends with morphology and type: early-type galaxies have a mean B-K color hovering near 4.0 mag, while late-type disk galaxies exhibit bluer colors (with much larger scatter), ranging from 2 to 4 mag (probably related to appreciable and variable extinction/reddening in late-types). The late-type irregular galaxies have the bluest colors, ranging from ~1 to 2 mag, and the largest color scatter. It is clear that the NIR is mostly insensitive to extreme late-type, irregular and dwarf galaxies. Note that barred spirals show slightly different trends than the normal spirals for the late-type spirals, particularly the Scd vs. SBcd groups. This difference is probably related to reddening due to extinction in the bars and to the difference in stellar populations between bars and spiral arms.

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