6.5 Opt-IR Diameters

The isophotal and total diameters are compared in Figure 24. The lower panel shows that the NIR isophotal diameter is typically 60-70% that of the blue isophotal diameter for early types, and can be as small as 20-30% for late-types, dwarfs and irregular galaxies. The NIR "total" diameter (upper panel) evidently captures most of the blue light for early-types, but is only 50% that of the blue light for extreme late-types. This stands as a cautionary note to users of the 2MASS extended source catalog¨galaxies appear much smaller in the NIR bands, but their total flux is still, for the most part, recovered using the radial surface brightness profile.

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