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The optical colors of globular clusters (GCs) in most large early-typegalaxies are bimodal. Blue and red GCs show a sharp difference in the radialprofile of their surface number density in the sense that red GCs are morecentrally concentrated than blue GCs. An instant interpretation is that thereexist two distinct GC subsystems having different radial distributions. Thisview, however, was challenged by a scenario in which, due to the nonlinearnature of the GC metallicity-to-color transformation for old ($\gtrsim$10 Gyr)GCs, a broad unimodal metallicity spread can exhibit a bimodal colordistribution. Here we show, by simulating the radial trends in the GC colordistributions of the four nearby giant elliptical galaxies (M87, M49, M60, andNGC 1399), that the difference in the radial profile between blue and red GCsstems naturally from the metallicity-to-color nonlinearity plus the well-knownradial metallicity gradient of GC systems. The model suggests no or littleradial variation in GC age even out to $\sim$20${R}_{\rm eff}$. Our resultsprovide a simpler solution to the distinct radial profiles of blue and red GCsthat does not necessarily invoke the presence of two GC subsystems and furtherfortify the nonlinearity scenario for the GC color bimodality phenomenon.

Nonlinear Color-Metallicity Relations of Globular Clusters. IX. Different Radial Number Density Profiles between Blue and Red Clusters