2012ApJ...746...88B

We combine new Wide Field Camera 3 IR Channel (WFC3/IR) F160W (H₁₆₀) imaging data for NGC 1399, the central galaxy in the Fornax cluster, with archival F475W (g₄₇₅), F606W (V₆₀₆), F814W (I₈₁₄), and F850LP (z ₈₅₀) optical data from the Advanced Camera for Surveys (ACS). The purely optical g₄₇₅- I₈₁₄, V₆₀₆- I₈₁₄, and g₄₇₅- z₈₅₀colors of NGC 1399's rich globular cluster (GC) system exhibit clear bimodality, at least for magnitudes I₈₁₄> 21.5. The optical-IR I₈₁₄- H₁₆₀ color distribution appears unimodal, and this impression is confirmed by mixture modeling analysis. The V₆₀₆- H₁₆₀ colors show marginal evidence for bimodality, consistent with bimodality in V₆₀₆- I₈₁₄ and unimodality in I₈₁₄- H₁₆₀. If bimodality is imposed for I₈₁₄- H₁₆₀with a double Gaussian model, the preferred blue/red split differs from that for optical colors; these "differing bimodalities" mean that the optical and optical-IR colors cannot both be linearly proportional to metallicity. Consistent with the differing color distributions, the dependence of I₈₁₄- H₁₆₀ on g₄₇₅- I₈₁₄ for the matched GC sample is significantly nonlinear, with an inflection point near the trough in the g₄₇₅- I₈₁₄ color distribution; the result is similar for the I₈₁₄- H₁₆₀ dependence on g₄₇₅- z ₈₅₀ colors taken from the ACS Fornax Cluster Survey. These g₄₇₅- z ₈₅₀ colors have been calibrated empirically against metallicity; applying this calibration yields a continuous, skewed, but single-peaked metallicity distribution. Taken together, these results indicate that nonlinear color-metallicity relations play an important role in shaping the observed bimodal distributions of optical colors in extragalactic GC systems.