Optical and infrared photometry of globular clusters in NGC1399: Evidence for color-metallicity nonlinearity

We combine new Wide Field Camera 3 IR Channel (WFC3/IR) F160W (H ₁₆₀) imaging data for NGC1399, the central galaxy in the Fornax cluster, with archival F475W (g ₄₇₅), F606W (V ₆₀₆), F814W (I ₈₁₄), and F850LP (z ₈₅₀) optical data from the Advanced Camera for Surveys (ACS). The purely optical g ₄₇₅ - I ₈₁₄, V ₆₀₆ - I ₈₁₄, and g ₄₇₅ - z ₈₅₀ colors of NGC1399's rich globular cluster (GC) system exhibit clear bimodality, at least for magnitudes I ₈₁₄ > 21.5. The optical-IR I ₈₁₄ - H ₁₆₀ color distribution appears unimodal, and this impression is confirmed by mixture modeling analysis. The V ₆₀₆ - H ₁₆₀ colors show marginal evidence for bimodality, consistent with bimodality in V ₆₀₆ - I ₈₁₄ and unimodality in I ₈₁₄ - H ₁₆₀. If bimodality is imposed for I ₈₁₄ - H ₁₆₀ with 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 I ₈₁₄ - H ₁₆₀ on g ₄₇₅ - I ₈₁₄ for the matched GC sample is significantly nonlinear, with an inflection point near the trough in the g ₄₇₅ - I ₈₁₄ color distribution; the result is similar for the I ₈₁₄ - H ₁₆₀ dependence on g ₄₇₅ - z ₈₅₀ colors taken from the ACS Fornax Cluster Survey. These g ₄₇₅ - 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.