Optical and IR Photometry of Globular Clusters in NGC1399: Evidence for Color-Metallicity Nonlinearity

TL;DR

This study combines optical and infrared photometry of NGC1399's globular clusters to demonstrate that nonlinear color-metallicity relations explain the observed bimodal optical color distributions, challenging the assumption of simple metallicity bimodality.

Contribution

It provides evidence that nonlinear color-metallicity relations are crucial for understanding globular cluster color distributions, using combined optical and IR data for NGC1399.

Findings

Optical colors show bimodality, IR colors appear unimodal.

Nonlinear color-metallicity relations explain the color distribution shapes.

Optical-IR color relations have inflection points affecting metallicity interpretation.

Abstract

We combine new Wide Field Camera~3 IR Channel (WFC3/IR) F160W (H) imaging data for NGC1399, the central galaxy in the Fornax cluster, with archival F475W (g), F606W (V), F814W (I), and F850LP (z) optical data from the Advanced Camera for Surveys (ACS). The purely optical g-I, V-I, and g-z colors of NGC1399's rich globular cluster (GC) system exhibit clear bimodality, at least for magnitudes $I_814 > 21.5$. The optical-IR I-H color distribution appears unimodal, and this impression is confirmed by mixture modeling analysis. The V-H colors show marginal evidence for bimodality, consistent with bimodality in V-I and unimodality in I-H. If bimodality is imposed for I-H 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-H on g-I for the matched GC sample is significantly nonlinear, with an inflection point near the trough in the g-I color distribution; the result is similar for the I-H dependence on g-z colors taken from the ACS Fornax Cluster Survey. These g-z 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.