OPTICAL AND INFRARED PHOTOMETRY OF GLOBULAR CLUSTERS IN NGC?1399: EVIDENCE FOR COLOR-METALLICITY NONLINEARITY*

摘要

We combine new Wide Field Camera 3 IR Channel (WFC3/IR) F160W (H 160) imaging data for NGC?1399, 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 NGC?1399'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.