We present wide field JHK _ { S } photometry of 16 Galactic globular clusters located towards the Galactic bulge , calibrated on the 2MASS photometric system .
Differential reddening corrections and statistical field star decontamination are employed for all of these clusters before fitting fiducial sequences to the cluster red giant branches ( RGBs ) .
Observed values and uncertainties are reported for several photometric features , including the magnitude of the RGB bump , tip , the horizontal branch ( HB ) and the slope of the upper RGB .
The latest spectroscopically determined chemical abundances are used to build distance- and reddening-independent relations between observed photometric features and cluster metallicity , optimizing the sample size and metallicity baseline of these relations by supplementing our sample with results from the literature .
We find that the magnitude difference between the HB and the RGB bump can be used to predict metallicities , in terms of both iron abundance \mathit { [ Fe / H ] } and global metallicity \mathit { [ M / H ] } , with a precision of better than 0.1 dex in all three near-IR bandpasses for relatively metal-rich ( [ M / H ] \gtrsim -1 ) clusters .
Meanwhile , both the slope of the upper RGB and the magnitude difference between the RGB tip and bump are useful metallicity indicators over the entire sampled metallicity range ( -2 \lesssim [ M / H ] \lesssim 0 ) with a precision of 0.2 dex or better , despite model predictions that the RGB slope may become unreliable at high ( near-solar ) metallicities .
Our results agree with previous calibrations in light of the relevant uncertainties , and we discuss implications for clusters with controversial metallicities as well as directions for further investigation .