The NICMOS View of M31's Metal Rich Globular Clusters

Abstract

We have obtained HST-NICMOS observations of five of M31's most metal-rich globular clusters: G1, G170, G174, G177 & G280. For the two clusters farthest from the nucleus, G1 and G280, we statistically subtract the field population and estimate metallicities using K-(J - K) color-magnitude diagrams (CMDs). Based on the slopes of their infrared giant branches we estimate [Fe/H] = −1.22 ± 0.43 dex for G1 and −0.15 ± 0.37 dex for G280. We combine our infrared observations of G1 with two epochs of optical HST-WFPC2 F-band data and identify at least one LPV based on color and variability. The location of G1's giant branch in the K, (V - K) CMD is very similar to that of M107, indicating a somewhat higher metallicity than our purely infrared CMD; [Fe/H]= −0.9 ± 0.2 dex.