The Metal-Poor Inner Spheroid Globular Cluster NGC 6558

Abstract

We discuss near-infrared photometric and spectroscopic observations of stars in and around the globular cluster NGC 6558, obtained with the FLAMINGOS-I imaging spectrograph on the Gemini South telescope. Efforts to estimate the metallicity of NGC 6558 from the slope of the red giant branch (RGB) are biased by contamination from bulge stars. The near-infrared spectroscopic properties of the two brightest stars projected against the main body of the cluster are consistent with them belonging to the bulge. After removing probable bulge members, we find that (1) the brightest metal-poor giant near the center of NGC 6558 has MK = -4.6, which is 1–1.5 mag fainter than the expected RGB tip brightness, and (2) [Fe/H] = -1.5 ± 0.5 based on the RGB slope measured from stars that are likely cluster members. This metallicity estimate is consistent with the nondetection of the first-overtone CO bands in the near-infrared spectra of giants midway down the RGB of NGC 6558. The K-band luminosity function (LF) of NGC 6558, corrected for contamination from bulge stars, parallels that of NGC 6121 on the lower RGB and subgiant branch (SGB), indicating that any depletion of giants in NGC 6558 is restricted to the upper portion of the RGB. When scaled according to the number of stars midway up the giant branch, the K LFs of NGC 6121 and NGC 6558 agree to within ±0.1 dex on the SGB, indicating that their ages do not differ by more than ±2 Gyr.