A Principal Components Analysis of the Lick Indices of Galactic Globular Clusters

Abstract

We present a principal components analysis (PCA) of high-quality Lick/IDS absorption-line measurements for 11 indices in the wavelength range 4100-5400 Ang for 39 Galactic globular clusters (GCs). Only the first principal component appears to be physically significant. We find that there is a tight linear relationship between this first component (PC1) and GC metallicity over a wide range in [m/H] (-1.8 <= [m/H] <= 0), suggesting that PC1 may be used to accurately estimate metallicities for old extragalactic GCs from their integrated spectra. The classic GC ``second parameter effect'' is noticeable in the Balmer indices, though it does not appear in our PCA. We find little evidence for substantial differences in broad abundance patterns among Galactic GCs. One implication is that the metal-poor and metal-rich GC subpopulations formed from very similar physical processes.