A principal components analysis of the Lick indices of galactic globular clusters

摘要

We present a principal components analysis (PCA) of high-quality Lick/IDS absorption-line measurements for 11 indices in the wavelength range 4100-5400 Angstrom 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 less than or equal to m/H less than or equal to 0), suggesting that PC1 can be used to accurately estimate metallicities for old extragalactic GCs from their integrated spectra. (Metallicity calibrations for other sets of indices in this wavelength range can be furnished upon request.) The classic GC "second-parameter effect" is noticeable in the Balmer indices, although 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.