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The horizontal branch (HB) morphology of globular clusters (GCs) is moststrongly influenced by metallicity. The second parameter phenomenonacknowledges that metallicity alone is not enough to describe the HB morphologyof all GCs. In particular, the outer Galactic halo contains GCs with redder HBsat a given metallicity than are found inside the Solar circle. Thus, at least asecond parameter is required to characterize HB morphology. Here we analyze themedian color difference between the HB and the red giant branch (RGB), d(V-I),measured from HST ACS photometry of 60 GCs within ~20 kpc of the GalacticCenter. Analysis of this homogeneous data set reveals that, after the influenceof metallicity has been removed, the correlation between d(V-I) and age isstronger than that of any other parameter considered. Expanding the sample toinclude HST photometry of the 6 most distant Galactic GCs lends additionalsupport to the correlation between d(V-I) and age. This result is robust withrespect to the adopted metallicity scale and the method of age determination,but must bear the caveat that high quality, detailed abundance information isnot available for a significant fraction of the sample. When a subset of GCswith similar metallicities and ages are considered, a correlation betweend(V-I) and central luminosity density is exposed. With respect to the existenceof GCs with anomalously red HBs at a given metallicity, we conclude that age isthe second parameter and central density is most likely the third. Importantproblems related to HB morphology in GCs, notably multi-modal distributions andfaint blue tails, remain to be explained. (Abridged)

The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon