Extremely alpha-Enriched Globular Clusters in Early-Type Galaxies: A Step towards the Dawn of Stellar Populations?

Abstract

We compare [alpha/Fe], metallicity, and age distributions of globular clusters in elliptical, lenticular, and spiral galaxies, which we derive from Lick line index measurements. We find a large number of globular clusters in elliptical galaxies that reach significantly higher [alpha/Fe] values (>0.5) than any clusters in lenticular and spiral galaxies. Most of these extremely alpha-enriched globular clusters are old (t > 8 Gyr) and cover the metallicity range -1 <~ [Z/H] <~ 0. A comparison with supernova yield models suggests that the progenitor gas clouds of these globular clusters must have been predominantly enriched by massive stars (>~20 M_sol) with little contribution from lower-mass stars. The measured [alpha/Fe] ratios are also consistent with yields of very massive pair-instability supernovae (~130-190 M_sol). Both scenarios imply that the chemical enrichment of the progenitor gas was completed on extremely short timescales of the order of a few Myr. Given the lower [alpha/Fe] average ratios of the diffuse stellar population in early-type galaxies, our results suggest that these extremely alpha-enhanced globular clusters could be members of the very first generation of star clusters formed, and that their formation epochs would predate the formation of the majority of stars in giant early-type galaxies.