Young Globular Clusters

Abstract

Star formation in starbursts appears to be biased toward compact clusters, with up to 20% of all stars formed in them. Observations with HST show that many of these clusters have luminosities (-9 > Mv > -16), UBVI colors, and half-light radii (R_eff = ~3 pc) consistent with their being young globular clusters. Although we know little about the long-term stability of the youngest clusters (<20 Myr), compact clusters older than ~10 core-crossing times (20 - 40 Myr) are bound gravitationally and very likely young globulars. The present review concentrates on recent progress achieved in dating such clusters and on evidence that mergers of spiral galaxies can produce relatively rich subsystems of young globulars of solar metallicity in the remnants' halos. Studies of such subsystems suggest a scenario in which second-generation globulars form from giant molecular clouds squeezed into collapse by the high-pressure environment of starbursts. These bursts, often driven by mergers, may explain ongoing cluster formation in NGC 4038/39, the young halo globular clusters found around protoellipticals like NGC 3921 and NGC 7252, and the subpopulations of red metal-rich globulars observed in many giant elliptical galaxies.