Ages and Metallicities of Young Globular Clusters in the Merger Remnant NGC 7252

Abstract

Ultraviolet-to-visual spectra of eight young star clusters in the merger remnant and protoelliptical galaxy NGC 7252, obtained with the 4 m Blanco Telescope on Cerro Tololo, are presented. These clusters lie at projected distances of 3–15 kpc from the center and move with a velocity dispersion of 140 ± 35 km s-1 in the line of sight. Seven of the clusters show strong Balmer absorption lines in their spectra [EW(Hβ) = 6–13 A], while the eighth lies in a giant H II region and shows no detectable absorption features. Based on comparisons with model cluster spectra computed by Bruzual & Charlot and Bressan, Chiosi, & Tantalo, six of the absorption-line clusters have ages in the narrow range of 400–600 Myr, indicating that they formed early in the recent merger. These clusters, and probably also the seventh absorption-line cluster, are globular clusters, as judged by their small effective radii and ages corresponding to ~102 core crossing times. The one emission-line object is 10 Myr old and may be a nascent globular cluster or an OB association. The mean metallicities measured for three clusters are solar to within about ±0.15 dex, suggesting that the merger of two likely Sc galaxies in NGC 7252 formed a globular cluster system with a bimodal metallicity distribution. Since NGC 7252 itself shows the characteristics of a 0.5–1 Gyr old protoelliptical galaxy, its second-generation solar-metallicity globular clusters provide direct evidence that giant elliptical galaxies with bimodal globular cluster systems can form through major mergers of gas-rich disk galaxies. A puzzling property of the observed young globular clusters are the high masses of (1–35)M(ω Cen) implied by their luminosities and ages (for an assumed Salpeter IMF). A spectrum of a candidate superluminous globular cluster in the elliptical galaxy NGC 1700, obtained with the Hiltner Telescope at MDM Observatory, shows this object to be a foreground star.