Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921

Low-resolution, ultraviolet-to-visual spectra of two candidate globular clusters in the merger remnant NGC 3921 (cz_LG = 6021 km s^-1) are presented. These two clusters of apparent magnitude V ≈ 22.2 (M_V ≈ -12.5) lie at projected distances of about 5 kpc (0.9 R_eff) from the center and move with halo-type radial velocities relative to the local galaxy background. Their spectra show strong Balmer absorption lines [EW(Hβ–Hδ) = 11–13 Å] indicative of main-sequence turnoffs dominated by A-type stars. Comparisons with model-cluster spectra computed by Bruzual & Charlot and others yield cluster ages in the range of 200–530 Myr, and metallicities about solar to within a factor of three. Given their small half-light radii (R_eff ≲ 5 pc) and ages corresponding to ∼10² core-crossing times, these clusters are gravitationally bound and hence indeed young globulars. Assuming that they had Chabrier-type initial mass functions, their estimated current masses are (2.3 ± 0.1) × 10⁶ _⊙ and (1.5 ± 0.1) × 10⁶ _⊙, respectively, or roughly half the mass of ω Cen. Since NGC 3921 itself shows many signs of being a 0.7 ± 0.3 Gyr old protoelliptical, these two young globulars of roughly solar metallicity and their many counterparts observed with the Hubble Space Telescope provide supporting evidence that, in the process of forming elliptical-like remnants, major mergers of gas-rich disks can also increase the number of metal-rich globular clusters.