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F. Schweizer (Carnegie Institution -- DTM)
Major mergers between gas-rich galaxies of comparable mass trigger some of the strongest starbursts known, leading to the formation of thousands of new star clusters. As the case of The Antennae (NGC 4038/4039) illustrates, many of these clusters have luminosities (MV~= -9 to -16), integrated UBVI colors, and half-light radii (median R\rm eff \la 4~pc) consistent with their being very young globulars (\la30~Myr). Some of the clusters feature extensive power-law envelopes (up to 450~pc in radius) that may later be tidally stripped. The main problem with clusters in ongoing mergers is how to estimate the fraction of them that will remain gravitationally bound and will survive the merger as globulars.
The situation is clearer in recent, 0.5--1~Gyr old merger remnants that resemble protoellipticals (e.g., NGC 3921 and 7252). There, up to three populations of clusters can be distinguished: (1) The old metal-poor globulars that predate the merger and belonged to the former spirals, (2) a residue of young clusters that are still forming from late infalling gas, and (3) a prominent halo population of young clusters that seem to have formed mostly early during the merger. Numbering in the few hundreds, the latter halo clusters have luminosities, colors, and radii strongly suggesting that they are ~300--700~Myr old globulars. Their ages, determined from V-I colors and Balmer absorption-line strengths, correspond to ~100--200 core-crossing times and show that the clusters are gravitationally bound. The metallicities of these second-generation halo globulars appear to be near solar.
The combined evidence suggests that after a few Gyr such merger remnants will appear as field ellipticals with bimodal globular-cluster populations, and the second-generation clusters will form the red peaks of these populations. NGC 3610 and 5128 are likely examples of such field ellipticals with old and intermediate-age globulars. Whereas major mergers explain the presence of bimodal cluster populations in early-type galaxies quite naturally, they do not explain the very large numbers of metal-poor globulars observed especially in central cluster ellipticals.
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