2007ApJ...668..168G

We present high-resolution (R∼24,600) near-IR spectroscopy of the youngest super star clusters (SSCs) in the prototypical starburst merger, the Antennae galaxies. These SSCs are young (3-7 Myr old) and massive (10⁵-10⁷ M_☉ for a Kroupa initial mass function), and their spectra are characterized by broad, extended Brackett γ (Brγ) emission, so we refer to them as emission-line clusters (ELCs) to distinguish them from older SSCs. The Brγ lines of most ELCs have supersonic widths (60-110 km/s FWHM) and non-Gaussian wings whose velocities exceed the clusters' escape velocities. This high-velocity unbound gas is flowing out in winds that are powered by the clusters' massive O and Wolf-Rayet stars over the course of at least several crossing times. The large sizes of some ELCs relative to those of older SSCs may be due to expansion caused by these outflows; many of the ELCs may not survive as bound stellar systems, but rather dissipate rapidly into the field population. The observed tendency of older ELCs to be more compact than young ones is consistent with the preferential survival of the most concentrated clusters at a given age.