  Long-slit spectra of several dozen young star clusters have been obtained at
three positions in the Antennae galaxies with the Space Telescope Imaging
Spectrograph (STIS) and its 52"x0.2" slit. Based on H-alpha emission-line
measurements, the average cluster-to-cluster velocity dispersion in 7 different
cluster aggregates ("knots") is <10 \kms. The fact that this upper limit is
similar to the velocity dispersion of gas in the disks of typical spiral
galaxies suggests that the triggering mechanism for the formation of young
massive compact clusters ("super star clusters") is unlikely to be high
velocity cloud-cloud collisions. On the other hand, models where preexisting
giant molecular clouds in the disks of spiral galaxies are triggered into
cluster formation are compatible with the observed low velocity dispersions.
These conclusions are consistent with those reached by Zhang et al. (2001)
based on comparisons between the positions of the clusters and the velocity and
density structure of the nearby interstellar medium. We find evidence for
systematically lower values of the line ratios [NII]/H-alpha and [SII]/H-alpha
in the bright central regions of some of the knots, relative to their outer
regions. This suggests that the harder ionizing photons are used up in the
regions nearest the clusters, and the diffuse ionized gas farther out is
photoionized by 'leakage' of the leftover low-energy photons. The low values of
the [SII]/H-alpha line ratio, typically [SII]/H-alpha<0.4, indicates that the
emission regions are photoionized rather than shock heated. The absence of
evidence for shock-heated gas is an additional indication that high velocity
cloud-cloud collisions are not playing a major role in the formation of the
young clusters.
