Space Telescope Imaging Spectrograph Observations of Young Star Clusters in the Antennae Galaxies (NGC 4038/4039)

Long-slit spectra of several dozen young star clusters have been obtained atthree positions in the Antennae galaxies with the Space Telescope ImagingSpectrograph (STIS) and its 52"x0.2" slit. Based on H-alpha emission-linemeasurements, the average cluster-to-cluster velocity dispersion in 7 differentcluster aggregates ("knots") is <10 \kms. The fact that this upper limit issimilar to the velocity dispersion of gas in the disks of typical spiralgalaxies suggests that the triggering mechanism for the formation of youngmassive compact clusters ("super star clusters") is unlikely to be highvelocity cloud-cloud collisions. On the other hand, models where preexistinggiant molecular clouds in the disks of spiral galaxies are triggered intocluster 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 anddensity structure of the nearby interstellar medium. We find evidence forsystematically lower values of the line ratios [NII]/H-alpha and [SII]/H-alphain the bright central regions of some of the knots, relative to their outerregions. This suggests that the harder ionizing photons are used up in theregions nearest the clusters, and the diffuse ionized gas farther out isphotoionized by 'leakage' of the leftover low-energy photons. The low values ofthe [SII]/H-alpha line ratio, typically [SII]/H-alpha<0.4, indicates that theemission regions are photoionized rather than shock heated. The absence ofevidence for shock-heated gas is an additional indication that high velocitycloud-cloud collisions are not playing a major role in the formation of theyoung clusters.Comment: 13 pages, 6 figures, 2 tables; accepted for November A