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

Abstract

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 and its 52'' × 02 slit. Based on Hα emission-line measurements, the average cluster-to-cluster velocity dispersion in seven different cluster aggregates (knots) is <10 km s-1. 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 in which 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 and coworkers 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 [N II]/Hα and [S II]/Hα 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 [S II]/Hα line ratio, typically [S II]/Hα < 0.4, indicate 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 young clusters.