We present optical/near-IR integral field unit observations of a gas pillar in the Galactic H ii region NGC 6357 containing the young open star cluster Pismis 24. These observations have allowed us to examine in detail the gas conditions of the strong wind–clump interactions taking place on its surface.
By accurately decomposing the Hα line profile, we identify the presence of a narrow (∼20 km s−1) and broad (50–150 km s−1) component, both of which we can associate with the pillar and its surroundings. Furthermore, the broadest broad component widths are found in a region that follows the shape of the eastern pillar edge. These connections have allowed us to firmly associate the broad component with emission from ionized gas within turbulent mixing layers on the pillar's surface set up by the shear flows of the winds from the O stars in the cluster. We discuss the implications of our findings in terms of the broad emission-line component that is increasingly found in extragalactic starburst environments. Although the broad linewidths found here are narrower, we conclude that the mechanisms producing both must be the same. The difference in linewidths may result from the lower total mechanical wind energy produced by the O stars in Pismis 24 compared to that from a typical young massive star cluster found in a starburst galaxy.
The pillar's edge is also clearly defined by dense (≲5000 cm−3), hot (≳20 000 K) and excited (via the [N ii]/Hα and [S ii]/Hα ratios) gas conditions, implying the presence of a D-type ionization front propagating into the pillar surface. Although there must be both photoevaporation outflows produced by the ionization front and mass loss through mechanical ablation, we see no evidence for any significant bulk gas motions on or around the pillar. We postulate that the evaporated/ablated gas must be rapidly heated before being entrained.