VLT/FLAMES-ARGUS observations of stellar wind--ISM cloud interactions in NGC 6357

TL;DR

This study uses optical/near-IR IFU observations to analyze gas interactions on a pillar in NGC 6357, revealing turbulent layers and ionization fronts influenced by stellar winds, with implications for understanding similar processes in starburst galaxies.

Contribution

First detailed analysis linking broad emission line components to turbulent mixing layers caused by stellar winds in a Galactic HII region.

Findings

Broad and narrow H_alpha components identified, with broad components linked to turbulent mixing layers.

The gas conditions indicate a D-type ionization front with no significant bulk motions.

Similar broad emission features are likely produced by the same mechanisms in starburst environments.

Abstract

We present optical/near-IR IFU observations of a gas pillar in the Galactic HII region NGC 6357 containing the young open star cluster Pismis 24. These observations have allowed us to examined in detail the gas conditions of the strong wind-clump interactions taking place on its surface. We identify the presence of a narrow (~20 km/s) and broad (50-150 km/s) component to the H_alpha emission line, where 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 O-star winds from 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 line widths found here are narrower, we conclude that the mechanisms producing both must be the same. The difference in line widths 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 (>20000 K), and excited (via [NII]/H_a and [SII]/H_a 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.