Hubble Space Telescope imaging of a peculiar stellar complex in NGC 6946

Abstract

The stellar populations in a stellar complex in NGC 6946 are analyzed on images taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The complex is peculiar by its very high density of stars and clusters and semicircular shape. Its physical dimensions are about the same as for the local Gould Belt, but the stellar density is 1 - 2 orders of magnitude higher. In addition to an extremely luminous, 15 Myr old cluster discussed in an earlier paper, accounting for about 17% of the integrated V-band light, we identify 18 stellar clusters within the complex with luminosities similar to the brightest open clusters in the Milky Way. The color-magnitude diagram of individual stars in the complex shows a paucity of red supergiants compared to model predictions in the 10-20 Myr age range for a uniform star formation rate. We thus find tentative evidence for a gap in the dispersed star formation history, with a concentration of star formation into a young globular cluster during this gap. Confirmation of this result must, however, await a better understanding of the late evolution of stars in the corresponding mass range (> 12 Msun). A reddening map based on individual reddenings for 373 early-type stars is presented, showing significant variations in the absorption across the complex. These may be responsible for some of the arc-like structures previously identified on ground-based images. We finally discuss various formation scenarios for the complex and the star clusters within it.