Young stars in the outer HI disc of NGC 6822

Abstract

We present wide-field optical imaging covering the entire neutral hydrogen disc of the Local Group dwarf galaxy NGC 6822. These observations reveal the presence of numerous blue, young stars at large galactocentric radii well beyond R25. Blue stars are also found that are associated with NGC 6822's companion H I cloud, indicating that star formation was triggered in the companion in the last 108 yr. In general, blue stars are present where the H I surface densities reach values ≳ 5 × 1020 cm-2. However, over one-third of the blue stars detected within the H I disc are found at lower surface densities. The young stars trace the distribution of the neutral hydrogen in the inner disc, but seem to be avoiding the supergiant H I shell in NGC 6822, setting a lower limit for its age of 108 yr. The extended distribution of young stars implies that stars can form at large galactocentric radii in dwarf galaxies; the H I is therefore not necessarily much more extended than the stellar population. This finding has important consequences for the chemical enrichment of the interstellar medium throughout (dwarf) galaxies.